Comparison of damage accumulation measures in human cortical bone

Elastic modulus degradation, strength reduction, and energy dissipation have traditionally been the properties of choice to monitor the damage process in cortical bone. However, these properties only provide limited insight into the damage process given the complex mechanical nature of bone. In the current study, alternative measures of the damage process were investigated for machined human cortical bone specimens loaded under torsion. Seventy-two bone specimens from 6 human femurs were subjected to a series of torsional relaxation cycles in which damage was induced during a single relaxation cycle and the effects of damage on the elastic, yield, viscous, and failure properties were determined from pre- and post-damage relaxation cycles. The results revealed that degradation of all torsion properties exhibited a significant twist magnitude effect. However, the yield stress and strain, the relaxation rate, and the total relaxation exhibited 5-10 fold greater degradation than both strength and modulus, when residual strength tests were conducted at high shear strain rates. For the loading conditions examined in this study, the results indicated that the relaxation and yield properties of cortical bone are more sensitive to shear damage accumulation and better measures of the damage process than either strength or modulus. Further, the results reveal an important interaction between damage and the viscous behavior of bone which provides new insight into the effects of damage on bone mechanical properties.