Experimental Demonstration of the Eigen-strain Method by Using X-ray Diffraction for Determining Three-dimensional Welding Residual Stresses

摘要

In order to predict crack propagation for welded structures in in-service inspection, it is required to evaluate three-dimensional welding residual stresses nondestructively. Today, diffraction methods such as X-ray and neutron diffractions are used as nondestructive measurement techniques. In X-ray diffraction, however, residual stresses just on surface can be obtained. Although it is possible to measure residual stress distributions even in the thickness direction by using neutron diffraction, it is available only in special irradiation facilities. It means it is impossible to use neutron diffraction as an on-site measurement method in in-service inspection. The bead flush method based on the eigen-strain methodology makes it possible to estimate three-dimensional welding residual stresses non-destructively. In this method, welding residual stresses are calculated by elastic FEM (Finite Element Method) from eigen-strains which are determined by an inverse analysis from released strains in removal of reinforcement of weld. Here, the removal process can be regarded as non-destructive essentially because toe of weld which may become crack starters can be eliminated. Estimation accuracy in the bead flush method is worsened by processing strains on the machined surface, but the first author has been developed the bead flush method to be free from the influence of the processing strains. In this method, eigen-strain distributions are estimated not from released strains by strain gauges but from residual strains by X-ray diffraction. The authors tried to demonstrate the effectiveness of the advanced bead flush method by evaluating estimation accuracy for an actual welded plate. But, the estimation accuracy was poorer probably because an analytical model to estimate residual stress was relatively different from actual model. In this study, eigen-strain distributions in the thickness direction are considered by using the response surface methodology to improve estimation accuracy of welding residual stresses. However, further improvements of estimation accuracy are required as a future work.