Modeling of the competition between shear yielding and crazing in glassy polymers

Fracture in amorphous glassy polymers involves two mechanisms of localized deformations: shear yielding and crazing. We here investigate the competiti on between these two mechanisms and its consequence on the material's fract ure toughness. The mechanical response of the homogeneous glassy polymer is described by a constitutive law that accounts for its characteristic softe ning upon yielding and the subsequent progressive orientational strain hard ening. The small scale yielding, boundary layer approach is adopted to mode l the local finite-deformation process in front of a mode I crack. The conc ept of cohesive surfaces is used to represent crazes and the traction-separ ation law incorporates craze initiation, widening and breakdown leading to the creation of a microcrack. Depending on the craze initiation sensitivity of the material, crazing nucleates at the crack tip during the elastic reg ime or ahead of the crack. As the crazes extend, plasticity develops until an unstable crack propagation takes place when craze fibrils start to break down. Thus, the critical width of a craze appears to be a key feature in t he toughness of glassy polymers, Moreover, the opening rate of the craze go verns the competition between shear yielding and brittle failure by crazing . (C) 2000 Elsevier Science Ltd. All rights reserved.