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Understanding hydration effects on mechanical and impacting properties of turtle shell
Author’s Accepted Manuscript Understanding hydration effects on mechanical and impacting properties of turtle shell Xu Zhang, Zhen-bing Cai, Wei Li, Min-hao Zhu
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S1751-6161(17)30476-9 https://doi.org/10.1016/j.jmbbm.2017.11.007 JMBBM2564
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 13 June 2017 Revised date: 6 October 2017 Accepted date: 3 November 2017 Cite this article as: Xu Zhang, Zhen-bing Cai, Wei Li and Min-hao Zhu, Understanding hydration effects on mechanical and impacting properties of turtle shell, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2017.11.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Understanding hydration effects on mechanical and impacting properties of turtle shell Xu Zhang, Zhen-bing Cai, Wei Li ,Min-hao Zhu Tribology Research Institute, Key Laboratory of Advanced Materials Technology, Ministry of Education, Southwest Jiaotong University, Chengdu, 610031, China. Corresponding authors.E-mail addresses: [email protected]
Abstract Study of the properties of natural biomaterials provides a reliable experimental basis for the design of biomimetic materials. The mechanical properties and impact wear behaviors of turtle shell with different soaking time were investigated on a micro-amplitude impact wear tester. The damage behavior of turtle shells with different soaking time and impact cycles were systematically analyzed, also the impact dynamics behavior was inspected during the impact wear progress. The results showed that the energy absorption and impact contact force were significantly different with varied soaking time. Under different impact cycles, the peak contact force of shell samples with same soaking time were approximate to each other in value and the values of impact contact time change in a small range. However, the damage extent of shells were distinct with varied impact cycles. It was found that impact worn scars of shells increase with impact cycles increasing. However, under the same impact cycles, energy absorption and contact time increased with the extending of soaking time, but the peak contact force decrease. Especially shell without soaking, the absorption rate is the lowest. Keywords: Turtle shell, Hydration effect, Mechanical property,Impact wear,Energy absorption.
1. Introduction Biomaterials, especially natural materials, have attracted much attention in recent years 1
because of their novel internal structure and excellent mechanical properties[1-6].Mimicking the mechanical properties and structure of natural materials can provide engineered multi-functionality [7-10]. In nature, many creatures form natural protective armor after billions of years of evolution[ 11 - 12 ].The study on natural armors provide inspiration for designing new synthetic armor materials. So far, it is found in fish scales [13-15], mollusk shells [16-17] , pangolin scales[18-19] and crab exoskeletons [20], which all provides a fairly good defense under impact- against predators’ attack. The turtles have existed dating to 200 million years ago and is an ancient creature. Turtle shell provides it from predators due to the shape and strength of shell [21]. Balani et al.[22] studied the microstructure and mechanical properties of the turtle shell, and pointed out that turtle shell was a kind of sandwich structure, then tested elastic modulus and hardness of different regions, the range of modulus 0.47-22.15GPa, the corresponding range of hardness was 53.7-522.2MPa. Damiens et al.[23] found the turtle shell structure can be divided into three regions according to the experimental compression curve: linear elastic, perfectly inelastic, and hardening of the dense region, and it found that the stress strain curve of turtle shell material and a metal foam was similar. Achrai et al.[24-25] investigated the three point bending properties of shell, and the rib strength was 300MPa and flexural modulus was about 7-8.5GPa. Also shell surface keratin and biomimetic synthesis of composite impact resistance were studied, the results showed the biomimetic synthesis of composite material also had excellent properties of natural biological materials. Turtles often go back and forth between water and land, therefore, when it is attacked, the turtle shell has a watery condition. Hence, studying the impact wear behavior effected by soaking in water is very important. In previous research, scientist most focused the material structure characterization and mechanical behavior of turtle shell, and pay no attention the impact response. In fact, an important function of the turtle shell is to resist the bite normal stress from the natural enemy tooth. In this study, the aim was focused on investigating the impact behaviors of natural turtle shells with different soaking time by a new impact wear tester for the first time. Why selected different immersed time? As an amphibian ,every once in a while, the turtles have to 2
surface to breathe and breathe oxygen. Two is ashore, it can dry the skin,and it was not conducive to bacterial growth. The turtle shell will have different soaking times in the actual process. It would inspire a material and mechanical design for new materials which possess anti-impacting properties.
2. Materials and methods The turtle shell used in this experiment are from the natural death of adult freshwater turtle (From turtle breeding center, Chengdu, China). Fig.1 shows the overall and internal topography of the carapace of the turtle shell. The shape of carapace is oval and seams appeared on the surface (Fig.1A). The length of shell is about 7~8cm, and width of 6~7cm. The morphology of the original turtle shell surfaces and the cross-section on longitudinal direction were analyzed by scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan). The microstructure of shell cross section displays the internal structure of partition, the outermost layer is shell and the inner layer is a bone plate structure which can be divided into dense layer and cancellous layer. The outermost layer covered in the bone plate is composed of keratin[26], also compact and hard, thickness of about 50μm. Its important function is to protection and anti-corrosion. While the structure of the bone plate is similar to that of the animal skeleton, and bone plate was divided into dense layer and cancellous layer, wherein the cancellous layer thickness is about 1000μm. These two layers have no obvious boundaries, but gradually transition(Fig.1B). The concrete structure of cancellous layer (Fig.1C) is a porous structure, and pores formed by collagen fiber winding and diameter of pores about 60 m-300 m. The distribution of pores is irregular, and the pores at the junctions of the dense layer are sparse and small in diameter, while the pores close to the soft tissues are dense and have a diameter angle. These pores play a key role in the transport of nutrients[21]. Air-dried and wet samlpes after soaking were used in tests. Air-dried sample was denoted as sample 1#, while the samples 2#, 3#, 4# were got by soaking scales in water for 12h, 24h and 48h, respectively. Drying method has been adopt to measured the moisture content of different samples,and moisture contents of samples are followed as 12.9%(sample 3
1#), 22.1%(sample 2#), 27.2%(sample 3#), 32.1%(sample 4#) by weight. Microhardness were performed on the scale surface which were carried out polishing treatment and detailed parameters are provided in Table 1. Both tension and bending tests were carried out on the HY-0580 test machine (Fig.2A) with speed of 2mm/min and 0.15mm/min at room temperature, respectively. In the three point bending experiment, the experiment was stopped when the experiment time was more than one hour. The impact wear tests were performed on the micro-amplitude impact wear tester as indicated in Fig.2B. The shell sample(8) is fastened by the holder(9). The Si3N4 ball friction pair(7), a force sensor(6), and the impact block(4) are connected together to form the impact unit. The block moves forward with approximately constant velocity because the impact block is guided by a linear rolling element(3) with a low friction coefficient. The initial impact velocity v1 and rebound velocity v2 are measured by displacement sensor (5). Energy changes are obtained via velocity analysis. The damping punch(2) and the voice coil motor(1) are the core power components of the impact-fretting wear rig and provide impact kinetic energy. The voice coil motor(1) which is controlled by motor control system undergo linear reciprocating motion. The impact wear test adopted ball-on-flat contact, the impact frequency was 5.6Hz. In the test, the initial impact velocity was 184mm/s, namely impact kinetic energy was 7.6 mJ. The selection of force was from the average pressure on the teeth of a tortoise when it is attacked by its predators when it bites the turtle shell. In China, tiger, wolf, leopard and jackal maybe attract turtle, so we selected a test situation with a normal load of peak value of 100N as the experimental parameters. It is worth noting that the contact force was an output value(measured), not an input (set parameter), so some preliminary experiments is needed before the formal experiment. For impact speed, the bite was a process that slows down with the bite,184 mm/s was a middle speed during the shell suffered attack and it is a better running speed of this tester. Tests were divided into four groups according to immersed time following by 0h, 12h, 24h, and 48h. Each group tests were performed in different impact cycles(103,104,105). The samples immered in water were continuously irrgated with water to keep the hydrated state 4
during tests. The tensile samples were shape of dog-bone with size of about 30×10×1mm3, which were excised from the scales along longitudinal direction, using the handsaw and file. Also the three-point samples were obtained from sacles along longitudinal direction, which in size of about 30×7×1mm3 and samples used in impact tests were in size of 20×10×1mm3 and the detailed dimensions shown in Fig.3C. In order to reduce the error caused by size of samples,each sample were measured by vernier caliper to ensure its size consistent with each other before tests. Meanwhile the Si3N4 ball with a diameter of 9.525mm was used as friction pair, and the all the tests were carried out under room temperature of 25°C. After tests, the fracture cracks and worn surfaces coated with gold and then were observed by scanning electron microscopy(SEM-JSM 6610LV) and 3D profiler (NanoMap-D). Its water content
specimens
structure
was
subjected
to
FTIR(PE
1710)
testing
and
XRD(D1-SYSTEM)analysis
3. Results The tensile stress-strain curve of turtle shell was showed in Fig.3A , the samples used in study contained the seams. The data collected by the experimental machine are processed and analyzed, and the stress-strain curves are obtained by corresponding formula (1)(2). In first formula, the σ, F, and A is stress, tensile force, and cross sectional area, respectively. In second formula, the ε, ΔL, and L respectively indicate strain, sample length difference, and sample length. From the trend of each sample tensile curve(Fig.3) indicate that fracture of tensile stress decrease with increasing the soaking time, and it take the highest point of the curve as the fracture failure. There are different properties between shell shell and bone plate material, the data is adopted the fracture point of bone plate material point. The slope of stress-strain curve of sample without soaking is larger, and the fracture occurs when the strain is 0.028. According to the calculation, the tensile strength is 23.4MPa, and the plastic deformation stage is not significant. The slope of stress-strain curve of specimens with soaking(2#, 3# and 4#) decrease with the increase of soaking time, While the fracture strain 5
increase. The curves showed that the tensile strength of samples is as follows: 23.4MPa (sample 1#)20.6MPa (sample 2#)16.1MPa (sample 3#)13MPa (sample 4#). This phenomenon maybe due to the water molecules enter the porous structure and physical combine with other molecular, which could decrease the bonding force between the molecules. With the increase of the soaking time, the turtle shell has lower strength and higher tenacity. As a results, the turtle shell soaking would be more easy to break compare to the dry. σ= ε=
F
(1)
A ∆𝐿
(2)
𝐿
From Fig.3B , it is found that the maximum bending force value decrease with soaking time increasing. The bending force of samples 1# without soaking is maximum, and the shell and bone plate fractured at the same time, because the curve reached the highest point after a sharp decline. While the bending force of sample 4# with soaking 48h is lowest. From the trend of all the curves, as the soaking time increased, the slope of the curve decreased, and the emergence of resistance fracture stage. However, the fracture resistance of sample 1# stage is relatively short and broke after reaching the highest point, while the rest of the sample stage is longer. When the bending force reached the maximum value, bone plate is broken, but shell is not broken, which is still in the stage of fracture resistance. The fracture value of bone plate is regarded as failure load in order of 7.9N (sample 1#) 7.3N (sample 2#)4.9N (sample 3#)3.8N (sample 4#). The mechanical properties are inherent properties of materials and are determined by the material structure. While the mechanical properties of the material are determined by the bond and the ability between the atoms (molecules). The tensile fracture of shell material is mainly caused by tensile stress, and it is observed that the tensile fracture is more flattening, so it can be judged that the fracture surface is perpendicular to tensile stress. When the load exceeds the range that the material can bear, the sample would break. Comparison of four fracture of sample with different soaking time, the fracture of sample without soaking is brittle fracture, and shell and bone plate are broken at the same time. The fracture of sample 6
with different soaking time is not found the morphology of shell due to the different strength between shell and bone plate. Also the fracture of the soaking sample are brittle fracture and fibers are pulled out as shown in Fig.4 In the three point bending test, the forces acting on different parts of the sample are not same. The loading surface is shell shell, namely shell and dense layers are subject to compression. However, the cancellous layer is affected by the stretching action. Therefore, the tensile region is extended in the direction perpendicular to the principal stress when the samples break down, and then extend to the surface. The toughness of shell after different soaking time is higher than bone plate, so the shell and bone plate do not break at same time. The fracture of sample 1# present shell, while the other sample without shell fracture. From the fracture surface of each sample, it can be seen that the fracture is flat and perpendicular to the principal stress, and both fibers are pulled out. The curve of contact force changing with time of the samples with different water immersion time under the same impact energy can be found in Fig.5. As shown in Fig.5A, When the impact cycle is 103, the maximum contact force of the sample 1# without soaking is the highest, and the value is 89N. Meanwhile, the impact contact time is the shortest, and the value is 2.55ms. And the peak contact force of 8# sample is minimum, and the value is 66N. However, the impact contact time is the longest, and the value is 3.37ms. Comparing the contact force curves of four samples, it can be seen that the contact force decreases with the increase of soaking time, while the impact contact time increase. As shown in Fig.5(B-C), when the impact cycles is 104 and 105, according to the distribution of contact force curve, the impact contact force and contact time are related to soaking time. As the soaking time increase, the peak force decrease, and the contact time increase. The outer layer of the shell are directly affected by external stimuli when the turtle shell sample is impacted, and the load gradually decrease in the process of load transfer due to the bone can absorb energy because of its porous structure. While the toughness of the samples is enhanced, so that more impact energy can be absorbed during the impact process. Fig.5(D-E) indicate the peak contact force and contact time of different impact cycles, respectively. The impact contact time fluctuation range of sample 1# is small when the impact cycles are different, while the 7
impact time of the rest of samples increase significant with the increase of impact cycles. The maximum contact force of the four specimens fluctuates little with the increase of the impact cycles, while there are slight upward trend. The peak force rise with the increase of the impact cycles, which may be due to the hardening of the material surface during the impact process.
Fig.6 shows the dynamic response curves of samples with different moisture content under constant impact energy are obtained by arranging and analyzing the output data of the test machine. Fig.6A display the relation curve of rebound speed versus time. According to the curve trend, under the same impact kinetic energy, the rebound velocity of samples with different soaking time is different. And the rebound velocity of sample 1# without soaking is the highest (about 155.8 mm/s), and the difference from the initial velocity is 28.2 mm/s. While the rebound velocity of sample 4# is the lowest, and the difference from the initial velocity is 62.5 mm/s. There has a certain relationship between the rebound speed and soaking time. The rebound speed is reduced with the immersion time increasing, and the rebound speed is in turn: 155.8 mm/s (sample 1#) 139.6mm/s (sample 2#) 129.2mm/s (sample 3#) 121.5mm/s (sample 4#). According to the relation curve of rebound speed versus time, the corresponding energy response curve can be obtained by Kinetic energy formula, as shown in Fig.6B. The difference between the rebound kinetic energy and the initial kinetic energy of the samples are different when the initial kinetic energy of impact is 7.6 mJ. And the values are arranged in order as follows: 2.2mJ (sample 1#) < 3.2mJ (sample 2#) < 3.85mJ (sample 3#) < 4.25mJ (sample 4#). The absorption energy and absorption rate of different samples are different when the initial kinetic energy of the impact is the same, as shown in Fig.6C. The initial energy of impact consists of two parts: (1)the absorbed energy and the(2) rebound energy, and the rebound energy decreased with the increase of soaking time, while the absorption energy of sample increased. The corresponding energy absorption rate can be calculated according to formula α=
∆E E
, and the energy absorption rate increase
with the increase of soaking time, and the values were arranged in order as follows: 32% (sample 1#) < 29.7% (sample 2#) < 40.6% (sample 3#) < 44.6% (sample 4#). 8
The 3D morphology of worn scars can be seen from the samples, the worn area increases gradually with the increase of impact cycle, and the wear depth increase gradually (Fig.7). When the impact cycle is 103 and 104, around the impact produced worn scars are smoother, and the impact cycle is 105. There is a bulge around the worn scar, which is debris accumulation. The cross-section profiles of worn scars of samples without soaking are compared at different impact cycles. With the impact cycle increasing, there is an obvious difference between the worn scars profiles. When the impact number is 103, the surface damage is slight and almost no difference with the original surface, and its depth is only 10m. However, when the impact cycles increased to 104 and 105, the worn scars are more obvious. In the same impact cycle effecting, with the soaking time increasing, the wear extent decrease. Especially the worn scar of soaking time 48 hours, there are no obvious difference between the original surface and worn surface, the depth is only 20 m.
It can be seen that with the increase of impact cycle, the depth and width of worn scars increase(Fig.8). The maximum depth and width of worn scar is 94m and 2.7mm, respectively. The depth of worn scars are as follows: 10 m(103)88 m(104)94 m(105), and the maximum value is about 9 times than the minimum. The width of worn scars are as follows:1 mm(103)2.1 mm(104)2.7 mm(105). When the number of impact cycles is the same, the depth and width of the worn scars shows a downward trend with the soaking time increasing. The concrete value of depth according to the following order: 94m (Sample 1#)48 m(Sample 2#)47 m(Sample 3#)20 m(Sample 4#), and the maximum value is about 4.7 times than the minimum. The width of worn scars are 2.7 mm (Sample 1#)2.2 mm(Sample 2#)1.9 mm(Sample 3#)1.7 mm(Sample 4#). The depth and width of the grinding mark of 6# and 7# samples are similar at the same impact times. To find out why soaking has such a large effect on the mechanical and tribology properties of the shell. For XRD test, the results (Figure.9(A)) showed that the diffraction peak of the turtle materials with 0 hours in water appears 2-theta of 13.512°, the turtle materials with 12 hours in water appears at 2-theta of 13.813°, the turtle materials with 24 9
hours in water appears at 2-theta of 14.080°, and the turtle materials with 48 hours in water appears at 2-theta of 14.314°. the 2-theta of peaks wear increased according with the materials soaking 0 hours, 12 hours, 24 hours and 48 hours in water. Based on the Bragg’s Law (ref), the d-space was 3.299 nm in turtle materials with soaking 0 hour in water, 3.229 nm in the materials with soaking 12 hours, 3.169 nm in 24 hours and 3.118 nm in 48 hours. The decreased d-space of the materials’ nano-structures was because of the increased water-mediated bonding between the nano-structures from hydration. The hydration can decrease the likelihood of nanostructure interfacial breakage under the normal crushing loads, resulting in the less debris chipping off the turtle materials surface. The FTIR spectra are shown in Figure 9(B), for comparison. The main difference appears in the 2800 cm −1 -320 cm−1 range. The bending vibrations of C-H bond in -CH2 are located at 2920 cm−1 and in -CH3 group are located at 2848 cm−1. With the increase of soaking time, the peak value of the corresponding position was weakened. As described above,the shell can be divided into three layers,
shell, middle and inner
layer are dense and cancellous layers. After soaking 12 hours, 24 hours and 48 hours, the water molecules enter the porous structure and physical combine with other molecular. The water molecules act as free water or water clusters. When the sample with external support and suffer the impact from external force, the outer layer of shell directly affected by external force, the load decrease in the process of the transfer layer by layer[28]. In the impact test, when the surface of turtle shell suffers the impact force, the porous structure of the shell can play the role of energy absorption. The water molecules in the shell are squeezed and can act as a function of buffer, which is similar to the sponge. In this way, the water molecules could reduce the damage caused by impact action. From the analysis results from the above Figure 9, the soaking processing does change the binding force of the shell material at molecular scale, which cause its mechanical properties were significantly affected .The schematic diagram to describe the role of water in turtle shell during the soaking function is shown in Fig.10.
4. Conclusion 10
In this study, the tensile and bending properties of turtle shell samples with different soaking time were studied, and impact wear tests were also carried out. After the experiments, through the analysis of the experimental data and characterization of worn scars the conclusions are as follows: (1) With the soaking time increasing, the impact peak contact force decrease and the contact time increase. The rebound velocity / energy, absorption energy and absorption rate increase with the soaking time increasing. (2) The worn area of sample without soaking is larger than that of other soaked samples and the damage is slight with soaking time increasing under the same impact cycle. (3) Though the tensile and three point bending experiments, with the increase of soaking time, the tensile strength decreased continuously. The porous structure of the turtle shell can play a very good role in reducing vibration. The water molecules into clusters or clusters in its interior act as a function of buffer, when the turtle shell is stimulated by external.
Acknowledgement This research was financially supported by the National Natural Science Foundation of China (Nos. 51375407, U1530136, 51627806) and the Science and Technology Innovation Research Team of Sichuan Province (2017TD0017).
References 1 Meyers, M. A., Lin, A. Y., Seki, Y., Chen, P. Y., Kad, B. K., & Bodde, S. (2006). Structural biological composites: an overview. JOM, 58(7), 35-41. 2 Lin, A. Y. M., Meyers, M. A., & Vecchio, K. S. (2006). Mechanical properties and structure of Strombus gigas, Tridacna gigas, and Haliotis rufescens sea shells: a comparative study. Materials Science and Engineering: C, 26(8), 1380-1389. 11
3 Menig, R., Meyers, M. H., Meyers, M. A., & Vecchio, K. S. (2001). Quasi-static and dynamic mechanical response of Strombus gigas (conch) shells. Materials Science and Engineering: A, 297(1), 203-211. 4 Chen, P. Y., Mckittrick, J., & Meyers, M. A. (2012). Biological materials: functional adaptations and bioinspired designs. Progress in Materials Science, 57(8), 1492-1704. 5 Wegst, U. G. K., Bai, H., Saiz, E., Tomsia, A. P., & Ritchie, R. O. (2015). Bioinspired structural materials. Nature Materials, 14(1), 23-36. 6 Greiner, C., & Schäfer, M. (2015). Bio-inspired scale-like surface textures and their tribological properties. Bioinspiration & Biomimetics, 10(4), 044001. 7 Lv, S., Dudek, D. M., Cao, Y., Balamurali, M. M., Gosline, J., & Li, H. (2010). Designed biomaterials to mimic the mechanical properties of muscles. Nature, 465(7294), 69-73. 8 Xia, F., & Jiang, L. (2008). Bio‐inspired, smart, multiscale interfacial materials. Advanced materials, 20(15), 2842-2858. 9 Wegst, U. G. K., Bai, H., Saiz, E., Tomsia, A. P., & Ritchie, R. O. (2015). Bioinspired structural materials. Nature Materials, 14(1), 23-36. 10 Vincent, J. F. V. (2008). Biomimetic materials. Journal of Materials Research, 23(12 (Biomimetic and Bio-enabled Materials Science and Engineering)), 3140-3147. 11 Yang, W., Chen, I. H., Gludovatz, B., Zimmermann, E. A., Ritchie, R. O., & Meyers, M. A. (2013). Natural flexible dermal armor. Advanced Materials,25(1), 31-48. 12 Yang, W., Gludovatz, B., Zimmermann, E. A., Bale, H. A., Ritchie, R. O., & Meyers, M. A. (2013). Structure and fracture resistance of alligator gar (atractosteus spatula) armored fish scales. Acta Biomaterialia, 9(4), 5876-5889. 13 Bruet, B. J., Song, J., Boyce, M. C., & Ortiz, C. (2008). Materials design principles of ancient fish armour. Nature Materials, 7(9), 748-56. 14 Ikoma, T., Kobayashi, H., Tanaka, J., Walsh, D., & Mann, S. (2003). Microstructure, mechanical, and biomimetic properties of fish scales from pagrus major. Journal of Structural Biology, 142(3), 327-333. 15 Bigi, A., Burghammer, M., Falconi, R., Koch, M. H., Panzavolta, S., & Riekel, C. (2001). Twisted plywood pattern of collagen fibrils in teleost scales: an x-ray diffraction investigation. Journal of Structural Biology,136(2), 137-143. 16 Liang, Y., Zhao, J., Wang, L., & Li, F. M. (2008). The relationship between mechanical properties and crossed-lamellar structure of mollusk shells. Materials Science & 12
Engineering A, s 483–484(1-2), 309-312. 17 Menig, R., Meyers, M. H., Meyers, M. A., & Vecchio, K. S. (2000). Quasi-static and dynamic
mechanical
response
of
haliotis
rufescens,
(abalone)
shells. Acta
Materialia, 48(9), 2383-2398. 18 Liu, Z. Q., Jiao, D., Weng, Z. Y., & Zhang, Z. F. (2016). Structure and mechanical behaviors of protective armored pangolin scales and effects of hydration and orientation. Journal of the Mechanical Behavior of Biomedical Materials, 56, 165-174. 19 Liu, Z. Q., Jiao, D., Weng, Z. Y., & Zhang, Z. F. (2016). Water-assisted self-healing and property recovery in a natural dermal armor of pangolin scales. Journal of the Mechanical Behavior of Biomedical Materials, 56, 14-22. 20 Taylor, J. R., & Patek, S. N. (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp's telson. Journal of Experimental Biology, 213(Pt 20), 3496. 21 Gilbert, S. F., Loredo, G. A., Brukman, A., & Burke, A. C. (2001). Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evolution & Development, 3(2), 47-58. 22 Balani, K., Patel, R. R., Keshri, A. K., Lahiri, D., & Agarwal, A. (2011). Multi-scale hierarchy of chelydra serpentina: microstructure and mechanical properties of turtle shell. Journal of the Mechanical Behavior of Biomedical Materials, 4(7), 1440-51. 23 Damiens, R., Rhee, H., Hwang, Y., Park, S. J., Hammi, Y., & Lim, H., et al. (2012). Compressive behavior of a turtle's shell: experiment, modeling, and simulation. Journal of the Mechanical Behavior of Biomedical Materials, 6, 106-112. 24 Achrai, B., Baron, B., & Wagner, H. D. (2015). Biological armors under impact--effect of
keratin
coating,
and
synthetic
bio-inspired
analogues.
Bioinspiration
&
Biomimetics, 10(1), 016009. 25 Achrai, B., Baron, B., & Wagner, H. D. (2014). Bending mechanics of the red-eared slider
turtle
carapace. Journal
of
the
Mechanical
Behavior
of
Biomedical
Materials, 30(7), 223-233. 26 Alibardi, L., & Toni, M. (2006). Immunolocalization and characterization of beta-keratins in growing epidermis of chelonians. Tissue and Cell, 38(1), 53-63. 27. P Christiansen, S Wroe(2007).Bite forces and evolutionary adaptations to feeding ecology in carnivores. Ecology, 88 (2), 347-358 28 Higuchi, A., & Iijima, T.. D.s.c. (1985) Investigation of the states of water in poly(vinyl 13
alcohol) membranes. Polymer, 26(8), 1207-1211.
Table.1 Table.1 The moisture content and hardness of samples
0(1#)
12(2#)
24(3#)
48(4#)
Moisture content/%
12.9
22.1
27.2
32.1
Hardness/HV
49.80.9
39.70.88
320.67
18.630.56
Soaking time/h
Table.2
Table2 The bite force of the carnivores and the force of the teeth Partial predators of tortoises[27]
Average bite forces at the canine tips(N)
carnassial eocone(N)
Acinonyx jubatus
338.8
509.1
Leopardus tigrinus
63.0
97.2
Prionailurus bengalensis
94.4 61.2
Panthera leo
1314.7
China emerged
√
2023.7
Pardofelis marmorata
185.3
√
773.9
√
--
√
117.2 Wolf(Canis lupus) 493.5 Proteles cristatus 122.2
14
Figure captions
Fig.1 The morphology of turtle shell. (A) Whole morphology of shell. (B)Cross-section. (C)Cancellous layer. Fig.2 Diagram of experimental sample and tester in this study. (A) The schematic diagram of material experiment machine. (B)The schematic diagram of the low-energy impact wear tester. (C)The size of the sample. (1.voice coil motor 2.damping punch 3.linear rolling section 4.impact block 5.displacement sensor 6.force sensor 7.friction pair 8.sample of test 9.fixture) Fig.3 The mechanical properties of turtle shell. (A) Tensile stress-strain with different moisture. (B) Bending force-deflection curves with different moisture. Fig.4 The fracture morphology of tensile and bending with different soaking time. Fig.5 The effects of hydration and impact cycle on contact peak force and time. (A)Cycle=103. (B)Cycle=104. (C)Cycle=105. (D)The peak contact force with different soaking time. (E)The contact time with different soaking time. Fig.6 The velocity and energy of samples with different soaking time, cycle=10 3. (A) The rebound velocity - time of different soaking samples. (B) The rebound energy - time of different soaking samples. (C) The energy response of samples with different soaking time under the 7.6mJ impact energy. Fig.7 The morphologies and cross-section profiles of worn scars. (A)Cycle=103,soaking time=0h.
(B)Cycle=104,soaking
time=0h.
(C)Cycle=105,
soaking
time=0h.
(D)Cycle=105,soaking time=12h. (E)Cycle=105,soaking time=24h. (F)Cycle=105, soaking time=48h. Fig.8 The width and depth of worn scars. (A)Varied impact cycle. (B) Varied soaking time. Fig.9 XRD and IR spectra of the shell with varied soaking time Fig.10 The schematic diagram showing the role of water in process of impact of soaked 15
turtle shell
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Figure.2
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Figure.3
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Figure.9
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PII: DOI: Reference:
S1751-6161(17)30476-9 https://doi.org/10.1016/j.jmbbm.2017.11.007 JMBBM2564
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 13 June 2017 Revised date: 6 October 2017 Accepted date: 3 November 2017 Cite this article as: Xu Zhang, Zhen-bing Cai, Wei Li and Min-hao Zhu, Understanding hydration effects on mechanical and impacting properties of turtle shell, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2017.11.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Understanding hydration effects on mechanical and impacting properties of turtle shell Xu Zhang, Zhen-bing Cai, Wei Li ,Min-hao Zhu Tribology Research Institute, Key Laboratory of Advanced Materials Technology, Ministry of Education, Southwest Jiaotong University, Chengdu, 610031, China. Corresponding authors.E-mail addresses: [email protected]
Abstract Study of the properties of natural biomaterials provides a reliable experimental basis for the design of biomimetic materials. The mechanical properties and impact wear behaviors of turtle shell with different soaking time were investigated on a micro-amplitude impact wear tester. The damage behavior of turtle shells with different soaking time and impact cycles were systematically analyzed, also the impact dynamics behavior was inspected during the impact wear progress. The results showed that the energy absorption and impact contact force were significantly different with varied soaking time. Under different impact cycles, the peak contact force of shell samples with same soaking time were approximate to each other in value and the values of impact contact time change in a small range. However, the damage extent of shells were distinct with varied impact cycles. It was found that impact worn scars of shells increase with impact cycles increasing. However, under the same impact cycles, energy absorption and contact time increased with the extending of soaking time, but the peak contact force decrease. Especially shell without soaking, the absorption rate is the lowest. Keywords: Turtle shell, Hydration effect, Mechanical property,Impact wear,Energy absorption.
1. Introduction Biomaterials, especially natural materials, have attracted much attention in recent years 1
because of their novel internal structure and excellent mechanical properties[1-6].Mimicking the mechanical properties and structure of natural materials can provide engineered multi-functionality [7-10]. In nature, many creatures form natural protective armor after billions of years of evolution[ 11 - 12 ].The study on natural armors provide inspiration for designing new synthetic armor materials. So far, it is found in fish scales [13-15], mollusk shells [16-17] , pangolin scales[18-19] and crab exoskeletons [20], which all provides a fairly good defense under impact- against predators’ attack. The turtles have existed dating to 200 million years ago and is an ancient creature. Turtle shell provides it from predators due to the shape and strength of shell [21]. Balani et al.[22] studied the microstructure and mechanical properties of the turtle shell, and pointed out that turtle shell was a kind of sandwich structure, then tested elastic modulus and hardness of different regions, the range of modulus 0.47-22.15GPa, the corresponding range of hardness was 53.7-522.2MPa. Damiens et al.[23] found the turtle shell structure can be divided into three regions according to the experimental compression curve: linear elastic, perfectly inelastic, and hardening of the dense region, and it found that the stress strain curve of turtle shell material and a metal foam was similar. Achrai et al.[24-25] investigated the three point bending properties of shell, and the rib strength was 300MPa and flexural modulus was about 7-8.5GPa. Also shell surface keratin and biomimetic synthesis of composite impact resistance were studied, the results showed the biomimetic synthesis of composite material also had excellent properties of natural biological materials. Turtles often go back and forth between water and land, therefore, when it is attacked, the turtle shell has a watery condition. Hence, studying the impact wear behavior effected by soaking in water is very important. In previous research, scientist most focused the material structure characterization and mechanical behavior of turtle shell, and pay no attention the impact response. In fact, an important function of the turtle shell is to resist the bite normal stress from the natural enemy tooth. In this study, the aim was focused on investigating the impact behaviors of natural turtle shells with different soaking time by a new impact wear tester for the first time. Why selected different immersed time? As an amphibian ,every once in a while, the turtles have to 2
surface to breathe and breathe oxygen. Two is ashore, it can dry the skin,and it was not conducive to bacterial growth. The turtle shell will have different soaking times in the actual process. It would inspire a material and mechanical design for new materials which possess anti-impacting properties.
2. Materials and methods The turtle shell used in this experiment are from the natural death of adult freshwater turtle (From turtle breeding center, Chengdu, China). Fig.1 shows the overall and internal topography of the carapace of the turtle shell. The shape of carapace is oval and seams appeared on the surface (Fig.1A). The length of shell is about 7~8cm, and width of 6~7cm. The morphology of the original turtle shell surfaces and the cross-section on longitudinal direction were analyzed by scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan). The microstructure of shell cross section displays the internal structure of partition, the outermost layer is shell and the inner layer is a bone plate structure which can be divided into dense layer and cancellous layer. The outermost layer covered in the bone plate is composed of keratin[26], also compact and hard, thickness of about 50μm. Its important function is to protection and anti-corrosion. While the structure of the bone plate is similar to that of the animal skeleton, and bone plate was divided into dense layer and cancellous layer, wherein the cancellous layer thickness is about 1000μm. These two layers have no obvious boundaries, but gradually transition(Fig.1B). The concrete structure of cancellous layer (Fig.1C) is a porous structure, and pores formed by collagen fiber winding and diameter of pores about 60 m-300 m. The distribution of pores is irregular, and the pores at the junctions of the dense layer are sparse and small in diameter, while the pores close to the soft tissues are dense and have a diameter angle. These pores play a key role in the transport of nutrients[21]. Air-dried and wet samlpes after soaking were used in tests. Air-dried sample was denoted as sample 1#, while the samples 2#, 3#, 4# were got by soaking scales in water for 12h, 24h and 48h, respectively. Drying method has been adopt to measured the moisture content of different samples,and moisture contents of samples are followed as 12.9%(sample 3
1#), 22.1%(sample 2#), 27.2%(sample 3#), 32.1%(sample 4#) by weight. Microhardness were performed on the scale surface which were carried out polishing treatment and detailed parameters are provided in Table 1. Both tension and bending tests were carried out on the HY-0580 test machine (Fig.2A) with speed of 2mm/min and 0.15mm/min at room temperature, respectively. In the three point bending experiment, the experiment was stopped when the experiment time was more than one hour. The impact wear tests were performed on the micro-amplitude impact wear tester as indicated in Fig.2B. The shell sample(8) is fastened by the holder(9). The Si3N4 ball friction pair(7), a force sensor(6), and the impact block(4) are connected together to form the impact unit. The block moves forward with approximately constant velocity because the impact block is guided by a linear rolling element(3) with a low friction coefficient. The initial impact velocity v1 and rebound velocity v2 are measured by displacement sensor (5). Energy changes are obtained via velocity analysis. The damping punch(2) and the voice coil motor(1) are the core power components of the impact-fretting wear rig and provide impact kinetic energy. The voice coil motor(1) which is controlled by motor control system undergo linear reciprocating motion. The impact wear test adopted ball-on-flat contact, the impact frequency was 5.6Hz. In the test, the initial impact velocity was 184mm/s, namely impact kinetic energy was 7.6 mJ. The selection of force was from the average pressure on the teeth of a tortoise when it is attacked by its predators when it bites the turtle shell. In China, tiger, wolf, leopard and jackal maybe attract turtle, so we selected a test situation with a normal load of peak value of 100N as the experimental parameters. It is worth noting that the contact force was an output value(measured), not an input (set parameter), so some preliminary experiments is needed before the formal experiment. For impact speed, the bite was a process that slows down with the bite,184 mm/s was a middle speed during the shell suffered attack and it is a better running speed of this tester. Tests were divided into four groups according to immersed time following by 0h, 12h, 24h, and 48h. Each group tests were performed in different impact cycles(103,104,105). The samples immered in water were continuously irrgated with water to keep the hydrated state 4
during tests. The tensile samples were shape of dog-bone with size of about 30×10×1mm3, which were excised from the scales along longitudinal direction, using the handsaw and file. Also the three-point samples were obtained from sacles along longitudinal direction, which in size of about 30×7×1mm3 and samples used in impact tests were in size of 20×10×1mm3 and the detailed dimensions shown in Fig.3C. In order to reduce the error caused by size of samples,each sample were measured by vernier caliper to ensure its size consistent with each other before tests. Meanwhile the Si3N4 ball with a diameter of 9.525mm was used as friction pair, and the all the tests were carried out under room temperature of 25°C. After tests, the fracture cracks and worn surfaces coated with gold and then were observed by scanning electron microscopy(SEM-JSM 6610LV) and 3D profiler (NanoMap-D). Its water content
specimens
structure
was
subjected
to
FTIR(PE
1710)
testing
and
XRD(D1-SYSTEM)analysis
3. Results The tensile stress-strain curve of turtle shell was showed in Fig.3A , the samples used in study contained the seams. The data collected by the experimental machine are processed and analyzed, and the stress-strain curves are obtained by corresponding formula (1)(2). In first formula, the σ, F, and A is stress, tensile force, and cross sectional area, respectively. In second formula, the ε, ΔL, and L respectively indicate strain, sample length difference, and sample length. From the trend of each sample tensile curve(Fig.3) indicate that fracture of tensile stress decrease with increasing the soaking time, and it take the highest point of the curve as the fracture failure. There are different properties between shell shell and bone plate material, the data is adopted the fracture point of bone plate material point. The slope of stress-strain curve of sample without soaking is larger, and the fracture occurs when the strain is 0.028. According to the calculation, the tensile strength is 23.4MPa, and the plastic deformation stage is not significant. The slope of stress-strain curve of specimens with soaking(2#, 3# and 4#) decrease with the increase of soaking time, While the fracture strain 5
increase. The curves showed that the tensile strength of samples is as follows: 23.4MPa (sample 1#)20.6MPa (sample 2#)16.1MPa (sample 3#)13MPa (sample 4#). This phenomenon maybe due to the water molecules enter the porous structure and physical combine with other molecular, which could decrease the bonding force between the molecules. With the increase of the soaking time, the turtle shell has lower strength and higher tenacity. As a results, the turtle shell soaking would be more easy to break compare to the dry. σ= ε=
F
(1)
A ∆𝐿
(2)
𝐿
From Fig.3B , it is found that the maximum bending force value decrease with soaking time increasing. The bending force of samples 1# without soaking is maximum, and the shell and bone plate fractured at the same time, because the curve reached the highest point after a sharp decline. While the bending force of sample 4# with soaking 48h is lowest. From the trend of all the curves, as the soaking time increased, the slope of the curve decreased, and the emergence of resistance fracture stage. However, the fracture resistance of sample 1# stage is relatively short and broke after reaching the highest point, while the rest of the sample stage is longer. When the bending force reached the maximum value, bone plate is broken, but shell is not broken, which is still in the stage of fracture resistance. The fracture value of bone plate is regarded as failure load in order of 7.9N (sample 1#) 7.3N (sample 2#)4.9N (sample 3#)3.8N (sample 4#). The mechanical properties are inherent properties of materials and are determined by the material structure. While the mechanical properties of the material are determined by the bond and the ability between the atoms (molecules). The tensile fracture of shell material is mainly caused by tensile stress, and it is observed that the tensile fracture is more flattening, so it can be judged that the fracture surface is perpendicular to tensile stress. When the load exceeds the range that the material can bear, the sample would break. Comparison of four fracture of sample with different soaking time, the fracture of sample without soaking is brittle fracture, and shell and bone plate are broken at the same time. The fracture of sample 6
with different soaking time is not found the morphology of shell due to the different strength between shell and bone plate. Also the fracture of the soaking sample are brittle fracture and fibers are pulled out as shown in Fig.4 In the three point bending test, the forces acting on different parts of the sample are not same. The loading surface is shell shell, namely shell and dense layers are subject to compression. However, the cancellous layer is affected by the stretching action. Therefore, the tensile region is extended in the direction perpendicular to the principal stress when the samples break down, and then extend to the surface. The toughness of shell after different soaking time is higher than bone plate, so the shell and bone plate do not break at same time. The fracture of sample 1# present shell, while the other sample without shell fracture. From the fracture surface of each sample, it can be seen that the fracture is flat and perpendicular to the principal stress, and both fibers are pulled out. The curve of contact force changing with time of the samples with different water immersion time under the same impact energy can be found in Fig.5. As shown in Fig.5A, When the impact cycle is 103, the maximum contact force of the sample 1# without soaking is the highest, and the value is 89N. Meanwhile, the impact contact time is the shortest, and the value is 2.55ms. And the peak contact force of 8# sample is minimum, and the value is 66N. However, the impact contact time is the longest, and the value is 3.37ms. Comparing the contact force curves of four samples, it can be seen that the contact force decreases with the increase of soaking time, while the impact contact time increase. As shown in Fig.5(B-C), when the impact cycles is 104 and 105, according to the distribution of contact force curve, the impact contact force and contact time are related to soaking time. As the soaking time increase, the peak force decrease, and the contact time increase. The outer layer of the shell are directly affected by external stimuli when the turtle shell sample is impacted, and the load gradually decrease in the process of load transfer due to the bone can absorb energy because of its porous structure. While the toughness of the samples is enhanced, so that more impact energy can be absorbed during the impact process. Fig.5(D-E) indicate the peak contact force and contact time of different impact cycles, respectively. The impact contact time fluctuation range of sample 1# is small when the impact cycles are different, while the 7
impact time of the rest of samples increase significant with the increase of impact cycles. The maximum contact force of the four specimens fluctuates little with the increase of the impact cycles, while there are slight upward trend. The peak force rise with the increase of the impact cycles, which may be due to the hardening of the material surface during the impact process.
Fig.6 shows the dynamic response curves of samples with different moisture content under constant impact energy are obtained by arranging and analyzing the output data of the test machine. Fig.6A display the relation curve of rebound speed versus time. According to the curve trend, under the same impact kinetic energy, the rebound velocity of samples with different soaking time is different. And the rebound velocity of sample 1# without soaking is the highest (about 155.8 mm/s), and the difference from the initial velocity is 28.2 mm/s. While the rebound velocity of sample 4# is the lowest, and the difference from the initial velocity is 62.5 mm/s. There has a certain relationship between the rebound speed and soaking time. The rebound speed is reduced with the immersion time increasing, and the rebound speed is in turn: 155.8 mm/s (sample 1#) 139.6mm/s (sample 2#) 129.2mm/s (sample 3#) 121.5mm/s (sample 4#). According to the relation curve of rebound speed versus time, the corresponding energy response curve can be obtained by Kinetic energy formula, as shown in Fig.6B. The difference between the rebound kinetic energy and the initial kinetic energy of the samples are different when the initial kinetic energy of impact is 7.6 mJ. And the values are arranged in order as follows: 2.2mJ (sample 1#) < 3.2mJ (sample 2#) < 3.85mJ (sample 3#) < 4.25mJ (sample 4#). The absorption energy and absorption rate of different samples are different when the initial kinetic energy of the impact is the same, as shown in Fig.6C. The initial energy of impact consists of two parts: (1)the absorbed energy and the(2) rebound energy, and the rebound energy decreased with the increase of soaking time, while the absorption energy of sample increased. The corresponding energy absorption rate can be calculated according to formula α=
∆E E
, and the energy absorption rate increase
with the increase of soaking time, and the values were arranged in order as follows: 32% (sample 1#) < 29.7% (sample 2#) < 40.6% (sample 3#) < 44.6% (sample 4#). 8
The 3D morphology of worn scars can be seen from the samples, the worn area increases gradually with the increase of impact cycle, and the wear depth increase gradually (Fig.7). When the impact cycle is 103 and 104, around the impact produced worn scars are smoother, and the impact cycle is 105. There is a bulge around the worn scar, which is debris accumulation. The cross-section profiles of worn scars of samples without soaking are compared at different impact cycles. With the impact cycle increasing, there is an obvious difference between the worn scars profiles. When the impact number is 103, the surface damage is slight and almost no difference with the original surface, and its depth is only 10m. However, when the impact cycles increased to 104 and 105, the worn scars are more obvious. In the same impact cycle effecting, with the soaking time increasing, the wear extent decrease. Especially the worn scar of soaking time 48 hours, there are no obvious difference between the original surface and worn surface, the depth is only 20 m.
It can be seen that with the increase of impact cycle, the depth and width of worn scars increase(Fig.8). The maximum depth and width of worn scar is 94m and 2.7mm, respectively. The depth of worn scars are as follows: 10 m(103)88 m(104)94 m(105), and the maximum value is about 9 times than the minimum. The width of worn scars are as follows:1 mm(103)2.1 mm(104)2.7 mm(105). When the number of impact cycles is the same, the depth and width of the worn scars shows a downward trend with the soaking time increasing. The concrete value of depth according to the following order: 94m (Sample 1#)48 m(Sample 2#)47 m(Sample 3#)20 m(Sample 4#), and the maximum value is about 4.7 times than the minimum. The width of worn scars are 2.7 mm (Sample 1#)2.2 mm(Sample 2#)1.9 mm(Sample 3#)1.7 mm(Sample 4#). The depth and width of the grinding mark of 6# and 7# samples are similar at the same impact times. To find out why soaking has such a large effect on the mechanical and tribology properties of the shell. For XRD test, the results (Figure.9(A)) showed that the diffraction peak of the turtle materials with 0 hours in water appears 2-theta of 13.512°, the turtle materials with 12 hours in water appears at 2-theta of 13.813°, the turtle materials with 24 9
hours in water appears at 2-theta of 14.080°, and the turtle materials with 48 hours in water appears at 2-theta of 14.314°. the 2-theta of peaks wear increased according with the materials soaking 0 hours, 12 hours, 24 hours and 48 hours in water. Based on the Bragg’s Law (ref), the d-space was 3.299 nm in turtle materials with soaking 0 hour in water, 3.229 nm in the materials with soaking 12 hours, 3.169 nm in 24 hours and 3.118 nm in 48 hours. The decreased d-space of the materials’ nano-structures was because of the increased water-mediated bonding between the nano-structures from hydration. The hydration can decrease the likelihood of nanostructure interfacial breakage under the normal crushing loads, resulting in the less debris chipping off the turtle materials surface. The FTIR spectra are shown in Figure 9(B), for comparison. The main difference appears in the 2800 cm −1 -320 cm−1 range. The bending vibrations of C-H bond in -CH2 are located at 2920 cm−1 and in -CH3 group are located at 2848 cm−1. With the increase of soaking time, the peak value of the corresponding position was weakened. As described above,the shell can be divided into three layers,
shell, middle and inner
layer are dense and cancellous layers. After soaking 12 hours, 24 hours and 48 hours, the water molecules enter the porous structure and physical combine with other molecular. The water molecules act as free water or water clusters. When the sample with external support and suffer the impact from external force, the outer layer of shell directly affected by external force, the load decrease in the process of the transfer layer by layer[28]. In the impact test, when the surface of turtle shell suffers the impact force, the porous structure of the shell can play the role of energy absorption. The water molecules in the shell are squeezed and can act as a function of buffer, which is similar to the sponge. In this way, the water molecules could reduce the damage caused by impact action. From the analysis results from the above Figure 9, the soaking processing does change the binding force of the shell material at molecular scale, which cause its mechanical properties were significantly affected .The schematic diagram to describe the role of water in turtle shell during the soaking function is shown in Fig.10.
4. Conclusion 10
In this study, the tensile and bending properties of turtle shell samples with different soaking time were studied, and impact wear tests were also carried out. After the experiments, through the analysis of the experimental data and characterization of worn scars the conclusions are as follows: (1) With the soaking time increasing, the impact peak contact force decrease and the contact time increase. The rebound velocity / energy, absorption energy and absorption rate increase with the soaking time increasing. (2) The worn area of sample without soaking is larger than that of other soaked samples and the damage is slight with soaking time increasing under the same impact cycle. (3) Though the tensile and three point bending experiments, with the increase of soaking time, the tensile strength decreased continuously. The porous structure of the turtle shell can play a very good role in reducing vibration. The water molecules into clusters or clusters in its interior act as a function of buffer, when the turtle shell is stimulated by external.
Acknowledgement This research was financially supported by the National Natural Science Foundation of China (Nos. 51375407, U1530136, 51627806) and the Science and Technology Innovation Research Team of Sichuan Province (2017TD0017).
References 1 Meyers, M. A., Lin, A. Y., Seki, Y., Chen, P. Y., Kad, B. K., & Bodde, S. (2006). Structural biological composites: an overview. JOM, 58(7), 35-41. 2 Lin, A. Y. M., Meyers, M. A., & Vecchio, K. S. (2006). Mechanical properties and structure of Strombus gigas, Tridacna gigas, and Haliotis rufescens sea shells: a comparative study. Materials Science and Engineering: C, 26(8), 1380-1389. 11
3 Menig, R., Meyers, M. H., Meyers, M. A., & Vecchio, K. S. (2001). Quasi-static and dynamic mechanical response of Strombus gigas (conch) shells. Materials Science and Engineering: A, 297(1), 203-211. 4 Chen, P. Y., Mckittrick, J., & Meyers, M. A. (2012). Biological materials: functional adaptations and bioinspired designs. Progress in Materials Science, 57(8), 1492-1704. 5 Wegst, U. G. K., Bai, H., Saiz, E., Tomsia, A. P., & Ritchie, R. O. (2015). Bioinspired structural materials. Nature Materials, 14(1), 23-36. 6 Greiner, C., & Schäfer, M. (2015). Bio-inspired scale-like surface textures and their tribological properties. Bioinspiration & Biomimetics, 10(4), 044001. 7 Lv, S., Dudek, D. M., Cao, Y., Balamurali, M. M., Gosline, J., & Li, H. (2010). Designed biomaterials to mimic the mechanical properties of muscles. Nature, 465(7294), 69-73. 8 Xia, F., & Jiang, L. (2008). Bio‐inspired, smart, multiscale interfacial materials. Advanced materials, 20(15), 2842-2858. 9 Wegst, U. G. K., Bai, H., Saiz, E., Tomsia, A. P., & Ritchie, R. O. (2015). Bioinspired structural materials. Nature Materials, 14(1), 23-36. 10 Vincent, J. F. V. (2008). Biomimetic materials. Journal of Materials Research, 23(12 (Biomimetic and Bio-enabled Materials Science and Engineering)), 3140-3147. 11 Yang, W., Chen, I. H., Gludovatz, B., Zimmermann, E. A., Ritchie, R. O., & Meyers, M. A. (2013). Natural flexible dermal armor. Advanced Materials,25(1), 31-48. 12 Yang, W., Gludovatz, B., Zimmermann, E. A., Bale, H. A., Ritchie, R. O., & Meyers, M. A. (2013). Structure and fracture resistance of alligator gar (atractosteus spatula) armored fish scales. Acta Biomaterialia, 9(4), 5876-5889. 13 Bruet, B. J., Song, J., Boyce, M. C., & Ortiz, C. (2008). Materials design principles of ancient fish armour. Nature Materials, 7(9), 748-56. 14 Ikoma, T., Kobayashi, H., Tanaka, J., Walsh, D., & Mann, S. (2003). Microstructure, mechanical, and biomimetic properties of fish scales from pagrus major. Journal of Structural Biology, 142(3), 327-333. 15 Bigi, A., Burghammer, M., Falconi, R., Koch, M. H., Panzavolta, S., & Riekel, C. (2001). Twisted plywood pattern of collagen fibrils in teleost scales: an x-ray diffraction investigation. Journal of Structural Biology,136(2), 137-143. 16 Liang, Y., Zhao, J., Wang, L., & Li, F. M. (2008). The relationship between mechanical properties and crossed-lamellar structure of mollusk shells. Materials Science & 12
Engineering A, s 483–484(1-2), 309-312. 17 Menig, R., Meyers, M. H., Meyers, M. A., & Vecchio, K. S. (2000). Quasi-static and dynamic
mechanical
response
of
haliotis
rufescens,
(abalone)
shells. Acta
Materialia, 48(9), 2383-2398. 18 Liu, Z. Q., Jiao, D., Weng, Z. Y., & Zhang, Z. F. (2016). Structure and mechanical behaviors of protective armored pangolin scales and effects of hydration and orientation. Journal of the Mechanical Behavior of Biomedical Materials, 56, 165-174. 19 Liu, Z. Q., Jiao, D., Weng, Z. Y., & Zhang, Z. F. (2016). Water-assisted self-healing and property recovery in a natural dermal armor of pangolin scales. Journal of the Mechanical Behavior of Biomedical Materials, 56, 14-22. 20 Taylor, J. R., & Patek, S. N. (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp's telson. Journal of Experimental Biology, 213(Pt 20), 3496. 21 Gilbert, S. F., Loredo, G. A., Brukman, A., & Burke, A. C. (2001). Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evolution & Development, 3(2), 47-58. 22 Balani, K., Patel, R. R., Keshri, A. K., Lahiri, D., & Agarwal, A. (2011). Multi-scale hierarchy of chelydra serpentina: microstructure and mechanical properties of turtle shell. Journal of the Mechanical Behavior of Biomedical Materials, 4(7), 1440-51. 23 Damiens, R., Rhee, H., Hwang, Y., Park, S. J., Hammi, Y., & Lim, H., et al. (2012). Compressive behavior of a turtle's shell: experiment, modeling, and simulation. Journal of the Mechanical Behavior of Biomedical Materials, 6, 106-112. 24 Achrai, B., Baron, B., & Wagner, H. D. (2015). Biological armors under impact--effect of
keratin
coating,
and
synthetic
bio-inspired
analogues.
Bioinspiration
&
Biomimetics, 10(1), 016009. 25 Achrai, B., Baron, B., & Wagner, H. D. (2014). Bending mechanics of the red-eared slider
turtle
carapace. Journal
of
the
Mechanical
Behavior
of
Biomedical
Materials, 30(7), 223-233. 26 Alibardi, L., & Toni, M. (2006). Immunolocalization and characterization of beta-keratins in growing epidermis of chelonians. Tissue and Cell, 38(1), 53-63. 27. P Christiansen, S Wroe(2007).Bite forces and evolutionary adaptations to feeding ecology in carnivores. Ecology, 88 (2), 347-358 28 Higuchi, A., & Iijima, T.. D.s.c. (1985) Investigation of the states of water in poly(vinyl 13
alcohol) membranes. Polymer, 26(8), 1207-1211.
Table.1 Table.1 The moisture content and hardness of samples
0(1#)
12(2#)
24(3#)
48(4#)
Moisture content/%
12.9
22.1
27.2
32.1
Hardness/HV
49.80.9
39.70.88
320.67
18.630.56
Soaking time/h
Table.2
Table2 The bite force of the carnivores and the force of the teeth Partial predators of tortoises[27]
Average bite forces at the canine tips(N)
carnassial eocone(N)
Acinonyx jubatus
338.8
509.1
Leopardus tigrinus
63.0
97.2
Prionailurus bengalensis
94.4 61.2
Panthera leo
1314.7
China emerged
√
2023.7
Pardofelis marmorata
185.3
√
773.9
√
--
√
117.2 Wolf(Canis lupus) 493.5 Proteles cristatus 122.2
14
Figure captions
Fig.1 The morphology of turtle shell. (A) Whole morphology of shell. (B)Cross-section. (C)Cancellous layer. Fig.2 Diagram of experimental sample and tester in this study. (A) The schematic diagram of material experiment machine. (B)The schematic diagram of the low-energy impact wear tester. (C)The size of the sample. (1.voice coil motor 2.damping punch 3.linear rolling section 4.impact block 5.displacement sensor 6.force sensor 7.friction pair 8.sample of test 9.fixture) Fig.3 The mechanical properties of turtle shell. (A) Tensile stress-strain with different moisture. (B) Bending force-deflection curves with different moisture. Fig.4 The fracture morphology of tensile and bending with different soaking time. Fig.5 The effects of hydration and impact cycle on contact peak force and time. (A)Cycle=103. (B)Cycle=104. (C)Cycle=105. (D)The peak contact force with different soaking time. (E)The contact time with different soaking time. Fig.6 The velocity and energy of samples with different soaking time, cycle=10 3. (A) The rebound velocity - time of different soaking samples. (B) The rebound energy - time of different soaking samples. (C) The energy response of samples with different soaking time under the 7.6mJ impact energy. Fig.7 The morphologies and cross-section profiles of worn scars. (A)Cycle=103,soaking time=0h.
(B)Cycle=104,soaking
time=0h.
(C)Cycle=105,
soaking
time=0h.
(D)Cycle=105,soaking time=12h. (E)Cycle=105,soaking time=24h. (F)Cycle=105, soaking time=48h. Fig.8 The width and depth of worn scars. (A)Varied impact cycle. (B) Varied soaking time. Fig.9 XRD and IR spectra of the shell with varied soaking time Fig.10 The schematic diagram showing the role of water in process of impact of soaked 15
turtle shell
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Figure.2
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Figure.3
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Figure.4
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Figure.5
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Figure.6
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Figure.9
(A)XRD spectra
(B)IR spectra 24
Figure.10
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