Structure And Properties Of The Shell Of The Tortoise Trchemys Scripta Elegans

The turtles and tortoises often contact sand flow during their motion in water or on sand beach the movement. Their body surfaces are abraded by sand particles frequently. Their shell materials are naturally abrasion-resistant materials. The turtles and tortoises are most representatives in animals with shell. Their surface textures are free-form surface which have many advantages, such as, high strength, lower fluid resistance and brief form surface. The microstructures of the surfaces and cross-sections of the tortoise Trchemys scripta elegans were analyzed and the tensile and bending properties of the shell were examined. The reverse engineering technique was used for determining the geometry of the shell surfaces. The finite element simulation of the bearing capacity of shell structure were performed. The above research work will provide some clues to find effective methods for designing some construction structures and some military parts or products.The surface texture and morphologies of the back shell and abdomen shell of the tortoise were examined and the surface structure and scutes arrangement on the shell were analyzed by stereomicroscopy. The correlational angles of the scutes on the back shell were measured using the OLYCIAp3 image analysis system. The results showed that the correlational angles are close and the tortoises shell are similar in geometry, without relationship with their age and the demenssions. The outline of the tortoise shell's carapace were measured and the outline of the carapace hind part and the fore part were fitted by polynomial function using the Matlab software.The cross-section structure of turtle shell was observed using field emission environmental scanning electron microscope (model, XL30). It showed that, the cross-structure of turtle shell is divided into 3 layers from outside to inside: scutes, the dens layer and the loose layer. The dens layer belongs to the Haversian bone, and its structural unit is Haversian system. The thickness of the dens layer is about 600μm and its structure is compact, with many tiny holes of the diameter of 20μm -40μm on the surface. The thickness of the loose layer is about 1000μm; this layer is a porous structure with the holes of diameter the 60μm– 300μm; and the holes are formed by the collagen fibbers. The arragement has no rule. The composition of turtle shell was examined by scanning electron microscopy and energy spectroscopy. It was found that the scutes mainly contain carbon, nitrogen and oxygen, and trace calcium. The bone plates of the turtle shell mainly contains carbon, nitrogen, oxygen, calcium and phosphorus, the contents of calcium and phosphorus gradually increases from the outside to the inside and there was different between different individuals.The microhardness of tortoise shells'outside surface was examined using a digital micro indenter. It was found that the microhardness value was different between the different individuals and the microhardness value of its carapace and plastron's outside surface is close for the identical tortoise shell.The hardness value of shells'outside surface of tortoises increases as their age was increased. The hardness value of the air-dried shells was 2 times higher than the in vivo tortoise shells.Tensile properties of turtle shell were examined by INSTRON-1121 testing machine and the tensile stress-strain curve was obtained. The turtle shell displayed elastic-plastic deformation behavior. Stress-strain character of the turtle shell can be expressed by Sine function and Gaussian function respectively, and it was shown that Gaussian function fits better than Sine function. The tensile strength of shell is related to the bone plate's joints. The average tensile strength of samples with joints is 20.3 MPa and that without joints is 35.0 MPa. Elastic modulus is different for the different sampling positions. The average elastic modulus of shell is 1563 MPa.Bending strength and bending elastic modulus of the tortoise shell were measured using three-point bending mode. It was shown that the average vertical and transverse bending strength of samples without joints are 32.5MPa and 51.6MPa respectively. The average vertical and transverse bending strength of samples with joints are 24.2MPa and 53.5MPa respectively. Bending elastic modulus of the tortoise shell is different at different position and the average value of the bending elastic modulus is 967.6 MPa.The free-abrasive wear property of the tortoise shell was examined on JMM type free-abrasive wear tester, simulating the soil condition in field. It was demonstrated that the free-abrasive wear property is a function of the sliding velocity and the grain size of the abrasive material. The abasive wear depth of the shells was increased with the sliding velocity of the abrasive material against the testing specimen surface and with the particle size of the abrasive material. It was also found that the main abrasive wear mechanisms of the tortoise shell are mainly micro-plowing, micro-cracking and brittle fatigue. The surface of the tortoise shell was colored with white colorant in order to quantificationally measuring the geometry the tortoise shell surface by reverse engineering. The shell was measured using reverse engineering measurement equipment, a 3D laser scanning system (model LSV50). The point clouds of the surface of the tortoise shell were obtained. Based on the reverse-engineering technology, the structure of the tortoise shell was reconstructured using lofting surface function in Catia software. Using point clouds and their boundary curve, the surface of the tortoise shell structure was constructed by the Imageware software.The surface of the tortoise shell structure was imput to the finite element analysis software ANSYS and was simulated with plastic large strain shell unit SHELL43; the finite element model of the tortoise shell was established. The deformation and stress of the tortoise shell under the concentrated force was examined. The rectangular plate with the same length and width as the tortoise shell was used as the comparative object; the material of the rectangular plate was defined alloy steel as the finite element model of the tortoise shell. Under the 2 kN concentrated force, the largest stress of the tortoise shell's finite element model is 0.66 GPa, the largest stress of the rectangular plate is 3.59 GPa. The largest displacement of the finite element model of the tortoise shell is 0.065 mm, but it is 3.36 mm for the rectangular plate's finite element model. The distribution scope of stress of rectangular plate's finite element model is much larger than the tortoise shell's finite element model. The tortoise shell structure has higher supporting ability to concentrated force than the rectangular plate.