Guided ultrasonic waves in steel pipe with welded bend with defect

The propagation behaviour of guided ultrasonic waves in a steel pipe with welded bend is studied by finite element analysis. The effectiveness of the longitudinal L (0, 2), torsional T (0, 1) and flexural F (l, 1) guided waves in detecting circumferential cut near the weld is investigated. In order to identify the presence of the defect, the reflection strength due to the cut is studied. The geometry of the weld is constructed based on common V-bevel butt joints and the anisotropy of the 31 6L stainless steel weld is included to correctly predict the scattering of ultrasonic waves. The finite element model is built to allow high accuracy. The time steps are discretised such that there are 16 nodes in one period of the investigated frequency and the spatial element sizes are discretised such that there are at least 16 nodes in one wavelength corresponding to the interested modes. Detection of small circumferential cut (up to 60° circumferential extent) can be achieved with longitudinal L (0, 2) mode. Detection of moderate to large circumferential cut can be achieved by torsional T (0, 1) or longitudinal L (0, 2) modes, with T (0, 1) mode preferred due to its less mode conversion to higher order modes. Detection of defect by flexural F (l, 1) mode is difficult as reflection is weaker than torsional and longitudinal modes. F ( 1, 1) mode that is excited in different orientation also gives different results but is also ineffective in detecting defects.