Fracture surface analysis to understand the failure mechanisms of collagen degraded bone.

Fracture surface analysis is a powerful technique to investigate bone failure mechanisms. Previously, emu tibiae were endocortically treated with 1 M potassium hydroxide (KOH) solution for 14 days. This treatment caused in situ collagen degradation rather than removal, with no differences in geometrical parameters, but with significant changes in mechanical properties. KOH-treated tibiae showed significant decreases in failure stress and increased failure strain and toughness. The fracture surfaces of untreated and 14-day KOH-treated failed specimens were examined to further identify differences in the failure process to explain the previously observed increase in toughness. Areas of 'tension,' 'compression,' and 'transition' were identified using digital images of the fracture surfaces. Within these areas, the degree of 'roughness' and 'smoothness' was identified and estimated, using an optical profiler and SEM images. The fracture surfaces of 14-day KOH-treated bones showed a significantly higher 'roughness' compared to untreated bones. Furthermore, additional toughening mechanisms, which are important features for dissipating energy during the failure process, were observed in KOH-treated samples, but were absent in untreated samples. These results indicate that the significant increase in toughness of KOH treated bones is the result of structural alterations that enhance the ability of the microstructure to dissipate energy during the failure process, thereby slowing crack propagation. Fracture surface analysis has helped explain why KOH-treated bones have increased toughness compared to untreated bones, namely via toughening mechanisms on the compressive failure side.