Growth of fatigue cracks in polycarbonate

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Abstract
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 effects 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 already 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 ȧN = β λn 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 K̇( = ΔK/half the periodic time) exceeded 4000 lbf in. −3/2 s−1, the influence of the mean level of stress intensity factor, Km, became negligible in comparison to the effect of ΔK.

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