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Hierarchical composite materials design in biological exoskeletons achieves penetration resistance through a variety of energy-dissipating mechanisms while simultaneously balancing the need for damage localization to avoid compromising the mechanical integrity of the entire structure and to maintain multi-hit capability. Here, we show that the shell of the bivalve Placuna placenta (~99 wt% calcite), which possesses the unique optical property of ~80% total transmission of visible light, simultaneously achieves penetration resistance and deformation localization via increasing energy dissipation density (0.290 ± 0.072 nJ µm-3) by approximately an order of magnitude relative to single-crystal geological calcite (0.034 ± 0.013 nJ µm-3). P. placenta, which is composed of a layered assembly of elongated diamond-shaped calcite crystals, undergoes pervasive nanoscale deformation twinning (width ~50 nm) surrounding the penetration zone, which catalyses a series of additional inelastic energy dissipating mechanisms such as interfacial and intracrystalline nanocracking, viscoplastic stretching of interfacial organic material, and nanograin formation and reorientation. [ABSTRACT FROM AUTHOR]

Nanomechanical heterogeneity is expected to influence elasticity, damage, fracture and remodelling of bone. Here, the spatial distribution of nanomechanical properties of bone is quantified at the length scale of individual collagen fibrils. Our results show elaborate patterns of stiffness ranging from ∼2 to 30 GPa, which do not correlate directly with topographical features and hence are attributed to underlying local structural and compositional variations. We propose a new energy-dissipation mechanism arising from nanomechanical heterogeneity, which offers a means for ductility enhancement, damage evolution and toughening. This hypothesis is supported by computational simulations that incorporate the nanoscale experimental results. These simulations predict that non-uniform inelastic deformation over larger areas and increased energy dissipation arising from nanoscale heterogeneity lead to markedly different biomechanical properties compared with a uniform material. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically inspired materials technologies. [ABSTRACT FROM AUTHOR]