Polymers Resist Fatigue Crack Growth by Deconcentrating Stress

When a material is cyclically loaded, an amplitude of load exists, called the threshold, below which a crack does not grow. In a polymeric material, physical interactions between polymer chains are much weaker than covalent bonds between repeat units along an individual chain. Consequently, when a crack impinges on a chain, high tension transmits along a long length of the chain. Breaking a single covalent bond dissipates the energy stored in that long length. The longer the length over which high tension transmits, the higher the threshold. Here we review how stress deconcentrates in diverse polymeric materials, including polymer networks, particle-reinforced elastomers, glassy polymers, semicrystalline polymers, phase-separated polymers, and composites. Ample opportunities exist for investigation and innovation.