Lamb-like wave tomography of pipe structures

The adapation of the tomographic reconstruction to the guided wave inspection of pipe structures is investigated theoretically and experimentally.The geometry is formed in multiple pitch-catch configurations when transmits and receivers are respectively laid along two parallel circumferential belts around the pipe.Among the various guided wave modes propagating in pipe,it is those called Lamb-like modes that are of the interest in our investigation.These modes see the pipe more like a curved plate and take the helical direction of the shortest path around the pipe to the receiving transducer.The method calculating the arrival times of ray paths of Lamb-like waves is theoretically given, and the simultaneous iterative reconstruction technique(SIRT) is carried out to realize slowness imaging reconstruction for the given set of arrival times.Reconstruction for both numerical and experimental data acquired on pipes with through hole Haws is presented,and the arrival times of Lamb-like waves in healthy and damaged pipe are extracted and compared,it is found that Lamb waves that pass through the flaw zone indicate different arrival time information.This shows up as a longer arrival time because the guided waves tend to skirt around the flaw.Lamb-like wave tomographic reconstructions of pipes with two flaws are also demonstrated numerically and experimentally.Compared to the case of single flaw,reconstructed image quality of the flaw where fewer rays go through degrades.These results provide a theoretical basis for quantitative nondestructive evaluation of pipe damage using Lamb-like wave tomography.