단일 모드 램파 가진을 위한 전방향 전자기 트랜스듀서 개발

Abstract

This thesis is concerned with the development of two types of omnidirectional electromagnetic transducers tuned for generation of a single mode Lamb wave in a plate-like structure. Omnidirectional Lamb wave transducers may be preferred for efficient defect imaging in a plate. With an array consisting of them, the entire region can be effectively covered by the same inspection system and the imaging algorithm can be simple. One of the major issues in them is generation of only a single mode Lamb wave. When an excitation frequency is high, several Lamb wave modes are simultaneously generated to complicate signal processing and lower the accuracy of defect detection. Therefore, newly-designed omnidirectional transducers tuned for a single mode Lamb wave need to be developed.

To generate a single mode Lamb wave, two types of transducers, called an omnidirectional Lamb wave magnetostrictive patch transducer (OL-MPT) and an omnidirectional Lamb wave electromagnetic acoustic transducer (OL-EMAT) are developed. Firstly, the OL-MPT which can generate the S0 or A0 modes Lamb wave selectively is proposed. Because the circular magnetostrictive patch of the OL-MPT works as a wavelength filter, the relation between the wavelength of the generated Lamb wave and the diameter of the patch is found from modal analysis and a set of experiments.

Secondly, the OL-EMAT tuned for S0 mode Lamb wave is developed. A spiral double coil is applied to the OL-EMAT for maximizing the S0 mode and suppressing the accompanied A0 mode. Because the diameters and the widths of the double coil are the key factors determining the amplitudes of the S0 and A0 modes, a parameter study on the double coil is performed theoretically.

Finally, the defect imaging with an array consisting of the developed transducers is performed in a plate. For this, the defect imaging system including hardware and software is developed. And then the system is applied for crack detection in a plate.