Fracture phenomena in polystyrene

Abstract

Fracture mechanics concepts, in terms of stress intensity factors, have been used in a study of the growth of cracks and crazes in Crystal Polystyrene in air at 293°K. Single edge notch tension specimens and tapered cleavage specimens have been tested over a wide range of strain rates and also at constant load. It was initially found that the critical stress intensity factor K Ic (evaluated at crack instability) which should be a constant, independent of test method and specimen geometry, could apparently vary over a wide range. This phenomenon was shown to be caused by the presence of craze bunches at the crack tips; the size of these bunches being dependent upon the method of notching employed. A notching technique producing pure cracks was devised and as a result the lower bound value of K Ic at instability for the material was shown to be 1.05 MN/m3/2. A curve showing the relationship between crack speed and crack toughness K c was also obtained and a lower value of K c for crack initiation was extrapolated as being 0.78 MN/m3/2. The role of inherent flaws in the fracture of unnotched tensile specimens and the consequences of crazing are discussed in terms of fracture stresses predicted from the notched tests using a Dugdale model.