||Among the non-destructive testing techniques, guided waves has the characteristics of propagating long distance and being hard to attenuate, and it can also detect quickly and widely for the entire pipelines. However, identifying the signals of defects during the test is frequently difficult as a result of its multimodal and dispersive characteristics. Pipelines system is widespread use in petrochemical industry to transport gas or fluid. In virtue of restriction of space and pipelines planning, elbow parts will certainly provide to connect pipes, and this kind of complex pipe feature will bring about not only difficult to recognize signals but change direction of wave energy, that is, the energy will gather together outside of the elbow because of its geometry, then rest of the elbow will be blind area for the testing. In order to reduce error probability on recognizing signals, this study applied finite element method to simulate the propagation of T(0,1) torsional guided wave through the defect on the elbow, proposing time reversal method to analyze in accordance with defects signals, comparing difference of defect echo with and without this method so as to evaluate the feasibility of focusing ability of time reversal method on the elbow and also observing the influence of multiple defects exist in pipes on the focusing results. Time reversal method, a self-focusing technique, it can effectively focus on the spatial and temporal domain.|
The study results showed that it is beneficial to apply time reversal method to the improvement of signal-to-noise ratio on defect inspection for guided wave system. For instance, if defect exists at the end of elbow, and by comparing with defect reflective amplitude without and with time reversal method, it is clearly to show that the defect reflective amplitude with time reversal method is enhanced compared with that of without time reversal method. In addition to improving the signal-to-noise ratio and making defect easier to identify, it also showed that the wave energy will not be affected by the elbow, and can focus to the defect instead. Besides, when multiple defects exist in pipes, it will not make focusing behavior off the work either. For example, when there are three defects on the elbow pipeline, by using finite element method, the results showed that the focusing ability of wave energy will not be influenced even if it transmit through multiple defects on the pipe of straight part in advance, and can focus to the defect on the elbow part. The thesis also bring up the focusing oriented illustration which can easily to display the wave energy with time reversal method focus on elbow defects direction.