A study of fracture mechanisms in biological nano-composites via the virtual internal bond model

As organic–inorganic hybrid composites, natural biological materials exhibit superior mechanical properties such as toughness and strength in comparison with their constituent phases: the organic phase is usually very soft and the inorganic phase very brittle. Understanding the mechanisms by which nature designs strong composites with weak materials could provide helpful insight and guidance for the development and synthesis of novel materials for industrial applications. In our recent studies on nanostructure of biological materials (Proc. Natl. Acad. Sci. U.S.A. 100 (2003) 5597; Eng. Fract. Mech. 70 (2003) 1777), the nanometer size of mineral platelets is interpreted as a result of strength optimization, in that an intrinsically brittle material reaches the theoretical strength at the nanoscale despite of crack-like flaws. The mineral nanoparticles are embedded in a soft protein matrix, resulting in an organic–inorganic composite with high toughness and strength. The present study is focused on the effects of protein and protein–mineral interface. A Virtual-Internal-Bond (VIB) model, which incorporates an atomic cohesive force law into the constitutive model of materials, is adapted to model deformation and failure in the nanostructured biocomposite. We show that the protein layer can effectively enhance the toughness of biocomposites through crack shielding and impact protection.