Study on Fatigue Crack Propagation under Random Loading (1st Report)

Abstract

The fatigue crack propagation tests under K-controlled program loading and under random loading are executed on two kinds of steels and an aluminum alloy in order to establish the crack propagation life estimation method for actual structures, machines and so on. As for the K-controlled program test, transitional behavior of crack propagation after the acceleration or retardation was observed in order to improve the crack propagation model proposed previously by the authors. Subsequently, the quasi-random loading tests where the stress ratio is constant and the amplitude varies randomly every ΔN, cycles were performed to investigate the effects of stress distribution, ΔN, and truncate level of load on three kinds of materials. As a result, it was made clear that the fatigue crack propagation life under the random loading is roughly calculated by means of the linear estimation method using the constant amplitude test results. In addition, when the life estimation is made by means of either crack propagation model which takes into consideration the crack closure behavior or acceleration-retardation model obtained by the K-controlled tests, the estimated lives are more accurate than those by linear estimation method.