Crack growth monitoring using low-frequency guided waves

Over the last decade, the level of interest in the use of permanently-installed monitoring systems to track the progress of damage has increased dramatically. Cracks in pressurised operational pipelines can lead to structural failure and could ultimately lead to a loss of structural integrity and, in some instances, endanger life. Guided wave testing (GWT) is a relatively new and advanced non-destructive method used to inspect pipelines. Using permanently-installed guided wave transducers and baseline comparison techniques, it has been possible to detect defects as small as 1% in pipe cross-sectional change (CSC) in operational piping. This paper investigates, through finite element (FE) modelling and experiments on steel pipes, the interaction of torsional guided waves with circumferentially-oriented cracks. The FE modelling results show that circumferential cracks at welds that have a larger circumferential extent and deeper profile will produce a bigger change in amplitude of the reflected torsional T(0,1) signal, since it will lead to a larger change in the cross-section of the pipe wall. The results indicate that higher inspection frequencies produce a larger reflection coefficient and can be approximated as a linear function of frequency for narrow cracks with a characteristic length-to-wavelength ratio of <= 4.7%. The FE model was validated experimentally on an 8" pipe loop, where a centreline weld crack was artificially introduced using the spark erosion method. It was shown experimentally that a signal change of < 1% CSC can be reliably detected using a permanently-installed monitoring system and advanced baseline comparison algorithms.