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

Abstract

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 = εy + 1.2λx, where εx and εy are the local strains at the specimen axis, and λ is the strain range ratio Δεy/Δεx. 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 λ. Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.

Method of Detecting Unexpected Load Leading to Crack Growth Acceleration

“…Figure 3 shows the relationship between half-crack length a and number of stress cycle N. It is known that the crack growth rate accelerated when overload was applied with critical conditions [3]. In this study the acceleration and deceleration were obsereved depending on the conditions of overload or underload, in comparison with baseline data.…”

“…The background of this method is that the moving of the crack closure points related to the crack growth rate after the changing loading conditions. Also, another method for detecting the effects of loading conditions on crack growth behavior in a cracked material was shown in the previous study [3].…”

“…In the present study, the method of detecting whether a hazardous load was applied or not was examined by using the waveform of local strains. Just application of overload could be detected by the method of reference [3], too. However, the occurrence of the acceleration or deceleration of crack growth can be distinguished with strain waveform by the present method.…”

A Method for Detecting an Unexpected Application of a Hazardous Load During Operation

“…The crack initiation from a notch can be detected by using a strain gage during the operation [9,10]. That method was applied to detect the application of the overload during fatigue test with constant stress amplitude in the previous study [11]. This work was improved in the present study, and was understood that crack opening and closing behavior is strongly related to the acceleration of crack growth.…”

Effect of unloading on crack growth rate of Zr–2.5Nb tubes

“…A considerable decrease in the fatigue crack growth rate due to overload is suggested to occur due either to the crack closure [31,32] or to compressive stresses or strains [33,34] arising from unloading of the overload. However, the role of the crack closure or the compressive stress in the crack growth rate remains yet to be understood because of incomplete understanding of crack growth kinetics.…”