Defect imaging with guided waves propagating in a long range

Abstract

Pipe inspection with guided waves is highly expected as an efficient screening technique that enables us to inspect a few dozen meters of a pipe. Although inspection equipments widely introduced in a practical inspection site adopt easy-to-use axisymmetric modes, authors have developed defect imaging technique using non-axisymmetric modes as well. In our previous paper, defect images were shown in short range of only 1.2 meters away from transducers. In this study, therefore, effects of long propagation to defect images are discussed. Long range imaging was experimentally done using source signals and their multiple reflections between both edges of a 4-meter-long pipe, instead of reflected waves from defects. In the experimental investigation, large degradation of images was shown in far field. A computer simulation of guided wave propagation reveals that a major reason of the degradation is the difference between dispersion curves for an actual pipe and theoretical ones. To reduce the degradation, images were obtained by neglecting higher modes with larger differences in phase velocity. As a result, images were obtained in correct location up to about 20 meters.