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Article Self-Reinforced 6 Composite for Smart Vibration

Bidita Salahuddin 1,* , Rahim Mutlu 1,2,*, Tajwar A. Baigh 3, Mohammed N. Alghamdi 4 and Shazed Aziz 5,*

1 ARC Centre of Excellence for Electromaterials Science and Intelligent Research Institute, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2522, Australia 2 School of Mechanical, Materials, Mechatronic and Biomedical , University of Wollongong, Wollongong, NSW 2522, Australia 3 Department of Mechanical and Production Engineering, Islamic University of Technology, Gazipur 1704, Bangladesh; [email protected] 4 Department of Technology, Yanbu Industrial College, Yanbu Al-Sinaiyah City 41912, Saudi Arabia; [email protected] 5 School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia * Correspondence: [email protected] (B.S.); [email protected] (R.M.); [email protected] (S.A.)

Abstract: Passive using polymer composites has been extensively investigated by the engineering community. In this paper, a new kind of vibration dampening polymer composite was developed where oriented nylon 6 fibres were used as the reinforcement, and 3D printed unoriented nylon 6 was used as the material. The shape of the reinforcing fibres was modified to a coiled structure which transformed the fibres into a smart thermoresponsive actuator. This novel self-reinforced composite was of high mechanical robustness and its efficacy was demonstrated as

 an active dampening system for oscillatory vibration of a heated vibrating system. The blocking  generated within the reinforcing coiled actuator was responsible for dissipating vibration

Citation: Salahuddin, B.; Mutlu, R.; and increase the magnitude of the damping factor compared to samples made of non- Baigh, T.A.; Alghamdi, M.N.; Aziz, S. reinforced nylon 6. Further study shows that the appropriate annealing of coiled actuators provides Self-Reinforced Nylon 6 Composite an enhanced dampening capability to the composite structure. The extent of crystallinity of the for Smart Vibration Damping. reinforcing actuators is found to directly influence the vibration dampening capacity. Polymers 2021, 13, 1235. https:// doi.org/10.3390/polym13081235 Keywords: self-reinforced composites; nylon 6; polycaprolactam; polymer coil actuator; 3D extrusion printing; vibration damping; polymer crystallinity Academic Editor: Luciano Feo

Received: 13 March 2021 Accepted: 8 April 2021 1. Introduction Published: 11 April 2021 Vibrations in structural assembly reduce structural stability, position precision, flexi-

Publisher’s Note: MDPI stays neutral bility, and control. When a mechanical or material system encounters high- with regard to jurisdictional claims in vibration, a substantial quantity of passive heat can be produced due to the viscoelastic published maps and institutional affil- losses in the system [1]. Active heating is also a major issue when vibration is produced iations. within a complex mechanical system. This is a concerning issue for large structures such as aircraft or automobiles, as well as small assemblies like electronics. Polymers, when engineered appropriately, are excellent materials for damping passive vibration [2]. For instance, the unique properties of viscoelastic polymers include their enormous flexibility and excellent energy dissipation capability, though the elastic modulus and damping ca- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. pacity of such polymers are very delicate when the vibration frequency is changed (i.e., This article is an open access article the frequency of loading) and [3]. A vibration damper fabricated from such distributed under the terms and viscoelastic polymers can be formed either as an insert to place in a cavity within the struc- conditions of the Creative Commons ture or a laminate that is positioned between two surfaces of the structure [4–6]. However, Attribution (CC BY) license (https:// a self-heating effect due to mechanical energy dissipation within a polymeric damper is creativecommons.org/licenses/by/ also a critical problem when the damper experiences prolonged cyclic loading or excessive 4.0/). vibration [7,8]. This self-heating effect typically originates from the internal of the

Polymers 2021, 13, 1235. https://doi.org/10.3390/polym13081235 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1235 2 of 11

polymer molecules, which leads to hysteretic behaviour [9]. The heat resulting from the dissipative processes triggers the temperature increase of the loaded structure causing the self-heating effect. In the case of vibration damping by composite materials, the presence of fillers or reinforcement creates complex internal structures in the overall composite system. The damping behaviours depend on the properties of the material, as well as on the volume or weight fraction of fillers, the interfacial phenomena, filler/reinforcement spread, and dispersion, loading direction, and plasticisation of the matrix material. Several energy dissipation mechanisms exist in fibre reinforced composites such as viscoelasticity of matrix and fibre materials, the friction generated in the fibre/matrix interfacial region, energy dissipation at cracks and interfacial delamination produced at damaged locations, vis- coplasticity, and thermoelastic damping [10,11]. It has been shown that the self-reinforced composites (similar fibre and matrix materials) can withstand unwanted impact or vibra- tional force with less vulnerability to fibre/matrix delamination or cracks [12]. Coupling such a system response and behaviour is a challenging task but an important subject for any smart system design. In the context of composite fabrication, 3D printing offers many advantages including high precision, cost-effectiveness, and customised geometry [13]. This offers the ability to change the mechanical properties of engineering structures, such as stiffness which can be altered with internal fabrication patterns of the engineering structure [14]. polymers are the most used precursor materials for the extrusion-based fused filament 3D fabrication process, due to their inexpensiveness and suitable melting [15]. Several popular choices of exist such as acrylonitrile butadiene styrene, , , , and nylon [16,17]. Nonetheless, the printed materials often lack the mechanical properties to fabricate functional engineering components, which has limited the broader acceptance of this technology in practical applications [18]. To overcome this problem, reinforcements, such as fibres, are incorpo- rated with the polymer matrix while printing, to manufacture a composite structure with improved strength and stiffness. Moreover, the effect of mechanical vibration on the crack generation and propagation of structural composites has been overlooked in terms of introducing an adaptive technology. Self-reinforcing 3D printed composites can diminish the risk of crack generation with high interfacial shear strength. Universally, several techniques can damp mechanical vibrations, for example, by using external dampers that are highly effective for heavy assemblies. For lightweight materials and their assemblies, vibration reduction using an external damper is challenging to achieve. Exploring new materials or systems with combined characteristics of high strength, lightweight, inexpensiveness, and controlled damping capacity has now become crucial. Actuators such as piezoelectric polymers have also been widely researched for controlled vibration damping and have been found useful for the spacecraft near-DC bias . However, real- control of the vibration requires voltage applied from the external supply with feedback from an external sensing unit such as a sensor. A magnetorheological elastomer-based vibration damper has been demonstrated as a smart system to synchronise itself with the time-dependent harmonic oscillatory force to reduce the vibration of the steady-state system [19]. However, it requires an external magnetic field to apply. No active use of the self-heat produced due to the mechanical vibration has been demonstrated, which can be a practical solution to trigger an on- demand vibration damping system without requiring an external facility. Researchers have manipulated the vibration of epoxy composite beams by reinforcing them with stimuli-responsive shape memory alloy fibre actuators. When electrically heated, the fibres have generated a blocking force due to their restrained condition by the constraints of the matrix. However, this blocked force has led to a rise in the free vibration frequency of the full composite shaft. Shape memory elastomeric fibres have attracted significant interest for use as vibration dampers in lightweight engineering composite structures. However, these polymers exhibit a lack of control and environmental adaptation issues in providing Polymers 2021, 13, 1235 3 of 11

effective vibration resistance. Recently, highly oriented fibres were shown to be capable of acting as thermo-responsive contractile actuators while structurally reformed to twist-induced coils/springs [20]. Scalable and controlled actuation of coiled actuators can be obtained by following the specific fabrication parameters [21,22]. They also exhibit many other inherent practical advantages such as handleability, durability, and high robustness, which make them attractive reinforcement for structural devices. Therefore, these coiled actuators reinforced elastomeric composite can be used as the reinforcing filler to damp active vibration during the heat transmission process. In a previous study, researchers have reported the damping behaviour of coiled polyamide fibre by applying a step load without actively inducing the sample [23]. How- ever, there has been no research done that uses stimuli-responsive polymer fibres for the on-demand triggering of mechanical vibration damping using the thermal energy pro- duced or delivered within a vibrating system. This research paper utilises commercially available highly oriented polycaprolactam (commercially known as nylon 6) fibres as the twisted/coiled actuators for mechanical vibration damping in a heated mechanical assembly. The coiled structure of the fibre will be restrained by annealing the coiled sam- ple by heating at a temperature over the temperature(Tg)[24] and then reinforcing it within a continuous nylon 6 matrix. The novelty of such composite lies in the integration and operational domain of the embedded thermoresponsive actuators that actively participate in absorbing system heat (heat sink), making the overall system a smart vibration damper. Coiled actuators will mostly be responsible for vibration damping due to the generation of thermoresponsive blocking force, and the unoriented thermoplastic matrix will act as the solid platform. A set of self-reinforced composites was prepared by using the nylon 6 coils heat set at different temperature. Heat setting determines the extent of crystallinity and stiffness of the coiled actuators, and they eventually manipu- late the magnitude of damping factor obtained from the composites. The samples were characterised via the impact produced free vibration of a heated bar. The resulting vibration response is continuously observed using a LASER displacement analyser focused on the tip of the heated plastic bar. The data is then processed and analysed based on the logarithmic decrement method to evaluate the damping ratio [25]. Based on the concept developed, it is expected that the developed self-reinforced composites can damp the vibration more effectively than that obtained from a control pure polymer damper. The current mostly deals with the development of a novel smart composite system for adaptive vibration damping in heated assemblies. However, with further optimisation and improvements, this kind of damper can be used for real-world applications such as bushings for vibrating boilers, HVAC systems, as well as small assemblies like electronics. Coupled with motion sensors, it can also be used for ultra-stable and smart response systems. Potential future work may also include an investigation using high thermally conductive fillers such as CNT with Nylon 6 dampers for heat sink and/or self-heating system applications.

2. Materials and Methods 2.1. Materials Commercially sourced nylon 6 fibre (Sport Fisher monofilament, 300 µm diameter, Bris- bane, Australia) was used for coiled fibre fabrication. KODAK nylon 6 filament (2.85 mm diameter) was used for 3D extrusion printing. A square Teflon mould was used as the platform for the 3D printed material. Conductive silver paint (Jaycar Electronics, Brisbane, Australia) was sourced locally having an electrical conductivity of 0.02–0.1 Ω/cm2.

2.2. Fabrication of Coiled Fibres Figure1 shows a schematic illustration of fabricating a twisted/coiled fibre used as composite reinforcement. The fibre was suspended from a motor shaft at its upper end and supported by a fixed hanging on the bottom end applying 10 MPa axial to the fibre. The bottom mass was tied opposing to the motor rotation, and each turn from the Polymers 2021, 13, x 4 of 11

and supported by a fixed mass hanging on the bottom end applying 10 MPa axial stress to the fibre. The bottom mass was tied opposing to the motor rotation, and each turn from the motor formed one turn in the fibre. The coiled section of twisted/coiled fibre was then placed within a convection oven and heat set above the glass transition temperature (Tg) of nylon 6 (approx. 47 °C) [26,27], for 60 min, while both ends were still clamped to prevent any deformation. Several heat set temperatures of 50, 100, 150, and 200 °C were used to Polymers 2021, 13, 1235 4 of 11 fabricate a set of four coiled samples. The fibre was then taken out from the oven and relaxed at room temperature for 2 h while still clamped. Heat setting at a temperature over Tg benefits the newly formed coiled shape to be permanently set. Different elevated heatmotor setting formed temperature one turn inwas the chosen fibre. as The time-b coiledased section creep ofand twisted/coiled stress relaxation fibre effects was are then placedless for withinhigh-temperature a convection annealed oven and twisted/coiled heat set above polymer the glass fibres transition [28]. Heat temperature setting at (Thighg) of temperaturenylon 6 (approx. could 47 also◦C) have [26,27 a ],significant for 60 min, effect while on boththe polymer ends were crystallinity, still clamped hence to preventvibra- tionany deformation.damping, which Several is one heat of the set major temperatures aims of this of 50,study. 100, 150, and 200 ◦C were used to fabricateMorphological a set of four and coiled physical samples. properties The fibreof the was coiled then section taken of out the from fibre the were oven then and performed.relaxed at room Scanning temperature electron microscopy for 2 h while (SEM) still clamped.was used to Heat evaluate setting the at uniformity a temperature of coilsover Tandg benefits their geometrical the newly formedstructure. coiled Differential shape to scanning be permanently calorimetry set. Different (DSC) test elevated was conductedheat setting both temperature as-received was and chosen coiled as fibres time-based to evaluate creep their and melting stress relaxationpoints andeffects confirm are theless sustainability for high-temperature of semicrystalline annealed structure twisted/coiled after fibre polymer coiling. fibres Thermogravimetric [28]. Heat setting anal- at ysishigh (TGA) temperature was performed could also to determine have a significant the drying effect temperature on the polymer of the coiled crystallinity, fibres before hence vibrationuse as composite damping, reinforcement. which is one of the major aims of this study.

Figure 1. FabricationFabrication method method of of a a coiled coiled fibre. fibre.

2.3. FabricationMorphological of Self-Reinforced and physical Composites properties of the coiled section of the fibre were then performed.3D printing Scanning was performed electron microscopyin an open-sourc (SEM)e fuse was filament used to extrusion evaluate 3Dprinter the uniformity with aof heated coils andprinting their stage, geometrical which helps structure. control Differential the covering scanning material calorimetry temperature (DSC) during test wasprinting. conducted Both the both printing as-received filament and and coiled coiled fibres fibres to were evaluate vacuum their dried melting for 3 points hours andat 125confirm °C to the remove sustainability the moisture of semicrystalline before fabrications structure of afterthe composite. fibre coiling. The Thermogravimetric printing nozzle wasanalysis 400 µm (TGA) in diameter was performed and the tonozzle determine temperature the drying during temperature extrusion was of the set coiled to 250 fibres °C. Abefore Teflon use mould as composite (100 mm reinforcement. × 100 mm × 10 mm) was used as the printing stage where 16 coiled fibres (each 10 mm long) were vertically placed in a horizontal pattern using the 2.3. Fabrication of Self-Reinforced Composites melt-adhesion technique as depicted in Figure 2. The 3D printer was programmed to fol- low the3D ‘line’ printing infill was pattern performed while fabricating in an open-source the inside fuse matrix filament structure. extrusion When 3Dprinter deposited with a heated printing stage, which helps control the covering material temperature during on the Teflon mould, the temperature of the nylon 6 matrix was ensured to be just enough◦ printing.for material Both flow, the such printing that filament the coiled and fibres coiled were fibres not were affected vacuum by the dried molten for 3 nylon h at 125 6 ma-C to remove the moisture before fabrications of the composite. The printing nozzle was 400 µm trix. This was possible due to a slightly higher of the oriented coiled◦ fibres comparedin diameter to that and of the printed nozzle nylon temperature 6 matrix. Once during the extrusion printed material was set was to 250cured,C. the A piece Teflon × × ofmould the composite (100 mm was100 removed mm 10 from mm) the was Teflon used mould as the and printing polished stage using where fine-grade 16 coiled sand- fibres (eachpaper. 10 Composite mm long) samples were vertically with 50, placed 100, 150, in aand horizontal 200 °C heat pattern set usingcoiled thefibres melt-adhesion were pro- ducedtechnique and named as depicted Composite in Figure 1, Composite2. The 3D 2, printer Composite was 3, programmed and Composite to follow4, respectively. the ‘line’ Compositeinfill pattern samples while fabricatingwill be referred the inside to in the matrix rest of structure. this manuscript When deposited unless indicated on the Teflonoth- erwise.mould, the temperature of the nylon 6 matrix was ensured to be just enough for material flow, such that the coiled fibres were not affected by the molten nylon 6 matrix. This was possible due to a slightly higher melting point of the oriented coiled fibres compared to that of printed nylon 6 matrix. Once the printed material was cured, the piece of the composite was removed from the Teflon mould and polished using fine-grade sandpaper.

Composite samples with 50, 100, 150, and 200 ◦C heat set coiled fibres were produced and named Composite 1, Composite 2, Composite 3, and Composite 4, respectively. Composite samples will be referred to in the rest of this manuscript unless indicated otherwise. Polymers 2021, 13, x 5 of 11

The prepared composite samples were ready for a set of characterisations upon sand- paper-polishing to reduce fatigue effects of surface cracks. The sample was then cut into rectangular pieces (100 mm × 30 mm × 10 mm) for tensile and (30 mm × 30 mm × 10 mm) for vibration damping tests. 3D printed nylon 6 only samples were also fabricated without using any coiled fibres using the process explained above, and these were used as control samples. Evaluation of tensile properties gives an estimation of interfacial adhesion in between the coiled fibre and matrix materials, which is crucial for the ultimate tensile Polymers 2021, 13, 1235 properties of the composites. Although the orientation of the fibres could also5 play of 11 a significant role in the tensile properties of the composites, due to the scope of the current work, this factor was not considered for investigation.

Figure 2. 3D printing of self-reinforced composite matrixmatrix withwith embeddedembedded coiledcoiled fibrefibre reinforcementreinforce- mentas depicted as depicted in a Teflon in a Teflon mould mould (image (image for illustration for illustration only—not only—not to scale toand scale quantify). and quantify).

2.4. VibrationThe prepared Damping composite Test samples were ready for a set of characterisations upon sand- paper-polishing to reduce fatigue effects of surface cracks. The sample was then cut into The vibration damping measurement setup consists of a conductive silver-coated rectangular pieces (100 mm × 30 mm × 10 mm) for tensile and (30 mm × 30 mm × 10 mm) (PP) beam (100 mm × 3 mm × 1 mm), two adjacently placed as-prepared for vibration damping tests. 3D printed nylon 6 only samples were also fabricated without composite bars (30 mm × 30 mm × 1 mm) clamping the plastic beam at the fixed end, and using any coiled fibres using the process explained above, and these were used as control LASER displacement sensor (SICK OD2-P250W150I0, SICK Pty Ltd, Melbourne, Aus- samples. Evaluation of tensile properties gives an estimation of interfacial adhesion in tralia) focused on the tip of the cantilever polypropylene beam. The plastic beam was Joule between the coiled fibre and matrix materials, which is crucial for the ultimate tensile heated to 50 °C during the vibration tests to mimic a random temperature that can pro- properties of the composites. Although the orientation of the filler fibres could also play a duce within a vibrating mechanical system. The free, untethered end of the plastic beam significant role in the tensile properties of the composites, due to the scope of the current was subjected to a transverse direction impact force, and due to that, oscillatory vibration work, this factor was not considered for investigation. were produced and recorded by an e-corder data recorder unit (ED 410, e-DAQ Pty Ltd,2.4. VibrationSydney, DampingAustralia). Test Calculating the damping factor of the system, including the PP beamThe and vibration the novel dampingself-reinforced measurement composite setup padding, consists would of abe conductive sophisticated. silver-coated However, polypropylenewe were able to (PP) conduct beam a (100comparative mm × 3 analysis mm × 1 with mm), a twosimplified adjacently dynamic placed system as-prepared model usingcomposite logarithmic bars (30 decrement mm × 30 mm to calculate× 1 mm) the clamping damping the factor plastic of beam the effective at the fixed system. end, andThe dampingLASER displacement ratio of the sensorharmonic (SICK vibration OD2-P250W150I0, of the PP beam SICK was Pty Ltd,calculated Melbourne, by considering Australia) upperfocused displacement on the tip of limits the cantileverof 20 successive polypropylene peaks that beam.obtains The the period, plastic beam. The was damped Joule naturalheated tofrequency, 50 ◦C during the of the vibration system tests can tobemimic calculated a random using temperature that can produce within a vibrating mechanical system. The free,2 untethered end of the plastic beam was = (1) subjected to a transverse direction impact force, and due to that, oscillatory vibration waves were produced and recorded by an e-corder data recorder unit (ED 410, e-DAQ Pty Ltd, The damping ratio, ζ from the logarithmic decrement, measured from the LASER Sydney, Australia). Calculating the damping factor of the system, including the PP beam displacement sensor using Equations (2) and (3). and the novel self-reinforced composite padding, would be sophisticated. However, we were able to conduct a comparative analysis with a simplified dynamic system model using logarithmic decrement to calculate the damping factor of the effective system. The damping ratio of the harmonic vibration of the PP beam was calculated by considering upper displacement limits of 20 successive peaks that obtains the period, τd. The damped natural frequency, ωd of the system can be calculated using 2π ωd = (1) τd Polymers 2021, 13, 1235 6 of 11

The damping ratio, ζ from the logarithmic decrement, δ measured from the LASER dis- placement sensor using Equations (2) and (3).

2πζ δ = p (2) 1 − ζ2

1  d  δ = ln 0 (3) n dn

where n depicts the number of periods (i.e., 20 which is arbitrary), d0 and dn are initial (the first peak corresponds to the time at zero), and 20th displacement peaks. Effective damping factor, be of the system can be calculated with Equation (4). p be = 2ζ Meke (4)

= 33 = 3EI where Me 140 m and ke L3 that m depicts the mass of the beam, E is Young’s modulus bh3 of PP, I is the second of area (i.e., I = 12 ) for the PP beam.

3. Results and Discussion 3.1. Properties of Coiled Fibres Nylon 6 fibres of 300 µm diameter were used for the fabrication of coiled actuators. The microscopic morphology and shape of the actuators were investigated via SEM analysis with 10 kV accelerating potential, 35–100× magnification, and ~8 mm working distance. Figure3A shows SEM micrographs of as-received in Figure3A-I and coiled in Figure3 A-II nylon 6 fibres. The coiled fibre possesses some gaps between each coil, which is critical for the matrix material to strongly bond with them during composite fabrication. Figure3 A-III shows the cross-sectional view of a free-standing coiled fibre showing the persistence of coils without double-side tethering. This overtwisted coiled fibre consists of an axial hollow that permits the melt-flown matrix material to effectively cover the helical fibre structure. The combination of mechanical twisting/coiling and heat setting ensures this new permanent shape to the fibre. The coiled fibre was then taken for DSC analysis. DSC tests (ISO 11357-1:2016) were performed on the coiled fibres from 25 to 290 ◦C at a heating rate of 10 ◦C /min in nitrogen gas (50 mL/min). Figure3B shows the DSC curves of relevant coiled fibres, where a slight decrease of the melting points was noticed for coiled fibres compared to the precursor fibre. Such a decrease may be the result of the loss of partial crystallinity of the fibres during mechanical twisting. Table1 shows the useful results obtained from the DSC analysis, including the deviations obtained from three samples of each type. The degree of crystallinity was estimated by considering the heat of fusion for a fully crystallised nylon 6 sample (239 J/g) [29,30]. Although the degree of crystallinity has decreased after fibre coiling, the difference is much lower with the sample heat set at 150 ◦C. This can be related to the classical examples of increasing crystallinity in semicrystalline polymers using an annealing technique [31,32]. The 150 ◦C temperature was also just enough that it had a negligible effect on material degradation, which is probably the case of having less crystallinity from a sample heat set at 200 ◦C. TGA (ISO 11358-1:2014) was performed to determine the drying temperature of the coiled fibres before use as composite reinforcement. Three samples of coiled fibres (heat set at 150 ◦C) were heated from 30 to 150 ◦C at a heating rate of 10 ◦C/min in nitrogen gas (20 mL/min). When the sample was vacuum dried for 3 h at 125 ◦C, there was negligible moisture left in the sample; therefore, this drying condition was used for further works. PolymersPolymers2021 2021,,13 13,, 1235x 7 of7 of11 11

Figure 3. 3. PropertiesProperties of of coiled coiled fibre fibre (actuator): (actuator): (A (A) SEM) SEM images images showing showing the the coil coil structure, structure, (B) (DSCB) DSC thermogramsthermograms of of fibres fibres with with diffe differentrent heat heat set settemperature, temperature, (C) TGA (C) TGA thermograms thermograms showing showing the the amount of of moisture moisture is is no non-driedn-dried and and dried dried fibres. fibres.

Table 1. DSC analysis results results of of nylon nylon 6 6 fibres. fibres.

Melting TemperatureMelting Area ofArea Melting of Melting Peak Degree ofDegree Crystallinity of SampleSample Temperature(°C) (◦C) (J/g)Peak (J/g) Crystallinity(%) (%) 85.23 (±0.21) As-receivedAs-received 225.3( 225.3(±0.08)± 0.08) 85.23 (±0.21)35.7 35.7 (±0.05) (±0.05) Coiled: Heat set @ 50 ◦C 221.96 (±0.06) 77.32 (±0.18) 32.3 (±0.1) Coiled:Coiled: Heat Heat set set @ @50 100°C ◦C 221.96 221.87 (±0.06) (± 0.08)77.32 78.63 (±0.18) (±0.22) 32.3 32.9 (± (0.1)±0.09) Coiled:Coiled: Heat Heat set set @ 100 @ 150 °C ◦C 221.87 223.47 (±0.08) (± 0.1)78.63 82.43 (±0.22) (±0.27) 32.9 34.5 (±0.09) (±0.1) Coiled:Coiled: Heat Heat set set @ 150 @ 200 °C ◦C 223.47 222.31 (±0.1) (± 0.12)82.43 80.83 (±0.27) (±0.23) 34.5 33.8 (± (0.1)±0.11) Coiled: Heat set @ 200 °C 222.31 (±0.12) 80.83 (±0.23) 33.8 (±0.11) 3.2. Properties of Self-Reinforced Composites 3.2. Properties of Self-Reinforced Composites The 3D printed composite was mechanically tested to evaluate its tensile properties. A universalThe 3D printed tensile testercomposite and ISOwas 527-1mechanically test method tested (test to evaluate , 5 its mm/min) tensile properties. were used Ato universal evaluate thetensile tensile tester modulus and ISO and 527-1 strength test method of the (test composite speed, samples5 mm/min) (three were samples used to of evaluateeach type). the When tensile the modulus polymer and was strength in the of molten the composite state during samples printing, (three all samples the chains of each were type).in an amorphousWhen the polymer state. Once was in a linethe molten was printed, state during it took printing, several minutesall the chains to decrease were in to anthe amorphous room temperature, state. Once with a line a similar was printed, time taken it took (for several cooling) minutes for other to decrease lines that to werethe roomprinted temperature, above the with already a similar printed time lines taken [33 (for,34 ].cooling) This coolingfor other happens lines that in were a very printed short aboveperiod the for already most polymers printed tolines crystallize [33,34]. toThis their cooling complete happens potential in a andvery to short have period the desired for mostmechanical polymers properties. to crystallize Figure to4 Atheir shows comple the tensilete potential stress-strain and to have curves the of desired the self-reinforced mechani- calnylon properties. 6 composites. Figure 4A Both shows the tensile the tensile strength stress-strain at yield curves and ultimate of the self-reinforced tensile strength nylon was 6found composites. to be the Both highest the tensile for Composite strength at 3 yield (also shownand ultimate as Comp. tensile 3 in strength Figure4 wasB), signifyingfound to bethe the best highest fibre/matrix for Composite compatibility 3 (also shown amongst as allComp. four 3 composite in Figure 4B), samples. signifying Figure the4B best also fibre/matrixshows the comparative compatibility tensile amongst results all offour each composite composite samples. sample Figure together 4B also with shows that from the a comparativecontrol 3D printed tensile nylon results 6 of specimen. each composite The Young’s sample modulus together of with the compositethat from a samplescontrol 3D was printedrelatively nylon higher 6 specimen. due to the The contribution Young’s modulu from highlys of the stiff composite oriented samples nylon 6 was fibres relatively (Young’s highermodulus due 2700 to the MPa) contribution reinforcement. from highly Composite stiff oriented 3 (sample nylon with 6 fibres coiled (Young’s fibre heatmodulus set at °C 2700150 ◦ C)MPa) showed reinforcement. the highest Composite increase of modulus3 (sample owing with tocoiled the excessfibre heat crystallinity set at 150 from the) effective annealing process.

Polymers 2021, 13, x 8 of 11

Polymers 2021, 13, 1235 8 of 11 showed the highest increase of modulus owing to the excess crystallinity from the effec- tive annealing process.

FigureFigure 4.4.Properties Properties of self-reinforcedof self-reinforced composites: composites: (A) Tensile (A) stress-strainTensile stress-strain curves, (B )curves, Comparative (B) Comparativetensile properties. tensile properties.

3.3. Vibration Damping Test Test Results The damping damping behaviour behaviour of of an an impact-produc impact-produceded vibration vibration on a on heated a heated plastic plastic bar was bar wasestimated estimated using using a LASER a LASER displacement displacement analyser analyser.. Figure 5A Figure shows5A the shows schematic the schematic illustra- illustrationtion of the test of the setup test used setup for used measuring for measuring the damp the harmonic damp harmonic oscillatory oscillatory vibration. vibration. Three Threesamples samples each of each the control of the control samples samples and compos and compositesites 1-4 were 1-4 tested were to tested ensure to the ensure repro- the reproducibilityducibility of the ofresult. the result.The vibrating The vibrating plastic plasticbar was bar subjected was subjected to resistive to resistive heating (50 heating °C) (50by applying◦C) by applying a constant a 5V constant DC voltage. 5V DC The voltage. heat was The then heat conducted was then through conducted the adjacent through thecomposite adjacent matrices composite and activated matrices the and coiled activated actuators the coiled for force actuators generation. for force The bars generation. of the Theself-reinforced bars of the self-reinforcedcomposites (Figure composites 5) helped (Figure to damp5) helped the tooscillation damp the of the plastic of bar the plasticmuch faster bar much than faster the bars than of the the bars printed of the cont printedrol sample control (Figure sample 5B). (Figure It should5B). It shouldbe noted be notedthat the that initial the initialdisplacement displacement input in input the control in the control sample sample was much was smaller much smaller (~1.8 mm) (~1.8 than mm) thanthe composite the composite samples samples (approx. (approx. 3.5 3.5mm). mm). When When heated heated and and vibrated, vibrated, the the reinforcing reinforcing coiledcoiled fibresfibres triedtried toto generategenerate lengthlength contractioncontraction thatthat originatedoriginated fromfrom a a fibre fibre untwist untwist during dur- heatinging heating [20, 35[20,35].]. Due Due to the to the restrained restrained condition condition of of fibres fibres by by the the constraints constraints of of the the matrix, ma- ourtrix, coiledour coiled fibres fibres generated generated a blocked a blocked force forc insteade instead of lengthof length contraction. contraction. This This blocking block- forceing force actively actively repeals repeals the the mechanical mechanical energy energy produced produced byby vibrationvibration and and delivers delivers faster faster damping. Among the composites composites used, used, Compos Compositeite 3 3 exhibited exhibited the the best best performance performance with with a vibration-dampingvibration-damping factor of 1.31 ((±0.09)±0.09) compared compared to to that that of of 1.08 1.08 (±0.1) (±0.1) obtained obtained for for a a controlcontrol experiment. This This is is because because the the reinforcing reinforcing coiled coiled fibres fibres in in Composite Composite 3 3are are of of the the highest crystallinity and and able able to to produce produce th thee highest highest blocking blocking force force [24]. [24]. Controlled Controlled and and scalablescalable dampingdamping behaviour behaviour can can be be obtained obtained by changingby changing the geometrical, the geometrical, physical, physical, and/or mechanicaland/or mechanical conditions conditions of the coiledof the coiled fibres, fi suchbres, assuch coil as angle, coil angle, fibre diameter,fibre diameter, annealing an- temperature,nealing temperature, and pre-stretching and pre-stretching the coils. the coils.

PolymersPolymers2021 2021, 13, 13, 1235, x 9 of9 11 of 11

FigureFigure 5.5. Vibration dampingdamping ability self-reinforced composites: ( (AA)) Schematic Schematic illustration illustration of of vibra- vibration dampingtion damping measurements, measurements, (B) ( ControlB) Control sample, sample, (C ()C Composite) Composite 1, 1, (D (D)) Composite Composite 2,2, ((EE)) CompositeCompo- 3, (siteF) Composite 3, (F) Composite 4. 4.

4.4. ConclusionsConclusions InIn thisthis paper,paper, aa self-reinforcedself-reinforced nylonnylon 66 compositecomposite hashas beenbeen developeddeveloped andand itsits vibrationvibra- dampeningtion dampening behaviour behaviour was was investigated. investigated. The The reinforcing-orientedreinforcing-oriented nylon nylon 6 fibres 6 fibres were were inin coiled shape shape with with a thermoresponsive a thermoresponsive actu actuatingating property. property. A continuous A continuous of phaseunor- of unorientediented nylon nylon 6 matrix 6 matrix was 3D was printed 3D printed impregnating impregnating the coiled the fibres. coiled The fibres. slightly The higher slightly highermelting melting point of point coiled of fibres coiled (222 fibres °C) (222ensured◦C) that ensured there thatwas thereno property was no damage property of damagethese offibres these while fibres 3D while printing 3D the printing matrix the nylon matrix 6 with nylon a melting 6 with point a melting of 218 point °C. The of 218blocking◦C. The blockingforce generated force generated within the within reinforcing the reinforcing coiled actuator coiled is actuator responsible is responsible for dissipating for dissipating vibra- vibrationtion energy energy and increasing and increasing the magnitude the magnitude of the damping of the damping factor compared factor compared to a non-rein- to a non- reinforcedforced nylon nylon 6. The 6. most The promising most promising result was result found was when found the reinforcing when the coils reinforcing were heat coils wereset atheat 150 °C set before at 150 composite◦C before compositefabrication. fabrication. These fibres These could fibres produce could a higher produce blocking a higher blockingforce due force to their due enhanced to their enhanced polymer crystal polymerlinity crystallinity and damping and dampingproperties, properties, thereby show- thereby showinging higher higher vibration vibration dampening dampening capability. capability. The Thecoiled coiled actuator actuator provides provides an anenhanced enhanced dampening capability to the composites. The extent of crystallinity of the reinforcing actuator is found to directly influence the vibration dampening capacity.

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The current study mostly focused on the development of new smart composite ma- terial for adaptive vibration damping in a heated system and further investigations and improvements of the composite systems are necessary to suit them to real-world applica- tions. Our future investigations will be on maximising the vibration damping capacity by altering the geometry of the coiled fibres as this directly influences the capacity of produc- ing blocking force. Another approach to maximise the damping performance will also be studied by fabricating self-reinforced composites with different thermoplastic polymers and changing the infill geometries during the 3D printing of matrix material. It is also well-established that the vibration damping performance has a dependency on the system temperature, and we anticipate conducting such investigations in our extended works on the currently developed novel vibration damper.

Author Contributions: Conceptualization, B.S.; methodology, B.S., T.A.B., and S.A.; formal analysis, R.M., T.A.B. and M.N.A.; investigation, B.S. and R.M.; resources, R.M. and S.A.; writing—original draft preparation, B.S.; writing—review and editing, R.M., T.A.B., M.N.A., and S.A.; visualization, B.S. and R.M.; supervision, S.A.; project administration, S.A.; funding acquisition, M.N.A. and S.A. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding authors. Acknowledgments: The authors gratefully acknowledge the technical and intellectual support from The University of Wollongong (Australia), Yanbu Industrial College (Saudi Arabia), and the University of Queensland (Australia). Conflicts of Interest: The authors declare no conflict of interest.

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