Single torsional guided wave excitation in pipes by frequency selection using magnetostrictive sensor technology

Guided waves have been widely used for the efficient long-range nondestructive structural health inspection of pipes. Among the various guided wave modes, the torsional guided wave is most preferred since its first branch is nondispersive. Using magnetostrictive sensor technology, torsional guided wave can be excited in pipes, while longitudinal guided wave will be excited along, which brings some difficulties in following defect detecting. The objective of this work is to experimentally investigate the possibility of elimination of longitudinal guided wave by excitation frequency selection. The dispersion of guided waves in pipe was studied, and the cutoff frequencies of L(0,1) and L(0,2) mode guided waves were calculated. The frequency range between the cutoff frequencies of L(0,1) and L(0,2) mode guided waves was called dead zone, for the longitudinal guided waves can not propagate in this range. Detected signals in aluminum pipes at different frequencies were obtained in experiments, which were performed time-frequency analysis using the short-time Fourier transform(STFT). Experimental results showed that when the excitation frequency was in the range of or at the neighborhood of dead zone, L(0,1) and L(0,2) mode guided wave could be eliminated, and single T(0,1) mode wave was excited. This frequency range for single torsional wave excitation corresponded to the severely dispersive frequency range of L(0,1) and L(0,2) mode waves, which was called “trough zone” in this study, as the form of it was like a trough in dispersion curves. Different aluminum pipes were detected in experiments, and the results were consistent.