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Nacre, the iridescent material in seashells, is one of many natural materials employing hierarchical structures to achieve high strength and toughness from relatively weak constituents. Incorporating these structures into composites is appealing as conventional engineering materials often sacrifice strength to improve toughness. Researchers hypothesize that nacre's toughness originates within its brick-and-mortar-like microstructure. Under loading, bricks slide relative to each other, propagating inelastic deformation over millimeter length scales. This leads to orders-of-magnitude increase in toughness. Here, we use in situ atomic force microscopy fracture experiments and digital image correlation to quantitatively prove that brick morphology (waviness) leads to transverse dilation and subsequent interfacial hardening during sliding, a previously hypothesized dominant toughening mechanism in nacre. By replicating this mechanism in a scaled-up model synthetic material, we find that it indeed leads to major improvements in energy dissipation. Ultimately, lessons from this investigation may be key to realizing the immense potential of widely pursued nanocomposites.

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Image processing techniques provide access to full field measurements of different thermomechanical data (strain; strain-rate, Wattrisse et al., J Exp Mech, 41:29-38, 2001; temperature, Chrysochoos and Louche, Int J Eng Sci 38:1759-1788, 2000. These techniques have become increasingly useful for obtaining fine and local descriptions of material properties. As they can measure complete thermal and mechanical fields, they can be used to identify several parameters of constitutive equations during a single deformation process using specifically designed heterogeneous tests (Grédiac, Composites: Part A 35:751-761, 2004). In Geymonat and Pagano (Meccanica 38:535-545, 2003), surface strain fields obtained by digital image correlation were used to identify the distribution of elastic parameters and stress fields by minimizing a given energy functional. In this paper, the previous method is improved through a relevant choice for stress approximation, and then extended to a wider class of elastoplastic materials. Its reliability is then checked through applications on simulated data obtained under small perturbation and plane stress assumptions. In particular, the robustness of the method with respect to measurement noise is studied on the basis of numerical data. An experimental application to heterogeneous material identification is finally proposed.