Ultrasonic characterization of tubing pinch welds

Spatially-focused, high-frequency ultrasonic pulses have been used in the evaluation of the quality of stainless steel tubing pinch welds. Experimentally, echoes from the bond plane were generally too weak to observe, except in bond planes with macrocracking. Correlations were therefore sought between bond quality and the amplitude and spectral features of the back-surface echo that has passed through the bond plane twice. Linear scans of the back-surface echo parallel and perpendicular to the tubing axis were made on pinch welds of four classes of bond quality. Trends, with scatter, were observed between the ultrasonic parameters and bond quality. These experimental results were interpreted using the Stanke-Kino theory of ultrasonic attenuation due to grain scattering to relate the microstructural changes caused by the pinch-weld process to the observed ultrasonic signals. The results illustrate the importance of process-related changes in near-bond microstructure in attempts to nondestructively determine bond strength.