Bone plays a crucial role in providing the structural support, protection, and locomotion to our body. Fundamental understanding of its biomechanical functions is essential as it promises a two-fold benefit;​ first, addressing the bone fragility issues affecting a large portion of the society, and second, deriving inspiration for achieving advanced bio-inspired materials with enhanced properties. To achieve these goals, in this work first a computational model was proposed to account for the detailed ultrastructure of bone consisting of mineralized collagen fibrils, extrafibrillar matrix, non-collagenous proteins and interfacial water. Such a model is necessary to explain the ultrastructural origins of bone fragility, often manifested as deterioration of bone quality at the material level in addition to the negative changes occurring at the structural levels, such as bone loss or morphological remodeling. The cohesive finite element model of the sub-lamellar bone was analyzed in tensile and compressive loading modes. The contribution of each microstructural features in accomplishing the exceptional load-bearing capacity of bone was discussed in details by performing validated numerical experiments. The deformation/toughening mechanisms along with the failure modes were demonstrated to successfully explain the mechanics of bone at its nanoscale hierarchy (Essay I). In the next step (Essay II), the model was used to investigate the influence of nanoscale water in altering the mechanical response of the tissue such as brittleness. The finite element simulation results provided a mechanistic understanding of the hydration status induced changes, at the interface of mineral collagen phases, to the mechanical response of ultrastructural bone.

The second objective of this study was accomplished by mimicking the design motifs of bone ultrastructure to reveal how following this nature evolved material can lead to a material with improved toughness compared to conventional nanocomposite designs (Essay III). The toughness enhancement was attributed to the prevalent energy-dissipating damage occurring in the thin layer of organic non-collagenous materials binding the extrafibrillar mineral nanograins. Focusing further on the extrafibrillar matrix building block of the bone (Essay IV), a computational model of a three-dimensional hybrid organic-inorganic nanocomposite was analyzed using the cohesive finite element approach. It was demonstrated that although the organic adhesive accounts for only small volume fraction of the bulk nanocomposite, it can still play a pivotal role in tuning the mechanical properties of the hybrid nanocomposite. Overall, this study shed some light on the mechanics of bone ultrastructure and its capacity to help design new better materials.