Wall thickness measurement and defect detection in ductile iron pipe structures using laser ultrasonic and improved variational mode decomposition

A reliable non-destructive evaluation technique that accurately assesses the wall thickness and slag inclusions of ductile iron pipes prior to annealing is crucial to reducing costs in subsequent operations. The rough outer layer of the produced ductile iron pipes leads to considerable background noise, limiting the probability of detection of laser ultrasonic testing equipment. In this study, an improved variational mode decomposition approach based on particle swarm optimization is suggested initially to increase the signal-to-noise ratio. This technique develops a fitness function suitable for processing ultrasound signals. A scanning laser ultrasound technique is designed for simultaneous measuring wall thickness and finding slag inclusions in ductile iron pipes based on excitation locations and flight time. A finite element analysis simulation model is established to assess the feasibility of this scanning method. This experiment is conducted on a 4.2 mm thick sample with slag inclusion defects and a 6.2 mm thick sample without slag inclusion flaws. Experimental results demonstrate that the developed denoising technique may enhance the average signal-to-noise ratio from 34.83 dB to 43.15 dB; the accuracies of thicknesses measurement for two ductile iron pipes are 96.90% and 99.52%, respectively, and the slag inclusion defects can be recognized by ultrasonic amplitude or energy distribution in different B-scan images. It has potential implications for in-service inspection of ductile iron pipe structures.