Probabilistic prediction of crack propagation using the three-dimensional residual stresses estimation method based on the eigenstrain methodology for welded pipes

Statistical predictions of crack propagation are requested to evaluate remaining lifetime of operating welded structures. Today, crack growth rate for each observed crack cannot be evaluated accurately without neutron diffraction and synchrotron X-ray diffraction due to the difficulty of nondestructive measurements of welding residual stresses in the thickness direction. However, it is difficult to apply those nondestructive diffraction methods as on-site measurement techniques because the higher energy diffraction methods are available only in special irradiation facilities. To make things worse, measured results by diffraction methods cannot be directly applied to the FEM (finite element method) model for crack propagation prediction. From this view point, the methods based on the eigenstrain methodology have been proposed. In the bead flush method, for example, three-dimensional welding residual stresses are calculated by an elastic FEM analysis from eigenstrains which can be estimated by the inverse analysis from released strains during the removal of the weld reinforcement. Here, the removal of the excess metal is nondestructive treatment essentially because it is effective to eliminate stress concentration zone. In this study, numerical simulations for a welded pipe under SCC (stress corrosion cracking) were carried out to evaluate crack propagation statistically. As well, estimation accuracies of crack propagation using residual stresses estimated by the bead flush method were compared with the accuracy using residual stresses assumed to be measured by diffraction methods. Prediction accuracies of crack propagation estimated by this method were higher than that by diffraction methods. It is because estimated results base on the eigenstrain methodology satisfy the self-equilibrium condition of residual stress. (author)