Center frequency shift in pipe inspection using magnetostrictive guided waves

Abstract

Magnetostrictive guided wave technology is widely employed to inspect pipes. In general, it is assumed that the center frequency of the exciting signal is the same as that of the receiving signal; however, this assumption does not hold in practice. To elucidate this phenomenon, we investigate the reason behind the center frequency shift (CFS) based on the energy coupling mechanism of magnetostrictive guided waves; we construct a theoretical model to calculate the CFS of the receiving signal. The detection process for guided waves is separated into three stages: excitation, propagation, and reception. At the exciting stage, the width of the coil affects the alternating magnetic field distribution. The vibration generated by the alternating magnetic field with a certain width produces a superimposed strain, which results in the CFS of the generating wave. At the propagating stage, we consider that the center frequency of the guided wave remains unchanged because the propagation is a linear process. At the receiving stage, the coil length determines the size of the induction area and the magnetic induction at a point is determined by the strain around that point. The number of coil turns, the distance of two adjacent wires, and the differential during the process of the electromagnetic induction affect the frequency of the induction signal, which leads to the CFS of the receiving signal. The center frequency of the receiving signal decreases with increasing coil width. Experiments are performed to verify that the theoretical model and the experimental results are in good agreement with the theoretical results. The model developed in this study provides a reference for the design of magnetostrictive guided wave sensors.