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Abstract
Cortical bone has a complex hierarchical structure which is made of mineral crystals and collagen matrix. In this study, the mechanics of bone at four different material levels namely: (1) the atomic level (mineral crystal phase); (2) the nano level (collagen fibrils); (3) the micro level (lamellar structure); and (4) the macroscopic level (cortical bone) and their inter-relationship are investigated.
Combined wide-angle X-ray diffraction and small-angle X-ray scattering were used together with an in-situ mechanical test to investigate (1) the deformation and (2) relaxation processes of bone. Bone is stiff and tough structure. Partial failure in the nanostructures is found at 0.2% macroscopic strain and changes the macroscopic stiffness. Anisotropy and the microstructure of bone dominate the post-yield deformation process, either by sliding between the lamellar layers or by the growth of microcracks. The accumulation of microcracks, observed using microscopy, eventually leads to failure, and results in a rough fracture surface.
Bone displays viscoelastic behaviour. During stress relaxation, strain relaxation was observed in both the mineral phase and collagen fibrils. This relaxation process involves a combination of mechanical events and drying, and has distinctive fast and slow relaxation stages. Strain relaxation is fastest at the plane of maximum shear stress at the early stage of relaxation. Drying introduces shrinkage in the bone structure and is found to speed up strain recovery in the lateral direction. Anisotropy in bone is evident in strain relaxation pattern and drying rates.
Similarly, at the sub-micron material level, modulus and hardness of bone were found to be sensitive to loading rates upon nanoindentation. Different indenter sizes introduce different stress levels. At low and high stress levels, deformation is governed by elastic and plastic deformation, respectively. A viscous-elastic-plastic (VEP) mechanical model was developed to describe the deformation processes under spherical indentation. Subsurface cross-sectional imaging of the indents using transmission electron microscopy revealed extrinsic toughening of bone by non-symmetrical crack propagation along fibre boundaries and crack bridging.
A better understanding of the structure-property relationships of the hierarchical bone structure is developed from this research which may advance materials development and address orthopaedic issues.