Multiaxial Low Cycle Fatigue Life Prediction Under Irregular Loadings

In this paper, a series of fatigue experiments are conducted on 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 experimental results of 304 stainless steel show that cross hardening occurs when loading mode change and significant additional hardening is observed under out-of-phase loading. The stress states under two phases are analyzed and it shows that the stress in the second phase loading increases with the increasing of the first phase loading amount under the at and io loading path. The damage is less than 1 for all loading paths with the linear damage rule. The fatigue life predictions obtained by the linear damage rule, double linear damage rule, damage curve approach and the plastic work model of Morrow are on the unconservative side. A modified damage model is proposed, which considers nonproportional loading and material property and the prediction results are acceptable. Fatigue tests are performed on tubular specimens under variable amplitude, irregular axial-torsional loading and these tests consist of the proportional loading and nonproportional loading. Six damage parameters are employed for correlating fatigue data, i.e. SWT, KBM, CXH, VF, FS and LKN. A life computation procedure is employed in which rainflow cycle counting on suitable plane for different models, and a linear damage rule and the plastic work model of Morrow are used to calculate the damage. The prediction results show that the Morrow model can't give the better results. Because of uncertainty of the loading path and mode under random multiaxial loading, a new critical plane is defined by considering the loading path change with the time based on the weight function of critical plane. For KBM model, the weight function-critical plane approach presents good fatigue life prediction under irregular loading. The approach is also used for 304 stainless steel under two phases loading and the prediction results are acceptable. The validity of the weight function-critical plane approach is confirmed. The critical plane under two phase loadings is significantly affected by the loading mode, the loading amplitude and the loading time.