Knowledge of the structure-property-function relationships of dermal scales of armoured fish could enable pathways to improved bioinspired human body armour, and may provide clues to the evolutionary origins of mineralized tissues. Here, we present a multiscale experimental and computational approach that reveals the materials design principles present within individual ganoid scales from the 'living fossil' Polypterus senegalus. This fish belongs to the ancient family Polypteridae, which first appeared 96 million years ago during the Cretaceous period and still retains many of their characteristics. The mechanistic origins of penetration resistance (approximating a biting attack) were investigated and found to include the juxtaposition of multiple distinct reinforcing composite layers that each undergo their own unique deformation mechanisms, a unique spatial functional form of mechanical properties with regions of differing levels of gradation within and between material layers, and layers with an undetectable gradation, load-dependent effective material properties, circumferential surface cracking, orthogonal microcracking in laminated sublayers and geometrically corrugated junctions between layers.

The inner nacreous layer of gastropod mollusc Trochus niloticus is composed of ∼95 wt% planar arrays of polygonal aragonite-based tablets (∼8 μm wide, ∼0.9 μm thick, stacked ∼40 nm apart) and ∼5 wt% biomacromolecules. High-resolution tapping mode atomic force microscope images enabled nanoscale resolution of fractured tablet cross-sections, the organic component, and deformation of individual nanoasperities on top of tablet surfaces. Nanoindentation was performed on individual nacre tablets and the elastic modulus E and yield stress σy were reduced from elastic-plastic finite element simulations yielding E = 92 GPa, σy = 11 GPa (freshly cleaved samples) and E = 79 GPa, σy = 9 GPa (artificial seawater soaked samples). Images of the indents revealed extensive plastic deformation with a clear residual indent and surrounding pileup.

The nanoscale anisotropic elastic-plastic behavior of single-crystal aragonite is studied using nanoindentation and tapping mode atomic force microscopy imaging. Force-depth curves coaxial to the axis exhibited load plateaus indicative of dislocation nucleation events. Plasticity on distinct slip systems was evident in residual topographic impressions where four pileup lobes were present after indentation with a conospherical probe and distinct, protruding slip bands were present after indentation with a Berkovich pyramidal probe. A finite element crystal plasticity model revealed the governing roles of the {110}<001>slip system family, as well as the (100)[010], (100)[001], (010)[100], (010)[001], (001)[100] and (001)[010] systems.

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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 m icromechanical 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