Prediction of Welding Residual Stresses and Redistribution/Relaxation due to Cyclic Loading

A three dimensional gradient of residual stresses is inevitable for almost any welding process due to the thermal cycle. During subsequent cyclic loading, these residual stresses were expected to alter the fatigue life with trends similar to those observed for uniaxial experiments with mean stresses. However, plastic deformation during cyclic events can cause the residual stresses to redistribute and relax. Experiments were performed to determine the uniaxial fatigue performance of a low carbon steel transverse double-sided butt weld specimen. An automated laser speckle interferometry hole drilling system was utilized to determine both the initial residual stresses and those after a specified number of cyclic events. Finite element modeling was used to simulate the evolution of the residual stress during the welding thermal cycle and subsequent cyclic loading at room temperature. A user-defined subroutine was developed to include appropriate cyclic plasticity and time dependent creep mechanisms in the model. Excellent correlation between experimentally measured initial residual stress measurements and refined analytical modeling was demonstrated. During cyclic loading, the residual stresses were experimentally observed and analytically predicted to redistribute and relax nonlinearly with respect to applied nominal load and load ratio. A Findley critical plane damage parameter was employed to evaluate the interaction of a multidimensional residual stress field with uniaxial nominal loading after simple uniaxial theories were shown to be inadequate. By considering the amplitude and load ratio dependent stabilized residual stresses, the baseline weld data could be recast in terms of the Findley parameter, which collapsed the experimental constant amplitude data. The concepts developed allowed evaluation of a hypothetical high-low sequence, simulating an initial shakedown event, resulting in considerably longer fatigue lives for “normal” service events when redistribution and relaxation of the residual stresses were included in the durability analysis. The concepts presented are in a format that is easily extended to more complex geometries and loadings.