Investigations Of Multi-scale Microstructures And Mechanical Behaviors Of Cortical Bone

After natural evolution of tens of millions years,animal's bones have high strength,stiffness and fracture toughness.The excellent mechanical properties of the bones are closely related to their internal multi-level microstructures.The investigation of the relationship between the excellent mechanical properties and microstructures of the bones can provide important guidance for the design of artificial high-performance composites.In this dissertation,the relationships between mechanical properties of cortical bone and its microstructures are investigated with the combinational methods of mechanics tests,microstructural observations,theoretical analyses and numerical simulations.The main works and obtained contributions of this dissertation are listed as follows:(1)The macroscopic mechanical properties of the longitudinal and transversal specimens of cortical bone are tested by uniaxial compression experiment.The effects of the orientations,positions and strain rate of the specimens on their mechanical behaviors are analyzed.It is shown that the bone possesses obvious anisotropic properties.After the test,the fracture paths and the fracture surfaces of the fractured specimens are observed by macroscopic and microscopic experiments.It is indicated that the fracture path of the longitudinal specimens is along their longitudinal direction,and the fracture path of the transversal specimens makes about 45o from their longitudinal direction.The roughness of the fracture surface of the transversal specimens is larger than that of the longitudinal specimens.The mechanisms of elastic modulus and ultimate strength of the longitudinal specimens are larger than that of transversal specimens are analyzed based on micromechanics theory.It is revearled that the anisotropic property of the bone is related to the direction of the osteons in bone.(2)The relationships between fracture toughness of cortical bone,microstructural characteristics and fractal dimensions of fractured surfaces are investigated based on bending tests,microstructure observation of four different fracture surfaces,and the calculation of fracture toughness and fractal dimension.The test results shown that the fracture energy and fracture toughness of the four different specimens are obviously different,which indicates that the fracture behavior of cortical bone is related to its orientation.In bending process,the fracture path of the 0o specimen(transversal fracture)is obviously deflected and folded,and the fracture path of the 90o specimen(longitudinal fracture)approximates along the longitudinal direction of the bone,with almost no deflection.The fractal dimensions of the specimens of the four different directions are calculated by box dimension method.It is shown that the fractal dimension of the bone is positive correlated with its fracture toughness.(3)According to the results of the three-point bending experiment,it can be seen that the fracture paths of the fracture specimens have obvious crack deflection.Three different osteon and micro-crack models were established,and the effects of material parameters,osteon and cement line on crack propagation and its propagation path are analyzed based on linear elastic fracture mechanics and finite element method.The results show that the osteon blocks the propagation of microcracks when the modulus ratio of osteon and interstitial bone is greater than 1,the osteon promotes the expansion of the microcracks when the modulus ratio is less than 1.Osteons have a more pronounced effect on accelerating or blocking microcracks propagation in the model of multiple osteons.It is also shown that the cement line prevents the propagation of microcracks and deflects the propagation path of microcracks.(4)The Scanning Electron Microscope observation results of fracture surfaces of cortical bone show that the elliptic lacunae are unevenly distributed in osteons,and their long axes are along the circumferential direction of osteons(circumferential-elliptic lacunae).Based on the observational results,three kinds of osteon models with different lacunae(circumferential-elliptic lacunae,radial-elliptic lacunae and circular lacunae)are established.The effects of the direction and shape of the lacunae on the impact resistance and micro-damage of osteons are investigated,by comparing the stress distribution and damage evolution of the three models under impact and squeezing load.It is indicated that the circumferential-elliptic lacunae of the osteon are the result of natural optimization,which can effectively decrease the stress concentration and enhance the impact resistance of osteons.The results of damage analysis show that the circumferential-elliptic lacunae of osteons can specify the initiation position and propagation direction of microcracks,and making the microcracks propagation along the circumferential direction to avoid it entry into Haversian canal and caused the destruction of osteon.(5)Haversian canals are surrounded by mineralized collagen fibers of the annular bone lamellae,and the interval angle of the fibers in adjacent bone lamellae is about 30°.Four kinds of the composite models with different fiber spiral interval angles(15°,30°,45° and 90°)are established,and the four models are analyzed using low-speed impact damage analysis based on progressive damage method and Hashin failure criterion.The analysis results show that the composite model with the spiral interval angles of 30° has the greatest impact resistance.It is also indicated that the periodic spiral laminar structure of mineralized collagen fibers in osteons can effectively improve the ability of impact resistance of cortical bone.The results can provide useful guidance for the development of high performance biomimetic composites.(6)Based on the arrangement of hydroxyapatite crystals in mineralized collagen fibers of cortical bone,the mesoscopic model of staggered structure of hydroxyapatite crystals is established.The effects of the structural parameters and volume fraction of the hydroxyapatite crystals on the elastic modulus and fracture toughness of cortical bone were investigated.It is shown that the elastic modulus of cortical bone increases nonlinearly with the increase of the volume fraction of hydroxyapatite crystals.According to the crack propagation path and fracture bridging phenomenon of the staggered structure,the effects of the structure parameters and the volume fraction of the hydroxyapatite crystal on the fracture toughness of the cortical bone are investigated.The results show that the fracture toughness of staggered structure is obviously increased with the increase of the aspect ratio and volume fraction of the crystal plate.It also indicated that the staggered microstructure of hydroxyapatite crystals helps to enhance the fracture toughness of cortical bone.