Notch deformation and stress gradient effects in multiaxial fatigue

This paper investigates notch mechanics under multiaxial fatigue loading conditions with respect to notch stress-strain estimation rules and stress gradient effects. A pseudo stress-based plasticity modeling technique, incorporating a structural yield surface concept, was applied to predict local stress distributions for a 2024-T3 aluminum alloy notched tubular specimen, as well as for two different AISI 1141 steel alloy notched shaft specimen geometries. Stress-strain predictions were generated for several nominal loading conditions and compared to nonlinear FEA solutions. Most predictions were found to be within ±15% error for loading levels relevant to typical fatigue loading applications. To evaluate stress gradient models, fatigue life predictions were performed for each specimen geometry using Neuber's rule with fatigue notch factor, as well as several different interpretations of Theory of Critical Distances (TCD) approaches. The effect of critical distance value on life predictions was also studied. The Fatemi-Socie critical plane damage parameter was used to calculate fatigue lives. While the TCD approaches were found to provide improved multiaxial fatigue data correlation when compared to fatigue notch factor, negligible differences between the different TCD approaches were observed. Life prediction trends were found to be consistent regardless of notch geometry and material, thus suggesting some generality of the findings.