Hierarchical structures at multiple length scales are characteristic in a class of natural (e.g., bone) and synthetic (e.g., nano-composites) materials that are quasi-brittle in nature but allow for appreciable plastic deformation (1, 2). Since the bulk mechanical properties of the materials are heavily dependent on their nano/microscopic structures, micro/nano mechanics approaches are often required to study their behaviors. However, lack of an effective means to exemplify the post-yield and failure behavior directly at micro/nanometer scales has significantly precluded understanding the constitutive relationship of these materials. A compelling example is that such paucity has significantly hindered establishment of physically sound constitutive relationships for bone tissues. Recent progresses in nanotechnology have allowed for estimation of the stiffness and hardness of bone tissues at submicron and nano length scales (3–5). However, no methods are currently available to assess the post-yield and failure behavior of bone tissues at nano/microscopic levels. Our pilot study (6) has shown that nanoscratch tests could be used in assessing the in situ energy dissipation during the post-yield deformation of bone tissues. To this end, the objective of the present study is to establish a mechanistic model for the nanoscratch methodology based on a assumption that the in situ toughness of bone tissues or the capacity to dissipate energy until failure can be estimated based on the removal energy of the tissue consumed during a nanoscratch test.

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