| Pipes are widely used in petrochemical and power generation industries as one kind of key equipment for materials transmission and energy conversion. As the development of economy, the number of pipes increases rapidly, which means that more and more pipes need to be inspected. Therefore, how to inspect pipes rapidly becomes a big challenge in field of the nondestructive testing. Guided wave technology based on electromagnetic acoustic transducers (EMATs) has been applied for rapid screening of long lengths of pipes, due to its ability of inspecting the whole tube without moving the transducer and its advantage of non-contact. However, compared with the conventional piezoelectric transducers used in the guided wave inspection, the application of EMATs still faces the problem of low sensitivity. This study in this dissertation is systematically and deeply developed around the mechanisms and methods of signal strength enhancement in the longitudinal guided wave inspection for pipes based on EMATs.Firstly, based on the theory of classical electromagnetism and mechanics of elastic solid, the governing equations of Lorentz force, magnetization force and magnetostriction stain are obtained for the longitudinal guided wave inspection using EMATs. The mechanisms in the inspection process are stated in detail. The contribution of these mechanisms is analyzed based on both formulations and experiments. It is definite that the magnetization is dominated in the inspection process when the axial static magnetic field is used. While the radial static magnetic field is used, the direction of the field varies with location, which makes the effect of magnetostriction can not be ignored. These researches would be helpful for investigating methods of signal strength enhancement.For the longitudinal guided wave inspection of pipes which is carried out from the outside of the pipe, the influence of the region of the generating magnetic field on the signal strength is investigated. An improved transducer structure is designed to control the region of the generating magnetic field based on shielding effect. A new method for enhancing the signal strength of the guided wave inspection is achieved by adjusting the field region to a suitable value. The effectiveness of the presented method is verified by experiments and the applicability is discussed.Furthermore, the signal strength is enhanced through the receiving process. The expression of the magnetic vector potential is given, and the distribution of the induced magnetic field in the area is analyzed. Criterions of designing the receiver coil structure are stated based on the relationship between the magnetic vector potential and the induced voltage. The coil structure is optimized based on the criterions for maximizing the signal-to noise ratio (SNR) of the received signal. Both the simulation and experimental results show that, the SNR obtained by the optimal two-section coil increases by10%～40%compared with the conventional three-section coil.To solve the problems, which exist in the guided wave inspection of heat exchanger tubes, a new method of longitudinal guided wave inspection based on the open magnetic circuit is presented. Further investigations are carried out on the performance of the method on heat exchanger tubes with different materials. For inspecting the stainless steel heat exchanger tube, a method of suppressing the unwanted mode is obtained. For inspecting the ferromagnetic heat exchanger tubes, a new approach of enhancing signal strength is presented. Based on the above study, longitudinal guided wave transducers are designed for inspecting the heat exchanger tubes. The feasibility of the presented method is verified by field applications.All the work in this dissertation enriches the research of guided wave inspection of pipes based on EMATs, provides important support for the development of national standards GB/T31211-2014and GB/T28704-2012and can help promote the application of EMAT technique. |
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