Microtensile failure mechanisms in lamellar bone: Influence of fibrillar orientation, specimen size and hydration

A mechanistic understanding of bone fracture is indispensable for developing improved fracture risk assessment in clinics. Since bone is a hierarchically structured material, gaining such knowledge requires analysis at multiple length scales. Here, the tensile response of cortical bone is characterized at the lamellar length scale under dry and hydrated conditions with the aim of investigating the influence of bone's microstructure and hydration on its microscale strength and toughness. For individual lamellae, bone strength strongly correlates with the underlying mineralized collagen fibrils orientation and shows a 2.3-fold increase compared to the macroscale. When specimen size is increased to a few lamellae, the influence of fibril orientation and the size effect on strength are significantly reduced. These findings highlight the critical influence of defects, such as canaliculi and interlamellar interfaces, when assessing larger volumes. Hydration leads up to a 3-fold strength decrease but activates several toughening mechanisms enabling inelastic deformation. In axial specimens, toughening is seen through fibril bridging and crack kinking. In transverse specimens, water presence leads to a progressive but stable crack growth parallel to the fibril orientation, suggesting crack-tip plasticity at the fibrillar interfaces. This work offers a better understanding of the role of interfaces, porosity, and hydration in crack initiation under tensile loading, which is a crucial step towards improved clinical management of disease-related bone fractures through multiscale modeling approaches.