Fatigue life prediction method for AISI 316 stainless steel under variable-amplitude loading considering low-amplitude loading below the endurance limit in the ultrahigh cycle regime

To investigate the effect of cycle ratio of combined high- and low-amplitude loading corresponding to loading above and below the fatigue limit on fatigue life (cumulative fatigue damage), fatigue tests were conducted under repeated two-step amplitude loading condition with varying cycle ratios (the number of high amplitude cycles was kept at 10 cycles, the number of low amplitude loading cycle was set at four different values 10³, 10⁴, 5 × 10⁴, and 10/10⁵ cycles in one block) using AISI 316, a type of commercial austenitic stainless steel. The test results indicate that low-amplitude loading following high-amplitude loading contributed to the fatigue damage and affected fatigue life. Furthermore, both the EN (European) and KHK (The high pressure gas safety institute of Japan) standards, which describe predictive approaches for variable-amplitude loading conditions that consider stress levels below the fatigue limit in an S–N curve based on the oblique hyperbola model, gave inaccurate evaluation of fatigue life because of the effect of cycles of low-amplitude loading below the fatigue limit in one block. Fatigue damage at saturation in one block contributed by low-amplitude was proved. A reasonable predictive approach that considering the number of cycles of low-amplitude loading in one block using linear cumulative fatigue damage rule has been established based on repeated two-step amplitude loading test results, and its precision was confirmed through random-amplitude loading tests.