Abstract
Authors have been reported in previous paper that the S-N characteristic of high carbon-chromium hearing steel shows the "Duplex S-N Curve" which consists of two different S-N curves corresponding to the fracture modes, such as surface and internal failure mode in wide stress amplitude range. It is important to clarify the damage mechanisms under varying amplitude load cycling for the metals having quite different fatigue characteristics. In this study, fatigue behavior under high-low and low-high two-step loading fatigue tests was investigated. From the experimental results, the Palmgren-Miner linear cumulative damage rule was applicable for a bilevel loading sequence of which failure mode is same in the S-N curve. However, in case that the failure mode is different in each step amplitude loading, the problem was not quite simple and the linear-cumulative damage rule was able to apply only after obtaining the fracture mode at the second step amplitude loading. And also, it was found from the two-step loading fatigue tests that the granular-bright-facet (BCF) around a nonmetallic inclusion forms at an early stage of fatigue process.
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