The Effect of Variable Amplitude Strain Conditions on Low Cycle Fatigue Strength of Stainless Steel SUS316L

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Low cycle fatigue tests were performed for austenitic stainless steel, JIS SUS316L, used for pressure vessel and piping of engineering plants. This study investigated the effect of variable amplitude strain conditions on the fatigue strength. Fatigue tests were conducted under two-step and repeated two-step variable amplitude strain conditions, in which the influence of straining history on the hardening and softening behavior of the material had an important role on the fatigue strength. The experimental results were compared with predictions of the fatigue lives based on the fracture mechanics approach in conjunction with the hardening and softening behavior. These comparisons were performed for a fatigue usage factor, UF, calculated by the linear damage accumulation law using the fatigue strength diagram obtained under constant amplitude straining tests. The UF value at fracture, UF<font size="-1">f</font>, reduced under the variable amplitude straining conditions depending on the straining history. Variations of stress under the strain controlled tests were measured in detail and used for the fatigue life predictions. The results indicate that the minimum value of 0.7 for the UF<font size="-1">f</font> gives conservative evaluation of the low-cycle fatigue life.