Objective The damage detection technology based on ultrasonic guided waves has broad application prospects in the rapid damage detection of long-distance pipes. The technology can identify and locate the damages based on the analysis of signals of ultrasonic guided waves scattered by damages. However, for welded pipes, welds will also cause ultrasonic guided waves to scatter, influencing the extraction of waves scattered by damages and increasing the difficulty in pipe damage detection.

Methods To solve the problem that damage detection is affected by the welds, a method using ultrasonic guided waves based on mode conversion was proposed to track down the axial location of damages according to different characteristics of signals of waves scattered by welds and damages. A piezoelectric sensor was used to excite the single L(0, 2) mode ultrasonic guided waves as the incident waves. Based on the circumferential characteristics of axisymmetric and bending modes, the F(1, 3) mode signals generated by mode conversion were extracted from the signals received by the sensor, and the waves reflected by damages were identified. Based on the propagation velocity of ultrasonic guided waves in L(0, 2) and F(1, 3) modes, the axial location of damages in the pipes was realized. Numerical simulations and experiments were conducted on welded pipes with damages set at different locations to verify this damage location method.

Results The results of numerical simulations and experiments suggest that the damages in the whole pipe, including those in the weld zones and the non-weld zones, could be detected by extracting the mode conversion signals, and that the error in the axial location of damages was less than 3%. With the increase of damage depth, the amplitude of the F(1, 3) mode increased, which provided the basis for identifying the damage depth. It was difficult for traditional methods based on reflected waves in the L(0, 2) mode to identify the damages in the weld zones.

Conclusion Compared with the traditional damage detection methods based on the reflected waves, this method can, during the damage detection, reduce the interference of waves reflected by damages based on different characteristics of waves reflected by welds and damages, showing its potential advantages in damage detection of welded pipes, especially in weld zones.