Study on the Estimation Method for Welding Fatigue Mean Stress Effect Independently from Failure Mode and Joint Type

Abstract

Bending fatigue tests with stress ratio R = -1 and R = 0 of as welded (AW) and stress relieved (SR) root-failed plug weld (PW) specimens and toe-failed out-of-plane gusset weld (GW) and T-joint (TJ) specimens are performed. The welding residual stress (WRS) at the root notch and the weld toe of AW and SR specimens are measured by XRD method. The mean stress effect of the fatigue test results are analyzed by using modified MIL-HDBK-5D equivalent stress σ_eq based on hot spot stress (HSS) and 'root local stress (RLS)' determined by two-points stress extrapolation. It is found that all fatigue test results can be approximated by a single S-N curve regardless of failure mode, joint type, stress ratio of the external load and SR treatment when RLS can be determined so that the root-failed AW fatigue test results can be approximated by HSS S-N curve. As an alternative means for the root-failed cases where HSS-based σ_eq is inapplicable, 'EPLSC method', in which σ_eq is determined from the elastic-plastic local stress cycle (EPLSC) calculated by welding and cyclic loading thermal elastic plastic FE analyses with very fine FE mesh, is proposed. The fatigue test results of AW PW and GW specimens with various external stress ratio can be approximated by a single S-N curve when σ_eq is determined by EPLSC method. It can be considered that the proposed EPLSC method is effective in a large number of root-failure cases.