Fracture can be considered as a mutual competition between intrinsic resistance (induced largely by plasticity) to damage processes that operate ahead of a crack tip to promote crack advance and extrinsic crack-tip shielding mechanisms that act at, or behind, the tip to locally diminish crack-tip stresses and strains. We examine here how this mechanistic interplay is utilized to derive damage-tolerance in natural materials, e.g., bone, teeth, skin, sea shells and fish scales, compared to engineering structural materials such as aerospace ceramic-matrix composites, nuclear graphite, and advanced metallic materials, such as metallic glasses and high-entropy alloys.
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