Damage-tolerance in structural engineering and biological materials

The attainment of strength with toughness is a requirement for most structural materials; unfortunately, these properties are often mutually exclusive. Although the scientific quest continues for stronger materials, these materials have little utility as bulk structural materials without appropriate fracture resistance. Accordingly, developing damage-tolerant materials has traditionally been an exercise in compromise between hardness vs. ductility. Drawing examples from advanced metallic alloys, natural and biological materials, and structural and biomimetic ceramics, we examine here some of the newer strategies in dealing with this “conflict”. We focus on the interplay between the salient mechanisms that individually contribute to strength and toughness, that of plasticity and crack-tip shielding, noting that these phenomena can originate from very different length-scales (lessons from Nature are particularly relevant here). Furthermore, by focusing on mechanisms that permit the stable growth of cracks, and not simply on crack initiation, we show how these new and natural materials can defeat the strength vs. toughness conflict to achieve unprecedented levels of damage-tolerance.