The Influence of Loading Variables on Environment-Enhanced Fatigue Crack Growth in High Strength Steels

An experimental program was carried out to further determine the range of applicability of the superposition model, proposed by Wei and Landes, for estimating the effects of cyclic loading variables and chemical environment on fatigue crack growth. AISI 4340 steel, tempered at 200°F and 500°F, and RQ360A steel were used in this investigation. The influences of frequency, stress ratio, and cyclic-load waveform were examined. The results showed that the superposition model provides correct estimates of the trend and the order of magnitude for the influences of these variables within the applicable range (that is, for Kmax> KIsee) provided that steady-state crack growth data are used. Data on the AISI 4340 steel suggest the presence of some synergistic effect of fatigue and environmental attack. Modification of this model will be needed to incorporate this effect and to improve the accuracy of predictions.Environment can affect fatigue-crack growth at Klevels below KIsee. Data obtained on the RQ360A steel (below KIsee) indicate that the environmental effect can depend on frequency and cycli-cload waveform. These results are in agreement with published results on highly alloyed steels. Further basic studies will be needed to clarify these effects and to develop viable prediction procedures.Nonsteady-state crack growth in fatigue was observed. This stage of crack growth would occur at the start of testing and following test interruptions. This phenomenon must be recognized, and be taken into consideration in data acquisition and subsequent utilization.