Monitoring the evolution of localized corrosion damage under composite repairs in pipes with guided waves

Glass-fiber reinforced polymer (GFRP) repairs are often used in order to restore the strength of corroded pipelines. However, once they are on the pipe, it becomes difficult to identify continuing wall-loss due to on-going corrosion processes. This work aims at evaluating the possibility of monitoring the repair region with a permanently-attached array of piezoelectric sensors emitting ultrasonic guided waves. For this, a set of 8" pipes were prepared, on which GFRP repairs with different lengths, thicknesses and resin types were installed. Baseline signals of the undamaged samples were acquired before size-controlled defects, initially holes and then grinded patches, simulating localized corrosion, were machined in the pipes. Signals from the damaged conditions were processed along with the baselines by extracting their modal components up to the second flexural order, and then by processing the individual components with the Optimal Baseline Subtraction and Optimal Stretch methods. Results show that it is possible to detect localized wall loss increases greater than 0.5% of the cross-sectional area of the pipe in 1 m-long repairs, depending on the resin type which is used, and for a signal-to-noise ratio threshold of 3 dB.