Zheqi Chen, Zhigang Suo
A material tears, peels, and breaks by growing a crack. In a zone around the crack front, atoms undergo an irreversible process of breaking─and possibly reforming─bonds. Trailing behind the crack front are two layers of scars. Outside the irreversible zone and scars, atoms undergo the reversible process of elasticity. The irreversible zone is considered localized if it is small relative to the body. The idealization of localized irreversibility leads to a thermodynamic framework centered on the energy release rate. This crack driving force is defined using an ideal body in which a crack is stationary and deformation is elastic, and is applied to a real body in which a crack grows by an irreversible process. The irreversible zone scales with a material length: the fractocohesive length. We review recent advances in the development of crack-resistant elastomers and hydrogels as well as polymer networks reinforced by hard particles, fibers, or fabrics, subject to monotonic, cyclic, and static loading. Emphasis is placed on how molecular features, such as strand length, entanglements, noncovalent bonds, and chemical reactions, govern crack resistance. Design principles are highlighted that reconcile high toughness with low hysteresis through stress deconcentration. This review traces crack resistance to molecular origins, providing a foundation for designing next-generation crack-resistant materials.