Analysis And Characterization Of The Mechanical Properties And Behaviors Of The Bio-hierarchical Structures At The Micro/Nano Level

The natural biomaterials play the distinct functions in nature, reflecting their respective excellent mechanical properties, which is highly related to their specific hierarchical structure. The research on the mechanical behaviors of the hierarchical structure of biomaterials is the common topic in material science, mechanics and biology. The further understanding of the relationship among the macro/micro structure-property-function could not only be beneficial to recognize the biological evolution, but also be useful to design and fabricate the advanced biomimetic materials. Recently, the exploration of the excellent properties of natural materials becomes the research hotspot. In this thesis, the enamel, as a typical bio-hierarchical structure, is used for the experimental study. As the outermost material of the tooth, the enamel plays a role in biting and chewing in the mouth. The combination of the experiments, finite element analysis and theoretical modeling was used to analyze the micro-tribology, contact mechanics and cyclic contact. The main research contents and conclusions are listed as follows:In-situ nano scratch experiments were carried out on occlusal surface and axial section (on which axis is perpendicular to rods and axis parallels to rods) of human enamel using a sphero-conical indenter (～1μm tip radius) at different normal loads. The amount of elastic recovery as well as inelastic deformation during scratching process was obtained from three curves along the track:surface roughness, scratch depth, and post depth. Results reveal that the amount of elastic recovery and inelastic deformation of a single rod on the axial section is more than that on the occlusal surface. The load-dependence is attributed to the ploughing and pile-up. The maximum wear resistance of a single rod is observed on the occlusal surface. The location-dependence ascribed to the angle change of normal loading direction, hydroxyapatite crystals orientation and the scratching direction.The structure of hard tissue being surrounded by the relatively softer tissue is widespread in the bio-hierarchical structure. Considering the possible effect of the thin protein-rich sheath on the indentation deformation of an enamel rod, we analyzed the indentation response of an elastic cylinder with a compliant layer between the cylinder and rigid-surrounding material. The closed-form solutions were obtained between the indentation load and the indentation depth, which depends on the film thickness and the ratio of the Young’s modulus of the cylinder to the Young’s modulus of the film. Moreover, the finite element model was established to discuss the limitation of the Oliver-Pharr method in measuring the elastic modulus of the enamel rod. A concept of the threshold indentation depth was proposed, at which the percentage error of the measured modulus of the cylinder is±10%. Results reveal, the indentation load required to produce the same indentation displacement decreases with increasing the ratio of the Young’s modulus of the cylinder to the Young’s modulus of the film for compressible-elastic films. Incompressible-elastic films have no significant effect on the indentation response of the elastic cylinder. The normalized threshold indentation depth strongly depends on the modulus ratio of the film to the cylinder and the ratio of the film thickness to the cylinder radius. The results can be used to guide the use of the Oliver-Pharr method in characterizing the mechanical properties of tooth enamel and bio-composites with core-shell structures.Cyclic microindentations were performed on the occlusal surface and the axial section of tooth enamel, using the Berkovich indenter. Under the action of a cyclic indentation load, the indenter continuously penetrated into the material and reached a quasi-steady state at which the penetration depth per cycle was a constant. At the quasi-steady state, both the amplitude of the indentation depth and the penetration depth per cycle for the cyclic indentation of the axial section are larger than those for the indentation of the occlusal section under the same loading condition. The energy dissipation per cycle consists of two contributions; one is the plastic energy dissipated per cycle due to the propagation of the plastic zone underneath the indentation and the other is the energy dissipation due to the viscous flow during the cyclic indentation. Both the penetration depth and the plastic energy dissipated per cycle at the quasi-steady state are independent of the maximum applied load and increase with increasing the amplitude of the cyclic indentation load. Deviation of load-depth curve under multi-cycle from single-cycle could indicate the anisotropic fatigue damage during repeated indentations. On occlusal surface, fatigue damage is ascribed to the partial interface debonding between crystals and protein and incomplete refolding of domain bonds of protein molecules. While on the axial section, additional damages, i.e. fracture of crystals and debonding at the end of crystals are supposed to be dominant.Considering the variation of the rod arrangement in outer and inner enamel, the enamel was approximated as a bilayer structure. Finite element analysis of the cyclic indentation of the bilayer structure was performed to mimic the repeated contact of enamel during mastication. The results verifies the outer enamel serves as a protective layer due to its large resistance to the contact deformation, compared with the inner enamel. The finite element results of larger equivalent plastic strain and lower stresses in the inner enamel during cyclic indentation indicate better crack/fracture resistance of the inner enamel.For the general staggered arrangement of mineral platelets wrapped by soft matrix which exists in the load-bearing biomaterials, two analytical models,’Stress model’ and ’Displacement model’, were established from the ways of stress and displacement solution based on the modification of classical shear-lag model. Complementary finite element analysis (FEA) was used to verify the analytical models. Results reveal, compared to ’Displacement model’,’Stress model’gives a better prediction of the stress distributions within the staggered structure referring to FEA. The equivalent CTE predicted by both models reach asymptotic stable values as aspect ratio and volume fraction of platelets exceeding the critical values and agree well with the results of FEA. The relative error between the results from different models increases with the increase of the ratio of overlap to length of platelets.