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

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 = ε_y + 1.2λε_x, where ε_x and ε_y are the local strains at the specimen axis, and λ is the strain range ratio Δε_y/Δε_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 X Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.