Microstructure-Based Failure Mechanisms Encountered During Fracture Cutting of Age-Varying Bovine Cortical Bone

This article aims to investigate the characteristic microstructure-based failure mechanisms observed during the fracture cutting of age-varying bovine cortical bone. To this end, orthogonal cutting experiments are performed on cortical femoral bones harvested from three distinct bovine age groups, viz., young (∼1 month), mature (16–18 months), and old (∼30 months). Fracture cutting is induced at a depth of cut of 70 μm and a cutting velocity of 800 mm/min by using two distinct tool rake angles of +20 deg and 0 deg. The nanoindentation studies and porosity analysis show key differences between microstructural constituents, as a function of age. The high-speed camera images taken during the fracture cutting process provide insight into six dominant microstructure-specific failure mechanisms. These include primary osteonal fracture, woven fracture, and lamellar fracture observed in the plexiform region; and cement line fracture (i.e., osteon debonding), secondary osteonal fracture, and interstitial matrix fracture observed in the haversian regions. In addition to the conventionally reported specific cutting energy metric, a new metric of resultant cutting force per unit crack area and surface integrity are employed here. All cutting responses are seen to be sensitive to age-related microstructural variations and the tool rake angle. In addition to requiring more cutting force, the neutral tool rake angle also results in notable subsurface damage.