Investigation of nanoscale failure behaviour of cortical bone under stress by AFM.

The contribution of nanostructures of bone to the macroscale mechanical properties has received much attention, but most of nano-toughening mechanisms have remained in the theoretical stage or at static experimental observation. Our study shows that the medullary surface of the bovine femur provides a smooth natural surface ideal for observing nanostructures in bone. Mechanical loading is applied using an in situ mechanical device and the nanomechanical behaviours of the specimens are in situ recorded and imaged using an atomic force microscope (AFM). By the in situ observation of nanomechanical behaviours under stress, the existing nano-toughening mechanisms, such as fibril slippage and fibril bridging, are confirmed. Before the micro failure stage, mineralized collagen fibrils are strained with the increase of stress, followed by pre-separation (or slippage) due to stress concentration, resulting in cracked nanoscale interfaces. When micro-failure occurs (i.e. crack initiation), the nano-bridging mechanism contributes to resisting the formation of nanometre crack interface, the propagation of crack tip and the failure of crack bridging. Our study provides direct evidence for the connection between bridging-type mechanisms at different scale, which are composed of the corresponding bone structures at each level. Through the in situ observation of the microscopic failure in bone, some visual information are offered on the interaction between nanomechanical behaviours and nanostructures.