Residual stress determination in friction stir butt welded joints using a digital image correlation-aided slitting technique

This paper presents an incremental cutting method for evaluating the longitudinal residual stresses in a butt welded thin plate via combining the traditional residual stress measurement methods and the advanced optical technique. The proposed approach, which can be called digital image correlation (DIC)-aided slitting technique, introduces a successive extension slot to a specimen and employs the DIC technique to measure the released displacement profiles of the cutting sections after each cutting increment. Then the displacement profiles are used to directly calculate the residual stress distributions up to the slot tip and hence, a stress distribution can be obtained after a cutting increment. Finally, all of the stress distributions are averaged to ultimately determine the original residual stress field. This method does not include any complex experimental operations or tedious derivation, and the resolution of stress variation is greatly improved by the continuous measurement of the released displacements. The presented method has been preliminarily verified by a specimen with residual stress introduced by a four-point bending test. The results show that residual stresses determined by the DIC-aided slitting technique agree well with those from finite element (FE) prediction. The residual stress in a friction stir welded aluminum specimen obtained by the presented technique is also consistent with the evaluations given by X-ray diffraction. Furthermore, the residual stresses obtained by the DIC-aided slitting technique demonstrate higher accuracy and stability than the evaluations derived by the DIC-aided contour method.