Craze initiation in glassy polymers

In this work a method is presented that can be used to predict craze initiation in glassy polymers. The approach is based on the view that the development of a craze is preceded by de formation of a localised plastic deformation zone. As this zone develops, the hydrostatic stress increases, and, when exceeding a critical stress level, cavitation will take place, leading to local development of voids. The initiation of a localised plastic zone is numerically simulated using a constitutive model that incorporates an accurate description of the post-yield behaviour with the important phenomena of strain softening and strain hardening. Subsequent cavitation of the deformation zone is detected using a hydrostatic stress criterion. This criterion is identified and validated by confronting numerical simulations to experimental results of indentation experiments on polystyrene. A micro-indenter with a sapphire sphere of 150μm radius is used to produce indents that are later examined with an optical microscope. These observations lead to a critical force where crazes are initiated in polystyrene (PS). Combination of these experiments with a numerical study using the numerical model shows that the loading part of the indentation can be accurately predicted. A critical hydrostatic stress of 39MPa is extracted from the numerical model by analysis of the local stress field at the moment the indentation force reaches the experimentally determined force level at which crazes were found to initiate. This criterion is validated by application of the model to indentations on samples with different thermal histories, and at various loading rates. The influence of network on the value of the hydrostatic stress criterion is investigated by indentation of blends of polystyrene and poly(2,6-dimethyl-1,4phenylene-oxide). It is shown that the critical hydrostatic stress increases with network density.