Nano-scale mechanisms of bone toughening

Abstract

In this dissertation, a new three-dimensional bone ultrastructure finite element model is constructed to investigate the formation of dilatational bands at the nanoscale. The stochastic organization of bone mineral phase at mineralized collagen fibrils scale is considered in this model. Under tensile deformation, the local confinement from the extra-fibrillar mineral structure leads to large hydrostatic stress and stress fluctuation. The tensile hydrostatic stress at organic-inorganic interfaces causes the denaturation of non-collagenous proteins and the corresponding energy dissipation. Therefore, the formation of dilatational bands is claimed as a stress-induced protein denaturation mechanism. Also, the occurrence conditions for such interface protein denaturation process are discussed. The intrinsic toughening effect from this nanoscale energy dissipation mechanism is quantified to be on the order of 20%. Our model provides a new fundamental understanding of the intrinsic toughening mechanism of dilatational bands formation and its contribution to macroscopic bone toughness.

In the final track of the dissertation, we demonstrate the presence of eigenstress associate with nanoscale bone mineral phase. Firstly, X-ray spectroscopy and chemical analysis are used to substantiate the presence of eigenstress and OCP-citrates in bone. We proposed the eigenstress is associated with the lattice misfit between the bone mineral crystal and the epitaxial OCP-citrate layer. The fact that this eigenstress can be disrupted upon deproteinization and plastic deformation in the vicinity of a crack tip led to further investigations on the toughness mechanism from bone mineral phase.

Also, bone, as well as other composites with elastic-plastic damage regions, exhibits an R-curve toughening behavior due to extrinsic toughening mechanisms. In the extrinsic toughening mechanisms, the eigenstrains or eigenstress formed in the wake zone load back on the main crack tip leading to toughening. In the dissertation, we investigate the extrinsic toughening mechanism originated from nanoscale dilatational bands. The eigenstrain toughening effect increases linearly with the volume fraction of the toughening region. Also, the effect of bone elastic heterogeneity on such eigenstrain toughening is studied. It is observed that the elastic heterogeneity amplifies the extrinsic eigenstrain toughening effect.

Bone is a unique biomaterial with superior fracture toughness properties. The toughness of bone emerges from hierarchical structure and the interaction of multiscale toughening mechanisms. Recently, the formation of dilatational bands was suggested as a nanoscale energy dissipation processes in bone. However, a detailed mechanistic understanding of this nanoscale toughening mechanism and the toughening effect across multiple scales is lacking.