Digital Repository, ECF14, Cracow 2002

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Fatigue Life under Variable Amplitude Loading in a High Strength Steel Showing Step-Wise S-N Curve

Last modified: 2013-02-10


Fatigue tests under variable amplitude loading were conducted on a highcarbon chromium steel, JIS SUJ2. Tests were performed using cantilever-type rotatingbending fatigue testing machines in laboratory air. Load sequences employed were twostepblock loadings (high→low and low→high) and two-step multiple block loadings(high↔low). Fatigue damages under variable amplitude loading were estimated by thePalmgren-Miner rule. A step-wise S-N curve was obtained under constant amplitudeloading, in which the conventional fatigue limit was about 1275MPa. At stress levels aboveand below the fatigue limit, the fracture modes were different, i.e. surface-related crackinitiation and subsurface crack initiation, respectively. The linear cumulative damage lawcould evaluate the damages under two-step variable amplitude loadings where the fracturemodes were the same, i.e. for subsurface crack initiation. On the other hand, when thefracture mode was different, the cumulative damages, Σ(n/N), became much larger thanunity. Additional fatigue tests were performed using shot-peened specimens to establish thetime to subsurface crack nucleation. It was found that the subsurface crack nucleation tookplace at the early stage of fatigue life.

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