Investigation Of Elastoplastic Properties And Fracture Mechanism Of Dentin And Bone

Biological hard tissues,such as bone,tooth and nacre,possess unique microstructures and superior mechanical properties.Investigating the mechanical behaviors of the biological hard tissues can not only assist understandings for disease prevention and clinical treatment,but also provide guidelines for design of engineering materials.Although many studies have been conducted on the deformation and fracture of hard tissues,a good understanding of deformation mechanisms of such materials has not been achieved due to the complex hierarchical structure.In the present study,the mechanical properties of bone,dentin and other biocomposites are investigated using experimental evaluations and numerical simulations.The main contributions are summarized as follows:(1)The hardness and Young's modulus of human dentin were determined using nanoindentation,and the dependence of these parameters on the distance from pulp was obtained by a statistical method.Moreover,the effect of aging on the hardness and Young's modulus was explored.It was found that the middle and outer dentin show larger penetration resistance than inner dentin,and the Young's modulus and hardness of inner dentin are significant lower than that in middle and outer dentin.The gradients in mechanical properties of human dentin could promote resistance to penetration,which potentially protect the underlying pulp.In addition,the Young's modulus and hardness of human dentin increase with aging.(2)The nanoindentation experiments of dentin indicate that dentin undergoes plastic deformation accompanied by the reduction in stiffness.A plastic damage model was used to investigate the variation of such inelastic deformation behavior with thickness of dentin,as well as to identify the influence of aging.This model is capable of describing the stiffness reduction in the unloading process of nanoindentation experiment,and a good agreement between numerical simulation and experiment is achieved.The numerical simulations show that the outer and middle dentin have larger yield strength compared with the inner dentin,and that the inner dentin has the propensity of damage when subjected to loading.In addition the yield strength of dentin increases with aging.(3)Biocomposites are composed of hard mineral crystals embedded in protein matrix,which is a biopolymer providing toughness for biological hard tissues.In addition,the interfaces between minerals and protein are widely observed.The effects of the mechanical properties of interface between minerals and protein and the plastic deformation of protein on the mechanical performance of biocomposites were studied using the numerical simulations based on cohesive zone model.The complex interaction between the plastic deformation in matrix and the bonding properties at the interfaces has been studied,and the effect of such interactions on the toughness of biocomposites is identified.For the cases of the strong and intermediate interfaces,the toughness of biocomposites is controlled by the postyield behavior of the biopolymer.In detail,the matrix with low strain hardening exponent can undergo significant plastic deformation,and potentially promote fracture toughness of biocomposites.For the case of the weak interface,the toughness is governed by the bonding property of the interface.In contrast,the postyield behavior of the biopolymer shows negligible effect on toughness.Additionally,the mechanical properties of materials can be improved by tuning the microstructures.(4)The extended finite element method is adopted to characterize the fracture mechanisms of the mineralized collagen fibrils which are the basic building blocks of bones at the nanoscale.Crack initiation and propagation in two types of mineralized collagen fibrils,including the perfect microstructure and imperfect structure with an initial nanocrack,are investigated and the failure mechanisms of mineralized collagen fibrils are revealed.It is found that for the perfect microstructure,crack initiation takes place in the matrix near the short axes of mineral crystals and crack deflection is observed in the case of large load.For the two types of mineralized collagens,cracks mainly propagate in the matrix,which is accompanied by the formation of crack bridging.It should be noted that crack deflection and crack bridging are the major toughening mechanisms of bone.(5)Biocomposites exhibiting non-self-similar hierarchy possess superior mechanical properties.The extended finite element method is used to assess the influence of non-self-similar hierarchy on the crack propagation in the bone at the sub-microscale.It is found that initiation of microcracks occurs in the matrix in the vicinity of the short axes of reinforcements when the uniaxial tension is applied to the lamella,which is formed by protein reinforced by the mineralized collagen fibrils.With the increase in applied load,cracks deflect and grow along the direction of long axes of the reinforcements.These cracks can only propagate in the regions where the reinforcements are overlapped,and could not penetrate into the reinforcements.In addition,the composites with non-self-similar hierarchy undergo heterogeneous deformation,leading to the non-uniform distribution of cracks in the material,which further affects crack propagation.