Evaluation of Surface Residual Stress Field of Long Japanese Sword by X-ray Stress Measurement Using Optics of cosα Method

Abstract

In X-ray stress measurement using the sin²ψ method, there are directions and locations where residual stress cannot be measured due to interference between the stress equipment and long parts. In such cases, the long parts are often cut into small pieces before measurement. However, nondestructive measurement without cutting is indispensable for measurement objects with high cultural and historical values, such as long Japanese swords such as Wakizashi. On the other hand, X-ray stress measurement using the cosα method allows nondestructive evaluation of stress from the lattice strains measured from a Debye-Scherrer ring recorded by an area detector under a single X-ray exposure at an oblique angle. This method has an advantage that its simple optical system prevents interference between stress equipment and long parts. In this study, a nondestructive technique was developed for measuring the residual stress filed near the tang (Nakago-jiri) of a long Japanese Wakizashi sword using the optics of the cosα method without cutting the sword. As a result, using the Debye-Scherrer rings recorded by the cosα method, the 2θ - sin²ψ graphs were obtained from the Nakago surface, where the 2θ - sin²ψ graph parallel to the longitudinal direction of Japanese sword cannot be measured by X-ray stress measurement using the sin²ψ method. The obtained eight 2θ - sin²ψ graphs were used to evaluate the residual stress field near the Nakago surface. It was found that there was a clear stress gradient in the vertical stress component in the plane, which was not observed in the Nakago surface without Shinogi. The residual strain information in the directions that cannot be measured due to the interference between the stress equipment and parts is important, and the residual strain information should be measured as much as possible using this developed method.