Fatigue life prediction of type 304 stainless steel under sequential biaxial loading

Low-cycle fatigue tests are conducted on type 304 stainless steel with the loading sequence of: axial/torsional, torsional/axial, in-phase/90°-out-of-phase, and 90°-out-of-phase/in-phase. The results show that cross hardening occurs in axial/torsional loading but not in torsional/axial loading. Much additional hardening is observed under out-of-phase loading. Prior loading in sequential in-phase/out-of-phase loading (or vice versa) produces a small degree of additional hardening in stress response in the latter loading compared with pure in-phase or out-of-phase loading. The (n1/N1, n2/N2) data points fall below the linear damage line in all cases. The double-linear damage rule, the damage curve approach and the plastic work model are found to give nonconservative predictions. A modified damage model is proposed, which accommodates nonproportional loading effects in sequential loading. Fatigue lives predicted based on the new model are within a factor-2 scatter band for all test conditions.