High Local Deformation Correlates with Optical Property Change in Cortical Bone

Abstract

Bone is a biocomposite of collagen and apatite crystals which together constitute a striking hierarchical organization, though it can still become structurally compromised when external load exceeds its threshold. Mechanisms of bone damage have been proposed on different length scales corresponding to its hierarchical structure. However, the damage process was still not completely understood due to the complexity of bone’s hierarchy. We previously reported an opaque process zone feature in bone under tensile loading, which could be stained only when samples were kept in loaded condition, in contrast to the classical damage, which could be stained after the removal of loading. In this study, Digital Image Correlation (DIC) methods have been used to quantify the local strain value at the micro-scale upon dark zone emergence. The process zones observed under transmission illumination was found to colocalize with the high-strain (up to 14%) regions calculated by DIC, and overlap best with the shape of principal strain (e1). The average strain value recorded at the edges of the process zones was about 1.1%, around the proposed threshold for collagen interfibrillar sliding. Thus, we speculate that collagen interfibrillar sliding might be among the causes for this dark zone phenomenon.