| Summary: | 碩士 === 國立中山大學 === 機械與機電工程學系研究所 === 98 === The guided wave method is one of the non-destructive testing methods with the ability to inspect long length of pipeline. The presence of defects and pipe features can be indicated by analyzing the reflected echoes and the mode conversion phenomena of guided waves. To provide the profile of defects, the signals of guided wave are usually processed to form a B-scan or C-scan image. As for the characteristics of multimodes and dispersion, the C-scan image of defect shows a poor resolution both in the axial and cricumferential directions. Therefore, this study uses the synthetic aperture focusing technique (SAFT) to improve the resolution of the C-scan image. The propagation and scattering of the fundamental torsional mode T(0,1) in pipes are analyzed by the transient analysis of finite element method. Furthermore, the reflected signals are processed by SAFT to form a C-scan image with better resolution. Three types of defects including circumferential crack, axial crack and pitting were discussed in this study both by finite element method and experimental method. In the numerical study, the modification of the C-scan image of the circumferential crack showed a better consequent than the axial crack and pitting. The SAFT was also used to separate the images of two circumferential cracks with different axial location on the pipe succesfully. In the experimental study, the signals of the axisymmetric weld and non-axisymmetric defects were processed by SAFT to form the C-scan image. The results showed a nice resolution of circumferential cracks and the circumferential distribution of the weld in the modified C-scan image at the same time. The application of SAFT to guided wave on a pipe is valid to overcome the effects of guide wave multimode and dispersion characteristics, it provides a new way to defects identification.
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