Load sequence effects in the high cyde fatigue of two ferrite-pearlite microalloyed steels

Abstract

The high cycle fatigue (HCF) behaviour and load sequence effects in HCF of two microalloyed steels are presented. Dislocation structures that resulted from HCF loading were examined. The fatigue limits of the two microalloyed (MA) steels (non-failure after 2 × 10⁶ cycles) correlated well with the cyclic yield strength. The results of two-step loading showed that in both the steels after an initial life fraction of approximately 0.4, the residual life deviated significantly from that predicted by the Palmgren-Miner rule. Crack initiation lives of the MA steels had been determined from the two-step loading experiments. Fatigue functions, which can be used for life prediction under complex loading conditions using the consecutive Wöhler curve approach, were determined from the two-step loading tests. Dislocation cell structures were observed near the fracture surface in the HCF regime. At the fatigue limit, dislocation dipoles and loops were seen.