Plasticity retards the formation of creases

When an elastomer is compressed, its surface forms creases at a critical strain about 0.35. When a plastically deformable material is compressed, however, its surface remains smooth at much larger strains. As the smooth surface folds locally into a crease, the material around the crease loads and unloads. We show that the hysteresis of plasticity retards the formation of the crease. For a crease growing in an infinite body, the stress field is self-similar as the crease grows, and the critical strain e^c is the applied strain needed to maintain the self-similar growth. We simulate the formation of creases using an elastic-plastic model with linear hardening, and characterize the degree of plasticity of the model by the ratio of the tangent modulus to elastic modulus, E_t/E. A small value of E_t/E leads to a large critical strain for the onset of creases. We further show experimentally that creases can form at a strain of 0.49 for high-density polyethylene (HDPE) with E_t/E ∼ 0.025, but cannot form for metals (aluminum, copper, and stainless steel) with E_t/E ∼ 0.001.