Investigation of micro-cracking behaviors of human femur cortical bone during radial fretting

Radial fretting tests on human femur cortical bone with a ball-on-flat configuration were carried out in vitro under different normal contact loads. The kinetics behaviors and damage characteristics of the cortical bone were analyzed using the load versus displacement (F–D) curves. The fretting scars were examined using laser confocal scanning microscopy (LCSM) and scanning electron microscopy (SEM). A nano-indenter was used to characterize the mechanical property of the osteon of the cortical bone. Morphologies showed that the primary damage form was due to micro-cracking. Around the osteon, three kinds of micro-crack configurations were observed; i.e. annular cracks—located at the concentric lamellae; radial cracks—initiated from the Harversion canal; and interstitial cracks—initiated and propagated between the interstitial lamellae. Plenty of interstitial cracks occurred in the wear scar, which indicated that the interstitial tissue was the weak zone in the structure of cortical bone. In addition, the contact stresses under the radial fretting condition were calculated by the finite element method analysis (FEMs) and the behaviors of cracking were explained. The results demonstrated that the stress concentration that occurred around the Harversian canal and osteon system resulted in a better resistance than that from the interstitial tissue. The concentric lamellae presented a better radial-fretting resistance and contact fatigue resistance than that from the interstitial lamellae.