Fatigue tests under variable amplitude loading were conducted on a high carbon chromium steel, JIS SUJ2. The aim in the present study is to clarify the damage mechanisms under variable amplitude loading for a high strength steel showing the transition of failure mode in long life regime. Tests were performed using cantilever-type rotating bending fatigue testing machines in laboratory air. Load sequences employed were two-step block loading (high→low and low→high) and two-step multiple block loading (high↔low). Fatigue damages were estimated by the Palmgren-Miner rule. When failure mode was the same at two stress levels employed, i. e. subsurface crack initiation, fatigue damages were evaluated properly by the Palmgren-Miner rule. Also when the cycle ratio at low stress level became large and multiple block loading was applied, the estimation of fatigue damages was made successfully. According to SEM analysis, it was found that the morphology of fish-eyes formed under variable amplitude loading was the same as that obtained under constant amplitude loading.
