Fatigue Under Severe Nonproportional Loading

In order to extend our understanding to more generalized and practical cyclic loading conditions, severe nonproportional loading has been studied. Tension-torsion, low-cycle fatigue tests were performed on 1Cr-Mo-V steel at room temperature by applying intermittent axial half cycles. Tests with different loading paths but the same amplitudes of maximum shear strain and normal strain across the maximum shear plane were conducted. Different fatigue lives resulted from different loading paths even when amplitudes were held constant. Additional tests showed that the elimination of the compressive half cycles did not change fatigue life which suggest that the compressive half cycles were nondamaging. The principal implication of the work is that successful methods for predicting nonproportional loading must be selected from models which can show loading path sensitivity. Several models with this feature are discussed, and a procedure for predicting endurance from continuous cycling tests is proposed.