the work, the force is transferred from the action point to the entire work in accordance with the elastic property of each point. Residual stresses alter the local elastic constant of the material; as widely known, a compressive stress due to the thermal load from a welding torch can easily cause permanent plastic strain. Therefore, the response to the external force of the entire work containing residual stresses is substantially different from that without residual stresses. The aim of this study is to explore this exact question of “how do the residual stresses alter the response of welded work to external force?” To this end, a low-level tensile load is applied to a butt-welded thin-plate specimen and its response is compared with the response of a non-welded specimen of the same dimension and material. An electronic speckle pattern interferometer (ESPI) is used to analyze the specimen’s behavior as a full-field image. A strain gauges and a scanning acoustic microscope are used to analyze the residual stress. The results indicate that the change in the elastic constant due to the residual stress near the weld alters the pattern of overall deformation, making it less symmetric around the central line of the specimen (both parallel and perpendicular to the tensile axis). This generates strain concentration at a point away from the weld line, causing more bodily rotation in response to the applied tensile load. 15.2 Experimental The materials tested in this study were cold-rolled carbon steel plates. Two types of welded specimens were prepared. For the first type (specimen 1), two plates of 90 mm (wide) 50 mm (long) 0.4 mm (thick) were butt-welded into a plate of 90mm 100mm 0.4 mm, and cut into 20 mm wide 100 mm long specimens (Fig. 15.1a). The second type (specimen 2) was prepared in a similar fashion but the final dimension was 40 mm wide 150mm long 3 mm thick (Fig. 15.1b). For each type, a non-welded specimen of the same dimension was prepared as the control. The specimen was attached to a tensile machine for application of a tensile load. An ESPI setup sensitive to in-plane displacement was arranged in front of the tensile machine (Fig. 15.2). Fringe patterns were formed by subtracting the image taken before the application of the tensile load from the image taken after the application of the load. The phase associated with the displacement was evaluated by assigning the order to each fringe and interpolating the phase between fringes. A care was taken to apply as small load as possible to avoid plastic deformation caused by the tensile load. For comparison, a non-welded specimen of the same material and dimension was tested in the same fashion. A challenge in the application of the ESPI as stated above to analysis of residual stresses due to welding is that the phase analysis on the formed fringes is not easy. Often the welded specimen exhibits bodily rotation, which yields fringes parallel to the sensitivity vector of the ESPI interferometer. When the interferometer is sensitive to horizontal displacement, for example, the fringe pattern is approximately equidistant, horizontally parallel, as Fig. 15.3a indicates. This makes difficult to evaluate the normal strain, because the variation of the fringe order along the axis is small (in the case of Fig. 15.3a, the change in the fringe order in going horizontally from the left end of the image to the right end is two or less). As a solution to this issue, a system of carrier fringes was introduced by rotating the wedge placed after the beam expander for the left interferometric arm. The wedge was rotated after the tensile load was applied. The rotation of the wedge provided a constant phase variation along the tensile axis on the specimen as Fig. 15.2b indicates. The provision of the carrier fringe altered the fringe pattern from Fig. 15.3a to Fig. 15.3c. Now that the horizontal variation in the fringe order was high, the phase could be Fig. 15.1 Butt-welded specimen 1 (a) and specimen 2 (b). The box with dashed lines indicates approximate area of view. For specimen 2 strain was measured with strain gauges at 12 points; 5 mm and 20 mm away from the weld line along the tensile axis, 16 mm above/below the central line parallel to the tensile axis. Circles indicate these locations for theright sideof the weld line only. Strain gauge measurements were made on theleft side of the weld line at the locations symmetric to these six points. The circle and distances inserted in (a) indicate the locations where acoustic velocity is previously measured in a welded specimen similar to this study 144 S. Yoshida et al.
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