Low-cost instrumentation for high frequency ultrasonic guided wave laboratory research in free rock bolts

Abstract

• Ultrasonic guided wave measurements require specialized instrumentation. • 5 MHz measurements on rock bolts with low-cost (USD 1110) equipment. • Narrowband excitation, averaging and filtering used to increase SNR. • Achieved propagation distance of 9 m equivalent to 9000 wavelengths. • Pre-amplification used to overcome internal noise in low-cost USB oscilloscope. Guided wave ultrasound offers the ability to inspect slender structures from a single transducer location. However, the propagation of guided waves is more complex than the propagation of bulk waves used in conventional ultrasonic testing, and the experimental equipment required is more sophisticated. Typical equipment is expensive and may limit research and industrial application. This paper investigates what can be achieved by using low-cost instrumentation for laboratory measurements. The application is the measurement of guided waves in rock bolts (cylindrical rods) at relatively high frequencies (5 MHz). The dispersive wave propagation requires the use of narrowband excitation signals. The use of a bench-top waveform generator and oscilloscope is compared to the use of a compact and low-cost USB oscilloscope, which includes an arbitrary waveform generator. The introduction of a pre-amplifier mitigated the poorer noise performance of the USB oscilloscope, and comparable results were obtained from the two setups. It is demonstrated that waves propagated 9 m in a free rock bolt, with 10 dB/m attenuation, could be detected and analyzed with a measurement setup costing approximately USD 1110.