Investigation Of The Deformation And Fracture Mechanisms Of Biological Hard Tissues And Materials Design

Biological hard tissues possess unique structures and superior mechanical properties.Investigations of the mechanical properties of biological hard tissues can not only provideundestandings for preventing and curing disease, but also offer guidelines for materials design andmanufacturing. Cortical bones, enamel and dentin are important hard tissue of human body, andhave attracted extensive attentions owing to their important roles. In spite of many studies ofexploring the deformation and fracture properties of the hard tissues, the deformation and fracturemechanisms are still not understood thoroughly due to the complex hierarchical structure. In thisdissertation, the mechanical properties of cortical bones, enamel and dentin are investigated using ahybrid experimental and numerical approach, and the underlying deformation and fracturemechanisms are elucidated. In addition, based on the microstructure of enamel, bio-inspiredmaterials designs have been conducted. The main contributions are summarized as follows.(1) The crack growth resistance curves of cortical bones are determined through the compacttension (CT) experiment, and the fracture mechanisms in the transverse and longitudinal directionshave been investigated. Meanwhile, the crack propagation behavior of cortical bones has been alsosimulated using the virtual multidimensional internal bond model. Cortical bones exhibitanisotropic crack growth resistance properties. The fracture toughness in the transverse direction isgreater than that in the longitudinal direction, which is attributed to the anisotropic tougheningmechanisms. Crack bridging caused by uncracked ligaments in the crack wake is the majortoughening mechanism in longitudinal direction, while crack deflection and crack branching aremain toughening mechanisms along transverse direction. The numerical simulation based on virtualmultidimensional internal bond model can characterize the anisotropic crack growth behavior ofcortical bones, and capture the deflection of crack path in the transverse direction, demonstratingthat the numerical model is effective in studying crack growth properties of cortical bones. Whencrack length is larger than1mm, the fatigue crack growth behaviors in transverse and longitudinaldirections of cortical bones can be characterized by Paris law. However, the fatigue crack growthproperty of cortical bones is anisotropic. Fatigue crack growth rate in the longitudinal direction isgreater than that in the transverse direction. When crack length is less than1mm, the fatigue crackgrowth rate of cortical bones in the transverse direction decreases with increasing stress intensityfactor range, indicating that this behavior cannot be characterized by Paris law. (2) The remarkable feature of dentin is the tubules, which have significant influence on themechanical properties of dentin. Utilizing numerical simulation, the crack growth resistanceproperties of dentin along different orientations and in different regions have been explored. Dentinshows anisotropic crack growth resistance curves. The fracture toughness corresponding to crackgrowth along tubule axis is larger than that in the direction perpendicular to tubule axis. The crackgrowth resistance property of dentin is also region-dependent. The fracture toughness of thesuperficial dentin is greater than that of deep dentin, and the fracture toughness of middle dentin isbetween the two values.(3) The mechanical properties of enamel across enamel thickness have been characterized usingnanoindentation and microindentation, and the graded mechanical properties have also beeninvestigated using numerical simulations incorporating the experimental data. The elastic modulusand yield strength of enamel have been derived and the graded variations across enamel thicknesshave been discovered. The elastic modulus and yield strength of inner enamel are smaller than thecorresponding values of outer enamel. Compared with outer enamel, inner enamel has bettercapability of dissipating energy, but weaker resistance to deformation. The graded mechanicalproperties of enamel promote resistance to deformation, mitigation of fracture and enhance fracturetoughness.(4) The damage behavior of enamel is identified using nanoindentation and a numerical modelcharacterizeing this damage behavior has been proposed based on the experiment. It is foundthrough nanoindentation that the stiffness degradation occurs during the unloading stage, implyingthat enamel undergoes damage. To interpret the experimental results, a damage model has beenproposed, which takes into account the rupture of macromolecular chains in protein resulting fromlarge shear deformation when enamel is subjected to indentation loading. The numerical simulationbased on this model shows good agreement with the experimental results, indicating that this modelcan characterize the damage behavior of enamel. It is also found through numerical simulation thatthe damage behavior of enamel is region-dependent. Inner enamel undergoes larger damage thanouter enamel does. To interpret the region-dependent damage behavior, an analytical modelconsidering the deformation mechanism of protein has also been developed.(5) Enamel possesses complex hierarchical structure. At microscale, prisms and the prismsheathes which are rich in protein are the building blocks of enamel; while at nanoscale, prism is acomposite composed of mineral crystals and protein. The mineral crystals in prisms exhibit non-uniform arrangement. The effects of non-uniform arrangement of mineral crystals in enamelprisms on the mechanical properties of enamel are revealed by using multiscale finite elementsimulation. Based on the hierarchical structure of enamel, a micromechanical model is developedwhich takes into account the hierarchical structure of enamel and the inelastic deformation ofprotein. The numerical simulations based on this model show that the non-uniform arrangement ofmineral crystals in prisms enables large energy dissipation and meanwhile maintains sufficientstiffness. This important property of enamel promotes resistance to deformation, enhances thefracture toughness and retains structure integrity.(6) Based on the non-uniform arrangement of mineral crystals in enamel prisms, biomimeticmaterials design has been performed. According to the mechanical properties of the building blocksof enamel, i.e. mineral crystal and protein, magnesium alloy and polymethyl methacrylate (PMMA)have been chosen to design a composite. It is found that the designed composite possess highstiffness and large energy dissipation, and this property is independent on the volume fraction ofreinforcements, morphology of reinforcements and arrangement angle of magnesium alloy.(7) Prisms exhibit different morphologies in enamel. In the outer enamel which is close toocclusal surface, prisms are relatively straight; while in the inner enamel which is near dentinenamel junction, wavy morphology of prisms can be observed. Based on the enamel prism waviness,a biomimetic composite has been designed and the influences of geometric parameters ofreinforcement and reinforcement volume fraction on the mechanical properties of the biomimeticcomposite have been investigated. Numerical simulations show that the stiffenss and capability ofenergy dissipation of the designed composite are affected by the dimension and curvature angle ofreinforcements. By assigning appropriate geometries of the reinforcement, the composite canpossess high stiffness and large energy dissipation simultaneously, and this property is insensitive tothe volume fraction of reinforcements.