Development of a Process to Obtain a Graded Distribution of Particles Along the Overlap of Adhesive Single Lap Joints

ENGINEERING FACULTY OF UNIVERSITY OF PORTO

DEPARTMENT OF MECHANICAL ENGINEERING

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Supervisor: Lucas da Silva Author: Catarina Isabel Seixas da Silva Co-Supervisors: Ana Queirós José Marques

A thesis submitted for the degree of

MSc of Mechanical Engineering

june, 2019

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Departamento de Engenharia Mecânica

Faculdade de Engenharia da Universidade do Porto

Rua Dr. Roberto Frias

4200-465 Porto

Portugal

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Abstract

The constant need for improvement and innovation at all industries leads to the development of new technologies. Adhesive bonding is a joining technique in constant growth as, replacing conventional methods of fastening such as bolts and rivets, as it offers potential weight and cost savings.

The adhesive joint most studied and used in industrial applications is the single lap joint (SLJ), mainly due to its simplicity and efficiency. In typical SLJs, however, one of the problems associated with them is the nonuniform stress distribution (shear and peel), being concentrated at the ends of the overlap. This leads to early joint failure, especially if the adhesive is brittle. One of the main areas of investigation in the field of adhesive bonding is the development of methods for reducing such stress concentrations. In fact, functionally graded adhesive joints have been gaining attention as a technique to even those stress distributions along the joint, due to their potential high degree of customization, offering more solutions for application design. Essentially, this method consists of a modified adhesive with properties that vary gradually along the bondline, allowing an enhanced stress distribution along the overlap region.

Therefore, the main goal of this work is to propose a new method to decrease stress concentrations by applying the concept of functionally graded adhesive joints using magnetised cork microparticles. Thereupon, a customized apparatus was designed. With the appropriate application of tailored magnetic fields, using a set of magnets arrays, the magnetised microparticles were strategically placed along the bondline, then creating a particle concentration gradient from the ends of the overlap (higher) to the middle (lower).

Numerical models were developed to predict the particle distribution along the joint caused by the application of magnetic fields. Those models were based on the phenomenon of magnetophoresis of cork-magnetite microparticles. A characterisation of the magnetic behaviour of cork and magnetised cork microparticles was also performed. Early experimental tests using glass slides with the particles uniformly distributed within the adhesive selected were performed to validate the simulations.

Mechanical tests were executed to determine the properties of the following composite arrangements: neat resin and resin with magnetised cork microparticles (uniform and graded distributions), considering various amounts. Bulk specimens were used to perform tensile tests and glass transition temperature measurements. SLJs were also done to evaluate the influence of the reinforcement inclusions (magnetised cork microparticles). Simultaneously to mechanical testing, SEM analysis was made in regard to the particles themselves and the bulk specimens fracture surfaces.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Overall, it is concluded that the magnetised cork particles gradely distributed in the overlap region of the SLJ serve as reinforcement inclusions. This way, the newly developed method was validated, along with the apparatus conceived to produce such joints.

Keywords: Epoxy adhesives, functionally graded adhesive joints, magnetised particles, cork microparticles, magnetophoresis, numerical analysis, stress distribution, mechanical properties.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Resumo

A necessidade constante de melhoria e inovação em todas as indústrias conduz ao desenvolvimento de novas tecnologias. O uso de ligações adesivas está a ganhar destaque como método de ligação estrutural, substituindo métodos convencionais de fixação, tais como a utilização de parafusos e rebites, uma vez que este método potencia reduções de peso estrutural, assim como de custo.

A junta adesiva mais estudada e usada em aplicações industriais é a junta de sobreposição simples (JSS), devido principalmente à sua simplicidade e eficiência. Contudo, na maioria das JSSs, um dos problemas associados é a distribuição não uniforme de tensões (corte e arrancamento), estando estas concentradas nas extremidades da sobreposição. Por sua vez, tal desencadeia a rotura prematura das juntas, especialmente se o adesivo for frágil. Neste sentido, umas das áreas principais de investigação relativamente às juntas adesivas é o desenvolvimento de métodos para reduzir essas concentrações de tensões. Na realidade, as juntas adesivas funcionalmente graduadas têm vindo a ganhar mais relevância como uma técnica para igualar a distribuição de tensões, podendo ser facilmente modificadas para se melhor adaptarem ao carregamento e oferecendo uma vasta gama de soluções a nível de design. Essencialmente, este método consiste em ter- se um adesivo modificado com propriedades que variam gradualmente ao longo do comprimento de sobreposição, permitindo assim a existência de uma melhor distribuição uniforme de tensões nessa região.

Deste modo, o objetivo principal deste trabalho é propor um novo método para diminuir as concentrações de tensões, através da aplicação do conceito de juntas adesivas funcionalmente graduadas, usando micropartículas de cortiça magnetizadas. Portanto, foi projetado um mecanismo para este efeito. Através da aplicação apropriada de campos magnéticos cuidadosamente desenhados, obtidos usando um conjunto de magnetes, as partículas magnetizadas foram estrategicamente colocadas ao longo da sobreposição de uma junta adesiva, criando assim um gradiente de concentração de partículas desde as extremidades (elevado) até ao centro (baixo).

Modelos numéricos foram desenvolvidos de modo a prever a distribuição das partículas ao longo da junta, causada pela aplicação de campos magnéticos. Esses modelos basearam-se no fenómeno da magnetoforese de micropartículas de cortiça-magnetite. Por outro lado, foi também realizada a caraterização relativa ao comportamento magnético de partículas de cortiça pura e magnetizada. Testes experimentais preliminares usando lamelas de vidro com partículas uniformemente distribuídas no adesivo foram realizados de modo a validar as simulações feitas anteriormente.

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Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Testes mecânicos foram executados para determinar as propriedades básicas dos seguintes arranjos compósitos: resina e resina com micropartículas de cortiça magnetizadas (distribuições uniforme e graduada), considerando quantidades variadas. Provetes bulk foram usados para executar testes de tração e medições da temperatura de transição vítrea. JSSs foram também feitas de modo a avaliar a influência de inclusões de reforço (micropartículas de cortiça magnetizada). Simultaneamente aos testes mecânicos, foi realizada uma análise pelo SEM relativamente às partículas e às superfícies de fratura dos provetes bulk.

Globalmente, concluiu-se que as partículas magnetizadas de cortiça distribuídas gradualmente na zona de sobreposição de uma JSS funcionam como inclusões de reforço. Desta forma, o novo método desenvolvido foi validado, assim como o mecanismo concebido para produzir estas juntas.

Palavras-chave: Adesivos epóxidos, juntas adesivas funcionalmente graduadas, partículas magnetizadas, micropartículas de cortiça, magnetoforese, análise numérica, distribuição de tensões, propriedades mecânicas.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Acknowledgements

This thesis would not have been possible without the guidance from everybody listed next. So, for that, I am really thankful for all the help and support you all gave me.

To my supervisor, Professor Lucas, I would like to thank, whose expertise was invaluable in the formulation of the research topic.

I am especially thankful to Ana and Zé for their valuable guidance. You provided me with the tools that I needed to choose the right direction and successfully complete my dissertation.

I am grateful to all of those whom I have had the pleasure to work with during this semester. Each of the members of ADFEUP has provided me extensive personal and professional guidance and taught me a great deal about scientific research and life in general.

Also, I would like to thank Vítor Amaral, from CISM (Centre for Imaging and Structure of Materials), University of Aveiro, for providing me relevant content about the magnetic behaviour of the pure and magnetised cork microparticles.

In addition, I want to thank my closest friends, who were of great support in deliberating over our problems and findings, as well as providing happy distraction to rest my mind outside of my research. To Katie and Mário, you supported me greatly and were always willing to help me.

Nobody has been more important to me in the pursuit of my dreams than the members of my family. Therefore, I would like to thank my Parents, Sista, Grandma and Titi, whose love and guidance are with me in whatever I pursue.

Last but not least, this research would not have been possible without the financial assistance provided by Foundation for Science and Technology (POCI-01-0145-FEDER- 028035).

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Contents

Abstract ...... i Resumo ...... iii Acknowledgements ...... v List of Figures ...... ix List of Tables ...... x Nomenclature...... xi 1. Introduction ...... 1 1.1. Background and motivation ...... 1 1.2. Problem definition ...... 2 1.3. Objectives ...... 2 1.4. Outline of this thesis ...... 3 2. Theoretical background ...... 7 2.1. Adhesive bonding as a joining technique ...... 7 2.2. Functionally graded adhesive joints...... 9 2.2.1. Mixed adhesive joints (MAJs) ...... 10 2.2.2. Graded cure...... 12 2.2.3. Second phase inclusions ...... 15 2.3. Magnetophoresis with cork-magnetite microparticles ...... 18 2.3.1. Basic concepts of magnetism ...... 18 2.3.2. Magnetic characterisation of the particles ...... 24 2.3.3. Magnetophoresis ...... 25 3. Development of the experimental procedure...... 35 4. Development of an apparatus to produce FGA SLJs ...... 43 5. Conclusions ...... 45 6. Future work...... 45 References ...... 47 Paper 1 ...... 50 Patent ...... 50 Paper 2 ...... 50

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Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

viii

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

List of Figures

Figure 1. Example of an adhesive joint (adapted from [2])...... 7 Figure 2. Comparison between riveted joints and adhesive joints [2]...... 8 Figure 3. Undesirable load types of adhesive bonded joints: (a) cleavage, (b) peel stress (adapted from [6])...... 9 Figure 4. Schematic illustration of the shear stress distribution of a functionally graded adhesive joints versus a “conventional” adhesive joint...... 10 Figure 5. Mixed adhesive joint (Adapted from [2])...... 10 Figure 6. Schematic shear stress distribution at failure in mixed bi-adhesive joints [18]...... 11 Figure 7. Mixing ratio along the length of an adhesive joint (all dimensions in mm, adapted from [24])...... 12 Figure 8. Mechanical properties distribution along the overlap of a FGA joint obtained by graded cure [25]...... 13 Figure 9. Mould with the coil system that allows the graded cure in SLJs (adapted from [28])...... 14 Figure 10. Cell structure of a cork (a) board and (b) particle (adapted from [31])...... 16 Figure 11. Illustration of the effect of damping of cork cells: (a) cork cell with resin penetration; (b) cork cell without resin penetration [11]...... 17 Figure 12. Microstructure of cork microparticles with a size range of: (a)38-53 μm; (b) 125-250 μm [11]...... 18 Figure 13. Magnetic moment of a planar current loop of current I and area S...... 19 Figure 14. Field lines of the magnetic induction produced by a magnetic moment. Field lines diverge at the N magnetic pole and converge at the S magnetic pole [53]...... 20 Figure 15. Typical arrangements of magnetic moments of paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials...... 21 Figure 16. When a ferromagnetic divides into domains, its energy decreases as its induction field lines become confined essentially to its interior [55]...... 22 Figure 17. Typical magnetising cycle of a ferromagnet and corresponding magnetic domain structure...... 23 Figure 18. Magnetisation curves as a function of the applied magnetic field...... 24 Figure 19. Pure cork magnetisation curves as a function of the applied magnetic field. 25 Figure 20. Diagram of the forces acting on a magnetic particle within a viscous medium. The y components (out of the plane) are not represented...... 26 Figure 21. Surrounding atmosphere of the magnet model...... 28 Figure 22. Permanent magnet considered in the COMSOL model...... 28 Figure 23. Variation of the flux density across the magnet width obtained from COMSOL simulation with the “Magnetic fields, no currents” module...... 29

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 24. Variation of the flux density across the magnet length obtained from COMSOL simulation with the “Magnetic fields, no currents” module...... 30 Figure 25. First Halbach array used in the COMSOL simulations. The arrows indicate the directions of the magnetisation of each magnet...... 30 Figure 26. Simulations results for MA I: (top) magnetic flux density along the overlap length (xz plane); (bottom) colour map of the magnetic flux density gradient on the xy plane...... 31 Figure 27. Initial ( 푡 = 0푠 ) and final (푡 = 5푠 ) particles distributions obtained from magnetophoresis simulation for MA I...... 31 Figure 28. Initial (푡 = 0푠) and final (푡 = 5푠) particles distributions in xy plane, obtained from magnetophoresis simulation for MA I...... 32 Figure 29. Wood device used to constrain the magnets during the gluing process...... 32 Figure 30. Backlight image of a glass slide with particle distribution obtained using MA II...... 32 Figure 31. Magnet arrangement MA III...... 33 Figure 32. Backlight image of a glass slide with particle distribution obtained with MA III...... 33 Figure 33. Example of a glass slide used in preliminary tests...... 35 Figure 34.Cure cycle for the neat resin...... 36 Figure 35.Cure cycle of the adhesive with 1% of magnetised cork particles...... 36 Figure 36. Experimental representative curve 휎 = 푓(휀) for the adhesive BetamateTM UN3077...... 37 Figure 37. SLJ geometry, with 25 mm width (all dimensions in mm)...... 38 Figure 38. Failure load as a function of the overlap length, according to the Goland and Reissner classical analysis...... 38 Figure 39. Example of bad interface adhesion in an aluminium SLJ...... 39 Figure 40. Examples of aluminium substrates plastic deformation, for a 25 mm overlap width SLJ...... 39 Figure 41. SEM image of a fracture surface of a composite resin-magnetised cork microparticles plate...... 40 Figure 42. Comparison of the influence of the inclusion between sieved particles and non- sieved particles – Tensile Stress as a function of the strain...... 41

List of Tables

Table 1. Dimensions of a single BC-14 N52 magnet from K&J Magnetics...... 29 Table 2. Experimentally determined mechanical properties of BetamateTM UN3077. .. 37 Table 3. Mechanical properties of AW 7075 T651...... 38

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Nomenclature

Acronyms

FGA – Functionally graded adhesive

FGM – Functionally graded material

MAJ – Mixed adhesive joint

SI – International system of units

SLJ – Single lap joint

TEP – Thermally expandable particle

VMR – Variable mixture ratio

Symbols

B – magnetic induction field vector

E – Young’s modulus

F, P – Applied force

Fi – Magnetic field vector

G – Shear modulus

Gm – Magnetic field torque vector

h – Height

H – Applied magnetic field vector

I – Current

J – Angular momentum vector

l - Length

M – Magnetic moment per unit volume vector, also known as magnetisation

m – magnetic moment vector

r – Distance vector

R – Radius

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

S – surface vector t – Thickness

Tg – Glass transition temperature

Tgꝏ - Post-cure temperature

Uz – Zeeman energy v – Velocity

V - Volume w – Width

μ0 – Magnetic permeability of vacuum

μ – Magnetic permeability

ε – Strain

σ – Tensile stress

 – Shear stress

ρ - Density

 - Gyromagnetic ratio

ν – Poisson’s ratio

η – Viscosity

δ - Displacement

∇ - Gradient

휒 – Magnetic susceptibility

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Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

1. Introduction 1.1. Background and motivation

Adhesive bonding as a joining technique is a compelling method of joining materials and structures mostly because of its adaptability and ability to reliably join a large range of materials, as well as the combination of any of these [1-3]. Over the years, this method has been gradually replacing the traditional mechanical fixing techniques (e.g. bolts, rivets, welding), as it provides a smoother stress distribution [4], is relatively cheaper and offers lighter solutions than the others. Nowadays, structural adhesive bonding is a key technology for numerous industrial sectors, such as the aeronautical, aerospace, medical and civil industries [5], mainly because of the constant push for lighter, stronger, more resistant and environmentally friendly materials. For instance, the automotive industry, which has significantly grown its adhesive use in the past years, meets this demand by using customized materials such as composites, which combine different groups of materials to achieve components with excellent strength to weight ratios. Custom adhesives are no exception, with increased electrical, thermal, mechanical (toughness, fatigue, impact or wear) properties being sought.

One of the most used structural adhesives is the epoxy resin (a thermoset polymer) due to its good mechanical, thermal and chemical properties. The epoxy microstructure has a densely cross-linked molecular structure, which is very useful for applications in structural engineering, since it presents high modulus of elasticity and strength, as well as low creep and good thermal strength. However, the same microstructure that provides good properties to the epoxy resin, is responsible for the inherent brittleness (low ductility and toughness) with a low resistance to the initiation of cracks and their propagation. That is, one of the main problems associated with adhesive joints is the existence of stress concentrations (shear and peel) at the ends of the overlap [6], thus reducing joint performance. This is especially valid for the most common joint geometry – the single lap joint (SLJ) [7]. Therefore, a main area of investigation in the field of adhesive bonding is the uniformization of stress distribution along the adhesive bondline, in order to reduce the stress concentration at its ends, achieving stronger and lighter joints.

In the literature, several methods can be found to obtain a more even stress distribution throughout the joint [2, 6, 7], which also allow the improvement of the toughness of brittle adhesives. Among these methods, the most promising is the use of functionally graded adhesive joints [8]. Functionally graded adhesives (FGA) are defined as tailored adhesives which have gradually varying mechanical properties along a desired dimension, thus allowing a more uniform stress distribution along the bondline. Within the field of FGA, some promising new-era technologies are based on the inclusion of micro or nano inorganic (silicates, glass, alumina, etc.) [9] or organic particles [10].

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Additionally, natural materials (i.e. cork or wood fibres) [11-13] are gaining attention as reinforcements of polymeric matrices due to their unique properties: thermal insulation, low density, low cost and sustainability of the raw material [14].

1.2. Problem definition

The constant need for improvement and innovation at all industries leads to the development of new technologies. Therefore, it is of great interest to further develop new concepts in the adhesive bonding area, such as the theme of functionally graded adhesive joints, which are very promising due to their potential high degree of customization, offering more solutions and options in terms of application design.

The aim of this thesis is to propose an innovative method of producing functionally graded adhesive joints using magnetised cork microparticles.

With an appropriate application of magnetic fields, using a customized apparatus, the magnetised cork microparticles will be strategically moved/placed along the bondline of an adhesive joint, being then non-uniformly distributed along the entire overlap area. This results in a gradual variation of the mechanical properties along the overlap, decreasing the stress concentrations, then leading to a more uniform stress distribution on the overlap region.

Adhesive graded joints obtained by this method are expected to be stronger and more efficient, enabling the work with smaller areas and reducing the structural weight, which is a key factor in several industries.

1.3. Objectives

The main goal for this thesis is to develop and validate a new process to produce functionally graded adhesive joints using magnetised cork microparticles, in order to have mechanical properties that vary gradually along the bondline, thus allowing a more uniform stress distribution.

The specific objectives considered for this thesis were:

• Review the state of the art of functionally graded adhesive joints; • Characterisation of the magnetised cork microparticles; • Determination of the intrinsic mechanical properties of the selected adhesive; • Determination of the mechanical properties of the adhesive filled uniformly with magnetised cork microparticles;

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

• Development of an apparatus capable of manufacturing adhesive graded single lap joints with magnetised cork microparticles; • Compare, in quasi-static conditions, the graded joints with the reference homogenized joints.

1.4. Outline of this thesis

This thesis consists of a summary, a review paper, a patent and a mechanical characterisation paper.

Paper 1 - J.B. Marques, A.Q. Barbosa, C.I. da Silva, R.J.C. Carbas, L.F.M. da Silva, An overview on manufacturing functionally graded adhesives – Challenges and prospects, Journal of Adhesion – Accepted for publication in Journal of Adhesion.

Abstract of paper 1

Adhesive bonding is a constantly growing and compelling method of joining materials and structures mainly due to its cost-effectiveness, reliability and versatility. Its ability of joining a large range of materials and capability of reducing the stress concentrations in joining assemblies is preferred, in some situations, over the use of other mechanical joining methods such as riveting and bolting.

In recent years, adhesive bonding has become a key technology among the various industrial sectors, namely the automotive industry due to its constant demand for lighter, more resistant and environmentally friendly materials. Therefore, it is of great interest to further develop this kind of bonding, by developing functionally graded adhesive joints. Functionally graded adhesives (FGA) can be defined as tailored adhesives that have varying gradual mechanical properties along a desired dimension, allowing a more uniform stress distribution along the bondline. Application wise these joints are very promising due to their potential high degree of customization, offering more solutions and options regarding application design.

This overview aims to assess all the current experimental achievements and manufacturing processes in the field of FGA, as well as the complications and concerns that need to be addressed in order to achieve consistent, reproducible graded joints that can later be transferred to industrial applications.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Patent – C.I. da Silva, A.Q. Barbosa, J.B. Marques, R.J.C. Carbas, J. Abenojar, L.F.M. da Silva, M.A. Martinez, J.C. del Real, Method and apparatus to manufacture functionally graded joints using magnetised micro particles – PAT 20191000036260.

Abstract of the patent

An apparatus and method to manufacture graded single lap joints (SLJs) bonding with magnetic particle integrated adhesive, wherein the adhesive is a liquid adhesive and the substrates are non-magnetic substrates selecting from the group consisting of aluminium, polymers, ceramics, wood and derivatives. The apparatus comprising a mould with a bottom part (8) and a top part (5), a plate (7), screws (12) and a device that includes an upper (1) and a lower (1’) magnet holders, and an upper (9) and a lower (9’) male components; wherein the magnet holders (1;1’) contain at least one magnetic array, responsible for creating a static magnetic field, that acts on the adhesive layer to distribute the magnetic particles along the overlap length of SLJs. By use of the method, a beneficial reduction of the stress concentrations is provided to overlap ends, in order to bring more structurally efficient structures

Paper 2 – C.I. da Silva, A.Q. Barbosa, J.B. Marques, R.J.C. Carbas, E.A.S. Marques, J. Abenojar, L.F.M. da Silva, Mechanical characterisation of graded single lap joints using magnetised cork microparticles – to be submitted in an adhesive bonding field journal.

Abstract of paper 2

One of the main problems associated with adhesive joints is the existence of stress concentrations (shear and peel) at the ends of the overlap, reducing joint performance. This is especially valid for the most common joint geometry – the single lap joint. Therefore, a main area of investigation in the field of adhesive bonding is the uniformization of the stress distribution along the adhesive bondline, in order to decrease those stress accumulations at its ends, achieving stronger and lighter joints.

The main goal of this work was to develop a functionally modified adhesive, where the mechanical properties vary gradually along the overlap. With an appropriate application of magnetic fields, using a customized apparatus, magnetised cork microparticles, initially uniformly distributed within a resin, were strategically placed along the bondline of an adhesive joint, being then non-uniformly distributed along the entire overlap area. This results in a gradual variation of the mechanical properties along the overlap, decreasing the stress concentrations and leading to a more uniform stress distribution on the overlap region. The adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the borders of the overlap.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Therefore, the influence of the amount of magnetised cork microparticles was assessed. Tensile tests were performed for bulk specimens and SLJs, along with SEM analysis of the particles and correspondent bulk specimens fracture surfaces. Additionally, glass transition temperature measurements were done. From experimental tests, the inclusion of these particles enhances the joints performance for either graded joints or joints with a uniform particle distribution, when compared to those with neat resin. Also, it is possible to manufacture graded joints with different behaviours, depending on the amount of magnetised particles selected.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

2. Theoretical background 2.1. Adhesive bonding as a joining technique

Adhesive bonding is a material joining technique which is based on the bonding of two similar or dissimilar materials (adherends or substrates) through the solidification of a polymeric material (adhesive) between them, allowing then the transferring of mechanical force or work across the interface [15]. This geometry configuration is also known as adhesive joint and is illustrated in Figure 1. Therefore, an adhesive is a material that, when applied to the substrates, can join them together and resist separation [6].

Figure 1. Example of an adhesive joint (adapted from [2]).

The increasing application of adhesive joints as an alternative to mechanical joints is related to the numerous advantages presented over conventional mechanical fastening methods, such as bolting and riveting. That is, when establishing a comparison between traditional joining techniques and adhesive bonding, the following advantages can be considered [2]:

• Despite the fact that adhesives have a lower mechanical strength than metals, since it is possible to have a bigger bearing area between the materials, a higher stiffness and a more uniform stress distribution can be achieved, also allowing the reduction of stress concentrations caused by bolts and rivets (see Figure 2); • Design flexibility, with the possibility of bonding different types of materials, or materials whose properties differ a lot from each other; • Low weight, due to the polymeric nature of the adhesive; • Cost-effective technique, with the possibility of being automated.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 2. Comparison between riveted joints and adhesive joints [2].

However, despite its benefits, there are some disadvantages in adhesive bonding, that are [2]:

• Susceptibility of the adhesives to extreme environmental conditions (high temperature and humidity), due to their polymeric nature; • In order to avoid bad adhesion, the adherends surfaces that will bond need to be carefully prepared, with the appropriate surface treatments (mechanical abrasion, degreasing with a solvent, chemical etching or anodizing, among others); • Difficulty on providing quality control, since, in some cases, it requires destructive techniques; • One of the main goals in joints design is to avoid peel and cleavage stresses (see Figure 3), that are the types of load unappreciated in adhesive joints. Yet, this still is a challenge and an area of intense research; • The bonding process not instantaneous, thus requiring fastening tools in order to keep the parts in the right position.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 3. Undesirable load types of adhesive bonded joints: (a) cleavage, (b) peel stress (adapted from [6]).

In spite of the drawbacks outlined above, such as the non-perfectly uniform stress distribution in adhesive joints, there is enough room for improvements. So, in order to reduce the peel or cleave stresses, several methods have been proposed, over the years, to improve the joints strength. Among those are the use of fillets, adherend profiling and other geometrical solutions. Hybrid joints are another possibility to enhance the bonding performance, being the adhesives used in conjunction with rivets, bolts, or a spot weld, for example. Likewise, more approaches to obtaining hybrid joints have been introduced recently, those being functionally graded materials (FGMs) and functionally graded adhesives (FGAs) [2].

Henceforward, this thesis will be aim attention to the concept of FGAs, since this is the subject under study.

2.2. Functionally graded adhesive joints

Functionally graded adhesives are defined as tailored adhesives that have varying gradual mechanical properties along a desired dimension, providing a more even stress distribution along the bondline (see Figure 4) [16]. This technique for improving the joint’s performance has been an area of great research and development on the past years, due to not only its substantially high potential in future industrial applications, but also due to the ability of tailoring the stress distribution as desired in an adhesive layer. Therefore, having these features, it is theoretically possible to design high performance structural assemblies with customized properties.

In the literature, the current techniques for manufacturing FGAs are: Mixed adhesive joints (MAJs), Graded cure and Second phase inclusions.

On the next sub-sections, in order to better understand each concept, these three main types of FGAs, listed on the previous paragraph, will be presented in detail.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 4. Schematic illustration of the shear stress distribution of a functionally graded adhesive joints versus a “conventional” adhesive joint.

2.2.1. Mixed adhesive joints (MAJs) Mixed adhesive joints (MAJs) is a technique which combines more than one adhesive, commonly with different behaviours [17-21], along the overlap of an adhesive joint, resulting, ideally, in a gradual variation of properties along the bondline [2, 18, 19].

Accordingly, the concept of MAJs was first introduced with the study of bi-adhesive joints [19, 21], which can be considered a rough version of FAGs [2], since the variation of the properties along the bondline assumes a stepwise behaviour [18, 19, 21], instead of a gradual variation of the properties. In this approach, it is typically used a blend of a stiff adhesive (high modulus) with a ductile adhesive (low modulus), being the first one applied at the centre of the joint and the other one at the edges (see Figure 5) [18, 19]. This provides a synergetic effect [18, 20], allowing the load transfer from the overlap ends, which are the zones of the joint susceptible to stress concentrations [2, 18, 19], to the middle, where a higher strength adhesive actuates for the dissipation of those applied stresses.

Figure 5. Mixed adhesive joint (Adapted from [2]).

Several authors studied bi-adhesive joints [18-21] as a concept of MAJs. From all their experiments, it can be concluded that the performance of bi-adhesive joints is dependent on the ductile-brittle adhesive ratio and on the correspondent position along the overlap.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

In fact, brittle adhesives provide joint strength, yet they do not take into consideration the all joint length. On the other hand, using too much ductile adhesive, the joint’s strength may lower, but with the right quantity, the load is transferred along the overlap to the brittle adhesive [18]. That is, for a MAJ to be stronger than a brittle or ductile adhesive by itself, the load carried by the stiff adhesive must be higher than the load carried by the compliant one [2]. From those studies, researchers also found out that the quantity of brittle adhesive in the mixing ratio controls the mechanical properties of the joint. That is, for example, by increasing the amount of the stiff adhesive, the Young’s modulus, local hardness and tensile strength would increase too.

Although there have been improvements in the bi-adhesive joints section of MAJs, one of the main problems associated with them is the achieving proper separation of the adhesives (see Figure 6). Due to this fact, even in a joint bonded with several adhesives, the stress concentrations tend to occur locally at the borders between consecutive adhesives (due to the stepwise behaviour), thus leading to joint failure [18]. Therefore, bi-adhesive MAJs can only improve the joints performance up to a certain level.

Figure 6. Schematic shear stress distribution at failure in mixed bi-adhesive joints [18].

Furthermore, another method called Variable mixture ratio (VMR) of adhesives is a more recent technique to manufacture FGA joints based on the concept of MAJs [22-24]. This technique leans on a non-stepwise combination of different adhesives, enabling a gradual variation of the properties along the bondline, which results in a more uniform stress

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

distribution along the overlap [2]. Suitably, there is a continuously change of the mixing ratio of two distinct adhesives, one that is rigid and another one that is flexible, resulting in gradual modulus distribution. Therefore, the mechanical properties of the adhesive joint are greatly dependent on the ratio ductile-brittle of the mixture as well as the respective location along the joint, as it can be exemplified in Figure 7. That is, higher percentage of ductile adhesive leads to a lower Young’s modulus at the correspondent zone; otherwise increasing the quantity of the stiffer adhesive leads to more rigid zones (higher Young’s modulus).

Figure 7. Mixing ratio along the length of an adhesive joint (all dimensions in mm, adapted from [24]).

In opposition to bi-adhesive joints, with the VMR technique there are no local stress concentrations at the borders of consecutive adhesives, which positions this method as an innovative technique, worthy of investment for future industrial applications. Meanwhile, despite improved mechanical properties, thus enhanced joint performance, this method still needs to be better explored in the sense that it should become more general and simpler for future experimental validations and applications. In fact, for Variable mixture ratio of adhesives method, only customized and complex apparatus [1, 22-24] have been developed to produce functionally graded adhesive joints, which turn it difficult to reproduce new adhesive joints. Nevertheless, there is also the question of the difficulty on controlling the mixing ratio, once it is dependent on the adhesives proper selection for mixing, being those apparatus only developed for such adhesives combinations. Therefore, it still does not render feasible the reproducibility of this process, even less its industrial practice.

2.2.2. Graded cure An alternative way of joint improvement regarding joint strength was accomplished by Carbas et al. [25] through functionally graded SLJs with an adhesive functionally modified by induction heating (the graded cure method). Consequently, the mechanical properties, which are dependent on the adhesive and are function of the cure temperature,

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

vary gradually along the overlap, allowing not only a more uniform stress distribution along the bondline, but also the reduction of stress concentrations at the ends of the overlap. Thus, the adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the ends of the overlap – as depicted in Figure 8.

Figure 8. Mechanical properties distribution along the overlap of a FGA joint obtained by graded cure [25].

Induction heating is an electromagnetic heating method in which electrically conductive bodies absorb energy from an alternating magnetic field, generated by an induction coil. This is a fast technique, since it focuses heat at or near the adhesive bondline, and it is also considered to be very efficient, because the entire assembly does not have to be heated to cure only the adhesive, being the heat generated inside the workpiece [26, 27]. To achieve a tailored differential cure, so that an adhesive with graded properties along the length of the overlap is obtained, a specially developed apparatus for SLJs was designed. Once the induction heating system enables to obtain a graded cure with a focus temperature at the ends of the overlap accompanied by a gradual temperature decrease up to its middle, this apparatus consists of two heating coils (high cure temperature), located at the overlap ends, and a cooling coil (low cure temperature) in the middle part between those two – see Figure 9. Also, a positioning system was built in order to guarantee the correct alignment, geometry, position and thickness of the adhesive layer. The gradient of the temperatures between the middle and the edges can be modified either by changing the induction heating, also known as the alternating magnetic field (e.g. frequency and/or power), or the cooling system (e.g. flow and/or coolant).

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 9. Mould with the coil system that allows the graded cure in SLJs (adapted from [28]).

In particular, the apparatus enables the production of SLJs with high adhesive stiffness at the centre and minimum at the ends of the overlap, assuming that the adhesive, whose mechanical behaviour is sensitive to the cure temperature, exhibits a ductile behaviour for lower cure temperatures and a more brittle behaviour for higher cure temperatures. Therefore, as presented previously in the MAJs characterisation, this leads to a higher joint strength when compared to the strength of a joint made of an adhesive cured at only one temperature. For comparison, Carbas et al [25] used the isothermal cure process as a reference for the graded cure results. The adhesives tested in this study were Araldite®2011 and Loctite Hysol®3422. When cured at high temperatures (100, 120ºC), both adhesives showed ductile behaviour and, in opposition, the brittle behaviour appeared when they were subjected to low cure temperatures (23, 40ºC). Overall, the FGA joints were found to have a higher joint strength compared to the cases where the adhesive is cured uniformly at low or high temperatures. In detail, when compared to the corresponding isothermally cured counterparts, the failure load suffered an increase of 68.4% and 67% for Araldite®2011 cured at low and high temperatures, respectively, and an increase of 245.5% and 60.6% for Loctite Hysol®3422 cured at low and high temperatures, respectively. Similarly, regarding the strain, both FGA joints were better or just as good as their non-graded analogues were.

The effect of post-cure (Tg͚ ꝏ) on functionally graded adhesive joints obtained by induction heating was also analysed by Carbas et al. [29]. There were considered three different post-curing procedures, for the same adhesives used on the above study. In the first set, the joints were subjected to a curing process only; in the second set, the joints were exposed to a curing process pursued by a post-cure with a temperature bellow Tg͚ ꝏ; and lastly, in the third set, the joints were subjected to a curing process followed by a post- cure with a temperature above Tg͚ ꝏ. The analysis performed with these different conditions aims to understand the effect of the post-cure, at temperatures related to the glassy and rubbery region, respectively, on the mechanical behaviour of the adhesive joints. The

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

temperatures considered for the post-cure below Tg͚ ꝏ regarding the joints bonded with Araldite®2011 and Loctite Hysol®3422 were 60ºC and 40ºC, respectively. On the other hand, the temperature used on the joints bonded with both adhesives was 100ºC. The performance of the graded joints was measured in relation to the joints cured isothermally. Accordingly, it was found that, for the two adhesives, for a post-cure below

Tg͚ ꝏ the joints cured gradually had decreased failure load, instead of those cured isothermally which showed an increase of failure load value. On the other hand, above

Tg͚ ꝏ, the joints cured gradually and isothermally tended to present similar failure load values. Therefore, the authors concluded that the functionally customized adhesive properties had been lost when the joints were submitted to a post-cure above Tg͚ ꝏ. Additionally, the main problem that occurs with this technique is due to the thermal bond between dissimilar adherends. In these cases, the high conductivity adherend tends to be heated more by the induced flux than the other component with lower conductivity.

2.2.3. Second phase inclusions A usual method for enhancing the mechanical properties of adhesively bonded joints is adding a second phase to the adhesive layer which results in a composite material. A composite material is a combination of two or more materials which has better properties than its individual components [30].

This second phase can be a rubbery phase (reaction-induced phase separation) [31, 32] or a reinforcement inclusion, such as fibres, whiskers and particles [31-33], that can enhance either mechanical, electrical and/or thermal properties of the resin matrix.

In this section, reinforcement inclusions in the form of particles will be the main focus. Thus, according to size, particles can be categorized into micro or nano scale and, in terms of nature, they can be inorganic (silicates, glass, alumina, etc.) [9] or organic [10].

Since size is an important material parameter, it is of great importance to establish the difference between micro and nanoparticle inclusions. Therefore, for the same concentration, nanoparticles have higher surface area than microparticles, promoting more particle agglomeration. This is why particle mixing in nanoparticles requires more complex mixing methods (i.e. ultrasound or high shear mixing), instead of microparticle mixing that can be done by using simpler methods (i.e. three-roll milling) [2].

Reinforcement particle inclusions have already been studied in several works. Particularly, some works considered the application of reinforcement nano scale inclusions in the form of carbon based nanoparticles [34]; nano silica; nano clays; amongst others [33]. On the other hand, reinforcement micro scale inclusions investigated in another works have been thermally expandable particles (TEPs) [35], metallic micro materials [36], rubber powders [37] or glass particles [38].

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Currently, there is a growing interest from the scientific community as well as the industry in using new particles to enhance the joints performance. Depending on the material selected, working with microparticles can lead to less expensive and faster product developments, when compared with other techniques to increase the strength of the joints [32]. Additionally, microparticles are of great interest in the field of FGA joints since they have the potential of being displaced along the overlap, thus creating custom particle distributions, for the various purposes. Consequently, this drives for the obtainment of different properties along the bondline [2].

Furthermore, natural fibres or particles reinforced composites (i.e. cork or wood fibres) [11-13] are an emerging area as they are gaining attention as reinforcements of polymeric matrices, mainly due to their unique properties: thermal resistance, low density, low cost and sustainability of the raw material [14].

Cork is an organic material with unique combination of properties and can be found in the cork oak, constituting the external covering of its stem and branches [39]. A study made by the World Wide Fund for nature reveals that this biological material is obtained by a truly sustainable process, since it is a renewable and biodegradable source [40]. Macroscopically, cork is characterised by being lightweight, flexible, substantially impermeable to liquids and gases, with excellent thermal, electrical, acoustic and vibration insulation, as well as is an innocuous and imputrescible material (unaffected by microbial activity). In a microscopic scale, cork is described as an aggregation of sealed prismatic cells (see Figure 10a), that work together, displayed in an alveolar structure similar to a honeycomb shape [39, 41-45]. The main characteristics of cork are due from the interaction between the cells as a whole (plank; board or agglomerate). Nonetheless, it was noted that most of the cork properties can also be observed in particles (see Figure 10b) with smaller dimensions [32].

Figure 10. Cell structure of a cork (a) board and (b) particle (adapted from [31]).

Besides that, the properties of an adhesive/cork composite are dependent on various parameters, such as the materials properties; the interfacial adhesion properties between

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

the cork and the resin; size as well as the amount of cork particles and mixing conditions [14].

One possible reason for using cork particles in adhesives is that they act as obstacles to crack propagation, thus increasing the toughness of the adhesive, since the close cells could work together to absorb actively the impact [39, 41, 46]. According to Barbosa et al. [11, 47-49], the inclusion of cork microparticles has proven to be a good reinforcement in brittle adhesives. In the studies performed by the same authors, different amounts of cork and particle sizes were added to a brittle adhesive in order to analyse the influence of those on the behaviour of the cork/resin composite. The adhesive considered in these researches was Araldite2020®, an epoxy resin.

Firstly, it was concluded that the number of cells in a particle is deeply relevant for a good impact behaviour of the cork microparticles (see Figure 11) [11]. These particles are obtained by a milling process which can damage some cell walls, promoting the resin penetration in them. Therefore, the damping effect of the cork particles is decreased, lowering too the absorption of energy. Thus, as it is essential for the cork particles to have a few well-preserved cells, it was identified that above 30 μm, microparticles can work as an obstacle to crack propagation, due to cork cell structure; otherwise bellow this value cork has no effect on increasing the adhesive’s toughness, functioning as a defect.

Figure 11. Illustration of the effect of damping of cork cells: (a) cork cell with resin penetration; (b) cork cell without resin penetration [11].

Secondly, the effect of the cork microparticles on the impact properties of a modified epoxy was studied [11]. Experimental results showed that large particles (125-250 μm, see Figure 12b), with well-preserved cells walls, presented better results as reinforcement material compared to small particles (38-53 μm, see Figure 12a), with damaged cells walls. On the other hand, 1% of cork particles presents better impact absorption (higher joint strength), comparing to larger amounts, such as above 2% of cork particles in a

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

brittle resin, as well as the resin itself. So, the optimal combination found was to have cork particles ranging in size from 125-250 μm, considering a 1% amount (by volume) of them within the brittle resin [11]. Considering this optimal solution, the experimental results showed an overall improvement of the mechanical properties with better impact energy absorption, when compared to the properties of the epoxy resin itself.

Figure 12. Microstructure of cork microparticles with a size range of: (a)38-53 μm; (b) 125-250 μm [11].

Lastly, it was also proved that cork is able to improve the adhesive mechanical properties without detrimental effects on the curing process and on the hydrothermal degradation of the adhesive, regarding the interaction between cork microparticles and the resin [50].

Apart from being a viable technique to enhance the performance of an adhesive joint, this technology also allows to use a product (cork microparticles) that currently is not well exploited by the cork industry. However, new applications are starting to emerge for these particles, although most of them are related to their burning, as they are considered to be an industrial waste. A new purpose for this material would give a new and more sustainable perspective to the cork industry with potential benefits, specially to the Portuguese economy, since Portugal is the world’s leading market for this raw material, accounting for three-quarters of the world’s total production.

2.3. Magnetophoresis with cork-magnetite microparticles

2.3.1. Basic concepts of magnetism The fundamental concept in magnetism is that of magnetic moment, which is relevant both at the microscopic and the macroscopic scales (e.g., the magnetic moment of atoms or ions versus the magnetic moment of a sample). Magnetic moment is a vector quantity that can be associated to any loop of electrical current. A current loop can be characterised

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

by a surface vector 푺 (see Figure 13); the magnitude 푆 = |푺| represents the loop area; the direction of 푺 is normal to the loop and its sense is determined by the sense of the current circulation in the loop. For a current loop with surface vector 푺 and electrical current 퐼, the corresponding magnetic moment is defined as [51]

풎 = 퐼푺. (1)

풎

S 푰 Figure 13. Magnetic moment of a planar current loop of current I and area S.

Microscopic magnetic moments are due to the orbital motions of electrons and their spins. The total magnetic moment of a macroscopic sample is then the sum of all magnetic moments of its constituent atoms or ions. Therefore, a magnetised body has a nonzero total magnetic moment. The SI unit of magnetic moment is Am2 [51, 52].

There is a direct relationship between magnetic moment and angular momentum (푱),

풎 = 훾푱, (2) where 훾 is a constant known as the gyromagnetic ratio. This equation implies that any change in the orientation of a magnetic moment is constrained to obey the law of conservation of angular momentum [51, 52].

In the presence of a magnetic (induction) field 푩 (SI unit: Tesla, T), a magnetic moment 풎 has a magnetic energy, known as Zeeman energy, given by:

푈푍 = −풎 ∙ 푩. (3) This energy is minimum when the magnetic moment is aligned with the magnetic field. For a general orientation of the magnetic moment, the magnetic field will exert a torque on it given by:

푮푚 = 풎 × 푩. (4) By the law of conservation of angular momentum, this magnetic torque will cause the magnetic moment to precess around the direction of the magnetic field. If energy dissipation occurs during this precession, the magnetic moment will eventually reach the equilibrium state of full alignment with the magnetic field. There is, therefore, a natural tendency for magnetic moments to align with a magnetic field [51, 52].

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

When the magnetic field is nonuniform, the Zeeman energy will change with position, leading to the appearance of a magnetic force given by:

푭푍 = −∇푈푍 = ∇(풎 ∙ 푩) = (풎 ∙ ∇)푩 + (푩 ∙ ∇)풎. (5) It should be noticed that because, in general, magnetic moments depend on the applied magnetic field, the spatial derivatives of 풎 may be related to those of 푩. Magnetic moments are also sources of magnetic field. At distances 풓 much larger than the spatial extent of a particle or body with magnetic moment 풎, the magnetic induction is of dipolar type, as given by

휇 푩 = 0 [3(풓̂풎) − 풓̂풎], (6) 4휋푟3 where 휇0 is the magnetic permeability of vacuum, 풓̂ = 풓/‖풓‖ and 푟 = ‖풓‖. Figure 14 shows the typical distribution of field lines of the dipolar magnetic field produced by a magnet. In the traditional notation, a magnet can be represented by two poles, North (N) and South (S), which define the direction and magnitude of the corresponding magnetic moment [51, 52].

Figure 14. Field lines of the magnetic induction produced by a magnetic moment. Field lines diverge at the N magnetic pole and converge at the S magnetic pole [53].

Two interacting magnetic moments 풎푖 , 풎푗 (e. g., two magnetite microparticles) separated by a vector 풓풊풋 = 풓풋 − 풓풊 in vacuum have a magnetostatic energy of the form [52]

휇0 푈푖푗 = −풎푖 ∙ 푩푗 = 3 [풎푖풎푗 − 3(풓̂푖푗풎푖)(풓̂푖푗풎푗)]. (7) 4휋푟푖푗 where 푩푗 is the magnetic field produced by moment 풎푗 and 풓̂푖푗 = 풓풊풋/푟풊풋 . The force acting on 풎푖 by the magnetic field produced by 풎푗 can be calculated from the negative gradient of 푈푖푗 with respect to the spatial coordinates of 풎푖: 푭푚,푖푗 = −∇푖푈푖푗. The total magnetic force exerted on 풎푖 by its neighbouring magnetic moments is therefore the sum of pair contributions from all other magnetic moments [52]:

3휇0 푭푚,푖 = ∑푗≠푖 4 [(풓̂푖푗풎푖)풎푗 + (풓̂푖푗풎푗)풎푖 + (풎푖풎푗)풓̂푖푗 − 5(풓̂푖푗풎푖)(풓̂푖푗풎푗)]. (8) 4휋푟푖푗

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Since magnetic moment is an extensive quantity, it is usually more appropriate to use a related quantity, known as magnetisation (SI unit: Am−1), which is defined as magnetic moment per unit volume [52]:

푑풎 푴 = . (9) 푑푉 According to electromagnetism theory, magnetisation (as happens with magnetic moment) is a source of magnetic field. In a medium with magnetisation 푴 under an applied magnetic field 푯 (SI unit: Am−1) the resulting magnetic induction is given by:

푩 = 휇0(푴 + 푯). (10) Interactions between electrons are responsible for the large variety of magnetic behaviours of materials [52]. The main types of magnetic behaviour are depicted in Figure 15.

Figure 15. Typical arrangements of magnetic moments of paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials.

In a paramagnetic material, in the absence of an applied magnetic field (퐻 = 0), the atomic or ionic magnetic moments are oriented in random directions due to thermal disorder so that the net (spontaneous) magnetisation is zero. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, leading to a nonzero induced magnetisation. For a low applied field, the magnitude of the induced magnetisation is proportional to the applied field, 푀 = 휒퐻, where 휒 is known as the magnetic susceptibility [51, 54].

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

In a ferromagnet, due to strong electron interactions, the magnetic moments tend to orient parallel to each other, originating a nonzero spontaneous magnetisation. A ferromagnetic sample minimises its total magnetic energy by the formation of magnetic domains (see Figure 16). Inside each magnetic domain, magnetic moments are aligned along a specific crystallographic direction, which differs from domain to domain. As a result, in the absence of an applied field, the total magnetisation may be relatively small, which corresponds to the usually called non magnetised state [51, 54].

Figure 16. When a ferromagnetic divides into domains, its energy decreases as its induction field lines become confined essentially to its interior [55].

The magnetic domain structure can be suppressed by the application of a sufficiently strong magnetic field, resulting in a remarkable increase in magnetisation. Once the monodomain state is achieved and the final domain is aligned with the applied field, magnetisation remains almost constant at a value known as the technical saturation magnetisation (푀푠푎푡 ). After removing the external field, the sample acquires a new domain structure, which is dominated by domains whose orientation are more aligned with the applied field. This results in a remanent magnetisation (푀푟) and the sample is said to be magnetised. To demagnetise it, it will be needed to apply a reverse field whose value is known as magnetic coercivity (퐻푐) [51, 54]. A typical full magnetising cycle is shown in Figure 17.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 17. Typical magnetising cycle of a ferromagnet and corresponding magnetic domain structure.

Because of thermal disorder, every ferromagnetic material has a characteristic transition temperature, called the Curie temperature, or Curie point, above which it becomes magnetically disordered or paramagnetic. Common ferromagnetic materials include iron, nickel, cobalt, their alloys, and some alloys of rare-earth metals. The most powerful permanent magnets are made of rare-earth intermetallic alloys because these materials exhibit high values of remanent magnetisation and coercivity [51, 54].

In an antiferromagnet, electron interactions originate an anti-parallel alignment of neighbouring magnetic moments, giving rise to a zero net magnetisation. Typically, an antiferromagnetic material has two crystal interpenetrating sublattices; magnetic moments in one sublattice are oriented parallel to each other but opposite to the moments of the other sublattice. The anti-ferromagnetic ordering has a critical temperature, known as the Néel temperature, above which paramagnetism is observed [51, 54].

A ferrimagnetic material is similar to an antiferromagnet but the magnetic moments of its sublattices do not exactly compensate each other, so that it has a nonzero spontaneous magnetisation, like a typical ferromagnet. Ferrimagnetic materials also exhibit magnetic domains. Most naturally occurring ferrites, like magnetite, are ferrimagnetic [51, 54].

Every material has, to some extent, a weaker form of magnetism, known as diamagnetism, which is basically characterised by a small and negative contribution to the magnetic susceptibility. Such contribution is usually negligible in materials exhibiting one of the four main types of magnetism. In nonmagnetic materials, such as cork particles, however, the diamagnetic component is dominant [51, 54].

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

2.3.2.Magnetic characterisation of the particles Cork and magnetised cork microparticles of different sizes were magnetically characterized at CISM (Centre for Imaging and Structure of Materials), University of

Aveiro. Three particle size ranges were considered: 125-250 μm (sample A), 75-125 μm

(sample B) and 53-75 μm (sample C). The magnetic hysteresis cycles (see Figure 18) of the samples were measured at room temperature in the field range ± 70 kOe (± 7 Tesla), using a MPMS3 SQUID magnetometer. It should be noticed that an applied magnetic field of 퐻 = 10 kOe corresponds to a magnetic induction 퐵 = 휇0퐻 of 1 T. Particles containing magnetite (samples A, B and C) present a considerable magnetisation, which is fully compatible with the predominant ferrimagnetic behaviour of magnetite. The 푀(퐻) curves were normalized by the whole mass (cork + magnetite).

Fe O +Cork M(H) 25 3 4 A 20 B C 15 Pure Cork 10

-5 M (emu/g) M -10

-15

-20

-25 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 H(kOe) Figure 18. Magnetisation curves as a function of the applied magnetic field.

The saturation magnetisation, 푀푠푎푡 , of the cork-magnetite samples is size-dependent: samples A (125-250μm) and B (75-125μm) have a saturation magnetisation of about 18.1 and 17.6 emu/g, respectively, while sample C (53-75 μm) has a higher value of 22.6 emu/g. These saturation values correspond to only about 19, 20 and 25 % of the expected magnetisation of a pure bulk magnetite crystal, neglecting the mass of cork present. The changes in 푀푠푎푡 with particle size may be related with differences in the relative amount of magnetite and cork present for different particle sizes.

As expected (see Figure 19), the pure cork sample exhibits a diamagnetic behaviour, characterised by much lower values of magnetisation and a negative linear response to the applied field.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Pristine Cork M(H) 0.10 Pure Cork 0.08

0.06

0.04

0.02

0.00

-0.02 M (emu/g) M -0.04

-0.06

-0.08

-0.10 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 H(kOe) Figure 19. Pure cork magnetisation curves as a function of the applied magnetic field.

From these measurements in such broad magnetic field range, an estimate of the coercive field (needed for zero magnetisation) cannot be done with accuracy. Nevertheless, considering the typical field measurement errors in this procedure, one can estimate that the A, B and C particles have coercivity fields below 10 Oe (1 mT). This means that the particles of all three samples can be easily magnetised and demagnetised with magnetic fields as low as 0.01 T, which corresponds to ten times the coercivity. However, given the time constraint of this work, only one particle size range was used in the magnetophoresis studies. Particles A were selected because, as magnetisation results show, the magnetisation curve of these particles is the less size-dependent among the three size ranges.

2.3.3.Magnetophoresis The term magnetophoresis generally refers to any type of technique that exploits the induced motion of magnetic particles through viscous media under the influence of external magnetic fields [56]. This is a rapidly growing research area primarily due to its potential applications in biomedical research and clinical diagnosis and therapy [57]. The appealing feature of magnetophoresis is that it can provide conditions for controlled particle separation.

In this work, magnetophoresis is applied to change the spatial distribution of cork- magnetite microparticles immersed in a viscous epoxy resin. It is assumed that all particles are identical and their magnetic moments are due only to the magnetite

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

component. Regarding the magnetophoresis driving mechanism, there are several important simplifying assumptions that are usually made:

a) The magnetic torque, as given by Eq. (4), is very effective in quickly aligning the magnetic moments with the applied magnetic field, so that 풎 and 푩 may be considered parallel throughout the entire motion. This means that particle rolling can be neglected. b) For sufficiently low particle concentrations, the magnetostatic forces, as given by Eq. (8), can be neglected. Therefore, only the Zeeman force needs to be considered. c) Because common magnetophoresis applications use paramagnetic particles, commercially available simulation software packages (e. g., COMSOL Multiphysics) assume that the particle magnetic moment is a linear function of the

applied magnetic field, 풎 = 푉푝푴 = 휒푉푝푯, where 푉푝 is the particle volume and 휒 is the magnetic susceptibility. Considering 푩 = 휇푯, where 휇 is the magnetic

permeability ( 휇 = 휇0휇푟 ; 휇푟 is the relative permeability), the Zeeman force

becomes: 푭푍 = ∇(풎 ∙ 푩) = 2휇휒푉푝(푯 ∙ ∇)푯 = 2(풎 ∙ ∇)푩. Given the previous assumptions, this will be the only contribution considered to the magnetic driving force.

There are four relevant external forces acting on magnetic particles moving through a viscous medium: the hydrodynamic drag force ( 푭푑푟푎푔 ; 푥, 푦, 푧 components); the gravitational force (푭푔푟푎푣; −푧 component); the buoyant force (푭푏푢표푦; +푧 component); the magnetic force (푭푚푎푔푛; 푥, 푦, 푧 components). Figure 20 shows a schematic diagram of the forces acting on a magnetic particle.

Figure 20. Diagram of the forces acting on a magnetic particle within a viscous medium. The y components (out of the plane) are not represented.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

These forces are given by:

푭푑푟푎푔 = 6휋휂푅푝(풗푝 − 풗푓) (11)

푭푏표푢푦 = 휌푓푉푝품 (12)

푭푔푟푎푣 = 휌푝푉푝품 (13)

푭푚푎푔푛 = 푭푍 = 2(풎 ∙ ∇)푩 (14)

In these equations, η is the viscosity of the fluid, 푅푝 is the average radius of the particle,

풗푝 and 풗푓 are the particle and fluid velocities, respectively; 휌푝 and 휌푓 are the particle and fluid densities, respectively; 품 = −푔풌̂ is the acceleration of gravity.

In order to displace non-uniformly the magnetised particles along the bondline of the joint, it was decided to use permanent magnets, which can be arranged to produce a magnetic field of the required magnitude with favourable geometry and gradient.

The modelling of magnetophoresis of cork-magnetite microparticles for different magnet arrangements was made using the COMSOL Multiphysics software. This software has three important modules which can tackle different parts of a magnetophoresis model. The “Magnetic fields, no current” module is used to calculate the magnetic field distribution of a system of permanent magnets. Magnet geometry and dimensions, as well as corresponding magnetic properties, such as the remnant magnetisation and magnetic relative permeability, can be input. The “Creeping flow or laminar flow” module defines the type of fluid motion, the temperature and fluid’s (adhesive’s) properties, such as the density and dynamic viscosity. Finally, the “Particle tracing” module is used to simulate the motion of a set of magnetic particles within the adhesive fluid, driven by a static non- uniform magnetic field. Input parameters include the number of particles (not exceeding 1% of volumetric fraction for more accurate results), particle density, size and relative permeability; the time at which the particles are randomly released can also be specified. The forces acting on the particles are of the types described by Eq. (11-14).

A limitation of the “Particle tracing” module is that it does not allow the modelling of magnetic other than paramagnetic particles. The more complex magnetisation curve of ferrimagnetic cork-magnetite particles must, therefore, be approximated by a linear relationship typical of paramagnetic materials. A particle can be modelled by sketching a cork sphere and a shell of magnetite, but this is not a feasible solution since it would have to be replicated for more than 100 particles. In effect, for a typical adhesive layer of

25×50×0.5 mm and particles with a diameter of 125-250 μm, a 1% volumetric fraction of particles corresponds approximately to 1811 particles. Moreover, their initial positions would have to be handpicked and their magnetisation curves individually defined. Despite

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

these limitations, it is believed that simulations with COMSOL are still able to provide useful insight into the main aspects of the magnetophoretic behaviour of cork-magnetite particles.

COMSOL was first used to model a permanent magnet using the “Magnetic fields, no currents” module. The main purpose of this was to validate the numerical results of the simulation with the real values given by the magnet manufacturer. Apart from all the parameters already mentioned above, it was also necessary to stipulate a surrounding atmosphere (see Figure 21), serving as a boundary condition. Figure 22 shows the model for a single BC-14 N52 magnet from K&J Magnetics (Pipersville, PA), where a refined mesh was applied in order to obtain better results. Table 1 shows the corresponding dimensions of the magnet.

Figure 21. Surrounding atmosphere of the magnet model.

Figure 22. Permanent magnet considered in the COMSOL model.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Table 1. Dimensions of a single BC-14 N52 magnet from K&J Magnetics.

Length (l) Width (w) Height (h) [mm] [mm] [mm] 19.05 6.35 1.59

According to the supplier’s datasheet, the surface magnetic flux density at a reference 푙 ℎ point of coordinates ( , , 푤) , also represented in Figure 22, is 0.683 T and the 2 2 corresponding obtained value from the simulation was 0.675 T, which corresponds to an error of just 1.2%. The calculated variations of the magnetic flux density (B) across the width (푤) and length (푙) of the magnet are shown in Figure 21-Figure 21. Near the magnet borders, specially across the width, the flux density shows appreciable deviations from its central value. These variations of the magnetic flux density are symmetric with respect to the centres of the corresponding magnet faces.

Figure 23. Variation of the flux density across the magnet width obtained from COMSOL simulation with the “Magnetic fields, no currents” module.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 24. Variation of the flux density across the magnet length obtained from COMSOL simulation with the “Magnetic fields, no currents” module.

In this work, to create a magnetic field gradient, a set of permanent magnets (with a N40 grade, from K&J Magnetics) was used arranged in a configuration with alternate directions of their magnetisations. Such type of magnet arrangements are known as Halbach magnet arrays [58]. Figure 25 shows the first magnet configuration (Magnet Array I, MA I) used in this study, which features an adhesive layer with an overlap of

25×50×0.5 mm.

Figure 25. First Halbach array used in the COMSOL simulations. The arrows indicate the directions of the magnetisation of each magnet.

The magnetic flux density distribution for this magnet arrangement, calculated with COMSOL, is shown in Figure 26. The corresponding results of the magnetophoresis simulation are presented in Figure 27-Figure 28, in terms of initial and final particle distributions. It is seen that 5 s is a sufficient period of time to produce a significant change in the particles distribution.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

MA I was constructed by gluing the component magnets with super glue in a wood device (see Figure 29) used to prevent the effects of strong repulsion between magnetically misaligned adjacent magnets.

Figure 26. Simulations results for MA I: (top) magnetic flux density along the overlap length (xz plane); (bottom) colour map of the magnetic flux density gradient on the xy plane.

Figure 27. Initial (푡 = 0푠) and final (푡 = 5푠) particles distributions obtained from magnetophoresis simulation for MA I.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 28. Initial (푡 = 0푠) and final (푡 = 5푠) particles distributions in xy plane, obtained from magnetophoresis simulation for MA I.

Figure 29. Wood device used to constrain the magnets during the gluing process.

Several magnetophoresis tests were conducted with MA I, but it was found that the final particle distribution not only had poor repeatability but was also quite irregular. This was likely due to, among other factors, the high sensitivity of the particles motion to changes in the magnetic flux density during the positioning of the top row of magnets over the sample glass slide.

A second magnet array (MA II) containing only the bottom row of magnets of MA I was therefore tested. This new arrangement made the experimental procedures easier and increased the magnetic flux density. It provided more reproducible results, but the obtained particle distribution exhibited a somewhat irregular and stepwise profile (see Figure 30).

Figure 30. Backlight image of a glass slide with particle distribution obtained using MA II.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Thus, a decision was made to remove the two inner magnets, in order to increase the magnetic flux density slope from the middle to the edges. The new magnetic array (MA III), as depicted in Figure 31, features an air gap and magnets of grades N50 and N40.

Figure 31. Magnet arrangement MA III.

As shown in Figure 32, the obtained typical particle distribution exhibited already the required gradation and a better consistency.

Figure 32. Backlight image of a glass slide with particle distribution obtained with MA III.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

3. Development of the experimental procedure

For a correct implementation of the newly proposed method of producing FGA SLJs using magnetised microparticles, the correspondent experimental procedure can be divided into three parts, named:

• The adhesive selection; • The substrates selection; • The magnetised microparticles.

Therefore, the first step consists of determining which type of adhesive should be applied in order to visually verify a non-uniform distribution of the magnetised particles. With the objective of increasing the joint’s performance, an epoxy resin was selected, which is commonly known to have a brittle behaviour.

To assess whether the adhesive is suitable to produce such joints, preliminary tests were made using glass slides with a silicone frame, representing the overlap of a joint (see Figure 33). The adhesive used in this first attempt was BetamateTM UN3077, from DOW (Switherland). From the glass slide tests, it was visually concluded that when a magnetic field was applied on the magnetised particles uniformly distributed in the resin, they would not move. This could be explained by the fact that this resin is too viscous (45 Pa‧s) to let the particles displace freely within it.

Figure 33. Example of a glass slide used in preliminary tests.

On the other hand, in parallel to the BetamateTM UN3077 glass slide tests, the characterisation of the adhesive was performed, regarding its cure curve (푇푒푚푝푒푟푎푡푢푟푒 = 푓(푡𝑖푚푒)). This was performed for the neat adhesive itself and for the composite resin – 1% of magnetised particles to evaluate is there is any sort of influence of those particles on the cure regime (see Figure 34-Figure 35). From the obtained cure cycles, it was concluded that the particles do not influence them, being the adhesive cured at 100 ºC for 350 minutes (approximately 5.8 hours) and then submitted to cooling.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 34.Cure cycle for the neat resin.

Figure 35.Cure cycle of the adhesive with 1% of magnetised cork particles.

The glass transition temperature (Tg) was measured, having a value of 161.41 ± 3.78 ºC. Also, the tensile stress–strain curves were determined for this adhesive, being shown in Figure 36 a typical curve. Therefore, the average values of the intrinsic mechanical properties for the BetamateTM UN3077 experimental results are shown on Table 2.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 36. Experimental representative curve 휎 = 푓(휀) for the adhesive BetamateTM UN3077.

Table 2. Experimentally determined mechanical properties of BetamateTM UN3077.

Young’s Modulus Maximum Tensile Stress Maximum Strain [MPa] [MPa] [mm/mm] 3483.2 ± 26.5 57.8 ± 5.4 0.0233 ± 0.00290

Thus, as a next iteration, the adhesive selected needed to be less viscous than the previous one and, for this reason, Araldite2020®, from Huntsman Advanced Materials (Pamplona,

Spain), was chosen, having a dynamic viscosity approximately of 0.15 Pa‧s.

Once the adhesive had been chosen, the implementation to the joint was carried on – the proper selection of the substrates. So, since there can be no interference with the magnetic properties of the magnetised particles, the material used for the substrates must have nonmagnetic properties. For example, aluminium is a suitable alternative for the adherends. Therefore, the material used in the adherends was the AW 7075 T651 aluminium alloy, from Poly Lanema, Lda (Ovar, Portugal).

In terms of adherends thickness, aiming the reduction of peel loads, 3 mm was the adopted size for a 25×50 mm (width×length) overlap area with 0.5 mm of adhesive thickness, so that the plastic deformation of the substrates could get minimized. The mechanical properties of the aluminium alloy AW 7075 T651 are shown in Table 3.

Also, based on the materials properties and on the joint’s geometry (see Figure 37), a classical analysis was made using the Goland and Reissner formulation to assess whether the substrates suffer plastic deformation (see Figure 38). Indeed, for a 50 mm overlap

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

length, it was determined that it is the adhesive that is working under loading, being the adherends still in the elastic domain.

Table 3. Mechanical properties of AW 7075 T651.

Young’s Modulus Yield Stress Ultimate Stress Poisson’s Elongation [GPa] [MPa] [MPa] Ratio (%) 71 470 540 0.3 7

Figure 37. SLJ geometry, with 25 mm width (all dimensions in mm).

Figure 38. Failure load as a function of the overlap length, according to the Goland and Reissner classical analysis.

Regarding the surface treatment, for a good adhesion at the substrates interface, they were first abraded using sandpaper (± 45º orientation), followed by proper cleaning (to remove grease and other impurities), an phosphoric acid anodization process, according to the ASTM D3933 standard, and then by the application of a Scotch-WeldTM EW-5000 AS primer, from 3MTM.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

However, from experimental results regarding aluminium SLJs with a 0.5 mm Araldite2020® adhesive thickness, bad interface adhesion between the primer and the substrates was found, thus leading to the conclusion that the anodization process is not enough to provide good adhesion at the interface. These results can be visualized in Figure 39, where the resulting type of fracture is a reflection of this bad adhesion – near the interface or adhesive failure.

Figure 39. Example of bad interface adhesion in an aluminium SLJ.

Additionally, notwithstanding the previous conclusions, in all aluminium joints tested until this moment, some plastic deformation was detected, detectable by bent substrates. (see Figure 40). The peel loads induced by this bending have more influence on the failure of the joints than shear loads, which substantially decreases the correspondent failure loads.

Figure 40. Examples of aluminium substrates plastic deformation, for a 25 mm overlap width SLJ.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

In order to solve the bending of the substrates and to enhance the interfacial adhesion between them and the adhesive, the joints surfaces will be treated with a Sol-Gel anodizing replacement Surface Pre-Treatment AC-130-2, from 3MTM followed by the application of a Scotch-WeldTM EW-5000 AS primer, from 3MTM. Additionally, the overlap area will be reduced to 15×50 mm (width×length).

Magnetised cork microparticles with a size range of 125-250 μm were used. Over time, it was noticed that the particles tend to get magnetised (due to the remnant magnetisation), causing them to agglomerate. A SEM analysis was performed on a fracture surface of a composite resin-magnetised cork microparticles plate, which can be seen in Figure 41. These SEM images are based on backscattered electrons and the differences in colour are given by the atomic number of the correspondent constituent. Particularly, the lighter colour means it is ferrite, since its atomic number is bigger than of carbon, which is the main constituent of the cork and the adhesive. Ideally, only magnetised cork particles like the one seen highlighted in the green area of Figure 41 should be present, which exhibit a good magnetite coating. However, as it is shown in the red area of Figure 41, magnetite inclusions appear to have loosened from the cork particles, as they are undesirable, since the experimental results will be influenced either visually or through the mechanical properties of the composite resin-magnetised cork microparticles.

Figure 41. SEM image of a fracture surface of a composite resin-magnetised cork microparticles plate.

Therefore, from time to time, the magnetised particles batch should be sieved, removing inclusions or particles which size is smaller or bigger than 125-250 μm, such as mainly the agglomerated particles and the magnetite dust.

The influence of the inclusion of sieved particles and non-sieved particles was studied. In fact, huge differences have been duly noted from experimental results. Those were

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

obtained from bulk testing in quasi-static conditions (1 mm/min), considering the BS 2782 standard, for the adhesive Araldite2020® with 1 % amount (in volume) of the corresponding magnetised microparticles. The results regarding the tensile stress as a function of the strain are shown in Figure 42.

Figure 42. Comparison of the influence of the inclusion between sieved particles and non-sieved particles – Tensile Stress as a function of the strain.

Based on the curves shown in the previous figure, as expected, the addition of more particles (i.e. magnetite rigid inclusions and bigger particles) in the epoxy resin tends to increase the stiffness of the composite and, consequently, the strain leans to lower values. On the other hand, in contrast, the sieved particles composite has higher plastic deformation (higher values of strain), as expected, having then a lower maximum tensile stress.

To sum up, facing all the obstacles presented above, a solution can be excerpted for each of the problems pointed out:

• The adhesive used will be Araldite2020®; • Instead of a standard ASTM D3933 anodization, the joints surfaces will be treated with a Sol-Gel anodizing replacement Surface Pre-Treatment AC-130-2, from 3MTM, followed by the application of a Scotch-WeldTM EW-5000 AS primer, from 3MTM;

• The overlap area will be reduced to 15×50 mm (width×length), instead of 25×50 mm;

• The particles will be sieved, fulfilling the wanted size range of 125-250 μm.

Experimental results are shown in Paper 3.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

4. Development of an apparatus to produce FGA SLJs

The main goal of this thesis is to produce functionally graded adhesive single lap joints with magnetised microparticles, where the ends of the overlap have higher percentage of those particles than the middle area. The displacement of the magnetised particles is made by a set of magnets. Therefore, a customized apparatus (shown in Paper 2) capable of manufacturing in a practical manner such joints was developed. The patent (Paper 2) explains the developed apparatus the as well as the method that ensures a graded distribution of particles and thereupon graded adhesive properties along the overlap length of the joint by the application of magnetic fields created by the magnets’ arrays. In order to proceed with the development of the apparatus, it is first necessary to establish the design requirements, being the most relevant the following:

• It must assure the alignment of the joints and guarantee the correct overlap dimensions; • It must be compact as well as capable of withstanding various temperature and pressure conditions without damage; • It needs to have suitable slots for accommodating the magnets’ holders; • Cannot interfere with the magnetic fields generated; • It must able to manufacture two SLJs at the same time, without interference between adjacent magnetic fields; • It must be easy to use.

Aluminium was the material chosen for the manufacturing of the apparatus as it is a non-ferromagnetic metal. Images and detailed descriptions of the apparatus are seen Paper 2.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

5. Conclusions

The objective of this thesis was to develop a new technique to obtain a joint with functionally graded adhesive properties along the bondline. Magnetised cork microparticles were used in order to create a custom distribution along the overlap so that a more uniform stress distribution would be achieved, thus enhancing the joints performance. Numerical models were developed in order to find the best particle gradient distribution along the overlap of a single lap joint, considering various magnet set ups.

The joints gradually modified along the overlap were obtained by the application of tailored magnetic fields, using an apparatus designed for the purpose. It was experimentally proven that these joints have a different mechanical performance when compared to both the joints with uniform particle distributions and the joints using only neat resin. Additionally, the magnetic arrays are dependent on the amount and size of the magnetised particles, so that a graded distribution of particles can be obtained.

6. Future work

This thesis is an integral part of a Foundation for Science and Technology project (POCI- 01-0145-FEDER-028035). Within the scope of this project, as a reference to further developments, there should be taken into consideration the following aspects:

• Araldite2020® is a low viscosity adhesive and because of that particles deposition occurs. Therefore, new adhesive systems should be developed and tested in order to improve this issue; • A complementary study on the influence of magnetite-cork reinforcements on the fatigue behaviour of the adhesive would also provide valuable scientific data and enable the design of the long lasting structures with these materials; • Fracture toughness tests should be performed at high and low temperatures, which would allow to assess if these reinforcements alter the temperature dependence of the adhesive’s fracture toughness; • Hydrothermal degradation tests should be carried out so that the behaviour of the magnetite within the adhesive would be assessed under extreme conditions; • An intensive study regarding the magnetic characterization of the magnetised cork microparticles is in the line as one of the next steps to be done; • Another possible next step would consist in analysing the effect of the magnetised microparticles for different size ranges, determining their ideal size range; • Last but not least, this technique of producing functionally graded joints should be assessed for other types of magnetised particles.

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

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Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

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Paper 1 Patent Paper 2

Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Paper 1

Literature review article

51 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

52 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

An overview on manufacturing functionally graded adhesives – Challenges and prospects

J.B. Marques1, A.Q. Barbosa1, C.I. da Silva2, R.J.C. Carbas2, L.F.M. da Silva2

1INEGI, Rua Dr. Roberto Frias 400, 4200-465, Porto, Portugal

2Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias 400, 4200-465, Porto, Portugal

Abstract

Adhesive bonding is a constantly growing and compelling method of joining materials and structures mainly due to its cost-effectiveness, reliability and versatility. Its ability of joining a large range of materials and capability of reducing the stress concentrations in the parts to be joined is preferred, in some situations, over the use of other mechanical joining methods such as riveting and bolting.

In recent years, adhesive bonding has become a key technology among the various industrial sectors, namely the automotive industry due to its constant demand for lighter, more resistant and environmentally friendly materials. Therefore, it is of great interest to further develop this kind of bonding, by developing functionally graded adhesive joints. Functionally graded adhesives (FGA) can be defined as tailored adhesives, in which the change in composition and/or microstructure is continuous along a position, allowing a more uniform stress distribution along the bondline. Application wise, these joints are very promising due to their potential high degree of customization, offering more solutions and options regarding design.

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1. Introduction

Adhesive bonding is a growing and compelling method of joining materials and structures mainly due to its reliability and versatility, namely the ability to join a large range of materials [1-3]. Similarly to the tendons in our body, adhesive bonding allows a more gradual transfer of shear load from one substrate to another [4] and lower localized stress concentrations [5] due to not requiring holes such as riveting and bolting [1-3]. Adhesive bonds allow a better joining of dissimilar materials [1, 3, 6] when compared to welding for example. It is also more cost-effective and also lighter than other joining methods, thus reducing the overall weight of structures [7].

In recent years, structural adhesive bonding has become a key technology among the various industrial sectors, namely the automotive industry due to its constant demand for lighter, more resistant and environmentally friendly materials. For the reasons stated above, the interest in this type of bonding is not exclusive of the automotive industry but also the civil, aerospace, microelectronic and optoelectronic industries [8].

Currently in the automotive industry, the demand is met with customized materials such as composites, which combine different material groups in a single component. The same logic is applied to adhesive bonding, where adhesives with tailored electrical, mechanical and thermal properties are being sought.

However, adhesively bonded joints still face a number of issues. The overlap ends of the joints are under high shear and peel stress gradients and home to high stress concentrations due to stiffness changes [7], therefore reducing joint performance. This is especially valid for the case of the single lap joint (SLJ), which is the most common joint geometry [3]. Therefore, the uniformization of the stress distribution along the adhesive bondline is one possible answer to fix the concerns raised in the previous paragraph [9].

In the literature several methods can be found to obtain a more even stress distribution throughout the joint[3]. Most involve a geometrical modification of the joint such as adhesive filleting [10], adherend tapering [11], rounded adherend corners [12], novel joint geometries [13] and adherend denting [3]. Many of these, however, increase the design and manufacturing complexity.

Throughout history, the development of new materials has played a key role, allowing the creation of advanced polymers, ceramics, metal alloys, etc. One benchmark in this

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evolutionary curve was the development of composite materials, which are a class of advanced materials. Composites combine one or several materials with different physical and chemical properties that form a unique material, with different physical and chemical properties from their individual parent materials.

Functionally graded materials (FGMs) are advanced materials in the family of engineering composites [14], having properties that vary along a dimension in a continuous way, enabling a clean transition from one material to another [15] (see Figure 1 and Figure 2). The problem of combining two different materials, as for example a ceramic material bonded to a metal, is that the transition between the two materials can be too abrupt, requiring an accommodation of strain leading to plastic deformation or even cracking [16]. Using graded materials, the large deformations are held before yielding or failure since the stresses are lower [16]. Erdogan listed some of the advantages in using FGMs in contrast with conventional and composite materials, which include the increase in bond strength in dissimilar material joining, the reduction of thermal and residual stresses as well as crack driving force reduction [17].

This concept was initially proposed in the mid 1980’s, where researchers in Japan were faced with a requirement for the project of a thermal barrier, in a space shuttle, with a 1000 K temperature variation between its outside and inside, which was less than 10 mm thick [18]. The differences of the joined materials, more specifically in the thermal expansion coefficients between iron, aluminium and carbon composites lead to the deformation and separation of materials. This problem incentivised the development of more heat-resistant materials, and the concept of functionally graded materials was born to answer these problems, reducing the thermal stresses at the joined surfaces [19].

Figure 1 – Illustration of non-continuous (a) and continuous structure (b)

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Figure 2 - Material structural properties of ordinary composite materials vs functionally graded composite materials [19].

Several examples of these functionally graded materials already exist in nature, as it is the case of bones and teeth [15]. Researchers target this natural solution found in the real world in order to solve real life engineering problems.

In nature, material grading occurs as a response to the formation of stress concentrations in material interfaces. The beak of the Humboldt squid, for example, is one of the stiffest and hardest materials known, yet the surrounding muscle structure is compliant [20]. Due to the muscle mass being graded, the transition between the very stiff beak gradually decreases two orders of magnitude from the tip of the beak to its base [20]. Another, more relatable example is one of the many biological interfaces presented in our body, the tendon to bone joints. These have been found to have graded material properties, allowing a more balanced stress distribution across the joint [21, 22]. In Figure 3 from [22], the two different tissues that are present in the tendon to bone joint are shown, those being the tendon which has an Young’s Modulus of 0.4 GPa in the direction of muscular force in physiological loading conditions, and the bone with an Young modulus of 20 GPa, which represents a big material mismatch. This mismatch in mechanical properties is atoned by a functionally graded interface (tendon-to-bone insertion).

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Figure 3- Tendon to bone insertion, connecting two very different materials [22].

Similar to the tendon to bone joint, the development of functionally graded adhesive joints can bring improvements to the overall joint performance in structures. It should be noted that even with FGA the joint design could be optimized in order to further improve its performance [23].

FGAs can be defined as tailored adhesives that have varying gradual mechanical properties along a desired dimension, allowing a more uniform stress distribution along the bondline [24]. Joint improvement regarding joint strength has been achieved by manufacturing graded joints with microparticles [25], nanoparticles [26], tailored adhesives using adhesive mixes [5, 27] and graded cure processes [28, 29]. Improved flexural strength by graded micro particle distribution was achieved in [30]. The non- stepwise combination of adhesive mixes also leads to the fabrication variable Young’s moduli adhesive plates [31, 32] that further allow custom design of joints. The current techniques in obtaining FGA are presented in Figure 4.

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Figure 4 – Techniques uses to create FGA.

This review aims to list the ultimate enhancements, manufacturing processes and progress recently developed in the field of FGA.

In the first three sections (2, 3.1 & 3.2) a brief introduction of what can be considered the first approaches to adhesive tailoring is presented, those being the combination of adhesives with different behaviours (mixed adhesive joints) and adhesive reinforcements using micro/nanoparticles of different nature, liquid rubber and metallic fibres. The following sections (3.3, 3.4 & 3.5) list the current techniques that allow the manufacturing of FGA joints, those being through graded cure, by varying the ratio between two adhesives along the bondline length, and by strategic placement of micro and nanoparticles along the bondline.

2. Functionally graded materials – brief overview

In the past section, the concept of FGM was introduced and despite being a recent concept, it may provide clues towards the development of FGA. In this section, the main types of processing methods, grading and applications will be briefly discussed.

FGM can be divided into four types of grading, namely, fraction gradient, shape gradient, orientation gradient and size gradient (Figure 5). As expected, this field still requires a lot

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more growth since there are very few studies regarding not only FGM modelling and analysis but also manufacturing methods and experimental testing [33]. Nevertheless, this technology already has a variety of applications in the automotive, aerospace and commercial industries which may take form in thermal protection and wear resistance coatings [33].

Figure 5 – FGM grading type. a) Fraction gradient b) Shape gradient c) Size gradient d) Orientation gradient (adapted from [33]).

Currently, the main processing methods for FGM are additive manufacturing (AM), centrifugal casting (CC), powder sintering (PS) and material coating using chemical vapour deposition (CVD) and physical vapour deposition (PVD). CVD and PVD are used in surface coating, while CC only allows the creation of cylindrical components and PS involves the mix and sintering of premixed-powders [33].

Based on this information it can be stated that these processes are not interesting towards the adhesive field. However, some concepts from the previous methods can be adapted to the manufacturing of FGA, for example, in CC, the production of mixed metal composites (MMC) makes use of not only gravitational and centrifugal fields but also magnetic, electric and electromagnetic fields. These fields are responsible for influencing and organizing particles in the liquid composite slurry, this way tailoring the MMC final structure [34]. By creating a custom heterogenic magnetic field flux density gradients and superimposing fields, it should be possible to organize particles in a non-uniform manner

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along a certain direction, regarding their orientation and size, in different fractions/concentrations.

AM is a recent manufacturing method which includes stereolithography (SL), laser sintering (LS), fused deposition modelling (FDM) and laminated object manufacturing (LOM). Compared to the techniques mentioned above, it is probably the most promising approach regarding the production of a functionally graded adhesive bondline. AM bases itself in the emerging use of 3D printing technologies, where a component is made layer by layer, thus allowing creating a graded adhesive bondline increment by increment. The application of different adhesive mixes (by changing the volume fraction of two components) along an adherend through a printer head has already been done in some studies [5, 27]. Additionally, as mentioned in the previous section, the optimal design of a joint could be found in the use of FGA and functionally graded adherends which are realizable by also using AM [35].

3. Functionally graded adhesives manufacturing and experimental results 3.1 Background on mixed adhesive joints

Mixed adhesive joints (MAJs) are a combination of stiff and ductile adhesives, resulting in a stepwise variation of properties along the adhesive bondline [36-40]. The combination of a brittle adhesive at the centre of the joint and ductile adhesive at the edges (Figure 6) provides a synergetic effect [37, 39, 41], allowing the load transfer from the overlap ends which are prone to stress concentrations [3, 37, 40], to the middle higher strength adhesive.

Several authors focused on developing MAJs and their findings are reported bellow. All the studies target the combination of ductile and brittle adhesives, as described in the previous paragraph, where the main concerns are the brittle-ductile adhesive quantities and corresponding lengths placed along the joined substrates and its assembly during manufacturing.

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Figure 6 – Mixed adhesive joint (adapted from [3]).

Pires et al. [38] used a high modulus adhesive in the middle part of the overlap combined with a compliant adhesive in the overlap ends. After comparing the bi-adhesive joints with the single adhesive joints, it was noted a 22% increase in the overlap strength and peel stress reduction for the bi-adhesive scenario, when compared to the joint made exclusively out of either singular adhesive.

In 2004, Fitton et al. [40] conducted a series of numerical and experimental studies on the subject of bi-adhesive joints. SLJs with three different adherends were manufactured, one bonding unidirectional carbon fibre reinforced plastic (CFRP) to unidirectional CFRP parallel to the fibre, another with unidirectional CFRP to unidirectional CFRP perpendicular to fibre and the final with unidirectional CFRP parallel to the fibre to steel. For each combination of adherends the joints were made using standalone high modulus adhesive, low modulus adhesive and three combinations of bi-adhesive joints. All of these combinations consisted of using different amounts of low modulus and high modulus adhesives along the overlap length (50 mm), displayed on Figure 7.

Figure 7- Adhesive combinations from ductile to stiff (adapted from [40]).

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Experimental studies, in the unidirectional CFRP to unidirectional CFRP parallel to the fibre SLJ indicated that the highest shear strength joint corresponded to the high modulus adhesive only SLJ, and the lowest to the low modulus adhesive only SLJ. The bi-adhesive combo did not provide improvements regarding shear strength, but in terms of failure the 5-40-5 variable modulus joint exhibited less composite failure at the joint, indicating lower peel stress presence. For more than 5 mm of low modulus adhesive at the overlap ends, the joint shear strength reduces. In the second experiment, using the high modulus adhesive, the failure occurred completely in the composite and the joint shear strength was under half of the bi-adhesive 5-40-5 combination. The third experiment with dissimilar adherends indicated that the bi-adhesive joint shear strength was the same as the single adhesive joints, but an increase of approximately 25% in strain was noted. This can be explained either by the ability of the low moduli adhesive to deal with higher strains to failure because of the adherends stiffness mismatch, or due to the reported steel adherend yielding.

Marques et. al [39] in 2008 also experimented with the combination of a ductile and brittle adhesive in double strap joints. Single adhesive tappers and mixed adhesive double strap joints (DSJ) with and without tappers were manufactured, in which the tapered bi- adhesive did not provide the strongest joints but provided higher ductility, therefore valuable to situations where a compromise of strength and ductility is sought [39].

In another study da Silva et al. [36] showed that, if done properly, the mixed modulus concept is a big improvement for bonding dissimilar materials whose difference of coefficient of thermal expansion (CTE) is high. The same author tried to further optimize bi-adhesive SLJs in [37]. In his study, the same brittle adhesive was applied at the middle of the overlap and SLJs were produced with three other increasingly ductile adhesives at the overlap ends. Higher joint strength was achieved in all bi-adhesive combinations when compared to the standalone brittle adhesive.

Overall, bi-adhesive joints performance is dependent on the ductile-brittle adhesive ratio and position along the overlap. Brittle adhesives provide joint strength but do not take advantage of the whole overlap length, and on the flipside using too much ductile adhesive may lower joint strength, yet with the right amount the load is transferred along the overlap to the brittle adhesive [37]. For a mixed adhesive joint to be stronger than a standalone brittle or ductile adhesive joint, a higher section of the overlap as to work

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under load, this is achieved by the synergistic effects of the brittle-stiff adhesive combination, in where the load carried by the stiff adhesive has to be higher than the load carried by the compliant one [37].

The manufacturing of these joints is not straightforward due to the need of controlling the length and thickness of both adhesives. Marques et al. [39] used a very stiff epoxy adhesive (Araldite AV138/HV998) and a more flexible epoxy adhesive (Araldite 2015), where the brittle adhesive was placed over the lower patch, between the nylon fibre barriers and the ductile adhesive on the remaining parts. Both adhesives were cured simultaneously.

Marques et. al [39] found the most effective manner to set the boundaries between the adhesives was to use a nylon line glued to the adherends, which allows very good dimensional control at the expense of a small area when compared to partitioning made using straps of Teflon or silicone. This technique is a bit tricky manufacturing wise, since the addition of a glued thin nylon line adds another degree of complexity to the process.

3.2 Background on adhesive reinforcement using second phase inclusions

The inclusion of a second phase to improve fracture toughness in the field of epoxy polymers and adhesives has been a successful and proven method since the late 1960’s [42, 43]. This second phase may be a rubbery phase (reaction-induced phase separation) [44, 45] or reinforcement inclusion (fibres, whiskers, particles) [44-46].

Regarding the incorporation of a rubbery phase in epoxy polymers, authors in [47] used rubber particles to modify the epoxy matrices, such as liquid CTBN (carboxyl-terminated butadiene-acrylonitrile), methacrylated butadiene-styrene copolymer (MBS) particles and MBS with a few per cent of carboxyl groups in a poly(methyl methacrylate) (PMMA) shell showed improved fatigue crack growth resistance for certain sizes. In this technique, the epoxy polymers modified by CTBN or MBS rubber particles were manufactured for several volume fractions and particle sizes of modifiers. Afterwards, they were tested by performing fracture toughness tests on single-edge notched (SEN) and compact tension (CT) specimens. The biggest concerns when it came to manufacturing were the prevention of bubble formation in the specimens, where the resin was degassed prior and after the addition of either the MBS particles or liquid CTBN rubber.

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Metallic fibres were tested as an adhesive reinforcement in [48], where stainless steel metallic micro fibres were used for this purpose. End-notched flexure (ENF), single-leg bend (SLB) and double-cantilever beam (DCB) tests indicated that for certain fibre distances shear banding was observed, hinting an improvement of the system with regards to fracture energy. In this case, some concerns such as galvanic corrosion may arise from this solution if a similar material is not selected for the fibre and adherends. Aluminium fibres were also used as reinforcement in [49] in SLJs and shear strength improvement was reported.

Further in the adhesive field, Kinloch et al. [50] increased the Tg, shear strength and adhesive toughness by using a rubber-toughened adhesive in combination with low concentrations (1-8%) of silica nanoparticles (20 nm). The adhesive system was produced using a two-component epoxy, sol-gel manufactured SiO2 particles and liquid rubber. Kishi et. al [51] concluded that a toughened epoxy resin containing 20 parts per hundred rubber (phr) of pre-formed polyamide-12 particles presented a T-peel strength three times higher than the neat unmodified adhesive.

For nanoparticles and whiskers usage as reinforcing phases, a heat resistant polymer- based adhesive reinforced with carbon nanotubes (CNT) and silicon carbide whiskers (SiCw) for low and high temperature applications, investigated by Wang et al. [52], displayed increased strengthening and toughening performance between room temperature and 700°C. Both the SiCw and CNTs were pre-treated in order to prevent agglomeration, these pre-treatments included the mix of the said reinforcements in an ethanol solution of silane and several ultrasound washes.

Banea et al. [53, 54] increased joint performance by mixing thermally expandable microparticles (TEP), which are mainly used for recycling purposes [53], with two different adhesives, being one brittle and the other ductile. The authors showed that an increase in amount of TEP (wt%) lead to a decrease in adhesive strength, while maintaining stiffness [53]. The tensile strength reduction was explained by the chance of the TEPs acting as hollow voids in the adhesive, increasing the stress intensity factor and therefore increasing stress concentration. However, the ductile adhesive had an inferior strength decrease. Regarding ductility, the only improvements occurred for the brittle adhesive, with 5 and 10 wt% of TEPs, obtaining a 56% and 10% increase, respectively. DCB tests were conducted using the brittle adhesive with three different amounts of TEP

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concentrations (5, 10 and 15 wt%) and a significant increase of fracture toughness occurred for all concentrations (152, 57, 37% respectively) [54]. The presence of these particles generates crack bridging, delaying crack propagation, hence increasing toughness.

Barbosa et. al studied the use of micro cork particles to enhance fracture toughness of brittle adhesives [55-58]. This material was investigated due to cork’s several attractive properties such as good thermal, acoustic and vibration insulation, flexibility, substantial permeability to liquids and gases and impact absorption. Barbosa et al. found that micro cork particles successfully acted as crack stoppers for certain volume concentrations and particle size ranges [55], improving the adhesive’s maximum strain to failure, lowering the composites glass transition temperature. The FTIR analysis showed that the cork microparticles did not have any chemical reaction with the used adhesive[57]. This results in an overall increase in ductile behaviour [56] and increase in energy release rate (퐺퐼푐) [58].

Several authors also made use of microencapsulated healing agents included in the resin, which promoted crack shielding and thus increasing fatigue life extension and fracture toughness [59-64].

Fine-tuning of the distribution of micro and nanoparticles along the bondline is one of the cornerstones required to obtain reliable FGA using these particles. For this effect, some important aspects can be listed, such as, the chemical interaction between the added particles and adhesive, particle surface treatments to prevent agglomeration, optimal particle size and concentration, as well as what kind of particles act as the best reinforcement (particle nature). Manufacturing concerns also arise since a process that allows the fabrication of faultless adhesive bonds with custom particle distribution has not been developed. Appropriate inspection methods also have to be employed to verify if the particles are distributed as intended, these include, and are not exclusive to, transmission electron microscopy (TEM) and scaling electron microscopy (SEM).

3.3 FGA by graded cure

Carbas et al. [29] took a different approach and successfully manufactured functionally graded SLJs through a graded cure, obtaining higher joint performance than regularly

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cured SLJs. The graded cure was executed by using a specially developed apparatus for SLJs composed by two heating coils, located near the overlap ends and a cooling coil situated at the middle of the overlap, thus generating a temperature gradient along the overlap (Figure 8). A positioning system was also built in order to guarantee the correct alignment, geometry, position and thickness of the adhesive layer.

Since this technique uses high frequency electricity to heat materials, either the joint or the adherends need to have some kind of metallic properties, such as ferromagnetic particles in the adhesive or metallic adherends [65].

Figure 8 – Coil system that allows functionally graded cure in SLJs by [65]

When used with adhesives whose mechanical behaviour is sensitive to the cure temperature, assuming that for high cure temperatures ductile behaviour is displayed and for lower cure temperatures a more brittle behaviour occurs, the apparatus enables the production of a SLJ that has high adhesive stiffness at the centre and minimum at the ends of the overlap. Following the same reasoning presented in the MAJs description, this leads to a higher joint strength than a joint made of the adhesives individually. For comparison sake, the isothermal cure process was used as reference for the graded cure results. Both adhesives used in this study were Araldite®2011 and Loctite Hysol® 3422. The adhesives when cured at high temperatures (100 and 120 °C) showed ductile behaviour and, in contrast, when cured at room temperature (23 and 40 °C) displayed brittle behaviour.

The failure load of the functionally graded joints in contrast to the isothermal cured joints displayed an increase in failure load of 68% and 67% for Araldite®2011 cured at low and high temperature, respectively. Functionally graded SLJs of Loctite Hysol® 3422 displayed an increase of 245.5% and 60.6% when compared to their isothermally cured counterparts cured at low and high temperature respectively. Strain wise, both FGAs were

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better or just as good as their ductile non-graded counterpart was. Carbas et. al [29] also proved that an increase in adhesive thickness of the FGA SLJs was matched with a strength increase, which according to [3] matches the behaviour of ductile adhesives.

The graded cure apparatus was used by Carbas et. al [28] to repair wood beams, where they were patched with CFRP and tested under four point bending. Two common wood beam failures and different repair lengths were also tested (Figure 9).

Figure 9 – Tested specimens layout in [28].

Two cures were performed, the standard isothermal cure and the functionally graded cure. The results showed that the failure load values for functionally graded specimens improved for most specimens. Regarding the compression specimen for 20 and 30 mm of repair area, the maximum improvement in failure load was approximately of 18% and 15%, accordingly. The cross grain tension specimens patched with the graded adhesive displayed a maximum improvement of approximately 10% and 25% for 40 and 60 mm repair lengths, respectively.

This novel method and apparatus allow the manufacture of adhesive joints with varying mechanical properties along the overlap length through a differential cure process. This technique is encouraging and incentivises the study of more heating techniques such as infrared radiation, electric heating etc., as well as proper adhesive selection and characterization regarding cure schedules. However, in [66] it was concluded that the graded effect of these joints is lost when the joint is subjected to post-cure temperatures higher than the glass transition temperature (Tg), which limits the range of applications

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of this method, since there is a strict temperature range in where these kind of joints can be used.

3.4 FGA by variable mix ratio

Arguably, the idea of combining adhesives with different behaviours started with the study of bi-adhesive joints and, although these may lead to improvements in joint performance, they are considered a rough version of FGA [3].

Mix ratio induced FGA are one the most published theme in the experimental field of FGA. Their success relies on a clean and repeatable manufacturing process, as well as the proper selection of adhesives for mixing. Many room for experimentation and improvement exists, since there are numerous combinations that can be made regarding the adhesives used in this system and the optimal volume ratio of the said adhesives.

Kawasaki et al. [31], in 2016, studied the use of mixed adhesives to improve joint properties by developing a special apparatus to manufacture bulk specimens with varying mechanical properties along its length. In this study, two types of second-generation acrylic (SGA) adhesives were used, where one has a ductile behaviour (LDC-141) and the other a brittle one (G-672-15P). Each adhesive was a two component, with an agent A (oxidizer) and B (reducer). The oxidizers from the brittle and ductile adhesives were mixed and the same was done for the remaining reducers (Figure 10), both with the same mixing ratio. After mixing, each mixture was placed in a syringe.

Figure 10 – Schematic of the mixing procedure of two types of SGAs for bulk specimen manufacture in [31].

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For the production of dog bone specimens, the oxidizer and reducer mixes were placed in a static mixer and injected into a silicon mould shaped like a dumb-bell specimen. Mixing ratios between the brittle and ductile adhesives were between 0:10 and 10:0 (weight fraction) in increments of 10%. The bulk specimens cured at room temperature for 24hrs and post cured at 60°C for 24 hrs. To manufacture adhesive-layer specimens, two special applicators were used to apply the oxidizers mix and reducers mix in the matching adherends, those being aluminium and PTFE respectively (Figures 11 and 12). For these specimens, the mixing ratio of oxidizer varied along the adherends length and was matched by the same reducer mixing ratio, since the recommended mixing ratio of both agents was 1 to 1. Afterwards the adhesive covered surfaces of both adherends were placed in contact (Honeymoon adhesion). Bondline thickness control was assured by glass beads and, for each mix ratio, five tests were conducted where the strains were measured using optical extensometers instead of strain gages, due to high deformation.

Figure 11 – Schematic of the mixing procedure of two types of SGAs for bulk specimen manufacture in [31].

Figure 12 – Application of agents A and B on the matching adherend (left) and adherend contact (right)[31].

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Using a nanoindenter, the authors measured the Martens hardness at several points of the adhesive layer of each specimen. It was concluded that with the increase of brittle adhesive in the mixing ratio the Young’s modulus increased, proving that the Young’s modulus could be controlled over a big range, from 5.44 MPa to 1.66 GPa simply by controlling the mixing ratio of adhesives. The hardness measured along the length was also strongly dependent on the mixing ratio used at that point and corresponded local Young’s moduli. The same applies to the tensile strength, as with the increase in brittle adhesive in the mixture the tensile strength increased. For example, a bulk specimen with 30% brittle adhesive concentration displayed approximately an increase of 69% and 1170% in maximum strain and tensile strength, respectively, when compared to the adhesive with 0% brittle adhesive concentration. For the same situation (30% brittle adhesive concentration bulk specimen) but now compared to the bulk with 100% brittle adhesive concentration, the maximum strain rate increased approximately 49 times and the tensile strength decline was of approximately 67%. By these results, it can be stated that this technique allows adhesive manufacturing with high degree of flexibility, customization and resolution.

Kumar et al. [27] utilized 3D printing technology to create a varied modulus adhesive bond length through the control of the relative volume fraction of polymer. Similarly to Kawasaki et al. [31] apparatus, the 3D multi-material printer (Object Connex260 Polyjet 3D, Statasys Ltd., USA) allows the control of the relative volume fraction of polymer. Adherends were 3D printed using a rigid polymer VeroWhitePlus™ RGD835 and the adhesive was printed using a low Young’s modulus adhesive TangoPlus™ FLX930 (E1) and a digital material S40 (E2) with double the Young’s moduli. The SLJs were subjected to a single lap shear test and digital image correlation (DIC) was used to obtain the deformation of the joint and calculate the strain. Authors manufactured and tested SLJs with a solo adhesive, E1 (Figure 13a) and E2 (Figure 13b), and with a combination of both (Figure 13c and 13d). In one case the stiffer adhesive, E2, was used to add stiffness to the centre of the ductile adhesive E1, resulting in an increase of 8.58% in joint stiffness, with a 91.8% increase in failure load and a 50% increase in deflection in break representing a 169% increase in fracture toughness relative to the solo E1 adhesive. The opposite case was also tested. The addition of E1 to the stiff adhesive’s ends was done to the same degree, resulting in an increase of 5.12% in joint stiffness, with a 110% increase

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in failure load and a 48.8% increase in deflection in break representing a 169% increase in fracture toughness versus the stiffer E2 adhesive.

Figure 13 – a) SLJ using ductile adhesive E1. b) SLJ using stiff adhesive E2. c) SLJ using ductile adhesive E1 tailored with centre stiffness E2. d) SLJ using a brittle adhesive E2 tailored with edge compliance ([27]).

Both the ductile adhesive E1 tailored with centre stiffness and E2 tailored with edge compliance displayed a toughness increase of 169% for both cases while slightly increasing joint stiffness. Joint strength increased around by 100% when compared to the homogeneous adhesives E1 and E2, respectively. This compliance-tailoring approach, realizable in multi-material 3D printers, has proven to reduce shear strains and peel at the ends of the adhesive in SLJs.

Chiminelli et al. [5] manufactured a graded adhesive joint using dissimilar adherends, an aluminium 6005A-T6 (3 mm thick) and a composite laminate 2.8 mm thick comprised of an epoxy matrix (Epikote L-1100 and Epikure 294) reinforced with glass fibre. The adhesives used in this study were two epoxy resins, DP490 (brittle) and DP190 (more flexible). Similarly to Kawasaki et al. [31] a special apparatus was developed, only this time to manufacture a graded SLJ with an 50 mm overlap length, 20 mm width and 1 mm bondline thickness. The system is composed by a set of linear actuators that control the material flow, and therefore the mixing of both adhesives. The adhesive is squeezed and applied through a head dispenser, containing an additional actuator which is responsible for moving the lower adherend in order to control the deposition movement. The post testing fracture surfaces were inspected and the failure in most cases was cohesive. For the mono brittle adhesives, however, crack propagation was through the composite interface introducing fibre damage in the first layer. An increase of 70% of the ultimate load occurred, regarding the graded adhesive when compared to the flexible adhesive.

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Recently, Nakanouchi et al. [32] studied and developed a method to manufacture functionally graded adhesive joints. For this effect, an apparatus was specially designed and built consisting in two independent linear actuators, each equipped with an adhesive cartridge that is deployed/pushed by a shaft. The mix flows to a rotary mixer and is applied to the adherend from the mixer’s tip. Both cartridges contain different agents (A and B) of two Second-Generation Acrylic (SGA) adhesives, G-672-15P (brittle) and LDC-141 (ductile), and by controlling the feed rate of the shafts, the adhesive mixing is controlled. In this procedure, the agent A (oxidizer) from the brittle SGA was mixed with agent B (reducer) from the ductile SGA. The adhesive mix is then applied to a polytetrafluoroethylene (PTFE) mould, allowing the manufacturing of a graded plate. The grading starts with the application of an adhesive mix of 25% ductile adhesive and 75% brittle adhesive to 75%-25% brittle-ductile (Figure 14).

Figure 14 - Mixing ratio along length (all dimensions in mm, adapted from[32]).

After application, the mould with the graded mixture cured at room temperature for 24h and was post cured at 60°C for another 24h. From this first plate, eight short dog bone specimens were cut and punched from the plate and subjected to tensile testing. Other two graded plates were fabricated, one with soft-hard adhesive grading and the other with a hard-soft-hard mixture. From these last two plates, long dog-bone specimens were cut and punched. Stress-strain graphs (Figure 15) were consistent with the grading: as the ratio of brittle adhesive increased, the maximum strain decreased (154% to 9.92%); and as the ratio of ductile adhesive increased the specimens became more ductile. The Young’s modulus was calculated using the Secant modulus from the origin to the 2% strain point of the stress-strain curves and its variation from the softest specimen to the hardest was of 599.3 MPa.

72 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 15- Stress-strain curves for short dogbone specimens 1 to 8 [32].

For the long dogbone specimens, the strain in the parts with higher percentage of ductile adhesive was superior to the hard parts, as expected (Figure 16).

Figure 16 - Long bone specimens with the matching strain curves along length[32].

3.5 FGA using second phase inclusions

The next sections describe the studies done regarding the development of FGA by strategically placing micro or nanoparticles along the bondline length. It is important to

73 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

establish a difference between micro and nanoparticle inclusions from each other since size is an important material parameter.

For the same concentration, nanoparticles have a higher surface area than microparticles, thus promoting more particle agglomeration. Therefore, mixing nanoparticles requires more complex methods such as ultrasound and high shear mixing, whereas microparticle mixing can be done using simpler methods such as three-roll milling [3]. Besides that, the use of “big” particles or the presence of agglomerates may increase the local stress intensity factors in the composite system and, instead of acting as a reinforcement, the particles will instead act as a defect/void.

Particle nature is also coupled to the particles optimal size, for example Pearson et al. [67] found that the fracture toughness in rubber-modified epoxies increases for smaller size rubber particles (0.2 μm), in contrast to bigger particles (1-2 μm). Counter to this, Barbosa et.al reported that in order to use cork particles for adhesive toughening they should not be smaller than 38-53 μm to prevent damage to the cork’s honeycomb cell structure.

Considering the previous points, particle application/inclusion in FGA were divided by their prefix: micro and nano.

3.5.1 Microparticles

Stapleton et al. [25] manufactured several single strap joints to study functionally graded joints in order to reduce peel stress concentrations in near adherend discontinuities. In this study, the grading was executed by placing glass beads, which are normally used for thickness control, in handpicked positions along the adhesive. The adherends and doubler were composed by a single ply of 0/±45 triaxially braided composites, with a fibre volume faction of 52% and a matrix of Epon 862 epoxy resin. The adhesive used was AF 163-2k. Two specimens with no beads, two with uniformly spread beads and two with beads placed in handpicked positions (Figure 17) were manufactured. It should be noted that all specimens were manufactured as a plate and individual specimens were cut out of the plane.

74 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 17 - Representation of double strap joint with placed glass beads[25].

Results revealed that the beadless specimens performed the worst, followed by the uniformly beaded specimens and graded beads, which had and average strength 26.5% and 70.7% higher than the uniform beads and beadless specimens, respectively. Visual inspection of the surface fractures backed up the results, being that in the FGA joints the failure was mostly cohesive.

In a more recent study, Bonaldo et al. [30] also evaluated the use of the same type of TEP used in [53, 54] to create a FGA joint along the overlap by local mixing of particles using an high speed mixer to guarantee an bubble-free homogeneous particle distribution. It should be noted that during manufacturing, each adhesive with a certain particle concentration (5%, 3%, 1%, 0.5%) as to be subjected to high speed mixing. After joint manufacturing, tensile and four-point bend (4PB) tests were performed on SLJs to characterize the behaviour of TEPs-modified graded joints. Bonaldo et al. initially evaluated particle influence by manufacturing non graded DCB joints for two double component adhesives, Betamate™2096 and SikaForce®7888, with three different TEPs concentrations (0, 5 and 10 wt%). The TEPs-modified Betamate™2096 adhesive when compared to the unmodified DCB counterpart displayed an increase of 17% in 퐺퐼퐶 for 5 wt%, and a decrease of 6% for 10 wt% TEP modified specimens. The TEPs-modified

SikaForce®7888 adhesive exhibited an increase of 152, 57 and 37% in 퐺퐼퐶 for 5 wt%, 10 wt% and 15 wt% respectively. The authors argue that the unequal thickness of both adhesive DCB specimens (SikaForce®7888 1 mm and Betamate™2096 0.2 mm) explains the difference in behaviour since it was shown previously that SikaForce®7888 fracture toughness depends on the thickness of the adhesive layer [68]. SLJs of only Betamate™2096 were manufactured as control specimens, alongside SLJs with homogeneous mixes of adhesive with 1 and 5% of uniformly distributed TEPs. Lastly, SLJs with different particle concentrations of TEPs along the overlap length, or graded bondline, as displayed in Figure 18a and 18b, were produced. The overlap length was

75 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

divided into different regions, where the higher TEP concentrations were located at the overlap ends where the failure occurs due to high local strains.

a) 5 4 3 2

1 TEP amount [wt%] amountTEP 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Normalized overlap length

b) 1

0.5 TEP amount [wt%] amountTEP 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Normalized overlap length

Figure 18 – TEP distribution along the bondline in SLJs for tensile testing.

The lap-shear strength of the TEPs-modified Betamate™2096 adhesive did not match or outperform the unmodified adhesive’s lap-shear strength. The graded TEP-modified adhesive presented a decrease of approximately 10% and 19% for 1 and 5 wt% of TEPs, respectively. However, the graded adhesive had a lower decrease in lap-shear strength when compared to the homogenous mixtures of 1 and 5% TEPs, which were 17% and 23%. Four-point bending tests were carried out and, since it is known that bending moments play an important role in SLJs strength, overlap lengths of 12.5 and 25 mm were manufactured for 4 different TEP concentrations 0, 5, 10 and 20 wt% for homogeneous and graded conditions (Figure 19 a) and b)).

76 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

a) 10

5 TEP amount [wt%] amount TEP 0 0 0.2 0.4 0.6 0.8 1 Normalized overlap length

b) 20

5 TEP amount [wt%] amount TEP 0 0 0.2 0.4 0.6 0.8 1 Normalized overlap length

Figure 19 - TEP distribution along the bondline in SLJs in 4PB.

The homogenous joints with 5 wt% TEPs showed an increase in flexural strength by 5.1% when compared to the unmodified adhesive, while the graded and homogeneous joints with 10 wt% and 20 wt% showed a decrease of 4.5% and 8.9% respectively. The graded and homogeneous joints with 20 wt% showed a decrease of 24.5% and 31.1% respectively. Nevertheless, it can be seen that the graded joints with TEPs with 10 wt% and 20 wt% performed better than their counterparts, the homogeneous joints for 10 wt% and 20 wt%. In respect to the SikaForce®7888 SLJs, for a 12.5 mm overlap, the graded joints presented a 22% and 8.7% increase in flexural strength for 10 and 20 wt%, when compared to the homogenous joints modified with the same wt% TEPs content. For the 25 mm overlaps there were no significant differences regarding flexural strength, between

77 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

the graded and homogeneous scenarios, due to adherend plastic deformation. Despite this, homogeneous SLJs with 5 wt% TEPs did not fail.

3.5.2 Nanoparticles

Carbas et. al [26] manufactured functionally graded SLJs using carbon black nanoparticles for two different curing techniques, dielectric and thermal heating. Carbon black particles are used to improve the conductivity of reinforced polymers[69] and, due to their microwave absorption properties, they enable the dielectric processing of materials that are not eligible to heating by microwaves [70]. The adhesive used in this study was Araldite®2011, which has high strength and stiffness, and two types of black nanoparticles, Monarch®120 and Vulcan®XC72R, were used. High strength steel, DIN C65 heat-treated, was picked as adherend in order to prevent plastic deformation. The authors manufactured specimens with graded and uniform distribution of particles along the bondline (20% volume of nanoparticles) as well as neat adhesive specimens for comparison sake. For the FGA SLJ, several mixes with different particle concentrations were prepared and applied in lines using a syringe, thus allowing a gradual distribution of adhesive with different particle concentrations (20%, 10%, 5%) (Figures 20 and 21).

Figure 20 - Graded joint adhesive application process [26].

The carbon black distribution throughout the bondline is schematically represented in Figure 21.

78 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

10 [% /vol] [% 5

Carbon black particles amount particles black Carbon 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Normalized overlap length

Figure 21 - Black carbon nanoparticles distribution along the bondline(adapted from [26]).

The adhesive was heated in steps of 2 minutes, using a microwave (dielectric cure). Two hundred watts of power were set, until it reached the temperature of 100°C, remaining at this temperature for 6 min, equalling a total cure time of 10 min. Thermal heating cure was done at 100°C during 36 min. Regarding dielectric cured SLJs, results showed that the graded joints were up to 20% stronger than neat adhesive SLJs. Similarly to the previous case, graded specimens cured by thermal heating displayed an improvement in joint strength up to 30% when compared to neat resin SLJs. Being that the Monarch®120 black nanoparticles showed the biggest improvements for both cure processes and overlap lengths. For either cure processes, the joint strength increased linearly as a function of the overlap length for FGA, indicating ductility wise improvement. The main difference between the processes was relative to curing time, being that thermal heating took 36 min when compared to dielectric, which took 10 min. Dielectric curing may be advantageous for applications that require faster curing at the cost of the materials being eligible to heating by microwaves.

3.6 Discussion

As it can be seen from Table 1, several improvements regarding FGA have been made when it comes to failure load increase [25, 26, 28, 29], ultimate strength increase [5, 27], flexural strength [30], varying Young’s moduli, strain/stress along the adhesive’s

79 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

length [6, 32] and increased toughness [27]. Also, new manufacturing concepts and methods were developed [5, 27, 29, 31, 32].

80 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Authors Year Adhesive(s) Adherends Grading Type Results Stapleton et. 2012 AF 163 Composite Glass beads Graded 70.7% improvement over al. [25] (micro) single strap - non-functionally graded joint joint

Carbas et al. 2013 Aradite®2011 High Graded cure - Graded SLJ - Increased failure load, [29] strength specially from 60% to 245.5% for Loctite Hysol® steel developed graded SLJs compared to 3422 apparatus dummy SLJs - Overall ductility maintained and sometimes increased

S. Kawasaki 2015 LDC-141 Aluminium Mixing ratio Graded dog Graded mechanical et al. [31] PTFE induced by bone - properties along G-672-15P using specially specimens length such as developed Young’s Modulus, stress apparatus and strain

Carbas et al. 2015 Loctite Hysol® Wood Graded cure - Graded SLJ - Joint failure load increase [28] 3422 beams specially for most repair lengths developed apparatus

S. Kumar et 2016 TangoPlus™ Rigid Mixing ratio Graded SLJ Joint strength increased by al. [27] FLX930 polymer induced by - at least 91.8% and 169% VeroWhite using 3D toughness increase for Digital material Plus™ printing both graded specimens S40 RGD835 -

Carbas. et al. 2017 Araldite®2011 High Monarch®120 Graded Joint - Adhesive strength [26] strength (nano) improvements from 20% steel Vulcan®XC72R to 30% vs neat specimens (nano) Dielectric and thermal heating Chiminelli 2017 DP490 Aluminium Mixing ratio Graded SLJ 70% ultimate load et. al. [5] 6005A-T6 induced by - increase when compared DP190 Glass fibre specially to either single adhesive reinforced developed SLJ composite apparatus

J. Bonaldo et 2018 SikaForce®7888 High TEP Expancel Graded SLJ Improvements of al. [30] strength 031 DU 40 ( flexural strength in 4PB Betamate™2096 steel DIN particles Betamate™2 with graded 10 and 20 55Si7 (micro) 096) wt% for

SikaForce®7888 (22% 4PB (both and 8,7% respectively) adhesives) M. 2019 LDC-141 None Mixing ratio Graded dog Graded mechanical Nakanouchi Graded induced by bone - properties along et al. [32] G-672-15P adhesive using specially specimens specimens length such as plate was developed Young’s Modulus, stress manufactur apparatus and strain ed

81 Development of a process to obtain a graded distribution of particles along the overlap of Tableadhesive 1- Overall single display lap of jointsthe reviewed studies.

From the previous sections, it can be said that in a short span of years several improvements to regular joints have been made. Carbas et al. [28] successfully improved wood patching using functionally graded cured adhesives, being that the biggest gains in failure load were relative to the brittle patches. In SLJs the graded cure followed the same trend, with bigger improvements in failure loads specially when compared to the adhesive with brittle behaviour. In the field of FGA with microparticles, Stapleton et al.[25] showed a failure load increase using a FGA when compared to the beadless and uniformly spread beaded single strap joints, by 70% and 26%, respectively. Regarding nanoparticles Carbas et al. [70] also achieved increased failure load by using two different kinds of carbon black nanoparticles.

Several authors [27, 31, 32] proved that adhesive systems with high degree of customization, and therefore high application tailorability, are already manufacturable. The apparatus presented in this review have proven capable of producing a non-step wise mixture, promoting a FGA with varying Young’s modulus, strain, stress and thus graded behaviour along the bondline and increased toughness. Nevertheless, more studies on adhesive compatibility and fatigue behaviour are required for further process optimization.

Some of the studies are presented in Figure 22, in regards to gains relative to ultimate strength or failure load.

82 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 22 – Approximate results regarding some FGA studies.

4. Challenges and prospects

After the revision on the field of FGA, some challenges mainly regarding manufacturing and quality assessment have to be noted.

Regarding the use of micro and nanoparticles as adhesive reinforcements to create application tailored adhesives, one of the more obvious concerns is about particle sizes and concentrations used in the adhesive joint [67]. Several studies presented in this review article displayed tests with several adhesive-particle combinations in order to fine-tune adhesive-particle system to its full potential. The physical and chemical nature of the particles used in the system will dictate how beneficial or detrimental a particle can be. In Barbosa et al., for example, it was found that 1% volume fraction of cork particles

83 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

within the 125-250 µm range were the most beneficial in terms of fracture toughness due to the preservation of the inner cells structure of the cork.

Overall, particle size is a controllable material parameter that has to be fine-tuned, as it influences not only the fracture toughness but also the toughening mechanism of a modified adhesive [47]. Even tough nanoparticles are desired for their smaller size and therefore smaller individual stress intensity factor, it is important to bear in mind that with smaller particle size, for the same volume of particles, the specific surface area increases (large surface area to volume ratio) [3]. This fact is worrisome because probability of particle agglomeration might arise if no surface treatment is performed a priori [71]. Finally, regarding micro and nanoparticles, better methods for particle manipulation are required.

The dispersion quality of nanoparticles in the fluid is also difficult to assess and it should be done at three size scales, those being optically, SEM and TEM. Particle size may affect the accommodation of deformation considering the crosslinking of the adhesives molecular chain. Therefore, if the size of the particle is equal or superior to the free space in the molecular chain, the particle may act as a defect when subjected to deformation. Micro particle use is more appealing when it comes to inspection, surface area to volume ratio and inter particle distance. However, with increasing particle size, in the case of adhesive toughening, the particles may act like defects and promote crack initiation.

Reliability and reproducibility are also key concerns, since the development of a foolproof methodology to position the particles along the bondline to achieve the proper grading is not easy. Changes to the grading due to adhesive flow during manufacturing add another degree of freedom to this problem [25].

In the manufacturing process of mix ratio induced FGA, the cure cycle used on the mix is also an important factor, and for adhesive applications this technique requires a special apparatus either in the form of a 3D printer like in [27] or a custom one similarly to the studies made in [5, 31, 32]. Adhesives combined in this technique should have similar chemical composition and may present different cure schedules and pot life, so a healthy compromise has to be found. This can be achieved by proper characterization of each individual adhesive pre mixing as well as characterization of the final product [36]. For this effect, multi-adhesive compatibility studies would be very useful.

84 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

The numerical simulation of the manufacturing process before the cure could be employed alongside computational fluid dynamics (CFD) to better design and optimize these joints, regarding the squeezing process, flow pattern studies [5] and even the curing process [72, 73]. This kind of analysis, however, is not easy and implies proper characterization of the used adherend surfaces with and without pre-treatments and fluid rheology.

Multiphysics software that allow multiphysics models which include CFD, structural mechanics, chemistry, particle tracing and particle-particle and/or particle-fluid interaction may lead to process optimization, although this is all contingent on material characterization. The combined use of this, the mix adhesive ratio technique with micro or nanoparticle grading is yet to be explored, nevertheless it could be an interesting combination.

The functionally graded joints created by differential curing [28, 29] have proven successful and require a special apparatus for manufacturing reliability and repeatability. However, more studies are required on this approach regarding joint degradation and durability.

In Figures 23, 24 and 25 an overview of each method’s traits, regarding strengths, weaknesses, opportunities and threats is presented.

Figure 23 – Overview of FGA using micro/nanoparticles.

85 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 24 - Overview of FGA using graded curing.

Figure 25 - Overview of mix ratio induced FGA.

5. Conclusion

This work reviewed the latest methodologies, challenges and prospects in designing and manufacturing FGA alongside its results. Every approach such as grading through particle inclusion, cure and adhesive mixing showed encouraging results, displaying

86 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

improvements in several mechanical properties such as ultimate joint strength, failure load, toughness and flexural strength in a graded manner.

It is realistic to say that, similarly to what happened with functionally graded materials, with more research and fine-tuning, these approaches will be a great benefit towards the increasing use of adhesive joints in more applications and therefore to several industries such as automotive and aerospace industries. The ability of tailoring adhesive mechanical properties across a dimension opens more room for design solutions by enabling the use of more sophisticated adhesive bonds, which have higher performance.

ACKNOWLEDGEMENTS

Financial support by Foundation for Science and Technology (POCI-01-0145-FEDER- 028035) is greatly acknowledged.

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[43] Mcgarry, F. and Willner, A. Toughening of an epoxy resin by an elastomerer second phase. J. Am. Chem. Soc. Div. Org. Coating. Plast. Chem. Prepr. 1968, 28, 512-526 [44] Barbosa, A. Q. "Improved toughness of adhesives filled with cork micro particles." Thesis for degree of Doctor of Philosophy Ph.D. Dissertation, Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto, 2017. [45] Barbosa, A. Q., Da Silva, L. F. M., Banea, M. D., and Öchsner, A. Methods to increase the toughness of structural adhesives with micro particles: an overview with focus on cork particles. Materialwissenschaft und Werkstofftechnik. 2016, 47, 307-325. https://doi.org/10.1002/mawe.201600498 [46] Nemati Giv, A., Ayatollahi, M. R., Ghaffari, S. H., and Da Silva, L. F. M. Effect of reinforcements at different scales on mechanical properties of epoxy adhesives and adhesive joints: a review. The Journal of Adhesion. 2018, 94, 1082-1121. https://doi.org/10.1080/00218464.2018.1452736 [47] Azimi, H. R., Pearson, R. A., and Hertzberg, R. W. Fatigue of rubber-modified epoxies: effect of particle size and volume fraction. Journal of Materials Science. 1996, 31, 3777-3789. https://doi.org/10.1007/BF00352793 [48] Razavi, S. M. J., Ayatollahi, M. R., Esmaeili, E., and Da Silva, L. F. M. Mixed- mode fracture response of metallic fiber-reinforced epoxy adhesive. European Journal of Mechanics - A/Solids. 2017, 65, 349-359. https://doi.org/10.1016/j.euromechsol.2017.06.001 [49] Khoramishad, H. and Razavi, S. M. J. Metallic fiber-reinforced adhesively bonded joints. International Journal of Adhesion and Adhesives. 2014, 55, 114-122. https://doi.org/10.1016/j.ijadhadh.2014.08.005 [50] Kinloch, A. J., Lee, J. H., Taylor, A. C., Sprenger, S., Eger, C., and Egan, D. Toughening structural adhesives via nano- and micro-phase inclusions. The Journal of Adhesion. 2003, 79, 867-873. https://doi.org/10.1080/00218460309551 [51] Kishi, H., Uesawa, K., Matsuda, S., and Murakami, A. Adhesive strength and mechanisms of epoxy resins toughened with pre-formed thermoplastic polymer particles. Journal of Adhesion Science and Technology. 2005, 19, 1277-1290. https://doi.org/10.1163/156856105774784402 [52] Wang, M., Miao, R., He, J., Xu, X., Liu, J., and Du, H. Silicon Carbide whiskers reinforced polymer-based adhesive for joining C/C composites. Materials & Design. 2016, 99, 293-302. https://doi.org/10.1016/j.matdes.2016.03.060 [53] Banea, M. D., Da Silva, L. F., Carbas, R. J., and Campilho, R. D. Structural adhesives modified with thermally expandable particles. The Journal of Adhesion. 2015, 91, 823-840. https://doi.org/10.1080/00218464.2014.985785 [54] Banea, M. D., Da Silva, L. F., Carbas, R. J., and Campilho, R. Mechanical and thermal characterization of a structural polyurethane adhesive modified with thermally expandable particles. International Journal of Adhesion and Adhesives. 2014, 54, 191-199. https://doi.org/10.1016/j.ijadhadh.2014.06.008 [55] Barbosa, A. Q., Da Silva, L. F., Öchsner, A., Abenojar, J., and Del Real, J. C. Influence of the size and amount of cork particles on the impact toughness of a structural adhesive. The Journal of Adhesion. 2012, 88, 452-470. https://doi.org/10.1080/00218464.2012.660811 [56] Barbosa, A., Da Silva, L., and Öchsner, A. Effect of the amount of cork particles on the strength and glass transition temperature of a structural adhesive. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of

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Materials: Design and Applications. 2014, 228, 323-333. https://doi.org/10.1177/1464420713493581 [57] Barbosa, A., Da Silva, L., Abenojar, J., Del Real, J., Paiva, R. M., and Öchsner, A. Kinetic analysis and characterization of an epoxy/cork adhesive. Thermochimica Acta. 2015, 604, 52-60. https://doi.org/10.1016/j.tca.2015.01.025 [58] Barbosa, A., Da Silva, L., Abenojar, J., Figueiredo, M., and Öchsner, A. Toughness of a brittle epoxy resin reinforced with micro cork particles: Effect of size, amount and surface treatment. Composites Part B: Engineering. 2017, 114, 299-310. https://doi.org/10.1016/j.compositesb.2016.10.072 [59] Brown, E. N., White, S. R., and Sottos, N. R. Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite—Part II: In situ self- healing. Composites Science and Technology. 2005, 65, 2474-2480. https://doi.org/10.1016/j.compscitech.2005.04.053 [60] Jin, H., Miller, G. M., Sottos, N. R., and White, S. R. Fracture and fatigue response of a self-healing epoxy adhesive. Polymer. 2011, 52, 1628-1634. https://doi.org/10.1016/j.polymer.2011.02.011 [61] Brown, E. N., Sottos, N. R., and White, S. R. Fracture testing of a self-healing polymer composite. Experimental Mechanics. 2002, 42, 372-379. https://doi.org/10.1007/BF02412141 [62] Brown, E. N., White, S. R., and Sottos, N. R. Microcapsule induced toughening in a self-healing polymer composite. Journal of Materials Science. 2004, 39, 1703-1710. https://doi.org/10.1023/B:JMSC.0000016173.73733.dc [63] Brown, E. N., White, S. R., and Sottos, N. R. Fatigue crack propagation in microcapsule-toughened epoxy. Journal of Materials Science. 2006, 41, 6266- 6273. https://doi.org/10.1007/s10853-006-0512-y [64] Banea, M. D., Da Silva, L. F. M., Campilho, R. D. S. G., and Sato, C. Smart Adhesive Joints: An Overview of Recent Developments. The Journal of Adhesion. 2014, 90, 16-40. https://doi.org/10.1080/00218464.2013.785916 [65] Carbas, R. J. C. "Adhesively Bonded Functionally Graded Joints." Ph.D. Dissertation, Departamento de Engenharia da Universidade do Porto, Faculdade de Engenharia da Universidade do Porto, 2013. [66] Carbas, R. J. C., Silva, L. F. M. d., and Critchlow, G. W. Effect of post-cure on adhesively bonded functionally graded joints by induction heating. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 2014, 229, 419-430. https://doi.org/10.1177/1464420714523579 [67] Pearson, R. A. and Yee, A. F. Influence of particle size and particle size distribution on toughening mechanisms in rubber-modified epoxies. Journal of Materials Science. 1991, 26, 3828-3844. https://doi.org/10.1007/BF01184979 [68] Da Silva, L. F. M. and Campilho, R. D. S. G. The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive AU - Banea, M. D. The Journal of Adhesion. 2015, 91, 331-346. https://doi.org/10.1080/00218464.2014.903802 [69] Calberg, C., Blacher, S., Gubbels, F., Brouers, F., Deltour, R., and Jérôme, R. Electrical and dielectric properties of carbon black filled co-continuous two-phase polymer blends. Journal of Physics D: Applied Physics. 1999, 32, 1517-1525. http://dx.doi.org/10.1088/0022-3727/32/13/313 [70] Carbas, R. J. C., Da Silva, L. F. M., and Andrés, L. F. S. Effect of carbon black nanoparticles concentration on the mechanical properties of a structural epoxy adhesive. Proceedings of the Institution of Mechanical Engineers, Part L: Journal

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of Materials: Design and Applications. 2016, 232, 403-415. https://doi.org/10.1177/1464420716629046 [71] Haupert, F. and Wetzel, B., Reinforcement of Thermosetting Polymers by the Incorporation of Micro- and Nanoparticles; Springer US, Boston, MA, 2005. [72] Cebrián, A. S., Klunker, F., and Zogg, M. Simulation of the cure of paste adhesives by induction heating. Journal of Composite Materials. 2013, 48, 1459- 1474. https://doi.org/10.1177/0021998313487933 [73] Nakouzi, S., Pancrace, J., Schmidt, F., Le Maoult, Y., and Berthet, F. Curing Simulation of Composites Coupled with Infrared Heating. 2010, 3, 587-590. https://doi.org/10.1007/s12289-010-0838-5

92 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Patent PAT20191000036260

Invention to obtain functionally graded joints

94 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Method and apparatus to manufacture functionally graded joints using magnetized micro particles

C.I. da Silva1, A.Q. Barbosa2, J.B. Marques2, R.J.C. Carbas2, J. Abenojar3, L.F.M. da Silva1, J.C. del Real4, M.A. Martinez 1 Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal 2 INEGI, Rua Dr. Roberto Frias 400, 4200-465, Porto, Portugal 3 Materials Performance Group, Materials Science and Engineering Department, Universidad CarlosIII de Madrid, Leganés, Spain 4 Department of Mechanical Engineering/IIT, Universidad Pontificia Comillas, C/Alberto Aguilera 23, 28015 Madrid, Spain

Abstract An apparatus and method to manufacture graded single lap joints (SLJs) bonding with magnetic particle integrated adhesive, wherein the adhesive is a liquid adhesive and the substrates are non-magnetic substrates selecting from the group consisting of aluminium, polymers, ceramics, wood and derivatives. The apparatus comprising a mould with a bottom part (8) and a top part(5), a plate(7), screws(12) and a device that includes an upper(1) and a lower(1’) magnet holders, and an upper(9) and a lower (9’) male components; wherein the magnet holders(1;1’) contain at least one magnetic array, responsible for creating a static magnetic field, that acts on the adhesive layer to distribute the magnetic particles along the overlap length of SLJs. By use of the method, a beneficial reduction of the stress concentrations is provided to overlap ends, in order to bring more structurally efficient structures

Description Method and apparatus to manufacture functionally graded joints using magnetized micro or nano-sized particles

Technical field This invention relates a method and an apparatus for bonding a structural assembly and more particularly to bonding SLJs where the magnetic particles, preferentially micro or nano-sized, are placed along the overlap length with a specific disposition. The particle concentration will be higher at the edges of the overlap and steadily decreasing towards the middle. This way, mechanical properties vary gradually along the overlap using this tailored distribution of magnetic particles, provided by at least one specific magnet array, therefore creating a graded adhesive joint. This technique allows

95 for a smoother stress field, leading to a more efficient adhesive joint due to a more uniform stress distribution along the overlap reducing the typical stress concentrations at its the ends, and enabling a greater use of the overlap length. This technique can be applied in several industries in which structural adhesives are employed, such as, the aerospace, aeronautic, automotive and civil industries.

Background of the invention Adhesives are an emerging joining method, replacing traditional techniques. The application of adhesively bonded joints in the automotive industry has increased significantly mainly driven by the need for lighter vehicles, improved fuel economy and reduced emissions. Epoxy resins are one of the most used structural adhesives, due to their versatility as well as their mechanical, thermal and chemical resistance. Epoxies offer high strength and stiffness, low creep and good thermal resistance. The adhesive joint most studied and used in the industrial applications is the SLJ, mainly due to its simplicity and efficiency. However, in SLJs, the stress distribution is concentrated at the ends of the overlap, causing early failure of the adhesives. Several methods have been proposed to reduce these stress concentrations in order to more structurally efficient structures can be offered. Nowadays one promising approach is the development of functionally graded (FGA) adhesive SLJs joints, which allow for a large degree of control over the adhesive properties and can be tailored to adjust the stress levels along the overlap. FGAs can be defined as tailored adhesives that have varying mechanical properties along a desired dimension, allowing a more uniform stress distribution along the bondline. Recent research shows that the inclusion of particles can improve the mechanical properties of adhesives, improving the joint performance. The included particles increase the adhesives ductility, therefore by positioning them on the overlap ends, its compliance would increase and the load would be transferred to the middle of the adhesive which is stiffer, due to the lower particle concentration, thus allowing a more uniform load distribution and greater use of the whole overlap. For example, cork microparticles can be used to modify epoxy adhesives, improving their toughness and performance. There is also pressure on industries to use these types of materials, in order to reduce their carbon footprint. Cork can be used to reinforce a brittle adhesive and is able to improve the adhesive mechanical properties without detrimental effects on the curing process and on the hydrothermal degradation of the adhesive. It also has the advantage of being a sustainable material, which does not rot, with a low carbon footprint. At the moment three main techniques to obtain functionally graded SLJs can be found in literature: Through a differential cure process; by varying the adhesive mix ratio; through particle inclusion. In the differential cure technique, the employed adhesive must display ductile and brittle behaviour for different cure schedules, as in, when cured at high temperatures it behaves as a ductile adhesive and at low temperatures as a brittle adhesive. For this apparatus, the main components are the heating and cooling coils, responsible for the graded cure. Besides the limitation regarding cure schedule, the SLJs lose their grading when subjected to temperatures above the adhesive’s glass transition temperature (Tg). Functionally graded SLJs can also be obtained using an apparatus similar to those

96 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

used in additive manufacturing (3D printing). In this method a mixture of a ductile and brittle adhesives is made, and the volume fraction of the said adhesives is controlled, enabling the manufacturing of a graded SLJ or plate. This method also presents a limitation regarding the said adhesives cure schedule, since it must be similar, and also a limitation regarding the adhesive’s pot life. None of prior art documents disclose an apparatus and method to manufacture graded single lap joints, wherein the apparatus was designed and manufactured to achieve a graded distribution of magnetized particles using static magnetic fields. The apparatus contains at least one specific magnet array, capable of generating a tailored magnetic field, responsible for magnetic field gradients that force different particle concentrations throughout the overlap length, where the maximum amount of particles is located at the overlap ends and decreases towards the middle. Particle magnetization occurs by the application of a magnetite coating on the particle, being the magnetite adsorbed on the material surface. By using the tailored magnetic field and the matching method, this apparatus is capable of rearranging the magnetized particles in the desired fashion so that an adhesive, and therefore SLJ, with graded properties can be obtained. To use this methodology and apparatus non-magnetic substrates have to be used. The present apparatus and method proposed enables the production of adhesive joints with mechanical properties that vary gradually along the overlap, using magnetized particles, preferentially of micro or nano size. The adhesive layer of these joints will have varying stiffness along the overlap.

Summary of the invention One aspect of the present invention is directed to an apparatus for manufacturing graded single lap joints using magnetic particles, preferentially of micro or nano size, with non- magnetic substrates (aluminium, wood, polymers, ceramics and derivatives), for two ranges of adhesive viscosity, those being from 1 mPa∙s to 150 mPa∙s and from 150 mPa∙s to 10000 mPa∙s. The apparatus characterized in that it comprises a device that includes an upper and a lower magnet holders, and an upper and a lower male components; the magnet holders contains one or two sets of magnet arrays in the form of blocks, wherein the magnetic arrays is composed by at least 4 neodymium block magnets in which two first magnets display the magnetization vector vertically, in opposite directions, being each located at the edges of the magnetic array; and two second magnets display the magnetization vector horizontally, all in the same direction; each of these magnets is glued to their vertically magnetized pair. A second aspect of the present invention is directed to a method for manufacturing graded single lap joints. The concept is that by the application of at least one specially designed device that allows the coupling of at least 4 neodymium magnets, a non-uniform particle distribution is obtained. The magnetic array forms a non-uniform magnetic flux density through the bondline that promotes magnetophoresis, which is motion induced by a magnetic field on a particle of magnetic or magnetizable material in a fluid, to strategic

97 locations. This distribution displays a higher particle concentration at the overlap ends, which decreases towards the middle. Since the particles in hand promote an increase in the adhesive’s ductility, the resulting bondline will be more ductile at its ends, and stiffer at the centre, this way smoothing the stresses along it. This enables the load transfer from the overlap ends, by the ductile adhesive ends towards the middle, which is more brittle and withstands higher loads. This leads to the development of a functionally graded SLJ and promotes a greater use of the overlap area, leading to an increase in the SLJ performance. Other aspects, embodiments, advantages and mode of application of the disclosure are apparent from the detailed description of this document. Throughout the description and claims the word “comprise” and variations of the word, are not intended to exclude other technical features, components or steps. Additional objectives, advantages and features of the solution will become apparent to those skilled in the art upon reviewing this disclosure or may be learned by practice of the solution. It is intended that the various aspects of the invention can be practiced separately or in combination.

Brief description of the drawings The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings (also "figure" and "FIG." herein), of which: - FIG.1A-1B representative scheme of the main components of an apparatus according to the present invention; - FIG.2A-2M schematic representation of the working mechanism an embodiment of the apparatus; - FIG.3A-3C schematically illustrates an example of magnetic arrays; - FIG.4 shows adhesive plate obtained when it is using different magnet array configurations of the disclosure; and a schematic diagram of material composition distribution in FGM.

Detailed description of the invention The current invention is a method and apparatus (FIG. 1A and 1B) for manufacturing FGA joints using a non-uniform magnetic flux gradient caused by the magnet arrays (10) inserted in the magnet holders (1) (FIG. 3A-3C). The presented method requires a specially designed apparatus that, due to the magnets, enables ferromagnetic particle migration (magnetophoresis-motion induced by a magnetic field on a particle of magnetic or magnetizable material, in a fluid) to strategic zones in the adhesive layer. The intent is to achieve a higher particle concentration at the edges of the overlaps with a decreasing particle concentration towards the middle. The presented apparatus for manufacturing graded single lap joints, comprising:

98 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

- a mould that comprises a bottom part (8) and a top part (5), wherein the bottom part has a tear (11), a plurality of holes to place alignment pins (6) and a plurality of slots to place positioning blocks (3); wherein the top part has a tear (11’), a plurality of holes adapted to receive the alignment pins (6) and a plurality of slots to receive the positioning blocks (3) when both parts (8)(5) are arranged, one relative to the other; - a plate (7) to place over the bottom part (8) of the mould; - screws (12) to insert in the bottom part (8) for moving the positioning blocks (3); characterized in that the apparatus comprises: o a device for insertion in the tears (11)(11´), that includes an upper (1) and a lower (1’) magnet holders, and an upper (9) and a lower (9’) male components; o the magnet holders (1;1’) contains one or two sets of magnet arrays (10;10’) in the form of blocks, wherein the magnetic arrays (10)(10’) is composed by at least 4 neodymium block magnets in which two first magnets display the magnetization vector vertically, in opposite directions, being each located at the edges of the magnetic array (10)(10’); and two second magnets display the magnetization vector horizontally, all in the same direction; each of these magnets is glued to their vertically magnetized pair.

With the reference to the drawings and more specifically to FIG. 2A to 2M, the working mechanism an embodiment of the apparatus is schematically represented. Three method were performed with the aim to manufacture graded single lap joints bonding with a magnetic particle integrated adhesive, using the apparatus. Initially, the plate (7) is placed over the bottom part of the mould (8) as seen in FIG. 2A to 2B. Secondly, a plurality of alignment pins (6), displayed in FIG. 2C have to be placed in the holes contained by the bottom part (8), followed by the two tabs (4’) according FIG. 2D. Next the two substrates (2’) are placed as show in FIG. 2D, aligned with the tabs (4’) as show in FIG. 2E. At this stage the substrates (2’) are geometrically constrained by alignments (6) and the tabs (4’). It should be noted that the tabs (4’and 4) are also responsible for the length of the bonded area (overlap) as well as for the bondline thickness. Next, the same procedure is mimicked to the opposite side, the two tabs (4) are placed on top of the substrates (2’), in the opposite way of the previous tabs (4’) as seen in FIG.2F. The magnetic particle integrated adhesive (13) is applied to the overlap (FIG.2F). This adhesive-particle system, at this stage, contains a uniform distribution of particles due to the high-speed mixer used to mix the particles in the adhesive in order to prevent of air bubbles and ensure a homogeneous particle distribution. The substrates (2) are placed as show in FIG.2G. The mould is closed by pressing the top part (5) and bottom part (8) of the mould against each other as show in FIG.2H followed

99 by twisting the screws (12) move the positioning blocks (13), guaranteeing geometrical constriction of the entire SLJ. In the case of the applied magnetic particle integrated adhesive presenting a dynamic viscosity within the range of 1 mPa∙s to 150 mPa∙s only the magnet holders (1), which contain the pre-set array combination (10), are vertically inserted (FIG.2I) for a duration of 5 to 10 seconds. After the time lapsed, the magnet holders (1) are removed and the male components (9) are vertically inserted in the top part (5) and the male components (9’) are horizontally inserted in the bottom part (8), as seen in FIG.2J and FIG.2K. The final step requires inserting the remaining alignment pins (6) to fully assemble the mould. In the case of the applied magnetic particle integrated adhesive having a dynamic viscosity within the range of 150 mPa∙s to 10000 mPa∙s the magnet holders (1’) containing the pre-set array combination (10’) are horizontally inserted in the bottom part (8). Followed by the vertical fit of the magnet holders (1), which contain the pre-set array combination (10), in the top part (5) of the mould (FIG.2L), for a total duration of 15 to 20 seconds. After the elapsed time, both magnet holders (1)(1’) are removed and the male component (9) is vertically inserted in the top part (5) and the male component (9’) is horizontally inserted in the bottom part (8), as displayed in FIG.2M. The final step requires inserting the remaining alignment pins (6) to fully assemble the mould. The difference between the presented apparatus and the regular mould is the existence of tears, in its top (5) and bottom (8) parts (Figure 9), that allow the coupling of either or both the magnet holders (1)(1’) or the male components (9)(9’) (Figures 10 and 12), enabling the manufacturing of either SLJs with homogeneous and random particle distribution or graded SLJs. The above mentioned magnet arrays (10)(10’) can be composed from 4 to 8 neodymium magnets, which have specific magnetization orientations, as displayed in FIG.3A, FIG.3B and FIG.3C. Regardless of the number of magnets, the ones located at the edges display a vertical magnetization vector, in opposite directions (FIG.3A, FIG.3B). The remaining magnets, present in the middle, all display a horizontal magnetization vector in the same direction (FIG.3A, FIG.3B). When the two magnet arrays (10)(10’) are used, they follow the configuration displayed in FIG.3C. The grade (pull strength) of the magnets used to build the array may vary i.e N30, N40 or N50 (from weaker to stronger) and may include a combination of different grades or absence of magnets (air gaps). Each array is built by gluing the said magnets. The development of the magnet arrays was firstly done virtually by using a multiphysics software. With this software several types of magnet arrangements were tested, alongside the magnetic particles response to the static magnetic field generated by the magnet array. Using the above described magnet arrays, three distinct zones as clearly distinguishable regarding particle concentration (FIG.4) zone I, the region with lower particle concentration, or matrix rich region, zone II the transition region with intermediate particle concentration and zone III, the particle rich region that corresponds to the zone with the highest particle concentration. In the embodiments of the present disclosure, different types of non-magnetic substrates (2)(2’)can be used; the non-magnetic substrates can be selected from the group consisting of aluminium, polymers, ceramics, wood and derivatives.

100 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

In the embodiments of the present disclosure, the adhesive is a liquid adhesive. In the embodiments of the present disclosure, the magnetic particles integrated in the adhesive can be micro or nano-sized.

Claims 1. An apparatus for manufacturing graded single lap joints, comprising: - a mould that comprises a bottom part (8) and a top part (5), wherein the bottom part has a tear (11), a plurality of holes to place alignment pins (6) and a plurality of slots to place positioning blocks (3);

wherein the top part has a tear (11’), a plurality of holes adapted to receive the alignment pins (6) and a plurality of slots to receive the positioning blocks (3) when both parts (8)(5) are arranged, one relative to the other; - a plate (7) to place over the bottom part (8) of the mould; - screws (12) to insert in the bottom part (8) for moving the positioning blocks (3);

characterized in that the apparatus comprises: - a device for insertion in the tears (11)(11´), that includes an upper (1) and a lower (1’) magnet holders, and an upper (9) and a lower (9’) male components; - the magnet holders (1;1’) contains one or two sets of magnet arrays (10;10’) in the form of blocks, wherein the magnetic arrays (10)(10’) is composed by at least 4 neodymium block magnets in which two first magnets display the magnetization vector vertically, in opposite directions, being each located at the edges of the magnetic array (10)(10’); and two second magnets display the magnetization vector horizontally, all in the same direction; each of these magnets is glued to their vertically magnetized pair.

2. Method to manufacture graded single lap joints bonding with a magnetic particle integrated adhesive, using the apparatus of claim 1, characterized in that it comprises the following steps: a) placing the plate (7) over the tear (11) in the bottom part (8) of the mould; b) placing the displayed alignment pins (6) in the holes on the bottom part (8) of the mould; c) placing one or more tabs (4’) over the bottom part (8) of the mould; d) placing one or more substrates (2’) over the bottom part (8) aligned with the tabs (4’); e) placing other one or more tabs (4) on top of the substrates (2’) in the opposite way of the previous tabs (4’); f) applying a mixed adhesive with preferentially nano-sized or micro-sized magnetic particles (13) over the soon to be bonded area;

101 g) placing substrates (2) on the opposite way the substrates (2’) were placed; h) pressing the top part (5) and bottom part (8) of the mould against each other using the screw (12) for moving the positioning blocks (3); this way the tabs (4)(4’) and substrates (2)(2’) are geometrically constricted by the alignment pins (6) and the positioning blocks (3); i) inserting the magnet holders (1)(1’) containing the pre-set array combination (10)(10’) in the tears (11)(11’)of the mould, between 5 to 20 seconds; j) removing the magnet holders (1)(1’) and inserting the male components (9) (9’) in both parts of the mould; k) inserting the alignment pins (6) to fully assemble the mould.

3. Method to manufacture graded single lap joints bonded with a magnetic particle (preferentially micro or nano-sized) integrated adhesive, using the apparatus of claim 1, characterized in that it comprises the following steps: a) placing the plate (7) over the tear (11) in the bottom part (8) of the mould; b) placing the displayed alignment pins (6) in the holes on the bottom part (8) of the mould; c) placing one or more tabs (4’) over the bottom part (8) of the mould; d) placing one or more substrates (2’) over the bottom part (8) aligned with the tabs (4’); e) placing other one or more tabs (4) on top of the substrates (2’) in the opposite way of the previous tabs (4’); f) applying a magnetic particle integrated adhesive (13) with a dynamic viscosity within the range of 1 mPa∙s to 150 mPa∙s over the soon to be bonded area; g) placing substrates (2) on the opposite way the substrates (2’) were placed; h) pressing the top part (5) and bottom part (8) of the mould against each other using the screw (12) for moving the positioning blocks (3); this way the tabs (4)(4’) and substrates (2)(2’) are geometrically constricted by the alignment pins (6) and the positioning blocks (3); i) vertically inserting the magnet holders (1) containing the pre-set array combination (10) in the top part (5) of the mould, between 5 to 10 seconds; j) removing the magnet holders (1) and vertically inserting the male components (9) in the top part (5) and horizontally inserting the male component (9’) in bottom part (8); k) inserting the alignment pins (6) to fully assemble the mould.

4. Method to manufacture graded single lap joints bonding with a magnetic particle integrated adhesive, using the apparatus of claim 1, characterized in that it comprises the following steps: a) placing the plate (7) over the tear (11) in the bottom part (8) of the mould; b) b. placing the displayed alignment pins (6) in the holes on the bottom part (8) of the mould; c) c. placing one or more tabs (4’) over the bottom part (8) of the mould;

102 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

d) d. placing one or more substrates (2’) over the bottom part (8) aligned with the tabs (4’); e) e. placing other one or more tabs (4) on top of the substrates (2’) in the opposite way of the previous tabs (4’); f) f. applying a magnetic particle integrated adhesive (13) with a dynamic viscosity within the range of 150 mPa∙s to 10000 mPa∙s. g) g. placing substrates (2) on the opposite way the substrates (2’) were placed; h) h. pressing the top part (5) and bottom part (8) of the mould against each other using the screw (12) for moving the positioning blocks (3); this way the tabs (4)(4’) and substrates (2)(2’) are geometrically constricted by the alignment pins (6) and the positioning blocks (3); i) horizontally inserting the magnet holders (1’) containing the pre-set array combination (10’) in the bottom part (8), followed by vertically fitting the magnet holders (1), which contain the pre-set array combination (10) in the top part (5) of the mould, between 15 to 20 seconds; j) removing the magnet holders (1) and (1’), and vertically inserting the male components (9) in the top part (5) and horizontally inserting the male component (9’) in bottom part (8); k) inserting the alignment pins (6) to fully assemble the mould.

5. Method according to Claim 2, 3 or 4 wherein the substrates (2)(2’) are non-magnetic substrates selecting from the group consisting of aluminium, polymers, ceramics, wood and derivatives.

6. Method according to Claim 2, 3 or 4 wherein the adhesive is a liquid adhesive.

7. Method according to Claim 2, 3 or 4 wherein the magnetic particles integrated in the adhesive can be micro or nano-sized.

Porto, 08 de Julho de 2019

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104 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

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Paper 2

Mechanical properties article

115 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

116 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Mechanical characterization of graded single lap joints using magnetised cork microparticles C.I. da Silva1, A.Q. Barbosa2, J.B. Marques2, R.J.C. Carbas2, E.A.S. Marques1, J. Abenojar3, L.F.M. da Silva1 1 Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal 2 INEGI, Rua Dr. Roberto Frias 400, 4200-465, Porto, Portugal 3 Materials Performance Group, Materials Science and Engineering Department, Universidad CarlosIII de Madrid, Leganés, Spain

Abstract One of the main problems associated with adhesive joints is the existence of stress concentrations (shear and peel) at the ends of the overlap, reducing joint performance. This is especially valid for the most common joint geometry – the single lap joint. Therefore, a main area of investigation in the field of adhesive bonding is the uniformization of the stress distribution along the adhesive bondline, in order to decrease those stress accumulations at its ends, achieving stronger and lighter joints. The main goal of this work was to develop a functionally modified adhesive, where the mechanical properties vary gradually along the overlap. With an appropriate application of magnetic fields, using a customized apparatus, magnetised cork microparticles, initially uniformly distributed within a resin, were strategically placed along the bondline of an adhesive joint, being then non-uniformly distributed along the entire overlap area. This results in a gradual variation of the mechanical properties along the overlap, decreasing the stress concentrations and leading to a more uniform stress distribution on the overlap region. The adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the borders of the overlap. Therefore, the influence of the amount of magnetised cork microparticles was assessed. Tensile tests were performed for bulk specimens and SLJs, along with SEM analysis of the particles and correspondent bulk specimens fracture surfaces. Additionally, glass transition temperature measurements were done. From experimental tests, the inclusion of these particles enhances the joints performance for either graded joints or joints with a uniform particle distribution, when compared to those with neat resin. Also, it is possible to manufacture graded joints with different behaviours, depending on the amount of magnetised particles selected.

Keywords: Epoxy adhesives, functionally graded adhesive joints, magnetised particles, cork microparticles, magnetophoresis, numerical analysis, stress distribution, mechanical properties.

117 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

1. Introduction Over the years, the use of adhesive bonding as a joining method for structural applications has been the focus of intensive research and evolution, gradually replacing the classical mechanical fixing methods due to not only its improved mechanical performance, but also due to its versatility and ability to reliably join a large range of materials. This technique is also able to effectively join dissimilar materials, which cannot be easily performed with many of the alternative joining methods [1, 2]. Another key feature of adhesive joints is the fact that the stress distribution along the joint is more uniform than that provided by other conventional methods of fastening such as bolts or rivets [3]. Nowadays, adhesive bonding is a key technology to numerous industrial sectors, such as the aeronautical, aerospace, medical and civil industries [4], mainly because of the constant push for lighter, stronger, more resistant and more environmentally friendly materials. For instance, these demands are being met either with customized materials such as composites or even with custom adhesives. One of the most used structural adhesives is the epoxy resin (a thermoset polymer) due to its good mechanical, thermal and chemical properties. Therefore, the epoxy microstructure has a densely cross-linked molecular structure, which is very useful for structural engineering applications, since it presents high modulus of elasticity and strength, as well as low creep and good thermal resistance. However, the same microstructure that provides good properties to the epoxy resin, is responsible for the inherent brittleness (low ductility and toughness) with a low resistance to the initiation of cracks and their propagation. The single lap joint (SLJ) is the most studied joint geometry in literature and it is also the most widely used type of joint because of its simplicity and efficiency. In a SLJ the adhesive is usually loaded in shear, the type of loading under which adhesive joints perform best. Nevertheless, when analysing this type of joint, the main problem associated is the nonuniform stress distribution along the overlap, resulting in the presence of stress concentrations (peel – eccentric load path; and shear – unequal axial straining of the adherends) at the ends of the overlap [5-7]. This results in lower strength bonds, leading to a premature joint failure at the ends of the overlap, especially if the adhesive is brittle, such as the epoxy resin. Hence, in order to effectively increase the joints strength, the stress distribution must be as uniform as possible [2, 3, 8]. The development of methods for decreasing those stress concentrations and enhancement of joints performance is an area of interest that has increasingly been researched on the field of adhesive bonding. In the literature, several methods have been proposed to improve the joint strength, such as adhesive filleting [9], adherend tapering [10], adherend rounding [11], increasing thickness of the adhesive at the end [12] and other geometrical solutions (voids in the bondline, surface roughness or notches in the adherend) . However, with these methods, not only the complexity of the geometry increases, making it difficult to manufacture the joint, but also none of those gives an uniform stress distribution in the adhesive [2, 13]. A technique used to improve the stress distribution in an adhesive layer is the concept of mixed adhesive joints (MAJs) [14-22] which uses more than one adhesive on the same overlap. It consists of having a stiff and strong adhesive in the middle of the overlap and a flexible and ductile adhesive at its ends. However, one of the main problems associated with this technique is the lack of proper adhesive separation, as even in a joint bonded with several different adhesives, stress concentrations tends to occur at the borders

118 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

of adjacent adhesives [14]. That is, MAJs can only improve the bonding strength of a joint up to a certain level. Furthermore, there have been several studies to improve the joint strength by creating functionally graded materials (FGM) [23-25] and functionally graded adhesive (FGA) bondlines [26, 27]. More recently, Carbas et al. [28] proposed a method to relieve the high stress concentrations at the borders of the overlap which creates a continuously functionally modified adhesive along the overlap length of the joint by induction heating. However, the major drawback of this method is that the graded effect of these joints is lost when they are subjected to post-cure temperatures higher than the glass transition temperature [29]. Nowadays, micro or nano particles have been used as material reinforcements in several applications. They can be inorganic (silicates, glass, alumina, etc.) [30] or organic [31]. Depending on the material selected, the use of microparticles could be less expensive and allow faster product development, when compared to other techniques for improving joint performance. Additionally, natural materials (i.e. cork or wood fibres) [32-34] are gaining attention as reinforcements of polymeric matrices due to their unique properties such as the thermal insulation, low density, low cost and sustainability of the raw material [35]. The advantages of using natural microparticles present major competitive features, since two of the main concerns in several industrial sectors momentarily are the final weight of the structure and the reduction of the carbon footprint. Also, those particles are interesting because of the fact that they have the potential to be displaced in order to create custom particle distributions, for various purposes. Thus, this same principle can be applied to the fabrication of functionally graded joints, where the microparticles would be strategically placed along the overlap, so that different properties could be obtained along a certain direction. This results in enhanced joint resistance with minimal change in gross properties of the matrix resin [2], therefore increasing its toughness - the ability of a material to absorb energy and plastically deform without breaking. As a consequence of the many advantages this method can offer and since toughness is one of the main aspects that govern the strength of materials, currently there is a growing interest from the industry as well as the scientific community in the use of new particles to increase the toughness of brittle adhesives, especially for structural applications. Cork is a biological material with a remarkable combination of properties. A study made by the World Wide Fund for nature reveals that this is a truly sustainable product, since it is a renewable and biodegradable source [36]. In a macroscopic scale, cork is light, elastic and substantially impermeable to liquid and gas; it is also a thermal and electric insulator as well as an acoustic and vibration insulation absorber; innocuous and significantly imputrescible (unaffected by microbial activity), with the ability to be compressed without lateral expansion. Microscopically, cork may be described as a homogeneous tissue of thin-walled prismatic cells, displayed in an alveolar structure similar to a honeycomb [37-43]. However, this excellent behaviour arises for a particle with a considerable number of cells, exhibiting a damping effect and increased impact absorption of the resin [32, 44, 45]. This way, the cork structural properties can be very useful to reinforce a brittle adhesive. Cork could be used to improve the mechanical properties of an adhesive, especially its toughness, since the closed cells could work to absorb actively an impact [37, 38, 41, 42]. However, the properties of an adhesive/cork composite are not only dependent on the materials properties, but also on their interfacial adhesion properties between cork and resin, size and amount of

119 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

cork particles and mixing conditions [35]. In this context, Barbosa et al. [32, 46-48] studied the inclusion of cork microparticles as toughness promoter in brittle epoxy adhesives. Different particle sizes and amounts of cork were added to the resin to analyse the influence of those on the behaviour of the cork/resin composite. Firstly, the authors concluded that the particles dimensions must be above 30 μm, as below this value cork has no effect on increasing the toughness, working instead as a defect and not as an obstacle to the propagation of the cracks.

The optimal combination was found to be that using particles ranging in size from 125-250 μm, considering a 1% volumetric fraction. The experimental results showed an overall improvement of the mechanical properties (i.e. increased strength and strain), against the properties of the epoxy resin itself. Additionally, it was concluded that cork is able to improve the adhesive mechanical properties without detrimental effects on the curing process and on the hydrothermal degradation of the adhesive [49]. Thereupon, a new sort of application for cork powder was proposed, a material which currently is not well exploited by the cork industry, being typically considered as an industrial waste. The use of this material would give a new perspective to the cork industry with potential benefits, especially to the Portuguese economy, since Portugal is the world’s leading market of this raw material, producing three- quarters of the total production. In this sense, the present work aims to propose a new method to smooth the stress distribution along the bondline of a SLJ, through the application of the concept of FGA to joints using magnetised cork microparticles. Therefore, a customized apparatus was designed and with the appropriate application of tailored magnetic fields, using a set of magnets arrays, the magnetised microparticles were strategically placed along the bondline, then creating a particle concentration gradient from the ends of the overlap (higher) to the middle region (lower).

2. Experimental Details 2.1. Materials

Cork powder with 125-250 μm size was employed for this work. The cork used was supplied by Amorim Cork Composites (Mozelos, Portugal), without any treatment. The magnetised cork particles were manufactured by using the same cork powder and coating it with a magnetite layer, according to a patented process (P201730993 [50]). The selected adhesive was Araldite 2020®, from Huntsman Advanced Materials (Pamplona, Spain). This is a two-component adhesive (100/30 by weight), consisting of an epoxy resin

(component A) and a hardener (component B), with low viscosity (150 mPa‧s), transparent and that cures at 100°C, within 15 minutes. The Young’s modulus of this adhesive is typically 3100 MPa and its density is 1.1 g/cm3. This material was selected due to its brittleness, so that the improvements on the tensile strength after the magnetised cork microparticles inclusion can easily be perceived. The material used for the adherends was the AW 7075 aluminium alloy (see Error! Not a valid bookmark self-reference.), from Poly Lanema, LDA (Ovar, Portugal). The selection of this material was due to the nonmagnetic properties of aluminium, since this research is related to the application of magnetic fields in order to dislocate magnetised particles. The aluminium adherends will therefore have no interference on those fields.

120 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Table 1. AW 7075 mechanical properties.

Young’s Modulus Yield Stress Ultimate Stress Poisson’s Elongation [GPa] [MPa] [MPa] Ratio (%) 71 470 540 0.3 7

2.2. Manufacture of bulk specimens

According to the production plan shown in Figure 1, specimens with different amounts of cork and magnetised cork particles were manufactured, with neat referring to specimens which are composed only of adhesive without cork.

Figure 1. Bulk specimens production plan (% by volume).

The particles were initially mixed with the resin using a centrifugal mixing machine, model TM type SpeedMixer DAC 150 (Hauschild Engineering, Hamm, Germany), for 90 s at 1500 rpm. After that, the hardener was added to the mixture and the resulting blend was then submitted to the same centrifugal mixing machine, in the same conditions as previously mentioned. This procedure was the exactly the same for all the amounts of the particles considered. In order to ensure a better particle distribution after the mixing, the composite was heated to 50 ºC, for 10 minutes, to increase the adhesive viscosity. Thereafter, the composite was mixed again in the centrifugal mixing machine. By applying this method, particle agglomeration can be avoided in a simple, effective and reliable way [47]. After mixing the particles with the resin and the hardener, the mixture was poured in a pre- heated steel mould, where release agent had been previously applied to ensure an easy disassembly of the bulk specimen. A silicone rubber frame was used to apply a hydrostatic pressure to the adhesive, which was hot pressed (2 MPa), for 15 minutes at 100 °C (according to the manufacturer's recommendation cure schedule), as shown in Figure 2. Specimens were machined from the plates manufactured with the mould [51]. This manufacturing technique was used to produce the specimens used for tensile tests and those for glass transition temperature measurements.

121 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 2. Exploded view of the bulk specimens’ mould: steel plates, silicone rubber frame and the adhesive plate [51].

2.3. Manufacture of Single lap joints and Graded single lap joints In situ tests are one of the most reliable methods to characterize adhesives and adhesive joints. The single lap joint (SLJ) is one of the most commonly used joint configuration, being easy to manufacture and cheap to produce [2]. Specimens with and without microparticles, having different amounts and distributions, were manufactured, according to the production plan presented in Figure 3. Specimens with a uniform distribution of the particles and with a graded distribution were both produced.

Figure 3. SLJs production plan (% by volume).

For a better adhesion between the substrates and the adhesive, surface treatments were applied to the aluminium surface. Firstly, the surfaces of the adherends were abraded using sandpaper. Then, after cleaning with acetone (in order to remove any dust, oils or contaminants), a sol-gel anodising replacement was applied to the adherends, the 3MTM (USA) Surface Pre-Treatment AC-130-2. Lastly, in order to enhance the adhesion between the substrates and the resin, a primer was used, the Structural Adhesive Primer EW – 5000 AS, from 3MTM (USA). To aid in the manufacture of this type of specimens, an annealed carbon steel mould was used (see Figure 4). The mould ensures that the substrates’ alignment is correct, restricts their movement, controls the overlap length and defines the adhesive thickness, due to its especially designed alignment pins, shims and positioner blocks. To provide an easy release of the specimens after manufacture, mould release agent was applied to all surfaces of the mould [52].

122 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 4. Carbon steel mould for SLJs [52].

In order to manufacture graded SLJs, a patented apparatus was used (PAT 20191000036260). This apparatus and respective method enable the production of adhesive joints with mechanical properties that vary gradually along the overlap, using magnetised particles, preferentially of micro or nano size. The adhesive layer of these joints will have then varying stiffness along the overlap. The apparatus design provides slots for an upper and a lower magnet holders, as well as an upper and a lower male components; the magnet holders contain one or two sets of magnet arrays in the form of blocks, wherein the magnetic arrays are composed by at least 4 neodymium block magnets (see Figure 5).

Figure 5. Apparatus used to produce graded SLJs.

The curing process was the same as the one applied to the bulk specimens. After that, the specimens were carefully removed from the mould, separated with a saw and the excess adhesive on the side of the joint was manually removed with a file. The specimens were manufactured individually in a mould and the adhesive thickness was controlled using appropriately sized packing shims.

2.4. Particle size analysis Particle size analysis was a very important step to understand how the size of the pure and magnetised cork particles is distributed in the tested specimens. In order to analyse the particle size distribution, a Malvern Mastersizer 2000 apparatus (Malvern, United Kingdom) was used.

123 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

The particle size data obtained with this analysis complemented the results obtained in SEM analysis. Three tests were made for each condition. 2.5. Density measurement Knowledge of the particle density is essential for this study, as this information is fundamental to determine the amount of particles to be added to the epoxy resin and the assess the differences between cork and magnetised cork. The density of the particles was measured using a helium pycnometer, with the reference Micromeritics AccuPyc 1330 (DataPhysics, Neurtek Instruments, Eibar, Spain).

2.6. Tensile tests Failure strength tests are commonly used to determine the tensile stress-strain curve of bulk specimens. This test was selected because the stress-strain curve can be used to determine the tensile strength, failure strain and Young’s modulus. These mechanical properties are intrinsic to the material, being obtained under an uniform and uniaxial stress state, without influence of the adherends [52]. Therefore, for tensile tests, dog-bone specimens with 2 mm thickness were manufactured based on the specimen geometry defined by the BS 2782 standard (see Figure 6) [53]. The tensile tests were carried out in an Instron 3367 universal testing machine (Norwood,

USA), with a capacity of 30 kN, at room temperature and at test speed of 1 mm/min. Three specimens were tested for each condition.

Figure 6. Dog-bone tensile test specimens, according to the BS 2782 standard (dimensions in mm) [53].

2.7. Single lap joint tests SLJs specimens are usually used for gathering mechanical information of adhesively bonded systems such as the lap shear strength. This test was selected since the specimens are rather simple to manufacture and resemble the geometry of many practical applications [52]. The SLJ tests were carried out in the same testing machine as the tensile tests, under the same testing conditions (room temperature and test speed of 1 mm/min). Three specimens were tested for each condition. The geometry and dimensions of the SLJs are provided in Figure 7. In order to observe the effect of the magnetic fields gradients on the particles distribution, an overlap with

50 mm length was adopted.

124 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 7. Geometry of the SLJs according to ASTM D 1002 with 15 mm width (all dimensions in mm).

The SLJ test is standardized in ASTM D1002-99 and also in ISO 4587:2003 [54, 55]. The maximum loads were obtained from the experimental load-displacement curves.

2.8. Scanning electron microscopy Scanning electron microscopy (SEM) was employed to analyse the fracture surface from dog- bone bulk specimens, to determine the particles size and geometry, analyse the magnetic coating morphology and its chemical composition as well as to confirm if random particle distributions were successfully achieved. Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS) analyses were carried out in a JEOL JSM 6301F/Oxford INCA Energy 350/Gatan Alto 2500 microscope (Tokyo, Japan) at CEMUP (University of Porto, Portugal). This equipment was employed to analyse the pure and magnetised cork particles, and surface fractures from dog-bone bulk specimens. To do so, the samples were coated with a Au/Pd thin film, by sputtering, using the

SPI Module Sputter Coater equipment, for 120 + 120 푠푒푐, with a 15 mA current.

2.9. Glass transition temperature (Tg) measurement A dynamic mechanical type analysis method initially developed by Adams et al. [56] was used to determine the glass transition temperature (Tg) of the composite. The method involves excitation of the test specimen during both heating and cooling, thus being the Tg measured by registering the damping of the specimen as a function of the temperature, which is defined as the temperature at which the peak value of damping is observed.

This method of analysis is quite fast (19 °C/min) so that it does not alter the specimen during heating [47, 56] and is capable of ensuring a homogeneous temperature distribution along the specimen. The technique used to attach the specimen consists of using a pre-cured sheet of adhesive fastened between an aluminium beam and a constraining steel sheet (Figure 8). For the Tg measurements, the bulk adhesive was machined in the form of rectangular plates with dimensions of 30 x 10 mm and a thickness of 2.0 ± 0.1 mm.

125 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 8. Bulk specimen representation scheme for Tg measurement.

3. Results and Discussion 3.1. Particles characterization The particles size (Figure 9), shape and cellular structure were analysed, for particles with and without magnetic coating. Figure 10 presents the morphological and chemical differences between the two types of particles considered in this study. Cork particles with 125-250 μm size presented a honeycomb structure composed by several cells, which include some open cells on the edges of the particles and closed cells on the particle core (see Figure 10 a) and c)). The number of cells existing in a particle is fundamental for ensuring good mechanical behaviour of the cork particles. Due the milling process used to obtain these particles, some cells show damage on the cell walls. The selected particle size ensures that a sufficient number of closed cells are present in the particles. When analysing the chemical composition of the cork particles (see Figure 10 e)) it was observed that they are mostly composed of carbon (C) and oxygen (O), which is the typical composition of natural materials. The Au and Pd peaks derive from the fine conductive coating required for the observation of this type of material in SEM. In contrast, the magnetised cork particles do not present the typical cork cellular structure, since the ferrimagnetic coating covers the majority of cork cell (see Figure 10 b), d) and f)). It is possible to observe that the coating layer is rigid and not continuous, with several cracks being found. This discontinuity in the coating layer may be an advantage as it increases the interface area between the particle and the adhesive. Mapping of chemical components was performed (see Figure 11), allowing to identify the main chemical components of the magnetised cork particles, i.e. oxygen (seen in green), iron (seen in blue) and carbon (seen in red). This chemical mapping procedure is a very useful tool, as it allows to observe how these constituents are distributed. It was observed that, although the coating layer almost completely covers the cork particle (areas in blue and green), there are some areas that reveal the cellular structure of cork (red areas in the particle). Nevertheless, the results suggest that the coating layer covers the surface of the particles effectively, so that they can be displaced with the application of a magnetic field.

126 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 9. Particle size distribution of cork particles with 125-250 μm size range.

Cork particlesa Magnetised cork particlesa

a) b)

c) d)

127 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

e) f)

Figure 10. Cork particles characterization with and without magnetic coating, 125-250 µm size. a) shape of cork particles; b) shape of magnetised cork particles; c) cork cellular structure; d) detail of magnetic coating layer; e) Spectrum EDS of a cork particle; f) Spectrum EDS of a magnetised cork particle.

Figure 11. Chemical distribution map of a magnetised cork microparticle.

The magnetite coating, although very thin, as depicted in Figure 12, is responsible for an increase of the density of the magnetised particles comparing to cork particles, as can be observed in Figure 13.

128 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 12. Example of a magnetite coating layer in a cork particle.

Figure 13. Densities of pure and magnetised cork microparticles.

3.2. Tensile test results and fracture surface analysis In order to study the influence of the amount of magnetised cork particles, tensile tests were performed. Figure 14 shows typical tensile stress-strain curves of the neat epoxy resin and of the epoxy resin with 1, 2 and 5 % of magnetised cork microparticles.

129 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 14. Tensile stress – strain curves for specimens with different particles amounts.

Analysing these curves, it is evident that the presence of particles modifies the behaviour of the neat epoxy resin and that there are behavioural differences between the different amounts of particles under study. Therefore, for a 1% amount of magnetised particles with a random and uniform distribution among the resin, there is an increase in ductility, given by the increase of the strain, even though there is a slight decrease of the tensile stress, as expected. On the other hand, for the 2 and 5% amounts, the strain and tensile stress values are lower than those presented for neat epoxy resin and 1% of magnetised particles. Therefore, with the purpose of better understanding the effect of the particle amount on the fracture mechanisms, fractographic studies of the tensile bulk specimens fracture surfaces were performed using SEM. Figure 15 shows the fracture surface of the tensile bulk specimens for neat resin and for resin with 1, 2 and 5 % of magnetised cork microparticles. Accordingly, the results for the tensile stress-strain curves regarding 2 and 5% particle amounts are explained by the occurrence of particle deposition, as depicted in the SEM analysis, which tends to be more significant with the increase of the particle amount. As shown in Figure 13, the magnetised particles are heavier than pure cork particles, having both higher density than the adhesive. Mainly, this deposition is due to the high density of the magnetised particles, yet by increasing their amount in the resin, the magnetic component caused by their ferrimagnetic nature becomes relevant, making them attract each other. This way, instead of showing a uniform and random particle distribution, the specimens exhibit two layers, one with the resin itself and another with the deposited particles. However, the particles deposition can be avoided by the increasing of the adhesive’s viscosity. The crack propagation was also analysed with SEM analysis performed previously (Figure 15) to evaluate the particles distribution. Therefore, regarding the specimen with the neat adhesive, known to be a very brittle epoxy, the fracture is shown to be a rapid crack growth zone, where the instability criterion for crack growth is met with the continuously increased loading.

Comparing the images of the neat epoxy resin and the epoxy resin with 1 % of particles, considerable differences can be drawn. Firstly, the neat epoxy resin presents a relatively smooth

130 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

fracture surface. For specimens with 1 % of particles, a slow crack zone is noted at the beginning of the crack growth area (left) and a rapid crack growth zone (right), that led to the loss of material. As expected, the slow crack zone is near the crack initiation point and the rapid crack growth zone is away from the crack initiation point. With the increase of the amount of particles from 1% on, a deposition layer is progressively more perceptible, being more accentuated for the 5% amount of particles. The associated fracture is smoother on the layer composed by the adhesive, consistent to what succeeds to the neat epoxy specimen. However, in the deposition layer, the exact opposite occurs, having a less brittle fracture. Besides, for an amount of 5% it is perceptible that the particles act as defects and not as crack propagation stopping agents.

Figure 15. Fracture surface of tensile specimens for neat and epoxy with magnetised cork microparticles (1, 2 and 5% amounts).

Furthermore, as depicted in Figure 16, with the introduction of particles in the neat epoxy resin, the Young’s modulus decreases, as expected. However, for the various amounts of magnetised particles, the Young’s modulus variation is not significant, assuming a nearly constant value.

131 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

Figure 16. Young's modulus of specimens with different particles amounts.

3.3. Glass transition temperature measurements (Tg) Figure 17 shows the influence of particle inclusion in the neat epoxy resin regarding the correspondent Tg and strain values. The data shows that specimens with particles present a lower Tg in relation to neat resin specimens, being these results explained by the low Tg of the cork particles (approximately, 16.5 ºC [47]).

Figure 17. Glass transition temperature and strain as a function of the amount of particles (% by volume).

132 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

The Tg values obtained for specimens with neat epoxy resin and 1% of particles are in line with the tensile test results. It is known that as the Tg decreases, there is an increase in ductility of the adhesive [57]. However, for specimens with 2 and 5% of magnetised particles, Tg values are not conclusive, due to the particles deposition (see Figure 15). Thus, there is the need for new adhesive formulation in order to assess the effect of the magnetised particles when uniformly and randomly distributed in the epoxy resin.

3.4. Single lap joint tests Typical load-displacement curves obtained by tensile test of the SLJ specimens are represented in Figure 18, for both uniform and graded particle distributions. Since 2 and 5% particle amounts presented similar behaviours in the tensile stress-strain curves, SLJs were only produced for neat epoxy resin and 1 and 5% amounts.

Figure 18. Typical load-displacement curves of joints with neat epoxy resin and magnetised particles with uniform and graded distributions.

Both kinds of distributions for the two particle amounts considered present higher joint performance (judged by higher failure load and larger displacement at failure), when compared to the results for the neat adhesive. In fact, this can be explained by the high stress concentration of the joint at the ends of the overlap due to its high stiffness and the brittleness of the adhesive. The adhesive has a rather brittle behaviour which does not allow it to plastically deform all over the overlap. The failure strain of the adhesive at the ends of the overlap is thus reached before there can be global yielding of the adhesive along the whole overlap [58]. In this test, joints with 5% of uniform particle distribution exhibit higher displacement than joints with 1% of uniform particle distribution. This is the opposite to what occurs to the tensile test results. Such discrepancy can be explained by the fact that in the bulk specimens, the 2 mm

133 Development of a process to obtain a graded distribution of particles along the overlap of adhesive single lap joints

of thickness lead to a higher particle accumulation in the lower part of the specimen; while on the joints, where the thickness of the adhesive is 0.5 mm, the particles follow a more random and uniformly distribution within the total volume. Graded joints with a 1% particle amount are found to be stronger, having higher failure load and larger displacement, than those with where same particle amount is used but with uniform distribution. On the other hand, for graded joints with a 5% particles amount, although there is a decrease on the failure load value, an increase of displacement at failure is obtained. With this, one can conclude that by increasing the amount of magnetised particles in the neat epoxy resin, the graded joints tend to be more ductile, considering a graded distribution.

4. Conclusions In this paper, joints with a gradually modified adhesive, using magnetised cork microparticles, were studied and compared to the correspondent reference joints with a uniform particle distribution. The influence of the amount of magnetised cork microparticles added to a structural brittle epoxy resin was assessed by tensile strength, SLJ tests, Tg measurements and by SEM analysis. The following conclusions can be drawn:  The morphological analysis of cork microparticles shows that they contain a few closed cells, possibly improving the ductility of the material. From SEM analysis, the magnetised particles show a thin magnetite coating layer, which enables them to be displaced in the resin when a magnetic field is applied;  The magnetised cork microparticles can be used to enhance the mechanical properties of a brittle epoxy adhesive;  1% of magnetised cork particles incorporated in a brittle resin gives more ductility than other particle amounts. A different behaviour is observed for specimens above this amount, due to particle deposition;  Tg measurements are consistent with the tensile test results. For a 1% of particles amount, the Tg value is lower, corresponding to a more ductile behaviour;  For SLJs, the particles inclusion is responsible for the increasing of the failure load and displacement values, thus enhancing the joints performance (both graded joints and with a uniform distribution of particles);  The gradation of the particles distribution is responsible for the modification of the properties, when compared to the joints with a uniform particle distribution.

 5. Acknowledgments Financial support by Foundation for Science and Technology (POCI-01-0145-FEDER- 028035) is greatly acknowledged

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