The mode transformation of a T(0,1) guided wave in an elbow pipe and its influence on defect location

To correct the inaccurate location of elbow pipe defects, our study established an experimental platform for magnetostrictive T(0,1) guided wave detection. The experimental platform was used to detect circumferential grooving defects with the same specifications, which were present at three different locations, i.e., the front straight pipe segment of the elbow, the elbow segment, and the back straight pipe segment of the elbow. The echo signals were analyzed. We found that the echo signals of the first two defects were normal, and the defect location could be recognized accurately, while two defect echoes occurred continuously in the straight pipe segment behind the elbow. To determine the causes of the abnormal defect signals, we selected a pitch-catch detection sensor to collect the guided wave transmission signal of the straight section after the elbow. The finite element numerical model extracted the guided wave propagation time-domain signal, and then, snapshots of the wavefield propagation were generated. The analysis results suggest that T(0,1) guiding will gradually resolve with a new mode signal when it propagates to the straight section behind the elbow. Through the theoretical calculation and analysis of the dispersion curve, the signal was identified as an F(1,2) flexural guided wave because its group velocity was less than that of the T(0,1) guided wave. The former defect echo wave packet is the T(0,1) guided wave, and the latter is the F(1,2) guided wave. The orientation of the defects should be subject to the position of the T(0,1) guide wave.