Overload-induced retardation of fatigue crack growth in polycarbonate

The effect of single overload within an otherwise constant amplitude loading sequence on the retardation of fatigue crack propagation (FCP) behavior of polycarbonate (PC) was experimentally investigated in this paper. An improved compliance method was employed to measure the length of a fatigue crack, and optical and scanning electron microscopes (SEM) were used to observe the features of crack surface and crack tip deformation. It was shown that there was a small crack increment during the application of overload. After that, the crack growth rate reduced to a low level quickly, and then increased gradually until it reached the normal level of crack growth rate. The waveform and overload ratio of the single overload had different effects on the FCP of PC, with the former having slight effect, and the latter larger effect. The overload-induced retardation of fatigue crack growth was mainly due to the formation of crack tip craze zone. The relationship between overload retardation ratio R_r and overload ratio R_o had been obtained in the form of R_r = 0.95 + 0.056 R_o^2.72.