High cycle fatigue strength of 2.25Cr-1Mo steel under repeated two-step variable amplitude loading

Ultrasonic fatigue tests were conducted for medium strength steel, 2.25Cr-1Mo, under constant amplitude loading and repeated two-step variable amplitude loading conditions to evaluate the effect of loading conditions on fracture behavior. Most of the specimens were broken from an inclusion on the specimen surface. Cumulative damage value, D, calculated by Miner's rule, which ignored the damage by cyclic loading below fatigue limit, exhibited inappropriate prediction of fatigue life. The D values calculated by modified Miner's rule remarkably varied by the conditions of the repeated two-step variable amplitude loading. In some cases, specimen was not broken until D became larger than 60. Observations at fracture origin using replication technique revealed that stress amplitude below fatigue limit increased the resistance of fatigue crack initiation and/or growth. Mechanisms of these behaviors were discussed in detail.