Investigations Of Microstructures And Mechanical Behaviors Of Cuticle Of Lobster Claw

As a kind of natural biological composite material,the cuticle of lobster possesses excellent mechanical properties,such as high strength,stiffness and fracture toughness,after natural evolution over many centuries.The excellent mechanical properties of the cuticle are closely related to its microstructures.The investigation of the relationship between the excellent mechanical properties and microstructures of the cuticle can provide important guidance for the design of man-made high-performance composites.In this dissertation,the relationships between the mechanical properties and microstructures of the cuticle of lobster claw are investigated through the combinational methods of mechanical tests,microstructural observations,theoretical analyses,numerical simulations and bionic experiments.The main work and contributions of this dissertation are listed as follows:(1)The mechanical properties of the cuticle of lobster claw were tested.The test methods,contents and obtained results are:(1)The tensile mechanical properties of the specimens from different locations of the cuticle were tested.It was shown that the tensile strength,ultimate strain and fracture energy of the specimens from the middle part of the cuticle were larger than those of the specimens from its posterior and anterior parts.The results show that the nonuniform mechanical properties of the cuticle of lobster claw are closely related to the location.(2)The tensile mechanical properties of the hydrated and dehydrated specimens of the cuticle were tested.It was indicated that the tensile strength,ultimate strain and fracture energy of the hydrated specimens are larger than those of the dehydrated specimens.The results show that the mechanical properties of the cuticle of lobster claw are closely related to the water content.(3)The tensile mechanical properties of the longitudinal and transversal specimens of the cuticle were tested through tensile experiment.It was revealed that the elastic modulus,tensile strength,ultimate strain and fracture energy of the longitudinal specimens are obviously larger than those of the transversal specimens.The results show that the cuticle of lobster claw possesses anisotropic mechanical properties.(4)The mechanical properties of the longitudinal and transversal specimens of the cuticle are tested through four-point bending experiment.It is revealed that the elastic modulus,bending strength,ultimate strain,stress intensity factor,fracture energy and crack extension force are larger than those of the transversal specimens,but the ultimate strain of the longitudinal specimens is less than that of transversal specimens.The results also show that the cuticle of lobster claw possesses anisotropic mechanical properties.(5)The specimens from different locations of the cuticle were tested with nano-indentation instrument.It was exhibited that the hardness and elastic modulus of the middle of the cuticle are larger than those of its posterior and anterior.(6)The fracture surfaces of the speciemens of the cuticle were observed with scanning electron microscope.It was shown that the cuticle consists of mineral chitin-fiber layers and that the fiber layers are further composed of chitin-fiber sheets which consist of chitin-fiber strips with the nanometer scale in diameter.It was also observed that the fiber sheets in the fiber layers keep a small angle with their neighbouring fiber sheets,which composes a fiber-helicoidal structure.In addition,it is also observed that there are many small holes in the fiber layers of the cuticle.These holes are elliptic and their long axis are along the longitudinal direction of the cuticle(can be called as longitudinal hole structure).The longitudinal hole structure combines with fiber-helicoidal structure to form a hole-fiber helicoidal structure.(2)Based on the structural characteristics of the fracture surfaces of the longitudinal and transversal specimens,the fractal models of the crack-propagation path of the longitudinal and transversal fractures were presented.The crack extension forces of the fractal models was calculated.Further,the fractal dimension of the fracture surfaces of the longitudinal and transversal orientations were calculated through box-counting method and improved box-counting method.The calculated results are in good agreement with the experimental results.(3)Based on the characteristic of the longitudinal hole structure of the cuticle,following works were conducted.The effects of the hole direction on the stress intensity factors of the crack tips were investigated based on finite element method and the theory of fracture mechanics.It is shown that the stress intensity factor of the crack tip nearby the longitudinal hole is smaller than that of the crack tip nearby the transversal hole.(2)The effects of the hole direction on the crack-propagation path were investigated based on extended finite element method and the theory of fracture mechanics.It is indicated that the longitudinal holes make that the cracks occur more deflections than the transversal holes.Namely,the longitudinal holes will make that the material consumes more energy than the transversal holes and endow the material higher resisting-fracture ability.(3)The effects of the hole direction on the the stress distribution and damage evolution in the cuticle were investigated based on extended finite element method and the theory of progressive damage.It is shown that the maximum stress and damage region of the longitudinal holes are smaller than those of the transversal holes under the same load.It can be concluded that the longitudinal holes in the cuticle is an optimized structure which makes that the cuticle has high ability to prevent inner damages when a large outer load is applied on it.(4)Based on the structural characteristic of the fiber-helicoidal structure of the cuticle,the bionic composite laminates with different helicoidal angles were fabricated.Then,the impacting experiments of the bionic composite laminates were conducted.It was shown that the smaller the helical angle is,the larger the resisting-impaction ability of the material is.After the experiments,the relationship between the helicoidal angle and the resisting-impaction ability of the material was investigated with finite element model.It was also indicated that the smaller the helical angle is,the larger the resisting-impaction ability of the material is.(5)Based on the characteristic of the hole-fiber helicoidal structure of the cuticle,the effects of the hole size and the fiber-helicoidal angle of the structure on the resisting-impaction ability of the cuticle were investigated with the models of the hole-fiber helicodial structure and progressive-damage analytical method.It is indicated that the smaller the hole size and the helicoidal angle are,the larger the resisting-impaction ability of the material is.