Effect of Pre-Straining on High Cycle Fatigue and Fatigue Crack Propagation Behaviors of Precipitation Hardened Steel

Steel sheet is often shaped by straining to form a final product, and deformation process can influence the fatigue resistance. In this study, high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of precipitation hardened (PH) high strength steel with pre-straining levels of 0, 5 and 9% were investigated. Regardless of pre-straining level, fatigue cracks were found to be initiated at surface pits formed during pickling. The effect of pre-straining was not significant on the HCF resistance of PH steel, while the FCP rates increased with increasing level of pre-straining only in low and intermediate ΔK regimes. It was suggested that marginal increase on HCF resistance of PH steel was due to the combined effect of enhanced resistance to crack initiation and reduced resistance to crack propagation with pre-straining. The effect of pre-straining on HCF and FCP behavior of PH steel was discussed based on micrographic and fractographic observations.