Abstract
Guided wave ultrasound can be applied to inspect large volumes of structures from a single transducer location and is particularly effective in one-dimensional structures such as rods, pipes and rail. An application, which is of interest in South Africa, is the inspection rock bolts in the mining industry. The difficulty in inspecting embedded rods is that there is rapid attenuation of the signals with distance. While guided wave systems generally operate at low frequencies (20 kHz to 200 kHz), high frequency operation (1 MHz to 10 MHz) can offer some advantages when inspecting rock bolts. Guided wave ultrasound testing is complicated by multi-modal propagation and dispersion, which require the use of sophisticated excitation signals, which are not available in equipment designed for conventional ultrasonic testing. It was therefore necessary to develop a measurement setup and the approach taken was to use standard lab instruments wherever possible and to only purchase the items that are unique to this measurement, using the very limited budget available. The measurement setup was used to perform measurements on a rod and confirmed theoretical predictions. These measurements identified the frequencies where modes propagate with low attenuation and allow the attenuation to be quantified.
Highlights
Guided wave ultrasound offers the possibility of inspecting large volumes of structures from a single transducer location
While guided wave systems generally operate at low frequencies (20 kHz to 200 kHz) it was shown by Beard [5] that high frequency operation (1 MHz to 10 MHz) can offer some advantages when inspecting
Measurements were performed on a section of a rock bolt between 4 and 6 MHz and various guided wave modes were detected in good agreement with theoretical predictions
Summary
Guided wave ultrasound offers the possibility of inspecting large volumes of structures from a single transducer location It is effective in one-dimensional structures such as rods, pipes and rail. This has resulted in inspection and monitoring systems being developed for pipelines in the oil and gas industry internationally [1] and for a heavy haul train line in South Africa [2]. A power amplifier for driving a transmit transducer may cost between R100 000 and R 200 000, while a pulser-receiver for pulse-echo measurements could cost between R300 000 and R500 000 Such a system would still require additional instruments such as a two channel arbitrary waveform generator and an oscilloscope and ultrasonic probes and cables.
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