AN ANALYSIS OF MEAN STRESS IN MULTIAXIAL RANDOM FATIGUE

Abstract

Abstract— Generally engineering components are subjected to multiaxial variable amplitude loading, which may be non‐proportional. One of the best known low cycle fatigue design codes that addresses non‐proportional loading problems is the ASME Boiler and Pressure Vessel Code, but this does not include a cycle counting method and it does not allow for the presence of a mean stress. Traditional rainflow methods, incorporating a strain range and a mean stress, are applicable only where there is a single load varying with respect to time, or possibly where multiaxial loads are proportional. So a multiaxial non‐proportional cycle counting method and a fatigue damage calculation procedure are proposed here, based on plastic deformation response and a critical plane hypothesis that incorporates the effect of mean stress. A cyclic deformation model, based on the Ramberg‐Osgood equation and a multiaxial memory rule, is used to calculate the mean stress response under variable amplitude loading.The proposed procedure is assessed with combined tension/torsion tests on En15R steel under variable amplitude loading. Fatigue life predictions are compared for analysis with and without mean stress corrections, to assess the sensitivity to mean stresses of non‐proportional fatigue endurance in the low cycle regime.