Nacre-Like Aluminium Alloy Comosite Plates for Ballistic Impact Applications

NACRE-LIKE ALUMINIUM ALLOY COMOSITE PLATES FOR BALLISTIC IMPACT APPLICATIONS

A Thesis Submitted in Fulfilment of the Requirements for the Degree of Master of Philosophy In Civil Engineering By

Tingyi Miao 430070664

Supervisor: A/Prof. Luming Shen Auxiliary Supervisor: Dr. Yixiang Gan

School of Civil Engineering Faculty of Engineering and Information Technologies University of Sydney, NSW 2006 Australia 2019

Abstract

It is a major scientific challenge to develop the light-weight materials with high performance simultaneously in diverse applications ranging from civil engineering to defence. Nature, with billion years of evolution, has been inspiring engineers to fabricate novel materials with optimized microstructures and excellent mechanical properties.

In this study, motivated by the hierarchical structure of nacre, a multi-layered aluminium alloy

(AA) 7075-T651 based composite is being developed for ballistic impact applications. Engineers has extensively studied this type of nacreous shell due to its highly-organized microarchitecture, remarkable mechanical properties and toughness, which is 3000 times tougher than that of aragonite. The wavy surfaces are introduced to the tablets to enhance the interlocking effect between the layered tablets. To investigate the mechanical performance and the failure modes, numerical simulations and practical experiments were performed on the designed composite plates, in which the aluminium plates with different thickness and waviness, under high speed rate, varying from 300 m/s to 800 m/s.

Numerical results for composite plates of 5.4-mm, 7.5-mm and 9.6-mm thick bioinspired composite plates were compared with corresponding bulk plates under the impact of a rigid hemispherical projectile at same impact velocities. The most significant improvement was recorded for the 5.4-mm thick nacre-like aluminium alloy composite plate, which was attributed to the larger area of plastic deformation due to the tablet arrangement. Experiments data were collected to validate the numerical simulation. It has been found that the nacre-like composites of different thickness had better ballistic behaviour than the bulk ones. The proposed hierarchical structure is found to improve the ballistic performance by varying the failure mode from brittle and localized failure in the bulk plate to more diffuse failure in the composites.

Most of the existing work has focused on microstructure, while not many researches explored the macroscale (millimetre-size) behaviour of nacre-like engineering composites. Different ballistic testings have been done to prove the simulation results generated from Flores-Johnson that the nacre-like composites perform better than bulk plates. There are still some other structural parameters may affect the ballistic performance, such as the projectile nose shapes, interface properties and number of hierarchical levels.

The aim of this thesis is to give general background and research progress about natural nacre and the corresponding nacre-inspired artificial composites, to provide the basis for preparing the detailed experimental and numerical study on the ballistic performance of nacre-like aluminium alloy composites. My following ballistic experiments are to validate the numerical simulation results that the nacre-like composite plate has better ballistic performance at high velocity due to the tablets arrangement and plastic deformation. From previous simulation results, 5.4-mm nacre- like plate has shown a significant performance improvement compared with same thickness bulk plate owing to the hierarchical structure induced high energy absorptions. Hence, plate thickness and projectile velocity play a significant role on the performance improvement of the proposed nacre-like AA7075-T651 composites. Further experimental works are needed to assess other crucial parameters for modifying the mechanical behaviours of such bioinspired materials.

Declaration

This is to certify that to the best of my knowledge; the content of this thesis is my own work. This thesis has not been submitted for any degree or other purposes.

I certify that the intellectual content of this thesis is the product of my own work and that all the assistance received in preparing this thesis and sources have been acknowledged.

Tingyi Miao

Authorship Contribution Statements

This thesis contains material published as

Miao, T., Shen, L., Xu, Q., Flores-Johnson, E.A., Lu, G. and Zhang, J. Ballistic performance of bioinspired nacre-like aluminium composite plates. Composites Part B 177, 107382, 2019.

I co-designed the experiments with the co-authors, analyzed the data and wrote the drafts of the manuscript. Co-authors also revised the drafts and provided their feedbacks on the content of the manuscript.

In addition to the statements above, in cases where I am not the corresponding author of a published item, permission to include the published material has been granted by the corresponding author.

Tingyi Miao, 16/09/2019

As supervisor for the candidature upon which this thesis is based, I can confirm that the authorship attribution statements above are correct.

Luming Shen,

School of Civil Engineering, The University of Sydney, NSW 2006, Australia, 16/09/2019

Acknowledgements

I would like to say thank you to all the people who have helped me during my whole study session for the project. They gave me many useful suggestions and improvements for my simulations and experiment designs to make sure every step developed smoothly.

Prof. Guoxing Lu from Swinburne University of Technology has been helpful to give me advice and provide the gas gun system to complete the ballistic tests. His suggestions and requirements are very useful and necessary. Besides, his postgraduate students, Dr. Jianjun Zhang and Mr. Yu

Zhang have given up a lot of their time to assist me to set up the equipment and finish all impact tests for several days. With the help of their effort, we could finish the project in such a short time.

A special gratitude is for Dr. Emmanuel Flores-Johnson. This work would not have been possible without his previous work and achievements on nacre-like materials. He also helped me to solve some numerical modelling problems in time when creating my own models. I am honoured to have had the opportunity to cooperate with you on this project.

Most importantly, I would like to thank A/Prof. Luming Shen for his expertise and passion to provide various advices in the project. He has guided me to understand the theory and the use of finite element analysis. I really appreciate for his support and feel grateful to be his student.

In additional to that, the technical support team at the School of Civil Engineering was appreciated for helping me to fabricate experimental components sufficiently during my research period.

This work was supported in part by funding from the Faculty of Engineering & Information

Technologies, The University of Sydney, under the Faculty Research Cluster Program. This research was undertaken with the assistance of resources and services from the National

Computational Infrastructure (NCI), which is supported by the Australian Government, and the

University of Sydney HPC service at The University of Sydney.

I declare that the research presented here is my own original work and has not been submitted to any other institution for the award of a degree.

Contents

Abstract ...... 2

Declaration ...... 4

Authorship Contribution Statements ...... 5

Acknowledgements ...... 6

List of Tables ...... 10

List of Figures...... 11

1. Introduction ...... 14

2. Literature review ...... 16

2.1. Hierarchical structures ...... 17

2.2. Bioinspired composites ...... 19

2.3. Microstructures and mechanical properties...... 21

2.4. Numerical simulation of nacre-like material ...... 23

3. Methodology ...... 25

3.1. Experimental setup and testing procedure ...... 25

3.1.2. Ballistic testing procedure ...... 29

3.2. Numerical simulations ...... 32

4. Results and discussion ...... 38

4.1 Mechanical testing of aluminium alloy and epoxy specimens ...... 38

4.2. Ballistic impact tests ...... 39

4.3. Effect of hierarchical nacre-like structure of the plate...... 41

4.4. Effect of impact velocity ...... 44

4.5. Effect of plate thickness ...... 46

4.7. Numerical results ...... 52

5. Conclusion ...... 58

5.1. Summary ...... 58

5.2. Future Work and Plan ...... 58

Reference ...... 60

List of Tables Table 1. Dimension and areal density of the nacre-like composite and bulk aluminium plates. .. 27

Table 2. Nominal chemical composition (weight %) of aluminium alloy 7075...... 29

Table 3. Material properties for epoxy adhesive...... 37

Table 4. Material properties and Johnson-Cook model parameters for aluminium alloy...... 37

Table 5. Ballistic test results for both composite and bulk plates...... 39

List of Figures Figure 1. The hierarchical structure of nacre with some overlaps at microscale. [13] ...... 15

Figure 2. Nacre of the shell: (a). photo of the inner part of Haliotis laevigate; (b). scanning electron microscopy (SEM) image of a cross fracture surface of nacre. [29] ...... 17

Figure 3. Overview of nacre-like composites: (a). schematic with dimensions; (b). tablets under tension slide; (c). some stresses involved in progressive locking...... 20

Figure 4. AA7075-T651 nacre-like composite plate: (a) top view of a fabricated 100 mm × 100 mm plate; (b) 20 mm × 20 mm tablet; (c) 20 mm × 10 mm tablet; (d) 10 mm × 10 mm tablet; (e) schematic illustration of side view of the hierarchical structure (not to scale)...... 26

Figure 5. Detailed dimensions of the bi-concave aluminium tablets (unit: mm, tolerance:

±0.02 mm)...... 27

Figure 6. Detailed dimensions of the mould...... 28

Figure 7. Gas gun system set-up for ballistic test...... 30

Figure 8. Sketch of the gas gun system set-up for impact perforation and schematic representation:

(a) sketch for the entire system; (b) detailed illustration for the rear part of the testing facility. .. 31

Figure 9. Geometries and detailed dimensions of the hemispherical nosed stainless-steel projectile:

(a) cross-section view; (b) front view; (c) picture of the actual projectile...... 31

Figure 10. Detailed steel frames dimensions for clamping the specimen: (a) front/back view of the steel frame; (b) side view of the assembled frame...... 32

Figure 11. Abaqus/Explicit FE model: (a) solid geometry overview and (b) detailed mesh of the

5-layer plate under impact for the left half portion of the plate with mesh being refined towards to the centre...... 33

Figure 12. Independent meshes with tie constraints...... 36

Figure 13. Coupon tests using MTS criterion 50 kN: (a) AA7075 tension specimens after fracture;

(b) stress-strain curves obtained in three AA7075-T651 coupon tests...... 38

Figure 14. Experimentally observed and numerical simulated residual velocity versus impact velocity data for (a) 5-layer; (b) 7-layer; and (c) 9-layer composite plates as well as the corresponding bulk plates of the same weight...... 41

Figure 15. Typical penetration pattern of the 7-layer composite and the corresponding bulk plate subjected to an impact velocity of around 400 m/s...... 43

Figure 16. Optical microscope images of the failed (a) bulk plate in test No.8; and (b) composite plate in test No. 6...... 44

Figure 17. Penetration patterns for the 5-layer nacre-like composite plates subjected to different impact velocities...... 45

Figure 18. Penetration patterns for the 5-layer, 7-layer, and 9- layer composite plates subjected to an impact velocity of 430 m/s...... 47

Figure 19. High-speed camera images showing the perforation process of the projectile through the nacre-like composite plates of: (a) 5-layer; (b) 7-layer; (c) 9-layer with an initial impact velocity of around 430 m/s (time unit in s)...... 48

Figure 20. High-speed camera images showing the perforation process of the projectile through the bulk plates with thickness corresponding to that of (a) 5-layer; (b) 7-layer; (c) 9-layer nacre- like composite plates with initial impact velocity of around 430 m/s (time unit in s)...... 48

Figure 21. Penetration patterns for the 5.4-mm bulk plates subjected to different impact velocities.

...... 49

Figure 22. Penetration patterns for the 7.6-mm bulk plates subjected to different impact velocities.

...... 50

Figure 23. Penetration patterns for the 10.0-mm bulk plates subjected to different impact velocities.

...... 50

Figure 24. Penetration patterns for the 7-layer nacre-like composite plates subjected to different impact velocities...... 51

Figure 25. Penetration patterns for the 9-layer nacre-like composite plates subjected to different impact velocities...... 52

Figure 26. Predicted projectile residual velocities versus impact velocity for bulk and nacre-like plates [18]...... 53

Figure 27. Geometries and detailed dimensions of the long hemispherical nosed stainless-steel projectile...... 54

Figure 28. Comparison of short and long hemispherical projectile for bulk plates...... 54

Figure 29. Predicted projectile residual velocities versus impact velocity for bulk and nacre-like plates of 5.4-mm, 7.5-mm and 9.6-mm thickness...... 56

Figure 30. Penetration images for 5-layer nacre-like plate test No.4 at 428 m/s: (a) plastic deformation (PEEQ) in the FE simulation; (b) Experimental penetration...... 57

1. Introduction

Over the years, the development of energy-absorbing lightweight materials has been an interesting and popular topic in defence, aeronautics, automotive industry and civil engineering. Since many conventional structural materials have reached their performance limits, one of the major scientific challenges for engineers is to develop new high performance, multifunctional materials in various strategic fields [1]. After millions of years of evolution, natural materials have inspired engineers to design and fabricate novel materials with better structures and mechanical properties [2-5].

Spider silk, bone, and mollusc shells are typical natural examples that have inspired engineers to learn from the biological structures and the resulting excellent features to develop new bio-inspired materials with outstanding properties [6].

Among those remarkable natural materials, nacre has raised significant interest as a model system for materials science [7]. Owing to its well-constructed microstructures, i.e. hierarchical structures, organized over several length scales ranging from nanometres to millimetres [8, 9], nacre can achieve high strength and toughness. In particular, nacre from mollusc shell, known as mother of pearl, is a highly complex biological composite comprised of 95 wt% calcium carbonates

(microscopic polygonal tables of aragonite) and bonded together with 5 wt% organic materials

(proteins and polysaccharides) [9-13]. The submicron sized tablets are arranged in layers and tightly stacked to form a three-dimensional brick wall structure with some overlap as shown in

Figure 1 [13]. Although the organic matrix only occupies a small amount, it is quite important in spatial and chemical control of the crystal nucleation and growth, microstructure and toughness enhancement. The most remarkable property of nacre is its toughness, which is about 3000 times higher than that of aragonite [14]. This outstanding mechanical performance is attributed to the

14 brick arrangement of the structure, the waviness of the tablets and the multiple interfaces between tablets.

Figure 1. The hierarchical structure of nacre with some overlaps at microscale. [13]

So far, most of the existing work has focused on understanding and mimicking the chemical compositions and microstructure of nacre [8, 10, 15-17]. However, not many studies have explored the performance of nacre-like engineering composites at the macroscale (millimetre-size) [11, 18-

22]. The first prototype made of poly (methyl methacrylate) (PMMA) tablets in millimetre size based on the structure of natural nacre was developed by Barthelat and Zhu [14] for its unique mechanisms. They have demonstrated the effect of waviness of tablets on the toughness of nacre- like composite and presented theoretical models for the optimal design of this bio-inspired material.

Moreover, we recently performed a numerical study on the ballistic performance of the nacre-like composite plates made of 1-mm thick aluminium alloy (AA) 7075-T651 tablets bonded with toughened epoxy resin using Abaqus/Explicit under the impact of a rigid spherical bullet and observed a significant performance improvement for the nacre-like composite plate (compared to the same thickness bulk AA plate), especially for the thinner one, which was explained by the hierarchical structure facilitating both localized energy absorption (by deformation of the tablet) and more globalized energy absorption (by inter-layered delamination and friction) [18].However,

15 no experiments were done on nacre-like aluminium composite before, so to experimentally validate pervious numerical investigation of the ballistic behaviour of the bio-inspired aluminium alloy composites [18], we have designed and fabricated these nacre-like composite plates using many 1-mm thick AA7075-T651 wavy tablets bonded together with toughened epoxy adhesive for ballistic testing. Following our previous numerical study [18], composite plates with thicknesses of around 5.4-mm, 7.5-mm and 9.6-mm were fabricated and impacted by a hemispherical nosed steel projectile at impact velocities in the range of 300-500 m/s. The experimental results show that the measured residual velocity of the projectile through the composite plate was considerably lower than that through the corresponding bulk AA7075-T651 plate with similar areal density. The corresponding finite element simulations confirmed the above experimental observations.

2. Literature review

Since conventional structural materials have reached their performance limits, it is urgent and challenging to develop new lightweight structural materials with higher strength and toughness in building, transportation and energy [23]. Therefore, it because a new challenge to investigate substitutes by mimicking the architecture of natural/biological materials and structures [1]. The prospect of combining multiple materials with natural structure is increasingly attractive, but the understanding of how natural structures achieve their unique mechanical response has only been scarcely explored [23]. Compared with classical ceramics, nacre manages to overcome materials weakness to strengthen and toughen structures due to its special hierarchical architecture [24]. In this section, we will review the previous studies about the characteristics and modelling of the properties of both the natural nacre and nacre-like bio-inspired structures.

2.1. Hierarchical structures

There are three main layers in most molluscs of the bivalve and gastropods classes: the periostractum (outer layer, composed of hardened protein), the prismatic (middle layer, composed of columnar calcite) and nacre (inner layer, composed by aragonite and organic material) [25, 26].

As shown in Figure 2a, nacre can be found inside many species of seashells from gastropod and bivalve groups [27]. The shell of molluscs is grown by the mantle, which is a soft tissue covered the inside of shell. Among all the shell structures, nacreous structures are relatively stronger and stiffer [27]. Thus, nacre can survive external attack without structural integrity due to its high toughness and ductility to the mollusc shell [28].

Figure 2. Nacre of the shell: (a). photo of the inner part of Haliotis laevigate; (b). scanning electron microscopy (SEM) image of a cross fracture surface of nacre. [29]

The structure of nacre has been studied for many years. With the help of early scanning and transmission electron microscopy investigations in the 60s to 80s, the material structure has been clearly shown in Figure 2b. It contains pseudohexagonal aragonite mineral crystals (i.e., platelets or tablets) with a diameter of 5 to 10-휇푚 and a height about 0.5-휇푚. The confluent mineral layers are highly organised in up to 2000 vertical units [29]. The tablets are arranged in layers and tightly stacked to form a three-dimensional brick wall structure with some overlaps as shown in Figure 1

[13]. Although the organic matrix only occupies a small amount, it is quite important in spatial

17 and chemical control of the crystal nucleation and growth, microstructure and toughness enhancement [30]. The most remarkable property of nacre is its toughness, which is about 3000 times higher than that of aragonite [14]. In particular, nacre is formed by hierarchical structures, ranging from nanometre to millimetre length scale, similar to what is observed in other structural tissues [25, 28, 30, 31]. It is indicated that the layered structure of the abalone shell provides anisotropic mechanical strength that increases the toughness by increasing the resistance of crack propagation perpendicular to the surface and decreasing it correspondingly parallel to it [32].

Unlike manmade materials, natural materials show significant combinations of light-weight, stiffness, strength and toughness [27]. The mechanical behaviour of some shell materials has been delivered by testing the tensile strength, modulus of elasticity in bending and modulus of rupture

[33]. The mechanical properties of nacre have been studied from tension [9, 13, 34], compression

[35], shear [9, 13], bending [36, 37] and other different aspects [38]. Currey and Taylor [33] first demonstrated the high strength and toughness of nacre via mechanical tests on about 20 different species of seashells. Currey [34] used experiments to perform various mechanical and structural characteristics of nacre. Even though nacre is weak in tension as compared with steel, it possesses a relatively high tensile strength of the order of 70-100 MPa due to the small size of the crystallites, extremely uniform arrangement and the presumed relatively low stiffness of the matrix [34].

Afterwards, Jackson et al. [36] followed Currey’s study to measure and model three mechanical properties of nacre from the shell of a bivalve mollusc, Pinctada, including the Young modulus, tensile strength and fracture toughness. Nacre had better mechanical properties than other tested structures and it contains far more organic matrix in the shell compared with others. Nacreous structure is made of tablet-shaped aragonite crystals arranged in layers as in a brick wall, or in columns (sheet nacre and columnar nacre respectively) and the organic phase formed the “cement”

18 between the bricks [33]. Barthelat et al. [39] investigated the structure of the tablets and mechanical properties on nacre from Haliotis rufescens (red abalone). On the other hand, the dynamic nanomechanical behaviour of nacre, such as indentation hardness, was studied via dynamic nanoindentation (nano-DMA) experiments [40]. Hence, microstructure of nacre is the main reason why it has such high fracture toughness performed via many experimental results [30,

38].

2.2. Bioinspired composites

Owing to the limited quantity, relatively poor intrinsic properties of traditional materials and the protection of the environment and climate, scientists and engineers turn to mimic natural materials with their unique properties [41-43]. Traditional materials are those that directly being used from the nature or the surrounding environment. Biomaterials used to be only available in medical devices[44], but today they are widely applicable in a variety of strategic fields and have also inspired for next generation of light-weight nano-composites [45, 46]. With the help of existing biological micro-architectural designs, many investigators have studied the synthesis of various materials inspired from the structures of nacre and bone [47-50]. Mimicking of nacre is mainly focused on structure (including laminated structure, hierarchical structure, brick-like structure, or organic and inorganic multilayered structure), manufacturing process and components [30].

According to fabrication methods, the bio-inspired structures can be divided into two different categories, namely multilayered structure and self-assembled nanocomposite [24, 30]. The first method is about bulk multilayered materials process, using the coarse control of the layer thickness or material range limitations [24]. The later one is about gathering techniques, including crystallization beneath Langmuir monolayers etc., to control the microstructure at the nano- meterscale [51]. From nanoscale to millimetre scale, the design concept of the bio-inspired

19 structure of nacre is widely applied for fabrication, such as poly (vinyl alcohol) (PVA) [52] and poly (diallyldimethylammonium chloride) polycation (PDDA) via a layer-by-layer assembly technique and poly (methyl methacrylate) (PMMA) tablets [53] for its unique mechanisms [30].

Biomimicking has evolved from purely synthetic processes of natural features to molecular-based processing by using other techniques [32].

Figure 3 shows an overview of the nacre-like composites, including an example of the proposed nacre-like composite material in millimetre scale (Figure 3a), the anticipated tension behaviour of such a composite (Figure 3b) and some of the stresses involved in this mechanism (Figure 3c) [30].

The main design is directly borrowed from the features of natural nacre, i.e. the columnar arrangement of stiff and brittle tablets with well-defined overlap and core regions. Like this assembly method, our testing specimens will fabricate layer-by-layer so that the waviness on tablets could generate strain hardening and spread deformation and reinforcements in the core regions.

Figure 3. Overview of nacre-like composites: (a). schematic with dimensions; (b). tablets under tension slide; (c). some stresses involved in progressive locking.

Almost all methods related to the fabrication of nacre-like composites have used laminate processing [31]. However, the biomimetic structures of nacre have not included the subtle structures found in nacre, e.g. tablet waviness, mineral bridges and nano-asperities [30]. Biological materials and their construction processes play important roles in the research and development of bio-inspired materials in engineering science [42] and will affect future synthesis of materials with the improvement of techniques and assembly methods.

2.3. Microstructures and mechanical properties

Recently, scanning probe microscopy and nanoindentation techniques have been involved in testing mechanics of nano- and micro-scale entities in nacre [54]. The techniques include optical microscopy, scanning electron microscope (SEM), transmitting electron microscope (TEM), laser profilometry and atomic force microscopy (AFM) [9].

The development of new materials with increased mechanical properties and structure can be achieved after summarizing nacre’s compositions and structure [25, 55, 56]. Liddicoat et al. [57] have used novel high-resolution microscopy techniques and developed a new atom probe tomography (APT) analysis approach to observe nano-structural architecture of two different alloy systems: Al-Zn-Mg-based 7075 alloy and Al-Mg-Sc-based 5083 alloy. Therefore, the hierarchy and framework of solute have influenced the nano-crystallinity, grain texture and alloy strengthening, which is helpful for developing new generation of advanced alloys with new property-performance space [25].

Nacre has a tensile strength of between 35 and 110 MPa from all three classes with the maximum measured failure strain of 0.018 [34]. The small size of the crystallites enable nacre to have a high strength due to their extremely uniform arrangement and the presumed relatively low stiffness of the matrix. Many of the mechanical properties of nacre are produced by its structure, especially

21 for its importance of the precise and regular spatial arrangement of the plate. Inspired by the extraordinary natural structures, some simple analytical and numerical models have been put forward to explain the basic mechanical design principles of biocomposites. However, it is still at the basic and early stage for studying the role of hierarchy on the mechanical behaviour of biological materials [47].

Combined with a finite element model of three-dimensional hydrated nacre, Barthelat et al. [9] performed tensile, shear and shear-compression tests on dry and hydrated miniature nacre specimens. The experiments have proved the inelastic strains in tension (up to 1% strain at failure) and the large inelastic deformations and strain hardening of nacre under hydrated conditions.

As for the nacre-mimic composites, the property of AA7075-T651 is paid more attention and applied a lot in the experiments. 7075-T6 aluminium alloy is a cold finished aluminium, conducting the following composition (wt.%): 6.10 Zn, 2.90 Mg, 2.00 Cu, 0.50 Fe, 0.40 Si, 0.30

Mn, 0.28 Cr, and Al balance the rest [58]. 7075-T6 is ideally suited for primary airframe structure including fuselage and wing skin because it is the highest strength alloy of aluminium, possessing high fracture toughness and low fatigue crack growth [59, 60]. Material properties have been examined a lot via tension and compression tests for characterizing the stress-strain behaviour of the alloy[61-63]. This type of very high-strength alloy used in the testings are quite brittle, and fragmentation and delamination of the target occurred during impact [63]. For the coupon test, material was obtained in the form of bars of approximately 6mm in thickness and 100mm in length, from which several tensile and fatigue specimens are machined following the ASTM standards B

557 and E606, respectively [58, 64].

2.4. Numerical simulation of nacre-like material

Numerical parametric study and practical ballistic impact experiments are two basic approaches to explore the mechanical behaviour of nacre and its corresponding bio-inspired artificial materials.

Numerical simulation results are usually compared with experimental ones to validate and predict features of nacre and nacre-like composites. This section summarizes both the numerical modelling and ballistic performance testings reported in previous works, and expected outcomes is also prospected.

There are many types of modelling methods for simulating natural and bio-inspired materials, such as meshfree methods for modelling fracture behaviour in brittle materials [65, 66], smoothed- particle hydrodynamics (SPH) method for continuum media [67, 68] and the finite element method

(FEM) [18, 69, 70]. Among all, the finite element method has become one of the most common used methods for modelling the mechanism and behaviour of physical systems in engineering sciences nowadays [69]. It is also an important and effective tool to predict mechanical response by inter structural of materials for simulating the real continuous surroundings [70, 71] and modelling mechanical response of biological soft and hard tissues [8, 72, 73].

The mechanical behaviour of nacre has been analysed by the finite element modelling of the hierarchical micro- and nano-architecture [74-78]. Based on a simple brick and mortar architecture,

3D finite element models of nacre, using finite element software MARC™ (MSC MARC Corp.) with a pre- and post-processor, MENTAT™ (MSC MARC Corp.), were constructed by Katti and

Katti [75] to get the prediction of material properties, such as elastic modulus and yield stress of the organic materials, by applying tensile and compressive stress to the model. They have found that the nature of the organic phase and the organic-inorganic interface play key roles on the overall stress-strain response of nacre. After finishing nanoindentation simulations, Katti et al. [70] then

23 continued to conduct simulated tensile tests at low and high loads. The organic phase between the layers of aragonite platelets has exhibited a high value of elastic modulus of about 15 GPa based on their simulations [54, 74, 75].

The finite element analysis software ABAQUS/Explicit or LS-DYNA is widely used to create models for nacre-like composite materials. Knipprath et al. [19] investigated the ballistic performance of boron carbide layered plates using a triangular wave function in a simplified nacre- like structure design. Grujicic et al. [15] showed that the nacre microstructural features of B4C tablets and polyurea matrix considered play a critical role in the armor penetration resistance.

Besides, Tran et al. [79] studied the performance of composite panels subjected to underwater impulsive loading. In numerical simulations,, the Johnson-Cook constitutive law [80] is frequently used to describe the ductile metals such as aluminium to predict the mechanical behaviour and obtain stress-strain curve [59, 81-85]. The basic form of the constitutive model and data are suitable for computations because it use variables that are applicable in most computer codes by comparing results from cylinder impact tests of different materials (i.e. OFHC copper, 2024-T351 aluminium, Cartridge brass, S-7tool steel, etc.) [80]. It considers both kinematic strengthening and adiabatic heating of the material undergoing strains [86]. Together with Johnson -Cook (JC) constitutive model, the Johnson-Cook fracture criterion [87] is also taken into account to express the strain to fracture as a function of the strain rate, temperature and pressure [18, 88-91].

3. Methodology

3.1. Experimental setup and testing procedure

3.1.1 Nacre-like aluminium composite plates

Following our previous work in [18], the 100 mm × 100 mm composite plates contain 5, 7 or 9 layers with each layer containing many 20 mm × 20 mm sized 1-mm thick AA7075-T651 tablets, as shown in Figure 4a. In particular, the odd number layer of the plate consists of twenty-five 20 mm × 20 mm square tablets (Figure 4b) while for the even number layer, it contain sixteen 20 mm

× 20 mm square tablets in the middle, sixteen 10 mm × 20 mm tablets (Figure 4c) at the edges and four 10 mm × 10 mm tablets (Figure 4d) at the corners. As a result, individual tablets of each layer overlapped a quarter of the surface area with the neighbouring layers, which is close to the one-third of the surface area overlap in the natural nacre material [18]. An example of the brief schematic illustration of the hierarchical structure for a 5-layer aluminium plate is given in Figure

4e. The curved tablet is about 1 mm thick bonded together with 0.1 mm toughened epoxy adhesive.

Besides, the ballistic impact experiments contain three various plate thickness, namely around 5.4- mm, 7.5-mm and 9.6-mm compared with the corresponding aluminium bulk plates of the equal weight.

1.0mm

As proposed in [18], tablets with curved surfaces are used. Except for the tablets in the top and bottom layers of the plate, all tablets in the middle layers are bi-concave with 0.1-mm curvature on both sides, as shown in Figure 5. The surface curvature was manufactured by laser cutting with the radius of 2080 mm (R2028). For the top and bottom layers, each consists of 25 plane-concave tablets, which have a flat surface with the other side surface curved like the bi-concave plates. The small thickness difference between different composite plates with the same number of layers is mainly due to the assembly and compaction error during fabrication.

Top view

Figure 5. Detailed dimensions of the bi-concave aluminium tablets (unit: mm, tolerance:

±0.02 mm).

A toughened epoxy adhesive 3MTM Scotch-WeldTM 2216 B/A is employed to glue the tablets together. This adhesive is a flexible, two-part, room temperature curing epoxy with high peel and shear strength [92]. The mix ratio is based on weight scale with the value of 5 parts by 7 parts for base and accelerator, respectively. The work life at room temperature is about 90 minutes for 100 g and the handling strength time is about 8 to 12 hours. To get fully cured, the specimens were compacted under load of around 5 kg for 7 days before doing the tests. Table 1 shows the detailed information about the fabricated nacre-like composite plates and the calculated dimensions for the corresponding bulk plates (equivalent weight) based on the aluminium alloy density of 2754 kg/m3.

component part nacre-like composite plate bulk aluminium plate thickness (mm) areal density thickness (mm) areal density 2 2 (±0.2 mm) (kg/m ) (±0.1 mm) (kg/m ) 5-layer 5.6 15.0 5.4 14.9 composite plate 7-layer 8.3 21.0 7.6 20.9 composite plate 9-layer 10.6 27.4 10.0 27.5 composite plate

Table 1. Dimension and areal density of the nacre-like composite and bulk aluminium plates.

A mould demonstrated in Figure 6 was used to help fabricate the nacre-like composite plates with different layers. The four panels are flexible to make it easy to take out the plates within few hours after fabrication. Compacting was applied while gluing to decrease the epoxy thickness and gap between tablets. After the shape was formed, the plate would be taken out from the mould and cured for days.

Figure 6. Detailed dimensions of the mould.

The nominal chemical composition of the 7075 alloy is given in Table 2. Temper 651 indicates that the alloy is slightly stretched and aged to peak strength [63]. Coupon tests were carried out using standard tensile specimen according to ASTM E8 for aluminium alloy plates with a gauge length of 25 mm and a width of 6 mm. Three tests were performed using standard uniaxial tension specimens at room temperature using a universal testing machine (MTS criterion) with a maximum

28 loading capacity of 50 kN. The cross-head velocity was 1.0 mm/min, leading to loading strain rate of 0.0007 s-1. Axial compression tests were carried out on cubic specimens of the toughened epoxy adhesive 3MTM Scotch-WeldTM 2216 B/A with the dimension of 12mm × 12mm × 25mm. The tests were done at room temperature and with cross-head velocity 1.0 mm/min, resulting in loading strain rate of 0.0014 s-1.

Al Si Fe Cu Mn Mg Cr Zn Ti Others Balance 0.06 0.19 1.3 0.04 2.4 0.19 5.7 0.08 0.15

Table 2. Nominal chemical composition (weight %) of aluminium alloy 7075 [95].

3.1.2. Ballistic testing procedure

All the ballistic tests were performed using the gas gun system set-up shown in Figure 7, while

Figure 8a shows a schematic of the gas gun system setup. The impact velocity can be controlled by adjusting gas pressure in the charger chamber with the maximum gas pressure of 15 MPa. The ballistic impact system was instrumented with a laser velocimeter and two high-speed video cameras to measure the impact/incident velocity and residual/exit velocity of the projectile, respectively (Figure 8b). The light sensors were placed at the end of the barrel to measure the impact velocity. High-speed cameras were placed on both sides of the specimen to capture the penetration process and measure the exit velocity of the projectile. A high-speed camera was used to record both the impact and residual velocities of the projectile at a frame rate of 150,000 fps.

Calibrations were carried out before the tests. In the test, a composite or bulk AA7075 plate was impacted by a steel hemispherical projectile with a mass of 10.1g. The projectiles are made of standard stainless steel, fired in an indoor shooting range by a gas gun system of half inch (12.7mm) diameter barrel. The detailed geometry and dimension of the projectiles are presented in Figure 9.

Initially a slimmer and longer projectile wish sabot was designed and simulated and then this short one was substitute because it is easy to fabricate and exactly fit the gun barrel.

Figure 7. Gas gun system set-up for ballistic test.

Figure 8. Sketch of the gas gun system set-up for impact perforation and schematic representation:

(a) sketch for the entire system; (b) detailed illustration for the rear part of the testing facility.

Figure 9. Geometries and detailed dimensions of the hemispherical nosed stainless-steel projectile:

(a) cross-section view; (b) front view; (c) picture of the actual projectile.

The specimen was clamped between two centrally-opened steel frames (see Figure 10) before being fixed at the end of the gas gun barrel. The target specimen was 100mm × 100mm square plate. A series of ballistic tests were carried out using different plate thickness and projectile speed by manually adjusting the gas pressure to change the initial impact velocities. After perforating shots, projectile and target were collected for the failure modes analysis, and the impact and residual velocities were calculated via videos captured by the high-speed cameras.

(a) (b) 100*100mm Aluminium Plate 15 20 78 2*M16 Bolts 2*M16 Bolts 45 200 45

78 8 8 Steel Frame Edge Back Steel Frame Steel Frame Aluminiun Plate Front Steel Frame

Figure 10. Detailed steel frames dimensions for clamping the specimen: (a) front/back view of the steel frame; (b) side view of the assembled frame.

3.2. Numerical simulations

3.2.1 Problem description

To numerically investigate the ballistic impact behaviour of nacre-like aluminium composites with different thicknesses, finite element (FE) simulations of the plates impacted by a hemi-spherical steel projectile with a mass of 10.1g and initial impact velocities in the range of 300-900 m/s were performed using Abaqus/Explicit (Version 6.14) [93]. Instead of using a sinusoidal function [18], the waviness of the tablets was generated by mimicking the actual fabricated surface with a

32 waviness magnitude of 0.1 mm. The solid geometry was created using 3D computer-aided design software Rhinoceros and then imported into Abaqus/Explicit for pre- and post-processing (Figure

11). The target is fully clamped at all the edge boundaries. The mesh is comprised of reduced- integration linear hexahedral elements (C3D8R) for the solid tablets and cohesive model available in Abaqus for the cohesive epoxy layer [93]. Following the work of [18], the mesh convergence study confirmed that elements with an average size of 0.27 × 0.27 × 0.27 mm3 is sufficient. The mesh is refined towards to the centre of the plate.

Figure 11. Abaqus/Explicit FE model: (a) solid geometry overview and (b) detailed mesh of the

5-layer plate under impact for the left half portion of the plate with mesh being refined towards to the centre.

3.2.2 Material models

The constitutive response of the solid tablets was simulated using the Johnson-Cook constitutive model [94] and the Johnson-Cook fracture criterion [87]. The Johnson-Cook constitutive model is an empirical model where the von Mises flow stress, 휎, is expressed as follows:

휎 = (퐴 + 퐵휀푛)(1 + 퐶 ln 휀∗̇ )(1 − 푇∗푚) (1)

∗ where A, B, n, C and m are five material constants and 휀 is the equivalent plastic strain; 휀̇ = 휀̇/휀0̇ is the dimensionless plastic strain rate and 푇∗ is the homologous temperature, which is defined as

∗ 푇 = (푇 − 푇푟)/(푇푚 − 푇푟) with T being the absolute temperature, 푇푟 the room temperature and 푇푚 the melting temperature of the material [18, 94].

During transient plastic deformation (which would normally occur under ballistic impact), material softening may occur due to localised adiabatic heating [84]. Abaqus/ Explicit allows to include these effects by computing the increase in the heat flux per unit volume 푟푝푙,

푟푝푙 = 휂휎: 휀푝푙̇ (2)

Where 휂 is the inelastic heat fraction, 휎 is the stress and 휀푝푙̇ is the plastic strain rate. The heat change per unit volume can be expressed as,

휀 휂𝜎푑휀 ∆T = ∫ 푝푙 푝푙 (3) 0 𝜌퐶푃

The Johnson-Cook fracture criterion, which shows the relative effects of various parameters by accumulating damage as the deformation proceeds at an element integration point [87]. The basic form is defined as the ratio of the increment of equivalent plastic strain, ∆휀, during an integration

34 cycle, to the equivalent fracture strain, 휀푓, under the current conditions of strain rate, temperature, pressure and equivalent stress [18, 87]:

퐷 = ∑ ∆휀 (4) 휀푓

The general expression for the strain at fracture is then given by:

푓 ∗ ∗ ∗ 휀 = [퐷1 + 퐷2 exp(퐷3휎 )](1 + 퐷4 ln 휀̇ )(1 + 퐷5푇 ) (5)

Where 퐷1, 퐷2,…, 퐷5 are five material constants and the dimensionless pressure-stress ratio is

∗ 휎 = 휎푚/휎 where 휎푚 represents the average of the three normal stresses and 휎 is the von Mises equivalent stress. Material degradation starts when D = 1. Once the damage initiation criterion has been reached, the effective plastic displacement, 푢푝푙, is activated and is given as the following damage evolution equation [93]:

푢푝푙 = 퐿휀푝푙 (6)

where L is characteristic length of the element and 휀푝푙 is the plastic strain. 푢푝푙 is 0 before damage initiation. If a linear evolution of the damage variable d with the effective plastic displacement is assumed, the damage variable increases according to [93]:

푢푝푙 푑 = 푓 (7) 푢푝푙

푓 where 푢푝푙 refers to the effective plastic displacement at complete failure of the material defined by the user. When d = 1, the complete failure occurs and then the failed element is removed from the model mesh.

The constitutive behaviour of the cohesive interface between tablets is modelled by using the normal bilinear cohesive elements in Abaqus [93]. Both faces of the cohesive elements are

35 connected to the neighbouring components by using surface-based tie constraint as the discretization level in the cohesive layer is different (typically finer) from the discretization level in the surrounding elements (Figure 12). The constitutive behaviour of the cohesive elements in this model is defined by the user defined material model provided in Abaqus [93].

Figure 12. Independent meshes with tie constraints.

After mechanical tests, the experimental results were applied for numerical simulations in Abaqus.

Due to the shipment problem of epoxy adhesive, 3MTM Scotch-Weld 2216 B/A was used instead of Betamate 1044 (used in our previous paper [18]) with similar toughness. Therefore, some of the material properties for Betamate 1044 were input for the simulations listed in Table 3. Table 4 shows the input material porpeties for aluminium alloy AA 7075-T651.

Material Properties Epoxy Density ρ (kg/푚3) 1318* Elastic modulus in the normal direction E (GPa) 5.77* Poison’s ratio 0.25*

 Maximum normal traction 푡푛 (MPa) 85.5  Maximum shear traction 푡푠 (MPa) 70 2  Critical energy-release rate mode I 퐺Ic (J/푚 ) 1680 2  Critical energy-release rate mode II 퐺IIc (J/푚 ) 3570 * are coupon tests results for 3MTM Scotch-Weld 2216 B/A, and  are obtained from previous paper for Betamate 1044. Table 3. Material properties for epoxy adhesive.

Material Properties AA 7075-T651 Density ρ (kg/푚3) 2700 Young’s modulus E (GPa) 70 Poisson’s ratio ν 0.3 Inelastic heat fraction η 0.9 Specific heat 퐶푝 (J/kgK) 910 Stain hardening A (MPa) 520 B (MPa) 477 N 0.52 Stain rate hardening −1 Reference strain rate 휀0̇ (푠 ) 5 C 0001 Temperature softening Reference temperature 푇푟 (K) 293 Melting temperature 푇푚 (K) 893 M 1 Damage parameters 퐷1 0.096 퐷2 0.049 퐷3 -3.465 퐷4 0.016 퐷5 1.099 푓 0.0009 푢푝푙 (mm)

Table 4. Material properties and Johnson-Cook model parameters for aluminium alloy.

4. Results and discussion

4.1 Mechanical testing of aluminium alloy and epoxy specimens

All coupon tests were done at room temperature and with cross-head velocity 1.0 mm/min. The tensile specimens after fracture and the corresponding engineering stress-stain curves for the

AA7075-T651 are shown in Figure 13. The average values of yield strength and ultimate tensile strength are 505 ±1.5 MPa and 570 ± 3.5 MPa, respectively. The average Young’s Modulus and

Poison’s ratio for epoxy are 5.77 ± 0.1 GPa and 0.25 ± 0.01.

(a) (b)

Figure 13. Coupon tests using MTS criterion 50 kN: (a) AA7075 tension specimens after fracture;

(b) stress-strain curves obtained in three AA7075-T651 coupon tests.

4.2. Ballistic impact tests

To assess the ballistic performance of different configurations, a total of 28 specimens have been tested for both nacre-like AA 7075 composite plates and corresponding bulk plates. The details of the tested specimens are given in Table 5.

Composite Plate Test No. Thickness (mm) Mass (kg) Impact Velocity (m/s) Residual Velocity (m/s) 1 0.1515 352.61 225.81 2 0.1460 380.39 268.55 5.6 ± 0.2 3 0.1515 403.84 279.50 4 0.1515 428.32 289.14 5 0.2093 390.93 229.34 6 0.2066 412.84 248.57 8.3 ± 0.2 7 0.2121 427.19 258.09 8 0.2121 440.59 293.00 9 0.2726 399.45 205.23 10 0.2726 422.27 231.54 10.6 ± 0.2 11 0.2726 437.34 246.49 12 0.2782 451.50 285.94 Bulk Plate Test No. Thickness (mm) Mass (kg) Impact Velocity (m/s) Residual Velocity (m/s) 1 0.1570 342.11 242.17 2 0.1460 367.88 291.05 3 0.1515 380.39 308.33 5.4 ± 0.1 4 0.1515 409.16 320.19 5 0.1515 433.95 333.00 6 0.1487 447.52 346.25 7 0.2093 378.67 261.82 8 0.2093 399.71 270.00 9 7.6 ± 0.1 0.2066 410.70 278.23 10 0.2121 436.50 298.45 11 0.2121 464.66 309.11 12 0.2754 388.15 173.08 13 0.2726 398.23 236.45 10.0 ± 0.1 14 0.2726 420.99 242.84 15 0.2726 446.50 249.58

Table 5. Ballistic test results for both composite and bulk plates.

Figure 14 shows the experimentally observed residual velocity versus impact velocity data for the nacre-like composite plates with different layers and the corresponding bulk plates with the same number of layers. As can be seen from the figure, the nacre-like composite plates generally had better ballistic performance than the corresponding bulk ones except for the case with 9 layers. For the 5-layer plates, it is very clear that the nacre-like structure performed better than the bulk plate of similar thickness with an average of 8.4% reduction of the residual velocity for impact velocities in the range of 300-500 m/s. This is attributed to the hierarchical structure of the composite plate enables the increase of energy absorption during plate deformation and energy dissipation in the interface debonding and friction for the 5-layer nacre-like composite. The bulk plates failed by brittle fracture and fragmentation resulting in less energy absorption with relatively smaller penetration area, while there is a much larger impacted area and observed plastic deformation zone in the nacre-like plates.

A similar tendency can be found for the 7-layer nacre-like plates, which exhibit an average reduction in the projectile residual velocity of around 9.2% when compared to the corresponding bulk plates. However, the ballistic performance enhancement of the nacre-like composite plates is less obvious as the plate thickness increases to 9.6 mm. In one extreme case with an impact velocity of around 450 m/s, the 9-layer composite plate performed worse than the corresponding bulk plate.

This could be mainly due to the increased likelihood of ductile failure occurring prior to perforation in the thicker bulk plates.

(a) (b) (c)

400 400 380 Bulk_Test Bulk_Test Bulk_Test 340 Nacre_Test Nacre_Test 350 Nacre_Test 350 Bulk_Simulation Bulk_Simulation Bulk_Simulation Nacre_Simulation 300 Nacre_Simulation

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150 100 100 300 325 350 375 400 425 450 475 500 300 325 350 375 400 425 450 475 500 300 325 350 375 400 425 450 475 500 Impact Velocity (m/s) Impact Velocity (m/s) Impact Velocity (m/s)

4.3. Effect of hierarchical nacre-like structure of the plate

To study the effect of hierarchical structure on the ballistic performance of the plates, the typical failure patterns for a 7-layer composite plate and the corresponding bulk plate at an impact velocity of around 400 m/s are displayed in Figure 15. The hemispherical projectile (right at the bottom of the table) is slightly deformed at the front with plugging failure after the tests for the bulk plate.

As can be seen from the figures in Figure 15, the bulk plate experienced brittle failure and resulted in a very clean hole with a diameter similar to that of the projectile at the front face, as well as some cracks and a small amount of material loss at the back face. This is consistent with the video recorded by the high-speed cameras: the fragmentation from the back side of the plate is observed due to the brittle failure of the material under high speed penetration, while no fragments from the front side are observed. On the other hand, the failure patterns of the nacre-like composite plate are very different when compared to the bulk plate. It can be observed from the back-face image of the impacted composite plate (Figure 15) that petalling and petal bending was generated,

41 indicating more ductile failure. The damage of back face is more severe than the bulk plate, which results in a much larger petalling area and the delamination of many tablets.

In particular, tablets covering more than one third of the bottom layer and one tenth of the second layer have delaminated. Figure 16 shows the close-up views of the above-mentioned failed bulk plate and nacre-like composite plate obtained using an optical microscope. It appears from the figure that the cross section of the impacted area of the bulk plate is relatively smooth confirming brittle failure (Figure 16a). In addition to petalling and petal bending, the thin and sharp end of the tablet in the nacre-like composite plate induced by the large plastic deformation, suggesting ductile failure. Moreover, the delamination of the tablets can be clearly observed with the peeling off the epoxy from the tablet (Figure 16b).

bulk plate #8 composite plate #6 impact velocity (m/s) 399.7 412.8 residual velocity (m/s) 270.0 248.6

front face

back face

projectile deformation

before after before after

Figure 15. Typical penetration pattern of the 7-layer composite and the corresponding bulk plate subjected to an impact velocity of around 400 m/s.

Figure 16. Optical microscope images of the failed (a) bulk plate in test No.8; and (b) composite plate in test No. 6.

4.4. Effect of impact velocity

The failure patterns of three 5-layer composite plates under different impact velocities are given in Figure 17. The hemispherical projectiles after impact tests are only slightly deformed. For the plates, the front faces have experienced some ductile failure and exhibit a circular crater with nearly no global deformation for all the three impact velocities. The delaminated area for the back faces of the plates is almost the same for the three cases, covering one third of the bottom layer.

However, the damaged area decreases about 30% for the second bottom layer with the number of the delaminated tablets changing from 6 to 4 as the impact velocity decreases. Moreover, the top layer of the composite plate is more severely deformed at 푣1 = 352.6 푚/푠 for test No.1, which indicates that more projectile kinetic energy is transformed into plate kinetic energy and dissipated through plastic deformation of the tablets. Therefore, it is not just the interface failure that contributes to an increase of energy dissipation but also the in-plane load redistribution with a more diffused plastic deformation.

impact residual test projectile velocity velocity front face back face no. deformation (m/s) (m/s)

#1 352.6 225.8

before after

#3 403.8 279.5

before after

#4 428.3 289.1

before after

Figure 17. Penetration patterns for the 5-layer nacre-like composite plates subjected to different impact velocities.

4.5. Effect of plate thickness

Figure 18 displays the penetration patterns for the three nacre-like composite plates of different thickness subjected to an initial impact velocity of 430 m/s. It appears that all three hemispherical projectiles deformed a little at the front with no obvious difference among the three cases. As can be seen from the figures in the table, when the plate thickness increases, delamination zones on both sides of the plate become larger and more severe tablet delamination on the back face can be observed. No delamination occurred in the front face for the 5- and 7-layer plates, while three tablets delaminated in the front face of the 9-layer plate. Moreover, four layers of the tablets delaminated in the 9-layer plate in contrast to only two layers damage in the 5-layer plate. This is most likely since for a given impact velocity, the failure of the composite plate becomes more ductile as the plate thickness increases, which results in more delamination of tablets.

5-layer composite plate, #4 7-layer composite plate, #8 9-layer composite plate, #11 impact velocity 428.3 440.6 437.3 (m/s) residual velocity 289.1 293.0 246.5 (m/s)

front face

back face

projectile deformation

before after before after before after

Figure 18. Penetration patterns for the 5-layer, 7-layer, and 9- layer composite plates subjected to an impact velocity of 430 m/s.

Representative images of the penetration process of the projectile through the composite plates with different layers captured by the second high-speed camera are listed in Figure 19, while Figure

20 gives similar images during the ballistic testing of the corresponding bulk plates subjected to similar impact velocities. It can be seen in Figure 18 that more fragmentations from the rear side of the bulk plates can be observed in high-speed camera images when compared to their composite

47 counterparts, which is attributed to the relative low ductility of the bulk material. Also, the number of fragments increases with the thickness of the plate.

4.6. Summary for the rest experimental results

Similarly, Figure 21-23 are the penetration patterns for bulk plates of 5.4-mm, 7.6-mm and 10.0- mm thickness and Figure 24-25 are for 7-layer and 9-layer nacre-like composite plates. The delamination zones become bigger for both sides when the thickness is increasing at same initial impact velocity. For the same thickness plates, the impact area decreases with the rise of initial velocities. Almost similar tendency and deformation conditions as mentioned above.

Figure 21. Penetration patterns for the 5.4-mm bulk plates subjected to different impact velocities.

Figure 22. Penetration patterns for the 7.6-mm bulk plates subjected to different impact velocities.

Figure 23. Penetration patterns for the 10.0-mm bulk plates subjected to different impact velocities.

Figure 24. Penetration patterns for the 7-layer nacre-like composite plates subjected to different impact velocities.

Figure 25. Penetration patterns for the 9-layer nacre-like composite plates subjected to different impact velocities.

4.7. Numerical results

FE simulations of the nacre-like composite plates and the corresponding bulk plates impacted by the hemi-spherical steel projectile at different velocities were performed using Abaqus/Explicit.

Figure 14 shows the comparison of the numerically simulated and experimentally observed residual velocity versus impact velocity data for the nacre-like composite plates of different layers and the corresponding bulk plates with similar thickness. As can be seen from Figure 14, the trend of the simulated residual velocity versus impact velocity data is consistent with that of the experimentally observed one, although the simulated residual velocities are generally lower than those of the experimentally measured ones, particularly in the thinner plate cases.

From previous simulations, Emmanuel used a rigid 10-mm steel spherical projectile with a mass of 4.4g for ballistic tests. Figure 26 shows the graph of predicted projectile residual velocities versus impact velocity for bulk and nacre-like plates. Solid lines are for bulk plates while dash lines are for nacre-like plates with various thickness. Apart from the hemispherical nosed projectile used in the experiments (shown in Figure 9), the initial design was to use the longer projectile given in Figure 27 with a sabot to fit the gun barrel. Then simulations were done to compare short and long projectiles, one of the comparisons result for bulk plates is given in Figure 28.

Figure 26. Predicted projectile residual velocities versus impact velocity for bulk and nacre-like plates [18].

Figure 27. Geometries and detailed dimensions of the long hemispherical nosed stainless-steel projectile.

Figure 28. Comparison of short and long hemispherical projectile for bulk plates.

Figure 29 presents the plots of residual velocity versus impact velocity of the different configurations under a much wider impact velocity range. The lines through the data points were

54 fitted based on the Retch-Ipson model, and the projectile residual velocity 푉푟 can be predicted by

[63]:

푝 푝 1/푝 푉푟 = 푎(푉푖 − 푉푏푙 ) (8) where a and p are the empirical constants that can be calibrated by the numerical and experimental data points, and 푉푏푙 is the obtained ballistic limit.

When compared with the performance of its bulk aluminium counterpart, the 5-layer composite plate exhibits a 5.3% increase of the simulated ballistic limit and a 11.8% reduction of the simulated residual velocity at an incident velocity of 900 m/s. The observed enhancement of the

5-layer composite plate can be mainly attributed to the waviness of the tablets that leads to tablets interlocking, which in turn generates aluminium strain hardening and spreads deformation and reinforcements in the core regions.

On the other hand, for the 7-layer and 9-layer composite plates performed worse than the corresponding bulk plate for impact velocities lower than 250 m/s and 350 m/s, respectively, in the simulations. This is mainly due to the increased importance of the bending resistance of the bulk plates at the relatively low impact velocity range. Moreover, for the thicker 9-layer plate, the improvement brought about by the nacre-like structure was marginal, given that the ductile failure of the bulk plate was already fairly diffused before perforation when the impact velocity is lower than 450 m/s. According to the simulations, the performance improvement of the proposed nacre- like AA7075-T651 composite plate over bulk material is dependent on the plate thickness and projectile velocity. This is somehow consistent with the experiments.

Figure 30 compares the simulated and the real penetration patterns of the 5-layer nacre-like composite plate under impact velocity of 428 m/s. It can be seen from Figure 30b that the actual

55 penetration for the nacre-like composite plate is similar to the simulated result in terms of plastic deformation and tablet delamination. The overall plastic deformation of the aluminiun tablets appears to be widelydistributed throughout the composite structure due to several energy dissipation mechanisms activated during the ballistic impact.

Figure 29. Predicted projectile residual velocities versus impact velocity for bulk and nacre-like plates of 5.4-mm, 7.5-mm and 9.6-mm thickness.

Figure 30. Penetration images for 5-layer nacre-like plate test No.4 at 428 m/s: (a) plastic deformation (PEEQ) in the FE simulation; (b) Experimental penetration.

5. Conclusion

5.1. Summary

A series of impact perforation experiments and numerical analysis were carried out to test the ballistic performance of nacre-like AA 7075 composite plates against that of equivalent weight bulk plates. The nacre-like specimens consisted several layers of aluminium alloy tablets bonded together with toughened epoxy adhesive and impacted by a hemispherical projectile. The 5-layer thinner plate has showed significant improvement that leading a radical change in failure mode from brittle and localised failure in the bulk plate to more diffuse failure in the hierarchical structure to induce high energy absorptions. For the thicker 7-layer and 9-layer plates, on the contrary, the effect of layered structure is not that obvious at lower impact velocity due to the bending resistance of the bulk plate. Therefore, plate thickness and projectile velocity play a significant role on the performance improvement of the proposed nacre-like AA7075-T651 composites.

5.2. Future Work and Plan

There are several structural parameters may affect the ballistic performance of nacre-like composites, such as the projectile nose shapes, waviness, interface properties and number of hierarchical levels. In our designed experiments, we only have focused on the comparisons of hierarchical layers for aluminium alloy AA7075 while the numerical simulations only include the layer waviness and cohesive interface for such a low weight impact resistant material.

As listed before, the experimental results are not quantitatively enough to match the numerical ones, which is due to the limit amount of aluminium alloy tablets for fabrications. Each type of

58 aluminium specimens only contains four initial impact velocities. Therefore, more comprehensive experiments should be done for precise comparisons with the simulations.

In previous simulations, the aluminium alloy tablets were arranged with one type epoxy adhesive.

Additionally, future job can focus on various epoxy toughness and different tablet arrangement.

Since the designed projectile was heavier than expected, the maximum impact velocity reached for the gas gun system was around 450 m/s, lighter projectile with sabot can be designed for higher velocity tests. In conclusion, further experimental works are still challenging and necessary to assess other crucial parameters for modifying the mechanical behaviours of nacre-like bioinspired composites.

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