Evaluation of the Effectiveness of the Three-dimensional Residual Stresses Method Based on the Eigenstrain Methodology via X-ray Measurements

Abstract

In order to prevent fractures caused by fatigue or stress corrosion cracking in welded structures, it is important to predict crack propagation for cracks observed during in-service inspections. However, it is difficult to evaluate three-dimensional welding residual stresses non-destructively. Today, it is possible to measure residual stresses just on surface by X-ray diffraction. Neutron diffraction makes it possible to measure welding residual stresses non-destructively even in the thickness direction but it is only available in special irradiation facilities. Therefore, it is impossible to use neutron diffraction as an on-site measurement technique. As non-destructive method of three-dimensional welding residual stresses based on the eigenstrain methodology, the bead flush method has been proposed. In this method, three-dimensional welding residual stresses are calculated by an elastic FEM (Finite Element Method) analysis from eigenstrain distributions which are estimated by an inverse analysis from released strains by strain gauges in the removal of the weld reinforcement. Here, the removal of the excess metal contributes inhibition of crack initiation. Therefore, the bead flush method is a non-destructive technique essentially. However, estimation accuracy of this method becomes relatively poor when processing strains are added on the machined surface. The first author has been developed the bead flush method to be free from the influence of the processing strains. In this method, eigenstrains are estimated not from released strains but from residual strains on surface by X-ray diffraction. In this study, welding residual stresses on the bottom surface in an actual welded plate are estimated from elastic strains measured on the top surface using this method. To evaluate estimation accuracy, estimated residual stresses on the bottom surface are compared with residual stresses measured by X-ray diffraction. Here, eigenstrain distributions not only in the welding direction but also in the thickness direction are considered to improve the estimation accuracy.