Abstract
The effect of a single peak overload on the fatigue crack propagation was investigated in polyvinyl chloride (PVC), polymethyl methacrylate (PMMA) and polyamide (PA). The results obtained are summarized as follows:(1) A single peak overload was found to cause the acceleration of crack propagation during overloading and the retardation after overloading in all materials used.(2) The acceleration factor of crack propagation due to the overload was higher than that reported in metals. Especially PMMA which is a brittle material showed a remarkable acceleration.(3) The maximum retardation occurred immediately after the overload and the so-called delayed retardation was not observed in all the cases. The maximum retardation rate was fairly high in ductile PVC and PA, but extremely low in brittle PMMA.(4) The application of a single peak overload showed a tendency to prolong the fatigue life in PVC and PA since the retardation effect was larger than the acceleration effect, but in PMMA the retardation effect was small and the life was shortened under a certain condition.(5) From the examinations of fracture surface and crack closure, it was inferred that the craze formed at the crack tip and the blunting or orientation hardening at the crack tip played an important role for the acceleration and retardation, respectively.
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