Detecting Overload from Strain Information during Fatigue Crack Propagation under Negative Stress Ratio

摘要

In the present study, a detection method of an overload application during stress cycles under constant amplitude was investigated. Also, the effect of the tensile overload was shown at three Stress ratios: R = 0, -1, and -1.5, to understand the effects of R on crack propagation after an overload. At the baseline of R = 0, after the overload, retardation in the crack propagation was observed, and the crack growth rate decreased. However, in the case of R = -1.5, the fatigue crack growth rate actually accelerated after the tensile overload. The detection of that crack propagation behavior was attempted through the information of the strain waveform h; h = epsilon_y + 1.2 lambda epsilon_x, where epsilon_x and epsilon_y are the local strains at the specimen axis, and lambda is the strain range ratio DELTA epsilon_y/DELTA epsilon_x. The waveform shape of h was changed after the overloading. Also, the application of the overload could be detected by the variation of the strain range ratio lambda. Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.