Fatigue crack growth tests were carried out under constant amplitude, repeated two-step and random loadings by using the test specimens which have a longitudinal residual stress due to water cooling from a high temperature. Crack length and crack closure were measured by using the minicomputer-aided unloading elastic compliance method.
The crack opening stress intensity, Kop, under varying loadings was found to be controlled by the maximum stress intensity range-pair, (ΔrpK)max, and its actual stress ratio, R', which is defined as R'={(K)min+Kr}/{(K)max+Kr}, where (K)max and (K)min are the maximum and minimum applied peak stress intensity in the varying loading respectively, and Kr is the stress intensity factor induced by the residual stress. And also, Kop was found to coincide with the constant amplitude test results having the identical stress intensity range ΔK and stress ratio R with (ΔrpK)max and R'.
The linear accumulation law of crack growth in terms of the effective stress intensity rangepair, ΔrpKeff, was confirmed to give a good estimation of the fatigue crack growth rate under varying loadings even in the residual stress field.
