The effects of damage accumulation on the tensile strength and toughness of compact bovine bone

Damage accumulation in compact bovine femur subjected to uniaxial tensile loading was examined by strong light illumination effects of microcracking. Imaging was done using a high-speed camera capturing image at 200 to 1500 FPS. The tensile tests were performed in a multipurpose tensile testing system with cross-head speeds ranging from 0.5 to 10 mm/min which leads to strain rates of 0.0001 to 0.0012 s−1 (physiologically relevant to walking and running Hansen et al., 2008). The post-failure images were then examined in a scanning electron microscopy (SEM) and effects of microstructure, strain rate, and orientation were evaluated. Correlation of the high-speed images with stress-strain curves indicated that optically visible microcracks were most likely initiated at yielding, and the specimens with dispersed microcracks exhibited a higher energy-absorption capacity compared to the specimens with coalesced local cracks. It was found that damage accumulation negatively correlates to strain rate and that transverse specimens exhibited a different failure pattern compared to the longitudinal specimens. Strain hardening and softening were found in the longitudinal and transverse specimens respectively. The microcracking in the transverse specimens instantly increased to peak after yielding compared to the gradual growth until failure in the longitudinal specimens. The average Young's modulus (21.5 GPa) and ultimate stress (93.5 MPa) of the specimens loaded in the longitudinal direction were more than twice that of the specimens (10.9 GPa and 36.2 MPa respectively) loaded in the transverse direction. The current technique has shown potential in relating damage accumulation real time in bone samples subjected to tensile loading condition. This information will be helpful in relating the role of micro damage accumulation in initiating failure and/or remodeling in bone.