A signal enhancement method for magnetostrictive guided wave testing of pipeline based on mechanical attachments

摘要

The ultrasonic guided wave testing, an efficient technique operating in pulse-echo mode, is being widely used for Nondestructive Evaluation (NDE) and Structural Health Monitoring (SHM). It can realize the long-range inspection from a single sensor position, which is suitable for detecting defects located at inaccessible areas. Furtherly, the guided wave testing based on magnetostrictive effect can generate guided waves in the tested structure without physical contact. However, the non-contact characteristic determining the magnetostrictive sensor (MsS) has a relatively lower signal-to-noise ratio (SNR) when compared with piezoelectric guided wave sensors. In the literature, different methods have been applied for the MsS to improve the SNR. Some investigators focused on optimizing excitation parameters and sensor configurations to increase the magnetostrictive coupling efficiency. Furthermore, some signal processing methods were introduced to improve the SNR as well, such as Discrete Wavelet Transform, Wigner-Ville Distribution, etc. However, under the condition of invariable excitation parameters and specified sensor configurations, how to improve the SNR during the guided wave propagation process is seldom studied. In this paper, a new signal enhancement method based on mechanical attachments is purposed for the pipe inspection. Due to the fact that mechanical clamps change the stress boundary condition of the tested structure, these clamps produce large wave reflections and the guided wave energy mainly propagates between mechanical clamps (testing range). By placing the transmitter close to the clamp, waveform superposition occurs at the position of the receiver for the third defect echo signal, which is demonstrated from the simulation. Hence, the defect can be well identified by the enhanced defect echo signal, especially when the first defect echo signal can hardly distinguish from the noise. Considering the waveform superposition is independent of the defect position in testing range, the effect of the blind range for the guided wave inspection can be eliminated. A laboratory-based experimental study is carried out whose results indicate the amplitude of the third defect echo signal increases by 35% when compared to the first defect echo signal.