Investigating the relationship between fracture surface mismatch and the plastic J - Integral in bovine cortical bone: Application of stereophotogrammetry and shape from focus

The origin of fracture toughness in cortical bone is not well understood. In this study, the effects of plasticity on fracture toughness have been evaluated for bovine cortical bone. Evidence of plasticity was sought by studying the way fractured surfaces fit back together after mechanical testing. Also, the water and organic content dependence of plastic energy dissipation, J - Integral, was studied. Two sets of bovine cortical bone were characterized following ASTM E 1820 J - R fracture toughness experiments. In order to determine the crack size using the conventional elastic compliance method on small specimens (25 mm x 1.8 mm x 1.5 mm), an automatic multi-point optical clip gauge was developed. Two methods of 3D quantitative fractography were developed and calibrated to measure fracture surface mismatch. The 3D distribution of the residual deformation over the fracture surfaces has been obtained for the first time. Even when the load-displacement plots and the effective stress intensity factors showed small variations between specimens, the fracture mismatch could be related to the plastic component of the J - Integral. Toughening due to deflection was also observed in most of the specimens. It was found that water content is related to plasticity in cortical bone. For example, a previously dehydrated specimen recovered its plastic dissipation mechanism after being immersed in saline solution before testing. However, the mechanism by which water content affects plasticity is still not known. Additional experimentation with the 3D fractography system developed here should shed light into this question