Crack growth monitoring using fundamental shear horizontal guided waves

Monitoring cracks in critical sections of steel structures is a topic of growing interest. Existing high-frequency ultrasonic techniques have good detection sensitivities but poor inspection coverage, requiring an impractical number of transducers to monitor large areas. Low-frequency guided waves are used for corrosion detection in pipelines but are insufficiently sensitive for many crack detection applications. The sensitivity can be improved using higher frequencies and by placing the receiving transducers closer to the defect. This study evaluates the monitoring performance of an SH0 mode system at frequencies just below the high-order mode cut-off. Baseline subtraction with temperature compensation was applied to experimental data generated by a ring of transducers on a 6-in diameter pipe. It was found that the residual signals after baseline subtraction were normally distributed so the random fluctuations could be reduced by coherent averaging; it was thereby possible to reliably detect a 2 mm × 1 mm notch simulating a crack located one pipe diameter along the pipe from the transducer ring. The damage detection performance at different locations along the pipe was assessed by analysing receiver operating characteristic curves generated by adding simulated defects to multiple experimental measurements without damage. At a fixed standoff distance, the damage detection performance increases with the square root of the number of averaged signals and is also improved by averaging the signals received by transducers covering the main lobe of the reflection from the defect. When the defect is located more than about one pipe circumference from the transducer ring, the optimal performance is obtained by averaging across all the transducers in the ring, corresponding to monitoring the T(0,1) pipe mode. Therefore, an SH0 mode monitoring system has great potential for crack monitoring applications, particularly for welds in pipes.