Detection and discrimination of symmetric and asymmetric defects by using ultrasonic guided waves

Several methods are currently being used for nondestructive evaluation of pipelines. In the last few years, ultra-sonic guided waves have received much attention as a tool for inspection of oil and gas pipelines. This is due to the capability of these waves in propagating over long distances with little energy loss. In this paper, the possi-bility of detection of pipe defects, such as grooves and holes, is investigated using an array of angle-beam trans-ducers. Identification of guided wave modes is done by means of a 2-D FFT analysis. The analysis demonstrates that the asymmetric excitation leads to generation of longitudinal L(0,2) mode in addition to the undesirable flexural modes that make the interpretation of results difficult. In order to eliminate the undesirable flexural modes and to increase the amplitude of the longitudinal L(0,2) mode, the pipes were excited by axisymmetric guided waves. The results showed that the axisymmetric discontinuities, e.g. axisymmetric grooves, can be dis-tinguished from asymmetric defects, e.g. a hole, by checking the mode conversions that take place in each case.

Keywords:

Ultrasonic guided waves , Longitudinal L(0 , 2) mode , Mode conversion , axisymmetric excitation.