Fatigue crack initiation and propagation behaviors of bovine compact bones

An understanding of how fatigue cracks propagate in the bone is important because fatigue is thought to be the main cause of clinical stress fractures. In this study, fatigue crack initiation and propagation behaviors of bovine humerus and femur were investigated with respect to their microstructures. The initiation and propagation of fatigue cracks were observed in a specimen surface by using the replica method. Fatigue tests were also conducted on the bovine humeral and femoral compact bones in order to obtain S-N curves. Based on their microstructures, the bovine humeral and femoral compact bones are classified into the haversian and plexiform bones. In haversian bone, when the microstructure is inclined toward the bone axis, the fatigue crack initiates at the osteon-ossein interface and osteon lamellae interface. In case the microstructure is parallel to the bone axis, the fatigue cracks initiate at the haversian canals, Volkmann's canals, osteon-lamellae interface, and osteon-ossein interface. Among these, the fatigue crack that initiates at the Volkmann's canal may receive a relatively greater maximum shear stress; therefore, it has a tendency to be the main propagating fatigue crack that connects the fatigue cracks initiated at the other above mentioned sites. In the plexiform bone, when the microstructure is inclined toward the bone axis, fatigue crack initiates at the interface of the lamellae. When the microstructure is parallel to the bone axis, fatigue cracks initiate at the blood vessel and interface of the lamellae. The fatigue crack that initiates at the blood vessel may receive a relatively greater maximum shear stress; therefore, it has a tendency to be the main propagating fatigue crack that connects the fatigue crack initiated at the other site. In the haversian and plexiform bones, the crack propagation rate when the microstructure is inclined toward the bone axis is greater than that when the micro-structure is parallel to the bone axis.