Multiaxial fatigue evaluation using discriminating strain paths

Fatigue life and available cycle counting methodologies based on the critical plane approach are examined under discriminating axial-torsion strain paths with random and incremental changes in straining direction. Fatigue lives for quenched and tempered 1050 steel with no non-proportional hardening were found to be more sensitive to non-proportionality of loadings as compared to 304L stainless steel with significant non-proportional hardening. Proportional or in-phase axial-torsion cycles with different axial to shear strain ratios within an equivalent strain circle when applied in a random sequence resulted in significant additional hardening for 304L stainless steel, similar to the non-proportional cyclic hardening observed in 90 out-of-phase loading. In contrast, when such cycles are applied with a gradual increment of the axial to shear strain ratio, the stress response of 304L stainless steel is closer to that observed for in-phase loading. However, the sequence of loading did not significantly affect fatigue life for either material. Experimentally observed failure planes for all strain paths were in very good agreements with predicted failure planes based on the Fatemi-Socie critical plane parameter. Finally, fatigue lives for both materials under various strain paths were predicted satisfactorily employing Fatemi-Socie parameter, Palmgren-Miner linear damage rule, and either Bannantine-Socie or Wang-Brown cycle counting method. (c) 2010 Elsevier Ltd. All rights reserved.