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 Kic (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 Kic at instability for the material was shown to be 1.05 MN/m ~. A curve showing the relationship between crack speed and crack toughness Kc was also obtained and a lower value of Kc for crack initiation was extrapolated as being 0.78 MN/m ~.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.

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The rapid increase in the rate of application of thermoplastics in engineering design problems and the interest in the structural use of these materials have resulted in the requirement of comprehensive information about the behaviour of thermoplastics when subjected to cyclic loading conditions. In addition to the "total fatigue life" data already available for many materials, attempts have been made to analyse the crack initiation and steady crack growth processes and determine the efiects of parameters such as mean load, frequency and crack geometry on the rate of crack propagation. The results of an investigation of these aspects of fatigue crack growth in a brittle thermoplastic, polymethylmethacrylate (PMMA), have alreadv been reported. In this paper, the results of a test program devised to study the behaviour, at room temperature and in air, of a polycarbonate, (PC), under similar loading conditions, are presented. Fracture Mechanics concepts have been used to analyse the results. It was found that a relationship of the form a~ = An already shown to predict the cyclic fatigue crack propagation rate in PMMA, is also applicable to polycarbonate. However, when the effects of frequency and loading rate were studied, it was found that after the magnitude of parameter k( = AK/half the periodic time) exceeded 4000 lbf in.-3/2 s-l, the influence of the mean level of stress intensity factor, K,, became negligible in comparison to the effect of AK.

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