Methodology for circumferential localisation of defects within small-diameter concrete-covered pipes based on changing of energy distribution of non-axisymmetric guided waves

Concrete causes substantial attenuation on ultrasonic guided waves during their propagation within a pipe. The circumferential position of the defect is even more difficult to detect in a concrete-covered pipe with a small diameter. This paper reports that the axial and circumferential positions of a circumference defect in a small-diameter concrete-covered pipe can be identified by moving the half-covered axial magnetised magnetostrictive patch transducer (AM-MPT) around the circumference of the pipe for subsequent measurements. First, the theory of normal mode expansion (NME) and demagnetisation is presented for the proposal of the nondestructive test (NDT) method and half-covered AM-MPT. Second, the NDT method based on NME is explained. Subsequently, the finite element models and experiments are applied to reveal the propagation of triggered and reflected non-axisymmetric guided waves L (M,2) to prove that the proposed method is applicable. Third, several frequencies of L(M,2) are compared to determine the best fit. The experimental results of pipes tested using half-covered AM-MPTs with different length-width ratios of iron cobalt patches proved that the greater the length-width ratio, the greater the amplitude of L(M,2) signals. These results support the theoretical results for the demagnetising field. The signals received from different circumferential positions of the pipe using AM-MPTs reveal that increasing the coverage of the transducer can improve the consistency of the signals received from the different circumferential positions. Next, the group velocity and attenuation characteristics of L(M,2) within the concrete-covered pipes were studied through experiments, with results matching the theoretical results. Finally, the experimental results proved that the proposed method could be utilised to determine the axial and circumferential positions of the defect in concrete-covered pipes. Moreover, the precision of the detection results can be improved by adding the monitoring position around the circumference of the pipe. It was demonstrated that the proposed method has potential to detect defects in concrete-covered pipes. (C) 2020 Elsevier Ltd. All rights reserved.