Effect of Orientation on Elastic-plastic Fracture Toughness of Cortical Bone

Bone material is heterogeneous and anisotropic in nature. It also has a hierarchical structure that changes from nano-scale to macro-scale. Bone material contains good amount of non linearity during deformation. This nonlinearity may be due to several toughening mechanisms and presence of water in the bone material. Many researchers used critical stress intensity factor, critical energy released rate and crack growth-resistance curve approaches based on linear elastic fracture mechanics to examine toughness of bone. These approaches are inadequate to characterize fracture in presence of substantial nonlinearity preceding fracture. Crack tip opening displacement (CTOD) approach based on elastic plastic fracture mechanics has been applied in the present work to provide an estimate of fracture toughness of buffalo cortical bone for longitudinal as well as transverse orientation of cracking. The elastic modulus and yield strength of buffalo bone are also evaluated for the transverse as well as longitudinal orientation of loading and compared with the values available in the literature. The average CTOD toughness (delta(c)) for transverse orientation was found to be 63 pm, which is 61% more than that of longitudinal orientation of cracking (39 mu m). The equivalent K-fracture toughness values obtained from the delta(c) values in case of transverse orientation (12.68 MPa.m(1/2)) was found to be 141% more than that of longitudinal orientation of cracking (5.26 MPa.m(1/2)). The J-toughness values are calculated employing the corresponding delta(c) values in the two orientation of cracking and compared with the values of J-toughness available in the literature. It is suggested that the CTOD (delta(c)) and J-toughness are better parameters to predict the realistic fracture resistance of bone.