Micromechanics and Macromechanics of the Tensile Deformation of Nacre

Many natural materials exhibit extraordinary combinations of mechanical properties which are achieved through highly tailored and organized hierarchical microstructures. In particular, materials which function as natural body armor, such as mollusk shells, possess a structure with important features and properties at a variety of length scales, from the various constituent building blocks to the overall integrated and synergistic mechanical behavior of their complex assemblies. In this study, the mechanical behavior of the inner "brickand-mortar" nacreous layer of mollusk shells was modeled by taking into account both the mechanical behavior of organic matrix and the geometrical arrangement of the mineral-rich tablets. The protein, Lustrin A, which is present in Haliotis rufescens (red abalone) nacre, has been shown to possess a modular structure consisting of ~10 domains linked in series. Axial force-extension experiments on the full organic matrix of this same species exhibits an irregular "saw-tooth" type profile, whereupon numerous load drops are found to occur over the course of large axial extension (Smith, et al., 1999). This nanomechanical behavior has been attributed to the sequential unfolding of Lustrin A subunits and their corresponding rupture of sacrificial bonds. The micromechanical model developed here incorporates a new finite deformation constitutive law that assumes sequential force-induced unfolding of the individual protein domains in the organic matrix, as well as the complex spatial organization of the organic and inorganic components. Numerical simulations of tensile extension of representative volume elements of nacre show that progressive unfolding of the modules in the organic matrix provide a macroscopic “softening” mechanism, thus mitigating load transfer to the aragonite tablets, as well as averting early failure of the adhesive layers. This softening mechanism also enables larger deformation of nacre without catastrophic failure, and therefore offers an effective avenue for energy dissipation. 2 H.J. Qi, B.J.F. Bruet, J.S. Palmer, C. Ortiz, M.C. Boyce