Design of a miniaturised ultrasonic guided wave inspection instrument for steel strand flaw detection

摘要

Longitudinal mode ultrasonic guided waves (UGWs) in prestressed seven-wire steel strands have an inherent notch frequency property covering a frequency range of less than 100 kHz. If UGWs having such a frequency range are used to inspect the strand, the inspection instrument may fail to detect small defects due to difficulties in predicting the notch frequency. Therefore, it is more appropriate to design a miniaturised instrument that can excite and receive UGWs covering a frequency range of far more than 100 kHz for on-site strand inspection applications. In this study, an FPGA-controlled instrument is developed to test its UGW detection performance. The boost circuit operates in a frequency range of 0.1-1.5 MHz to produce a parametric burst of square waves with a low distortion content. The detected UGW signal is then filtered by a low-pass filter and amplified by a two-stage programmable gain preamplifier with a maximum amplification of 80 dB. The long-range strand inspection results show that the proposed instrument provides a signal-to-noise ratio for the detected UGW signal of 6.06 dB after the UGW has propagated for 56 m in the strand. The influence of a notch defect on the extent of mode conversion between the L(0,1) and F(l, 1) modes and the amplitude of the signal reflected from the notch are analysed based on the short-time Fourier transform (STFT) results from the detected UGW signals. The variation in the UGW signal spectrum that is induced by a notch with a cross-sectional area loss of 1.13% can be identified, indicating the high UGW detection performance of the developed instrument.