Defect imaging with guided waves propagating in a long range

Abstract

Pipe inspection with guided waves is expected to provide an efficient screening technique that will enable the inspection of lengths of pipe of up to a few dozen meters. In contrast with conventional inspection equipment widely used in practical inspection that employ easy-to-use axisymmetric modes, the present authors have developed a defect imaging technique that uses non-axisymmetric modes. In a previous paper, defect images were only available over a short range of 1.2 meters from the transducers. Therefore, in the present study, the effects of long propagation on defect images are discussed. Long-range imaging was performed experimentally using source signals and their multiple reflections between both edges of a four-meter-long pipe, rather than the reflected waves from defects. In the experimental investigation, large degradation of images was found in the far field. A computer simulation of the guided wave propagation reveals that a major reason for the degradation is the difference between dispersion curves for an actual pipe and the theoretical dispersion curves. In order to reduce the degradation, images were obtained by neglecting higher modes with larger differences in phase velocity. As a result, images were obtained at correct locations at distances of up to approximately 20 meters from the transducers.