A number of factors across multiple length scales (Table 1) may contribute to the mechanical competence of bone (i.e., the resistance to fracture). This structural complexity hinders a complete mechanistic explanation (model) for elderly bone fragility. Although age-related loss of bone mass has been viewed as the reason for the rising incidence of bone fractures in the elderly,(2) bone quality is more meaningful to provide insights into why individuals with ‘normal’ bone mineral density (BMD) suffer fractures. Bone is a hierarchical and hydrated composite of mineral and collagen phases,(3, 4) showing a rather complex response to the mechanical loading.(5, 6) While previous research has investigated the resistance of bone to crack propagation,(7, 8) fatigue behavior,(9–11) and mechanical changes to load-induced damage,(12–14) little is known about the mechanical behavior of bone as a function of progressive deformation (strain). In this study, by employing a previously developed progressive loading scheme,(15) we intend to investigate the progressive stress-strain behavior of bone with increasing strain levels, so as to gain more information on the underlying mechanisms for post-yield behavior of bone. It is hypothesized that the post-yield behavior of bone varies with the increasing strain, which plays a significant role in sustaining the toughness or quality of bone.

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