Structures, Properties And Biomimetic Models Of The Elytra Cuticle Of The Beetle Copris Ochus Motschulsky

The insects have optimized their structures and physiological functions (e.g., flying, jumping, crawling, drag reduction, anti-adhesion, self-cleaning, ) through their biological evolution over millions and millions of years. Biological materials are basically composite materials and the insect cuticle is a kind of biological composite. It will be significant and important to explore the microstructure features and strengthening-toughening mechanisms for developing the biomimetic designs of advanced composites.The surface morphologies, microstructures and the cross-secion structures of the elytra of the dung bettle Copris ochus Motschulsky was examined by stereoscopy and field emission scanning electron microscopy (FESEM). The anisotropic features were compared for the constitutional features of the cuticle of the dung bettle. It was shown that there is a ripple structure with a width of 0.95 mm along the longitudinal direction of the elytron. There is a prayer bead structure and some holes linking with the elytron interior in the grooves on elytra. The elytra of the dung bettle Copris ochus Motschulsky have a 0.05mm in thickness approximately, which is composed of the epicuticle, exocuticle and endocuticle respectively. The elytra cuticle of the dung beetle is a chitin-fiber-reinforced protein matrix composite with cascade structure feature, that is, multi-layer composite. The microfibers in each layer parallel to each other, while the fibers arrage spirally at fixed angle (approximately at 70°) in the adjacent layer.The thermogravimetric analyzer (TGA) and difference thermal analyzer (DTA) was used to determine the principal composition and thermal decomposition feature the elytra of the dung bettle Copris ochus Motschulsky. The diagram of TGA shows that water and proteins are removed from the untreated cuticle at temperatures between 20℃and 120℃. The degradation of chitin starts over 210℃.At the temperatures between 330 and 600℃, some high polymer and the inorganic minerals are removed. The diagram of DTA showed that a endothermic peak and two exothermic peaks can be seen clearly. The initial temperature and the final temperature on the diagram of TGA raises as the heating rate was increased, meanwhile, the peak temperature of the DTA was also increased.The EDX was used to determine the metal elements and their contents self- quantificationally. It was shown that, besides carbon (C), hydrogen (H) and oxygen (O) essential to the molecular structure, the cuticle of the dung beetle contains such metal elements as magnesium (Mg), aluminum (Al), iron (Fe) and potassium (K). The concentration of metal elements has differences between the forepart and the hindpart of the elytra cuticle considerably. While, the concentration of carbon and oxygen are approximately is close.The tensile behavior of the elytra cuticle was examined using a micro tensile tester. The results showed that the tensile procedure of the elytra cuticle divided to four stages: the elastic deformation stage, the yield stage, the strengthening stage and the neck shrinks stage, so as to the mechanical properties of the elytra cuticle of dung beetle were determined: the yield strength, the tensile strength, the elastic modulus and the elongation ratio are 1400.50±153.24 MPa, 1199.35±217.12 MPa, 14.56±4.20 GPa and 0.241±0.10 respectively. The elytra of the dung beetle can be considered as tough material since the elongation ratio ranging from 12.1% to 36.3%. The fracture surfaces of the elytra cuticle is similar to those of tough fracture surfaces. The paraboloid dimples and plastic deformation of the visco-liquid-like matrix were found, which reflects the tough fracture phenomenon of elytra cuticle. The water loss of the elytra cuticle can affect the fracture properties of elytra cuticle considerably, leading to the transformation from the gliding fracture to the brittle fracture.The nanomechanical properties of the elytra cuticle of the dung beetle were examined using a tribo-indenter (Hysitron). It was shown that the elastic modulus and nano-harness in vertical direction are 5.96±0.32GPa and 0.32±0.09GPa respectively; the elastic modulus and nano-harness in transverse direction are 7.34GPa and 1.66GPa respectively; the elastic modulus and nano-harness in longitudinal direction are 6.05GPa and 0.84GPa respectively.The curves of the elastic modulus and nano-harness in transverse and longitudinal direction have three obvious levels, which corresponds exactly with the chitin-fiber-reinforced composite construction of the elytron cuticle. It was also shown that the nanomechanical properties in transverse and longitudinal direction are better than that in vertical direction, the reason of which possibly is the effects of the entire internal composite structure.The finite element software ANSYS was used to simulate the elytra cuticle of the dung bettle. The finite element model of laminative plate structure was established for statics analysis to investigate the deformation and stress state under the uniform load, the concentrated force, the bending moment and the cyclic loading function.In order to investigate the deformation and stress state under the uniform load and shearing force, three different trabecular structure models were established for statics analysis. Under the uniform load, the deformation of the structure model concentrates in the upper part and the large stress distributes in trabecular structure, suggesting that the trabecular structure can be able to support construction and reduce stress effectively. The Stress diagram of the frustum trabecular structure model was compared to that of the other models. It was found that the destruction possibility of the frustum trabecular structure to be higher than others, such as the dehiscence between laminative plate and the trabecular, deguming and bruising. The stress of the model whose trabecular structure has a slender concentrate on contact surface between laminative plate and the trabecular, which results in a lower ability to resist compression. Under shearing force, the deformation of the structure model also concentrates in the upper part. It was found by compared with the stress diagram of the two cylindrical trabecular structure model that the model whose trabecular structure is lower possibility of cracking and peeling because of the lower ability to resist cutting, while, the model whose trabecular structure is thick and short is of good ability to resist cutting.