Demagnetization-based axial magnetized magnetostrictive patch transducers for locating defect in small-diameter pipes using the non-axisymmetric guided wave

The propagation of the non-axisymmetric guided wave in the small-diameter pipe is complicated, which makes the circumferential position of the defect difficult to be determined. This article reports on the design of a segmented axially magnetized magnetostrictive patch transducer array for efficient transduction of non-axisymmetric L(M,2) modes to determine defect's axial and circumferential positions in the small-diameter pipe. First, the background of the magnetostrictive patch transducer and non-axisymmetric guided wave in the pipe was presented. Moreover, the theoretical background to the influence of the length-to-width ratio of the magnetized rectangular patch on the demagnetizing factors was introduced. Second, the method of the pipe health monitoring using the designed segmented axially magnetized magnetostrictive patch transducer array was proposed. Third, the most suitable multi-belts of the flexible printed coils were chosen to provide the dynamic magnetic field by the comparison experiments. Then the signal amplitudes of the segmented axially magnetized magnetostrictive patch transducer array with different length-width ratios of magnetostrictive materials were compared with each other to prove the principle of demagnetization. Another two magnetostrictive patch transducer arrays employing permanent magnets were compared with the proposed segmented axially magnetized magnetostrictive patch transducer array. The experiments of pipe health monitoring were carried out to prove that the proposed method can realize pipe health monitoring over time. Fourth, the defect orientation experiments in a 304 stainless steel pipe with 48 mm inner diameter and 2 mm thickness were performed using the proposed segmented axially magnetized magnetostrictive patch transducer array at 650 kHz. The prediction of the circumferential position of the defects correlated well with the defect's true location through matching the angular profiles of the experimental results and the modulated numerical analysis for several axial distances. The experimental results for the segmented axially magnetized magnetostrictive patch transducer array demonstrated that the proposed segmented axially magnetized magnetostrictive patch transducer array could potentially be applied to detect the axial and circumferential positions of the defect in a small-diameter pipe.