RETARDATION AND ARREST OF FATIGUE CRACK GROWTH IN AISI 4340 STEEL BY INTRODUCING REST PERIODS AND OVERLOADS

— The aim of this study is to investigate the effects of an intermittent rest period at 300°C, an overload in tension, and the combination of an overload and a subsequent rest period, on fatigue crack growth in AISI 4340 steel. The intermittent rest period was found to stop fatigue crack growth completely near the threshold level of ΔK. The alleviation effect of a rest period on crack growth was more distinct at lower levels of ΔK. With overload, the higher overload ratio caused a greater effect on crack growth rate. The reduced stress intensity factor caused by crack branching and an enhanced roughness of crack surfaces probably contribute to the retardation and arrest of fatigue crack growth. The most distinct retardation of fatigue crack growth was found after the combined treatment of an overload and a subsequent rest period. Compressive residual stresses following an overload and strain-age hardening during the rest period at 300°C are suggested as assisting the arrest or retardation of fatigue crack propagation.