||Pipelines system is generally use in petrochemical industry to transporting fluid or gas. In order to further the efficiency of economic and space applications, elbow parts provide good benefit to connect pipes and change the direction of pipelines. The guided wave method has proved to be a useful and cost-effective NDE technique in inspecting pipelike structures. However, after the guided wave has come through the elbow, propagation path changes and mode conversion happens due of its asymmetric geometric, and will cause the wave characteristic become complex and hard to predict. The guided wave are no longer spread symmetrically but local focusing on part of pipelines. These factors may influence the testing results and estimate the wrong evaluation of defects.|
The thesis consists of two parts: the study of the T(0,1) guided wave focusing behavior beyond elbow, and the effect on defect testing by these wave focusing. To understand the focusing behavior after guided wave goes through the elbow, the transmission signal of guided wave are captured from pipe beyond elbow. By analyzing the signals on different distances and different circumference positions of pipes, the variations of guided wave signal amplitude shows the different intensity distribution while guided wave has passed through the elbow. After knowing the focusing behavior beyond the elbow, the defects are set to be proved the relation between the focusing behavior and the defect testing results. The study will proceed with both numerical simulation and experiment.
The study results show that factors cause the focusing behavior include the geometrical characteristics of elbows, the overlapping of different modes, and the dispersion phenomena. Since there are less modes exist at low frequency, and the difference between modes velocity are less as well. So the focusing behavior at low frequency are much simpler and also can be restored to stable quicker. While the frequency becomes higher, more modes appear to cause the focusing behavior that becomes more complex. The wave velocity of each mode come closer at high frequency also cause the focusing behavior keep longer. Furthermore, through the defect testing shows great directly related between guided wave focusing behavior and the defect reflection coefficient, which means the guided wave focusing results can use to predict if the defect signal were magnified or minified. The thesis bring up the focusing map which can easily to display the wave focusing distribution and its characteristic on pipe, that can provide inspectors as a reference when estimating the level of defects beyond the pipe.