Residual stress measurement in steel plates and welds using critically refracted longitudinal (LCR) waves

The application of acoustoelasticity using critically refracted longitudinal (LCR) waves is described for measuring residual stress in welded steel plates. Residual stresses are self-equilibrating and may exist in a material that has been deformed in a nonhomogeneous manner. When unknown residual stress is present in a structure, the true stress may become significantly greater than the working stress. In a corrosive environment, highly stressed areas that have not been properly stress relieved are prone to stress corrosion cracking. Areas near welds are particularly susceptible to stress corrosion cracking.Two welded plates were investigated for the present work: one hot-rolled and the other cold-rolled. Residual stresses are usually greatest after welding. Further, longitudinal stress (i.e., stress parallel to the weld bead) is typically greater than the component transverse to the weld bead. Since the acoustoelastic behavior of the LCR wave is largest when propagated parallel to a uniaxial stress, the LCR wave traveling parallel to the weld bead was used to investigate the stress changes after stress relieving of the welded plates. Both 1 MHz and 2.25 MHz probe frequencies were used in this study. The stress changes in the welds and in the cold-rolled plate were clearly indicated by the LCR data.Two verification methods were used: hole drilling (HD) and neutron diffraction (ND). The stress relief was verified by the hole-drilling technique. While the HD technique showed about the same stress magnitude as found by the LCR results, the orientation was reversed. The stress orientation was probably caused by the grinding process used to flatten the weld bead. Texture was also investigated using a neutron diffraction (ND) technique on the (001)[110] texture. The through-the-thickness technique yields an average of the orientation distribution of the (110) planes. At locations in the parent metal and in the weld, the distribution was found to be very similar, indicating uniform texture throughout the weld and parent metal zones.