Non-destructive evaluation of welding residual stresses in the thickness direction via X-ray diffraction

Abstract

For assessment of structural integrity of operating welded structures, non-destructive evaluation of a crack propagation direction and a crack growth rate for each detected crack is demanded in in-service inspection. For crack propagation prediction, three-dimensional welding residual stresses have to be evaluated quantitatively by non-destructive techniques. Today, synchrotron X-ray diffraction and neutron diffraction are known as non-destructive measurement methods of residual stresses in the thickness direction. However, these higher energy diffraction methods cannot be used as on-site measurement techniques because they are available just in special irradiation institutions. In the bead flush method based on the eigen-strain methodology, inner residual stresses can be estimated by using released strains during removal of reinforcement of weld. But, it is impossible to apply this method to welded structures without excess metal such as friction stir weld and spot weld joint. In this study, welding residual stress distributions for whole structure are calculated by an elastic FEM (Finite Element Method) analysis from eigen-strains which can be estimated by an inverse analysis from residual strains just on surface. Here, residual strains on surface can be measured non-destructively by X-ray diffraction that is widely used as an on-site measurement application. Although it was difficult to estimate inner welding residual stresses accurately from residual strains just on one-side surface, solutions with higher accuracy could be obtained by using measured strains on the both top and bottom surfaces in butt-welded plate. Additionally, it was shown that estimation accuracy in this method was good enough when measured strains near the weld line were used as additional information for estimation.