Mechanism Of Strength And Toughness Of Hierarchical Microstructure Of Cortical Bone

As a kind of natural biological composite material, cortical bone consists ofmineralized collagen fibers, hydroxyapatite and other amorphous collagen matters. Itpossesses high strength, stiffness and fracture toughness. The excellent mechanicalproperties of cortical bone are closely related to its hierarchical microstructure. Theinvestigation on the relationship between the hierarchical microstructure and theexcellent mechanical properties can reveal the mechanism of strength and toughness ofthe bone, and provide useful guidance for the development of high-performancebiomimetic composites.This thesis investigates the mechanism of strength and toughness of cortical bonethrough experimental observation, model analysis, numerical calculation, biomimeticfabrication and experimental verification. Following works are accomplished andprofitable conclusions are obtained.①The microstructure of cortical bone is observed with scanning electronmicroscope. It is founded that cortical bone possesses hierarchical microstructure.Cortical bone is first composed of osteons and interstitial matrix, and then the osteonconsists of long and thin collagen fibers which helically round Haversian canal at theircenter. There is a clear cement line boundary between the osteon and interstitial matrix.It is also observed that the osteons are pulled out form the interstitial matrix and thefibers are pulled out from the osteons.②The work of fracture of the osteons is analyzed based on the theory offiber-reinforced composite mesomechanics. It’s indicated that debonding, sliding andpulling out of the osteons bring more energy dissipation when the bone fractures alongits cross section and increase the capability resisting fracture of the bone. Based onshear lag model, the maximum pullout force of the collagen fibers in the bone and theosteons is analyzed. It is showed that the maximum pullout force and pullout energy ofthe collagen fibers is closely related to the embedded depth of the fibers. The criticallength of the fibers is obtained. Based on Euler’s formula, the maximum pullout force ofthe oblique collagen fibers in the osteons is also investigated. It’s indicated that themaximum pullout force and pullout energy will increase with the increase of theembedded depth and the oblique angle of the collagen fibers. It is indicated that thelarge helicoidal angle and the large length-thin ratio of the collagen fibers enhance the fracture toughness of the bone.③Based on the theory of linear elastic fracture mechanics and finite elementmethod, the models of the cortical bone with cement lines or without cement lines areput forward. The models are used for investigating the effect of the variety of osteon’smodulus, the distance from crack tip to the osteon, the crack length and the cement linesof the bone on the mechanical behaviors of the crack tip. From the analytic results onthe model without the cement lines it is revealed that the low-modulus osteons promotemicrocrack propagation toward the osteons in the cross section of the cortical bone. It’sbeneficial to avoid forming a macrocrack in fragile interstitial matrix. From the analysison the model with the cement lines it is indicated that the stress intensity factor, stressand strain of the crack tip decrease after a microcrack encounters an osteon. It isexposited that the cement lines restrain the propagation of the crack and enhance thefracture toughness of the bone.④Based on Abaqus software, both two-phase model and three-phase model aredeveloped for analyzing the propagation of microcrack. The two-phase model consistsof an osteon and interstitial matrix and the three-phase model consists of an osteon,interstitial matrix and cement line. The two models are used to study the role of thecement line in the microcrack propagation of the bone. It is shown that the cement lineenhances the applied strain initiating the microcrack, deflects the microcrack andreinforces the cortical bone in toughness. In addition, three models, containing manyosteons and respectively consisting of a single phase, two phases and three phases, arealso put forward. The two-phase model consists of the osteons and the interstitial matrixand the three-phase model is composed of the osteons, the interstitial matrix and thecement lines. Based on the extended finite element software, the stress and strain fieldsof these models are calculated and the propagation of the crack in the bone is simulated.It is indicated that the osteons and the cement lines enhance the applied stress and strainwhich initiate the microcrack to varying degrees, postpone the softening of stiffness,slow the crack growth and increase the total length of crack.⑤Biomimetic samples of the cortical bone are fabricated. The samples have themain structural characteristic of the cortical bone. The mechanical behavior of thebiomimetic samples is tested and compared. It is denoted that the strength, stiffness andtoughness of the biomimetic samples with helicoidal-fiber pipes are high than that of thesamples with non-helicoidal-fiber pipes.