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Mechanical characterisation of lignocellulosic fibres using toy bricks tensile tester
Journal of the Mechanical Behavior of Biomedical Materials 97 (2019) 58–64
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Mechanical characterisation of lignocellulosic fibres using toy bricks tensile tester
T
Ahmad Tarmezee Taliba, Mohd Afandi P. Mohammeda,∗, Azhari Samsu Baharuddina, Mohd Noriznan Mokhtara, Minato Wakisakab a b
Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM, Serdang, Selangor, Malaysia Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, 808-0196, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Toy-bricks tensile tester Anisotropic viscoelastic Lignocellulosic fibres
This paper demonstrates the potential use of toy-bricks as the building block of a mechanical tensile testing instrument for the mechanical characterisation of natural fibres. A table-top tensile testing instrument was developed using LEGO parts (Mindstorms EV3 and Technics) and a 2 kg capacity load cell, whereas deformation modes were programmed in an open source programming language. Experimental work was conducted on oil palm fibres under different tensile modes (i.e. constant deformation, triple-twisted-tension and deformationrelaxation modes), which showed anisotropic-viscoelastic behaviour, and microstructural damages due to deformation.
1. Introduction African oil palm (Elaeis guineensis Jacq.) empty fruit bunches (OPEFB) is a solid waste produced after oil extraction process in palm oil mills. In 2018, Malaysia produced approximately 98 million tons of oil palm fresh fruit bunches (Malaysian Palm Oil Board [MPOB], 2018). Even though oil palm has the highest vegetable oil yield per hectare of crop plantation and are the least expensive oil compared to other oil crops (Thomas et al., 2015), significant amount of waste are produced that need to be treated and utilised into products like biocompost, biocomposites and biofuel to achieve sustainable palm oil production (Ali et al., 2015; Mahmood et al., 2016; Thushari and Babel, 2018). Oil palm fibres (OPF) from OPEFB (obtained after palm oil milling process), like other natural lignocellulosic fibres, is a biomaterial with complex mechanical behaviour due to cellulose, hemicellulose, and lignin constituents, microfibrils arrangement and cellular structure (Qing and Mishnaevsky, 2009; Adler and Buehler, 2013; Omar et al., 2016). In addition, lignocellulosic fibres also exhibit time dependence viscoelastic behaviour (Del Masto et al., 2017). In oil palm biocomposites, the complex mechanical behaviour, along with physicochemical and morphological characteristics of the fibres, influenced the interfacial interaction between the fibres and the matrix used, which in turn affects the performance of the biocomposites mechanical properties (Sreekala et al., 2000). Mechanical tests such as uniaxial tensile loading, loading unloading cyclic, creep analysis and stress relaxation are conducted to
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understand the complex behaviour of lignocellulosic fibres like OPF (see works by Gunawan et al., 2009; Omar et al., 2014, 2016; Xiang et al., 2015; Hanipah et al., 2017). Often these mechanical tests were conducted in a laboratory using high-end commercial mechanical instruments such as the universal testing machine or micro-tensile tester. However, commercially available universal testing machines are expensive, in addition to being large and difficult to handle for small and thin samples such as lignocellulosic fibres. To overcome this problem, mechanical tests were conducted using customised mechanical testing instruments with a proprietary software (LabVIEW or Matlab programming language) or an open source software (with programming language like Python), where the hardware structures were fabricated from either metal-based construction (such as for testing hemp fibres by Placet et al., 2014) or assembled using toy brick parts (for testing stretchable electronics by Moser et al., 2016). The idea of using simple interlocking construction elements from commercial toy bricks for scientific instruments was reported by researchers, such as those conducted on bone coating experiments using LEGO toy brick parts (Strange and Oyen, 2011), transparent LEGO brick blocks for plant growth experiments (Lind et al., 2014), integration of LEGO bricks and 3D printed material for rapid prototyping of functional objects (Mueller et al., 2014), LEGO bricks in the investigation of photonic phenomena in metamaterial architectures (Celli and Gonella, 2015) and in STEM education such as construction of colorimeter (Asheim et al., 2014). The usage of LEGO bricks for the construction of
Corresponding author. E-mail address: [email protected] (M.A. P. Mohammed).
https://doi.org/10.1016/j.jmbbm.2019.05.010 Received 11 March 2019; Received in revised form 12 April 2019; Accepted 5 May 2019 Available online 09 May 2019 1751-6161/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the Mechanical Behavior of Biomedical Materials 97 (2019) 58–64
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the fibres strand were taken and then averaged. The fibres shape was assumed to be cylindrical. The average fibres distribution is shown in Fig. 1(a). Likewise, Fig. 1(b) illustrates frequency distribution OPF diameter with the highest frequency between 0.41- and 0.46-mm. Fibres with an average diameter ranging from 0.39 to 0.48 mm (10% within global average at 0.44 mm) were selected for mechanical tests. For scanning electron microscope (SEM) micrograph, samples were mounted on an aluminium stub using carbon tape and sputter coated with gold and viewed with 15 kV acceleration voltage. Tensile tests of the fibres were conducted using a custom-made toybrick tensile tester, where the structure of the tensile tester was developed using LEGO Technic components. The configuration of the tensile tester is shown in Fig. 2. The load cell used is Futek LSB200 (with a maximum load of 2 kg) (FUTEK Advanced Sensor Technology Inc, USA). This tensile tester was capable to test up to 50 mm in length and 2.0 mm in diameter for a cylindrical-shaped sample. Due to the small dimension of the tensile tester and the nature of sample analysed (approximately cylindrical in shape), the vice clamp used in commercial mechanical testing devices or the metal contraption used by Moser (2015) cannot be used to grip the OPF sample. Instead, for this tensile tester, a sample gripper at the rotating end was using a 7.5 mm inner diameter metal hook attached to a brass holder, whereas the stationary (load cell) end was using a 4.8 mm diameter screw and a flat 2.0 mm aluminium plate. The fibres sample was mounted as shown in Fig. 2(c) by referring to the method used by Nilsson (2006) and ASTM D3379-75 standard with modification, such that fibres strand with 30 mm length was mounted between two flat steel washers (with an inner diameter of 4.8 mm) at each end and then was applied with epoxy resin (Araldite Rapid Steel Epoxy, Belgium). This ensured that the sample ends remained intact and did not slip during tests. The tensile shear failure was minimized by making sure that during sample mounting preparation, the position of the fibre was perpendicular to the diameter of the flat washer used to mount the sample. The use of the washer to the sample holder ensured that the sample was in the same direction as the tensile loading direction. A shorter fibres length (30 mm) was used to ensure a more consistent fibres diameter range (with selected fibres had a standard deviation of less than 15%). To allow movements of the tensile tester crosshead, two LEGO Technic linear actuators (part number 61927) were connected to two LEGO Mindstorms EV3 large servo motors (part number 45502) where the linear actuators converted the rotary motion of the large motors into linear motion (following the approach by Moser, 2015) with the resolution of linear motion at 0.5 mm. The forward and backward linear movements of the actuators correspond to loading and unloading tensile movement of the crosshead attached to a tensile sample. This allows complex cyclic tensile tests to be conducted under different configurations. To study anisotropic behaviour of the fibres, a LEGO Mindstorms EV3 medium servo motor (part number 45503) was used to twist OPF sample clockwise or anticlockwise with a gear ratio between medium
scientific devices can be considered as part of frugal approach due to the abundance of the materials at a low cost with high precision nature (Drack et al., 2018). Apart from low cost, Lind et al. (2014) listed other benefits of using LEGO bricks in researches such as modularity, scalability, structural precision, simplicity and reproducibility. Likewise, the use of LEGO in scientific research helps researchers and students to design instrument/testing device, since the parts are easily assembled and having consistent bricks tolerance. In the case of EV3 Mindstorms robotics kit, the standardised electronic parts (servo motors, sensors and microcontroller) make it easier to construct scientific instruments without the needs to develop a complex control system. This helps to improve the first prototype of the design, before final design can be developed using a more advanced technique like 3D printing and electronic prototyping platform such as Arduino microcontroller or single-board computers like Raspberry Pi. The portable size of device developed using LEGO (such as those developed by Moser, 2015) can also be useful for engineering courses (educational purpose) to provide hands-on demonstration during classroom session (Cyr et al., 1997). For tensile tests of stretchable electronic polymeric film, Moser et al. (2016) developed a table-top tensile tester using LEGO parts with programmable LEGO brick NXT 2.0 package. They used the LEGO motors/sensors and a load-cell unit with a customised data-acquisition system to measure the forces obtained from the tensile testing. Therefore, the potential use of LEGO brick and motor/sensors for mechanical tests (such as rheological and mechanical testing) would benefit scientists and engineers. In this work, we aim to apply a similar concept to develop a prototype of portable toy brick tensile tester capable of testing non-woody lignocellulosic materials, i.e. OPF. To further reduce the cost, we also made use of the open source operating system to set up and conduct complex deformation tensile tests of OPF such as twisted uniaxial, loading unloading cyclic tensile and stress relaxation tests under various modes, where such approach is new for this biomaterial. Details on the tester development, mechanical tests conducted, microscopy and tensile analyses are shown in this work. 2. Materials and methods Fibres from the stalk region of oil palm empty fruit bunches (OPEFB) were used in this study. OPEFB were collected from Malaysian Federal Land Development Authority (FELDA) Besout Palm Oil Mill in Sungkai, Perak, Malaysia (GPS coordinates: 3°52′52.0″N 101°16′33.3″E). The OPEFB were stored at −20 °C to avoid microbial and fungal growth. The fibres from the OPEFB stalk region were manually separated into single fibre strands and washed with 2% detergent solution to remove contaminants. The fibres were then rinsed with tap water before being dried in an oven at 105 °C for 24 h (Xiang et al., 2015). The diameter of the fibres was determined by using a portable microscope (Dino-Lite AM 4113, Taiwan) where four measurements along
Fig. 1. (a) Distribution of OPF diameter with solid line shows mean of global diameter (0.44 mm) and error bars indicate standard deviation); (b) Frequency curve of OPF diameter. 59
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Fig. 2. (a) Dimension of toy-brick tensile tester frame and; (b) its main components; (c) OPF sample mounted between two flat washers.
Supplementary Material B.
motor and sample gripper is 1:1. The motor movements were controlled from EV3 Programmable Brick microcontroller and programmed using Python language (version 3.5) via the open source Linux Debian-based ev3dev operating system (kernel version 4.4.47–19-ev3dev-ev3). Details of the design and the parts used of this toy brick tensile tester are provided n Supplementary Material A. The tests were conducted by placing the sample in the horizontal position, where the fibres were applied with initial tension between 0.2 N and 0.3 N. All mechanical tests were performed at a crosshead speed of 1.0 mm/s (following Monteiro et al., 2011). Different deformation modes (tensile, twist, cyclic and relaxation) were shown in Fig. 3. Uniaxial test (Fig. 3(a)) was performed by stretching the fibres sample until failure. A similar test was also conducted on the commercial universal mechanical testing instrument, TA.XT Texture Analyser (Stable Micro System Ltd, UK) as a comparison with the toy-brick tensile tester proposed here. For analysis of anisotropic behaviour of OPF, the fibres were twisted at two different modes, i.e. one rotated anticlockwise for three times (T = −1080°), while the other was set three turns anticlockwise (T = 1080°). Cyclic test (Fig. 3(b)) was performed by loading and unloading the fibres at a specified deformation for five cycles at the deformation of 5.0% of total fibres length. On the other hand, cyclic loading under increasing deformation for each cycle (Fig. 3(c)) was conducted by increasing strain of OPF for each cycle by one percent of the length of the initial fibres (1%, 2%, 3%, 4% and 5%). The stress relaxation test (Fig. 3(d)) was performed by stretching the fibres to 5.0% strain and the stress decay was measure while the strain was held constant for 1000 s. Relaxation with constant strain step (Fig. 3(e)) was conducted by relaxing the fibres at 1% strain for 60 s and repeated the same steps after each 1% increments for five cycles. All tests were carried out at 25 °C, 80% relative humidity with at least five replications and a statistical analysis (one-way ANOVA and Pearson's correlation coefficient) was conducted using IBM SPSS Statistics for Windows Version 20.0 (IBM Corp., Armonk, NY) with α value set at 0.05. The summary of the statistical analysis is provided in
3. Results and discussion Table 1 shows a comparison of the fracture stress and strain for fibres tests in this work (OPF uniaxial and twists-clockwise and anticlockwise) with other findings in the literature. Although it seems that fracture stress for 1080° twist clockwise uniaxial test are lower than uniaxial and 1080° twist anticlockwise uniaxial test, as determined by the one-way ANOVA test, there are no statistically significant differences between tests. This shows that the results are inconclusive due to the large standard deviation of fracture stress and fracture strain, especially for uniaxial and 1080° twist anticlockwise uniaxial tests. All fracture stress and strain results reported here are comparable to those by Yusoff et al. (2009), Norul Izani et al. (2013), and Omar et al. (2014), but are lower than the results reported by Sreekala et al. (1997). It is interesting to observe that although the average of OPF diameter used in this study is 0.44 ± 0.05 mm, which is within the range of small OPEFB fibre diameter group by Gunawan et al. (2009), the value of fracture stress is lower than those stated in the same literature. The value of fracture strain for OPF (0.10 ± 0.04) however shows no significant difference with the other studies (between 0.10 and 0.16). All mechanical tests results (uniaxial tension, loading unloading cyclic and stress relaxation) on the OPF using toy-brick tensile tester is shown in Fig. 4. The variation of uniaxial tensile test results from the LEGO tester is within the same region with the results using the commercial texture analyser (TA-XT model), as shown in Fig. 4(a). Although the result curves seem to exhibit differences between the toybrick tensile tester and the commercial mechanical testing instrument, the overlapped standard deviation error bars show that there are no significant differences between the two tests. Previous works on oil palm fibres (oil palm mesocarp fibres by (Hanipah et al., 2017) also showed that the variations were still high using the same commercial 60
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Fig. 3. Schematic diagram for program modes for OPF mechanical test. T = twist (1080° anticlockwise or clockwise) with default value is 0° (uniaxial test), d = strain.
Table 1 Average values of fracture stress and fracture strain for OPF (mean ± standard deviation). Sample
Fracture Stress (MPa)
Fracture Strain (−)
References
OPF (Uniaxial) OPF (Uniaxial TA)a OPF (Uniaxial 1080° clockwise) OPF (Uniaxial 1080° anticlockwise) OPEFB OPEFB OPEFB OPEFB (small fibre diameter 0.4–0.475 mm) OPEFB (medium fibre diameter 0.475–0.575 mm) OPEFB (large fibre diameter 0.575–0.72 mm) OPEFB
97.00 ± 42.57 82.90 ± 65.22 54.57 ± 13.94 85.11 ± 30.94 74.4 ± 7 52 71 246.2 144.0 92.5 248
0.11 0.17 0.09 0.12 0.16 0.10 0.11 – – – 0.14
This study This study This study This study Omar et al. (2014) Norul Izani et al. (2013) Yusoff et al. (2009) Gunawan et al. (2009)
a
± ± ± ± ±
0.05 0.02 0.05 0.04 0.04
Sreekala et al. (1997)
= Uniaxial test using commercial mechanical testing instrument.
at 2% fibre strain (Fig. 4(c)). These effects can be clearly observed for results under incremental loading unloading cyclic mode in Fig. 4(d). Evidence of time-dependent behaviour (viscoelasticity) is shown in Fig. 4(e) from the steady reduction of forces at the holding deformation over time, which is consistent with previous studies on the mechanical tests of oil palm-based fibres (oil palm empty fruit bunch by Omar et al. (2016), oil palm mesocarp fibres by Hanipah et al. (2015), and oil palm stalk fibres by Xiang et al. (2015)). Furthermore, Fig. 4(f) shows step stress relaxation tests of the fibres, where the stress softening become more apparent after each stress relaxation steps. This can be due to microstructural damages that occurred within the fibres (will be
mechanical testing instrument. To elucidate the anisotropic behaviour of OPF, a twisted uniaxial test was conducted on the OPF sample by twisting the fibre for 1080° clockwise and anticlockwise, with the results shown in Fig. 4(b). Even though the average stress-strain curve of the twisted-OPF-clockwise sample is lower than the uniaxial and the twisted-anticlockwise curve, the results were not conclusive due to the large standard deviation of stress and strain (also shown in Table 1 previously), which makes it difficult to accurately investigate the anisotropic behaviour of OPF. The results from the loading-unloading cyclic test show permanent set (plastic deformation) and stress softening (Mullins effect) behaviour 61
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Fig. 4. Results for mechanical tests of OPF. Error bars indicate standard deviations. For a) UNI Lego and UNI TA represent uniaxial tests conducted using the toy-brick tensile tester and commercial mechanical testing instrument, respectively.
defects also occur on the fibre sample before mechanical test being conducted, as evidenced in Fig. 5(g). Fig. 6(a) shows that there is no statistically significant correlation between the diameter of the fibres with fracture stress and strain. This can be due to the uneven diameter of the fibres, where the microscopy analysis in Fig. 5(c) shows multiple localised damages due to the uneven diameter of OPF fibres along its axis (rotation of fibres was concentrated in an area with the smallest fibres diameter). Del Masto et al. (2017) reported that the geometry of elementary hemp fibres that have varying diameters and nonuniform shape along its strand affected the mechanical behaviour. Multiple localised damages coupled with the imperfection of OPF strand along with fibres splitting contribute to the unpredictable behaviour of OPF mechanical failure under uniaxial and twisted uniaxial tests, which results in the jagged strain-stress graph (an example is shown in Fig. 6(b)). This work, therefore highlighted the opportunity to improve the design and functionality of the tester, especially to further investigate the anisotropy behaviour and damage mechanism of lignocellulosic fibres. For example, the anisotropy behaviour can be studied through tensile tests under different modes, i.e. loading unloading cyclic test and stress relaxation test using twisted fibres strand using a more sensitive stepper motor rather than the servo motor by LEGO used here. Likewise, the previous model on the viscoelasticity of OPF (Hanipah et al., 2017; Omar et al., 2016) can be further improved by incorporating the anisotropy effect in the model. It can also be based on
explained later), which is similar to what happened during step cyclic loading-unloading test. The large deviation of true stress (and strain) of OPF can be due to the microstructural damage of the fibres, such as transverse cracking, crack branching and crack initiation at the interface of fibres. This is likely similar to the damage mechanism in bast fibres, as reported by Beaugrand et al. (2017) whose stated the damage mechanism in bast fibres was due to scale dependent tubular porosity combined with defects that randomly distributed along the fibres. Fig. 5(a) shows the SEM image of the fractured region of OPF after uniaxial test with Fig. 5(b) depicts a close-up image of the fractured region that shows exposed primary fibres, while Fig. 5(c) shows the SEM image for the sample that underwent 1080° twist clockwise uniaxial test and Fig. 5(d) sample that underwent 1080° twist anticlockwise uniaxial tests. Multiple localised damages are observed on some OPF samples (Fig. 5(c)) which occurred within the OPF from inter and intra-lumen damages. During tensile text (uniaxial and twisted fibre uniaxial), non-uniform fracture tips and longitudinal cracking occurred, where the OPF sample split into half along the longitudinal axis of OPF as shown in Fig. 5(d). This longitudinal cracks occurred along the fibres that started from the area of the fibres which has transverse defects (Aslan et al., 2011), as shown in Fig. 5(e), where micro-damages were propagated along the transverse axis of the fibre strand. Fracture also happened during OPF twisting before uniaxial tensile commenced, as shown in Fig. 5(f), that is likely due to the fibres over-twisting. Pre-damage due to kinks and 62
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Fig. 5. SEM images of (a); fractured region on the end of OPF after uniaxial test; (b) close-up of rectangular region in (a); (c) OPF after clockwise twist test with arrows showing localised damages; (d) test OPF after anticlockwise twist test; (e) propagated micro-damages along the transverse axis of OPF strand; (f) twisted OPF before uniaxial; (g) defects and kink on the OPF strand.
Fig. 6. (a) Fracture stress and strain for OPF uniaxial and twists tests across diameter; (b) composite images of OPF with a different mode of tests from left: uniaxial, twisted clockwise and twisted anticlockwise; (c) schematic diagram for twisted OPF with the theoretical value for rotated angle, θ1 = 10° and θ2 = −10°; (d) Example of tests results with a jagged stress-strain graph.
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the work by Placet et al. (2014) that described the finite transformation through material rotating frame formulation via anisotropic viscoelastic constitutive law. Shorter fibres around 10 mm or less can be used to ensure consistency of the diameter of OPF along the fibre strand. On the other hand, it would be interesting to study the fibres under compression mode, where the major issue is buckling of the sample during deformation. To solve this, Ueda et al. (2014) set the height of a fibres sample (carbon fibres) to be around 10 μm and a diameter within the range of 5 μm (diameter-height ratio of 0.5) inside an SEM microcompression tester. Finally, the understanding of mechanical behaviour of OPF can be applied in a different area of OPF applications, for instance, the mechanical aspect of OPF in solid fermentation and composting. It can be also applied in biocomposites, for example in the OPF-based 3D printing filaments as filler, which is interesting to be explored in the future. The understanding of OPF complex mechanical behaviour can also be expanded in other natural fibres-based application such as protective clothing, wound dressing or material reinforcement in the biomedical field.
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4. Conclusion A tabletop tensile tester was developed consisting of LEGO parts and sensors, as well as a miniature load cell. This followed with a specialised sample gripper and deformation procedure programmed in an open source operating system were developed. Mechanical tests on lignocellulosic fibres were then conducted under different deformation modes, such as cyclic tensile, stress relation and twisting motion. The tests revealed anisotropic-viscoelastic mechanical behaviour of the oil palm fibres, as well as multiple microstructural damages due to deformation. Acknowledgement Funding for this work was provided by Universiti Putra Malaysia Grant Scheme 2017 (UPM/700-2/1/GPB/2017/9521400 and GP-IPS/ 2017/9517900) and Kyushu Institute of Technology Educational Program 2017-2018. LEGO Mindstorms and LEGO Technic are trademarks of The LEGO Group of companies. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmbbm.2019.05.010. References Adler, D.C., Buehler, M.J., 2013. Mesoscale mechanics of wood cell walls under axial strain. Soft Matter 9 (29), 7138–7144. https://doi.org/10.1039/C3SM50183C. Ali, A.A.M., Othman, M.R., Shirai, Y., Hassan, M.A., 2015. Sustainable and integrated palm oil biorefinery concept with value-addition of biomass and zero emission system. J. Clean. Prod. 91, 96–99. https://doi.org/10.1016/J.JCLEPRO.2014.12.030. Asheim, J., Kvittingen, E.V., Kvittingen, L., Verley, R., 2014. A simple, small-scale Lego colorimeter with a light-emitting diode (LED) used as detector. J. Chem. Educ. 91 (7), 1037–1039. https://doi.org/10.1021/ed400838n. Aslan, M., Chinga-Carrasco, G., Sørensen, B.F., Madsen, B., 2011. Strength variability of single flax fibres. J. Mater. Sci. 46 (19), 6344–6354. https://doi.org/10.1007/ s10853-011-5581-x. Beaugrand, J., Guessasma, S., Maigret, J.-E., 2017. Damage mechanisms in defected natural fibers. Sci. Rep. 7 (14041), 1–7. https://doi.org/10.1038/s41598-01714514-6. Celli, P., Gonella, S., 2015. Manipulating waves with LEGO bricks: a versatile experimental platform for metamaterial architectures. Appl. Phys. Lett. 107 (8), 081901. https://doi.org/10.1063/1.4929566. Cyr, M., Miragila, V., Nocera, T., Rogers, C., 1997. A low-cost, innovative methodology for teaching engineering through experimentation. J. Eng. Educ. 86 (2), 167–171. https://doi.org/10.1002/j.2168-9830.1997.tb00280.x. Del Masto, A., Trivaudey, F., Guicheret-Retel, V., Placet, V., Boubakar, L., 2017. Nonlinear tensile behaviour of elementary hemp fibres: a numerical investigation of the relationships between 3D geometry and tensile behaviour. J. Mater. Sci. 52, 6591–6610. https://doi.org/10.1007/s10853-017-0896-x.
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Mechanical characterisation of lignocellulosic fibres using toy bricks tensile tester
T
Ahmad Tarmezee Taliba, Mohd Afandi P. Mohammeda,∗, Azhari Samsu Baharuddina, Mohd Noriznan Mokhtara, Minato Wakisakab a b
Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM, Serdang, Selangor, Malaysia Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, 808-0196, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Toy-bricks tensile tester Anisotropic viscoelastic Lignocellulosic fibres
This paper demonstrates the potential use of toy-bricks as the building block of a mechanical tensile testing instrument for the mechanical characterisation of natural fibres. A table-top tensile testing instrument was developed using LEGO parts (Mindstorms EV3 and Technics) and a 2 kg capacity load cell, whereas deformation modes were programmed in an open source programming language. Experimental work was conducted on oil palm fibres under different tensile modes (i.e. constant deformation, triple-twisted-tension and deformationrelaxation modes), which showed anisotropic-viscoelastic behaviour, and microstructural damages due to deformation.
1. Introduction African oil palm (Elaeis guineensis Jacq.) empty fruit bunches (OPEFB) is a solid waste produced after oil extraction process in palm oil mills. In 2018, Malaysia produced approximately 98 million tons of oil palm fresh fruit bunches (Malaysian Palm Oil Board [MPOB], 2018). Even though oil palm has the highest vegetable oil yield per hectare of crop plantation and are the least expensive oil compared to other oil crops (Thomas et al., 2015), significant amount of waste are produced that need to be treated and utilised into products like biocompost, biocomposites and biofuel to achieve sustainable palm oil production (Ali et al., 2015; Mahmood et al., 2016; Thushari and Babel, 2018). Oil palm fibres (OPF) from OPEFB (obtained after palm oil milling process), like other natural lignocellulosic fibres, is a biomaterial with complex mechanical behaviour due to cellulose, hemicellulose, and lignin constituents, microfibrils arrangement and cellular structure (Qing and Mishnaevsky, 2009; Adler and Buehler, 2013; Omar et al., 2016). In addition, lignocellulosic fibres also exhibit time dependence viscoelastic behaviour (Del Masto et al., 2017). In oil palm biocomposites, the complex mechanical behaviour, along with physicochemical and morphological characteristics of the fibres, influenced the interfacial interaction between the fibres and the matrix used, which in turn affects the performance of the biocomposites mechanical properties (Sreekala et al., 2000). Mechanical tests such as uniaxial tensile loading, loading unloading cyclic, creep analysis and stress relaxation are conducted to
∗
understand the complex behaviour of lignocellulosic fibres like OPF (see works by Gunawan et al., 2009; Omar et al., 2014, 2016; Xiang et al., 2015; Hanipah et al., 2017). Often these mechanical tests were conducted in a laboratory using high-end commercial mechanical instruments such as the universal testing machine or micro-tensile tester. However, commercially available universal testing machines are expensive, in addition to being large and difficult to handle for small and thin samples such as lignocellulosic fibres. To overcome this problem, mechanical tests were conducted using customised mechanical testing instruments with a proprietary software (LabVIEW or Matlab programming language) or an open source software (with programming language like Python), where the hardware structures were fabricated from either metal-based construction (such as for testing hemp fibres by Placet et al., 2014) or assembled using toy brick parts (for testing stretchable electronics by Moser et al., 2016). The idea of using simple interlocking construction elements from commercial toy bricks for scientific instruments was reported by researchers, such as those conducted on bone coating experiments using LEGO toy brick parts (Strange and Oyen, 2011), transparent LEGO brick blocks for plant growth experiments (Lind et al., 2014), integration of LEGO bricks and 3D printed material for rapid prototyping of functional objects (Mueller et al., 2014), LEGO bricks in the investigation of photonic phenomena in metamaterial architectures (Celli and Gonella, 2015) and in STEM education such as construction of colorimeter (Asheim et al., 2014). The usage of LEGO bricks for the construction of
Corresponding author. E-mail address: [email protected] (M.A. P. Mohammed).
https://doi.org/10.1016/j.jmbbm.2019.05.010 Received 11 March 2019; Received in revised form 12 April 2019; Accepted 5 May 2019 Available online 09 May 2019 1751-6161/ © 2019 Elsevier Ltd. All rights reserved.
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the fibres strand were taken and then averaged. The fibres shape was assumed to be cylindrical. The average fibres distribution is shown in Fig. 1(a). Likewise, Fig. 1(b) illustrates frequency distribution OPF diameter with the highest frequency between 0.41- and 0.46-mm. Fibres with an average diameter ranging from 0.39 to 0.48 mm (10% within global average at 0.44 mm) were selected for mechanical tests. For scanning electron microscope (SEM) micrograph, samples were mounted on an aluminium stub using carbon tape and sputter coated with gold and viewed with 15 kV acceleration voltage. Tensile tests of the fibres were conducted using a custom-made toybrick tensile tester, where the structure of the tensile tester was developed using LEGO Technic components. The configuration of the tensile tester is shown in Fig. 2. The load cell used is Futek LSB200 (with a maximum load of 2 kg) (FUTEK Advanced Sensor Technology Inc, USA). This tensile tester was capable to test up to 50 mm in length and 2.0 mm in diameter for a cylindrical-shaped sample. Due to the small dimension of the tensile tester and the nature of sample analysed (approximately cylindrical in shape), the vice clamp used in commercial mechanical testing devices or the metal contraption used by Moser (2015) cannot be used to grip the OPF sample. Instead, for this tensile tester, a sample gripper at the rotating end was using a 7.5 mm inner diameter metal hook attached to a brass holder, whereas the stationary (load cell) end was using a 4.8 mm diameter screw and a flat 2.0 mm aluminium plate. The fibres sample was mounted as shown in Fig. 2(c) by referring to the method used by Nilsson (2006) and ASTM D3379-75 standard with modification, such that fibres strand with 30 mm length was mounted between two flat steel washers (with an inner diameter of 4.8 mm) at each end and then was applied with epoxy resin (Araldite Rapid Steel Epoxy, Belgium). This ensured that the sample ends remained intact and did not slip during tests. The tensile shear failure was minimized by making sure that during sample mounting preparation, the position of the fibre was perpendicular to the diameter of the flat washer used to mount the sample. The use of the washer to the sample holder ensured that the sample was in the same direction as the tensile loading direction. A shorter fibres length (30 mm) was used to ensure a more consistent fibres diameter range (with selected fibres had a standard deviation of less than 15%). To allow movements of the tensile tester crosshead, two LEGO Technic linear actuators (part number 61927) were connected to two LEGO Mindstorms EV3 large servo motors (part number 45502) where the linear actuators converted the rotary motion of the large motors into linear motion (following the approach by Moser, 2015) with the resolution of linear motion at 0.5 mm. The forward and backward linear movements of the actuators correspond to loading and unloading tensile movement of the crosshead attached to a tensile sample. This allows complex cyclic tensile tests to be conducted under different configurations. To study anisotropic behaviour of the fibres, a LEGO Mindstorms EV3 medium servo motor (part number 45503) was used to twist OPF sample clockwise or anticlockwise with a gear ratio between medium
scientific devices can be considered as part of frugal approach due to the abundance of the materials at a low cost with high precision nature (Drack et al., 2018). Apart from low cost, Lind et al. (2014) listed other benefits of using LEGO bricks in researches such as modularity, scalability, structural precision, simplicity and reproducibility. Likewise, the use of LEGO in scientific research helps researchers and students to design instrument/testing device, since the parts are easily assembled and having consistent bricks tolerance. In the case of EV3 Mindstorms robotics kit, the standardised electronic parts (servo motors, sensors and microcontroller) make it easier to construct scientific instruments without the needs to develop a complex control system. This helps to improve the first prototype of the design, before final design can be developed using a more advanced technique like 3D printing and electronic prototyping platform such as Arduino microcontroller or single-board computers like Raspberry Pi. The portable size of device developed using LEGO (such as those developed by Moser, 2015) can also be useful for engineering courses (educational purpose) to provide hands-on demonstration during classroom session (Cyr et al., 1997). For tensile tests of stretchable electronic polymeric film, Moser et al. (2016) developed a table-top tensile tester using LEGO parts with programmable LEGO brick NXT 2.0 package. They used the LEGO motors/sensors and a load-cell unit with a customised data-acquisition system to measure the forces obtained from the tensile testing. Therefore, the potential use of LEGO brick and motor/sensors for mechanical tests (such as rheological and mechanical testing) would benefit scientists and engineers. In this work, we aim to apply a similar concept to develop a prototype of portable toy brick tensile tester capable of testing non-woody lignocellulosic materials, i.e. OPF. To further reduce the cost, we also made use of the open source operating system to set up and conduct complex deformation tensile tests of OPF such as twisted uniaxial, loading unloading cyclic tensile and stress relaxation tests under various modes, where such approach is new for this biomaterial. Details on the tester development, mechanical tests conducted, microscopy and tensile analyses are shown in this work. 2. Materials and methods Fibres from the stalk region of oil palm empty fruit bunches (OPEFB) were used in this study. OPEFB were collected from Malaysian Federal Land Development Authority (FELDA) Besout Palm Oil Mill in Sungkai, Perak, Malaysia (GPS coordinates: 3°52′52.0″N 101°16′33.3″E). The OPEFB were stored at −20 °C to avoid microbial and fungal growth. The fibres from the OPEFB stalk region were manually separated into single fibre strands and washed with 2% detergent solution to remove contaminants. The fibres were then rinsed with tap water before being dried in an oven at 105 °C for 24 h (Xiang et al., 2015). The diameter of the fibres was determined by using a portable microscope (Dino-Lite AM 4113, Taiwan) where four measurements along
Fig. 1. (a) Distribution of OPF diameter with solid line shows mean of global diameter (0.44 mm) and error bars indicate standard deviation); (b) Frequency curve of OPF diameter. 59
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Fig. 2. (a) Dimension of toy-brick tensile tester frame and; (b) its main components; (c) OPF sample mounted between two flat washers.
Supplementary Material B.
motor and sample gripper is 1:1. The motor movements were controlled from EV3 Programmable Brick microcontroller and programmed using Python language (version 3.5) via the open source Linux Debian-based ev3dev operating system (kernel version 4.4.47–19-ev3dev-ev3). Details of the design and the parts used of this toy brick tensile tester are provided n Supplementary Material A. The tests were conducted by placing the sample in the horizontal position, where the fibres were applied with initial tension between 0.2 N and 0.3 N. All mechanical tests were performed at a crosshead speed of 1.0 mm/s (following Monteiro et al., 2011). Different deformation modes (tensile, twist, cyclic and relaxation) were shown in Fig. 3. Uniaxial test (Fig. 3(a)) was performed by stretching the fibres sample until failure. A similar test was also conducted on the commercial universal mechanical testing instrument, TA.XT Texture Analyser (Stable Micro System Ltd, UK) as a comparison with the toy-brick tensile tester proposed here. For analysis of anisotropic behaviour of OPF, the fibres were twisted at two different modes, i.e. one rotated anticlockwise for three times (T = −1080°), while the other was set three turns anticlockwise (T = 1080°). Cyclic test (Fig. 3(b)) was performed by loading and unloading the fibres at a specified deformation for five cycles at the deformation of 5.0% of total fibres length. On the other hand, cyclic loading under increasing deformation for each cycle (Fig. 3(c)) was conducted by increasing strain of OPF for each cycle by one percent of the length of the initial fibres (1%, 2%, 3%, 4% and 5%). The stress relaxation test (Fig. 3(d)) was performed by stretching the fibres to 5.0% strain and the stress decay was measure while the strain was held constant for 1000 s. Relaxation with constant strain step (Fig. 3(e)) was conducted by relaxing the fibres at 1% strain for 60 s and repeated the same steps after each 1% increments for five cycles. All tests were carried out at 25 °C, 80% relative humidity with at least five replications and a statistical analysis (one-way ANOVA and Pearson's correlation coefficient) was conducted using IBM SPSS Statistics for Windows Version 20.0 (IBM Corp., Armonk, NY) with α value set at 0.05. The summary of the statistical analysis is provided in
3. Results and discussion Table 1 shows a comparison of the fracture stress and strain for fibres tests in this work (OPF uniaxial and twists-clockwise and anticlockwise) with other findings in the literature. Although it seems that fracture stress for 1080° twist clockwise uniaxial test are lower than uniaxial and 1080° twist anticlockwise uniaxial test, as determined by the one-way ANOVA test, there are no statistically significant differences between tests. This shows that the results are inconclusive due to the large standard deviation of fracture stress and fracture strain, especially for uniaxial and 1080° twist anticlockwise uniaxial tests. All fracture stress and strain results reported here are comparable to those by Yusoff et al. (2009), Norul Izani et al. (2013), and Omar et al. (2014), but are lower than the results reported by Sreekala et al. (1997). It is interesting to observe that although the average of OPF diameter used in this study is 0.44 ± 0.05 mm, which is within the range of small OPEFB fibre diameter group by Gunawan et al. (2009), the value of fracture stress is lower than those stated in the same literature. The value of fracture strain for OPF (0.10 ± 0.04) however shows no significant difference with the other studies (between 0.10 and 0.16). All mechanical tests results (uniaxial tension, loading unloading cyclic and stress relaxation) on the OPF using toy-brick tensile tester is shown in Fig. 4. The variation of uniaxial tensile test results from the LEGO tester is within the same region with the results using the commercial texture analyser (TA-XT model), as shown in Fig. 4(a). Although the result curves seem to exhibit differences between the toybrick tensile tester and the commercial mechanical testing instrument, the overlapped standard deviation error bars show that there are no significant differences between the two tests. Previous works on oil palm fibres (oil palm mesocarp fibres by (Hanipah et al., 2017) also showed that the variations were still high using the same commercial 60
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Fig. 3. Schematic diagram for program modes for OPF mechanical test. T = twist (1080° anticlockwise or clockwise) with default value is 0° (uniaxial test), d = strain.
Table 1 Average values of fracture stress and fracture strain for OPF (mean ± standard deviation). Sample
Fracture Stress (MPa)
Fracture Strain (−)
References
OPF (Uniaxial) OPF (Uniaxial TA)a OPF (Uniaxial 1080° clockwise) OPF (Uniaxial 1080° anticlockwise) OPEFB OPEFB OPEFB OPEFB (small fibre diameter 0.4–0.475 mm) OPEFB (medium fibre diameter 0.475–0.575 mm) OPEFB (large fibre diameter 0.575–0.72 mm) OPEFB
97.00 ± 42.57 82.90 ± 65.22 54.57 ± 13.94 85.11 ± 30.94 74.4 ± 7 52 71 246.2 144.0 92.5 248
0.11 0.17 0.09 0.12 0.16 0.10 0.11 – – – 0.14
This study This study This study This study Omar et al. (2014) Norul Izani et al. (2013) Yusoff et al. (2009) Gunawan et al. (2009)
a
± ± ± ± ±
0.05 0.02 0.05 0.04 0.04
Sreekala et al. (1997)
= Uniaxial test using commercial mechanical testing instrument.
at 2% fibre strain (Fig. 4(c)). These effects can be clearly observed for results under incremental loading unloading cyclic mode in Fig. 4(d). Evidence of time-dependent behaviour (viscoelasticity) is shown in Fig. 4(e) from the steady reduction of forces at the holding deformation over time, which is consistent with previous studies on the mechanical tests of oil palm-based fibres (oil palm empty fruit bunch by Omar et al. (2016), oil palm mesocarp fibres by Hanipah et al. (2015), and oil palm stalk fibres by Xiang et al. (2015)). Furthermore, Fig. 4(f) shows step stress relaxation tests of the fibres, where the stress softening become more apparent after each stress relaxation steps. This can be due to microstructural damages that occurred within the fibres (will be
mechanical testing instrument. To elucidate the anisotropic behaviour of OPF, a twisted uniaxial test was conducted on the OPF sample by twisting the fibre for 1080° clockwise and anticlockwise, with the results shown in Fig. 4(b). Even though the average stress-strain curve of the twisted-OPF-clockwise sample is lower than the uniaxial and the twisted-anticlockwise curve, the results were not conclusive due to the large standard deviation of stress and strain (also shown in Table 1 previously), which makes it difficult to accurately investigate the anisotropic behaviour of OPF. The results from the loading-unloading cyclic test show permanent set (plastic deformation) and stress softening (Mullins effect) behaviour 61
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Fig. 4. Results for mechanical tests of OPF. Error bars indicate standard deviations. For a) UNI Lego and UNI TA represent uniaxial tests conducted using the toy-brick tensile tester and commercial mechanical testing instrument, respectively.
defects also occur on the fibre sample before mechanical test being conducted, as evidenced in Fig. 5(g). Fig. 6(a) shows that there is no statistically significant correlation between the diameter of the fibres with fracture stress and strain. This can be due to the uneven diameter of the fibres, where the microscopy analysis in Fig. 5(c) shows multiple localised damages due to the uneven diameter of OPF fibres along its axis (rotation of fibres was concentrated in an area with the smallest fibres diameter). Del Masto et al. (2017) reported that the geometry of elementary hemp fibres that have varying diameters and nonuniform shape along its strand affected the mechanical behaviour. Multiple localised damages coupled with the imperfection of OPF strand along with fibres splitting contribute to the unpredictable behaviour of OPF mechanical failure under uniaxial and twisted uniaxial tests, which results in the jagged strain-stress graph (an example is shown in Fig. 6(b)). This work, therefore highlighted the opportunity to improve the design and functionality of the tester, especially to further investigate the anisotropy behaviour and damage mechanism of lignocellulosic fibres. For example, the anisotropy behaviour can be studied through tensile tests under different modes, i.e. loading unloading cyclic test and stress relaxation test using twisted fibres strand using a more sensitive stepper motor rather than the servo motor by LEGO used here. Likewise, the previous model on the viscoelasticity of OPF (Hanipah et al., 2017; Omar et al., 2016) can be further improved by incorporating the anisotropy effect in the model. It can also be based on
explained later), which is similar to what happened during step cyclic loading-unloading test. The large deviation of true stress (and strain) of OPF can be due to the microstructural damage of the fibres, such as transverse cracking, crack branching and crack initiation at the interface of fibres. This is likely similar to the damage mechanism in bast fibres, as reported by Beaugrand et al. (2017) whose stated the damage mechanism in bast fibres was due to scale dependent tubular porosity combined with defects that randomly distributed along the fibres. Fig. 5(a) shows the SEM image of the fractured region of OPF after uniaxial test with Fig. 5(b) depicts a close-up image of the fractured region that shows exposed primary fibres, while Fig. 5(c) shows the SEM image for the sample that underwent 1080° twist clockwise uniaxial test and Fig. 5(d) sample that underwent 1080° twist anticlockwise uniaxial tests. Multiple localised damages are observed on some OPF samples (Fig. 5(c)) which occurred within the OPF from inter and intra-lumen damages. During tensile text (uniaxial and twisted fibre uniaxial), non-uniform fracture tips and longitudinal cracking occurred, where the OPF sample split into half along the longitudinal axis of OPF as shown in Fig. 5(d). This longitudinal cracks occurred along the fibres that started from the area of the fibres which has transverse defects (Aslan et al., 2011), as shown in Fig. 5(e), where micro-damages were propagated along the transverse axis of the fibre strand. Fracture also happened during OPF twisting before uniaxial tensile commenced, as shown in Fig. 5(f), that is likely due to the fibres over-twisting. Pre-damage due to kinks and 62
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Fig. 5. SEM images of (a); fractured region on the end of OPF after uniaxial test; (b) close-up of rectangular region in (a); (c) OPF after clockwise twist test with arrows showing localised damages; (d) test OPF after anticlockwise twist test; (e) propagated micro-damages along the transverse axis of OPF strand; (f) twisted OPF before uniaxial; (g) defects and kink on the OPF strand.
Fig. 6. (a) Fracture stress and strain for OPF uniaxial and twists tests across diameter; (b) composite images of OPF with a different mode of tests from left: uniaxial, twisted clockwise and twisted anticlockwise; (c) schematic diagram for twisted OPF with the theoretical value for rotated angle, θ1 = 10° and θ2 = −10°; (d) Example of tests results with a jagged stress-strain graph.
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the work by Placet et al. (2014) that described the finite transformation through material rotating frame formulation via anisotropic viscoelastic constitutive law. Shorter fibres around 10 mm or less can be used to ensure consistency of the diameter of OPF along the fibre strand. On the other hand, it would be interesting to study the fibres under compression mode, where the major issue is buckling of the sample during deformation. To solve this, Ueda et al. (2014) set the height of a fibres sample (carbon fibres) to be around 10 μm and a diameter within the range of 5 μm (diameter-height ratio of 0.5) inside an SEM microcompression tester. Finally, the understanding of mechanical behaviour of OPF can be applied in a different area of OPF applications, for instance, the mechanical aspect of OPF in solid fermentation and composting. It can be also applied in biocomposites, for example in the OPF-based 3D printing filaments as filler, which is interesting to be explored in the future. The understanding of OPF complex mechanical behaviour can also be expanded in other natural fibres-based application such as protective clothing, wound dressing or material reinforcement in the biomedical field.
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4. Conclusion A tabletop tensile tester was developed consisting of LEGO parts and sensors, as well as a miniature load cell. This followed with a specialised sample gripper and deformation procedure programmed in an open source operating system were developed. Mechanical tests on lignocellulosic fibres were then conducted under different deformation modes, such as cyclic tensile, stress relation and twisting motion. The tests revealed anisotropic-viscoelastic mechanical behaviour of the oil palm fibres, as well as multiple microstructural damages due to deformation. Acknowledgement Funding for this work was provided by Universiti Putra Malaysia Grant Scheme 2017 (UPM/700-2/1/GPB/2017/9521400 and GP-IPS/ 2017/9517900) and Kyushu Institute of Technology Educational Program 2017-2018. LEGO Mindstorms and LEGO Technic are trademarks of The LEGO Group of companies. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmbbm.2019.05.010. References Adler, D.C., Buehler, M.J., 2013. Mesoscale mechanics of wood cell walls under axial strain. Soft Matter 9 (29), 7138–7144. https://doi.org/10.1039/C3SM50183C. Ali, A.A.M., Othman, M.R., Shirai, Y., Hassan, M.A., 2015. Sustainable and integrated palm oil biorefinery concept with value-addition of biomass and zero emission system. J. Clean. Prod. 91, 96–99. https://doi.org/10.1016/J.JCLEPRO.2014.12.030. Asheim, J., Kvittingen, E.V., Kvittingen, L., Verley, R., 2014. A simple, small-scale Lego colorimeter with a light-emitting diode (LED) used as detector. J. Chem. Educ. 91 (7), 1037–1039. https://doi.org/10.1021/ed400838n. Aslan, M., Chinga-Carrasco, G., Sørensen, B.F., Madsen, B., 2011. Strength variability of single flax fibres. J. Mater. Sci. 46 (19), 6344–6354. https://doi.org/10.1007/ s10853-011-5581-x. Beaugrand, J., Guessasma, S., Maigret, J.-E., 2017. Damage mechanisms in defected natural fibers. Sci. Rep. 7 (14041), 1–7. https://doi.org/10.1038/s41598-01714514-6. Celli, P., Gonella, S., 2015. Manipulating waves with LEGO bricks: a versatile experimental platform for metamaterial architectures. Appl. Phys. Lett. 107 (8), 081901. https://doi.org/10.1063/1.4929566. Cyr, M., Miragila, V., Nocera, T., Rogers, C., 1997. A low-cost, innovative methodology for teaching engineering through experimentation. J. Eng. Educ. 86 (2), 167–171. https://doi.org/10.1002/j.2168-9830.1997.tb00280.x. Del Masto, A., Trivaudey, F., Guicheret-Retel, V., Placet, V., Boubakar, L., 2017. Nonlinear tensile behaviour of elementary hemp fibres: a numerical investigation of the relationships between 3D geometry and tensile behaviour. J. Mater. Sci. 52, 6591–6610. https://doi.org/10.1007/s10853-017-0896-x.
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