Abstract
The effects of negative stress ratio and single compressive overload on the fatigue crack propagation rate were investigated in polyvinyl chloride (PVC), polymethyl methacrylate (PMMA) and polyamide (PA). The fatigue crack closure behavior and fracture surface appearance were also examined. The fatigue crack propagation rate, da/dN, was found to increase with decreasing stress ratio in the relationship of da/dN versus the maximum stress intensity factor, Kmax, for all polymers tested. This result implies that the compressive stress range in the cyclic loading causes the acceleration of fatigue crack propagation. The crack opening stress level decreased with decreasing stress ratio, and the fatigue crack propagation rate was found to be dominated by the effective stress intensity factor range, ΔKeff. These facts suggest that such an acceleration for these polymers can be explained on the basis of the plasticity-induced crack closure concept just as for metals. On the other hand, the fatigue crack propagation rate was not affected appreciably by single compressive overload in this experiment.
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