Study On Ultrasonic Dispersion Curves Based On The Finite Element Method

Because the ultrasonic nondestructive testing technology has the characteristics of fast, sensitive and economic, this technology has become one of the most important methods in NDT and widely used in aerospace, medical equipment, Marine exploration or other fields.Because the ultrasonic wave will occur dispersion phenomena and multi-modal phenomenon when it propagates in the structure, which brings a lot of trouble to detect. In order to avoid dispersion serious area and make the analysis process generate less modal, the center of the excitation signal frequency’s selection has become the urgent problem in the field of ultrasonic nondestructive testing. However, the appropriate center frequency’s selection must be according to the guided wave phase velocity and group velocity dispersion curve. For simple isotropic structures such as: plates, rods, tubes, etc. whose dispersion curves can be obtained by solving the traditional wave equation. However, the complex structures and composite materials,etc. are difficult to use analytic method to solve, in addition, the previous studies are relatively few, these problems severely restrict the development of ultrasonic nondestructive testing. Therefore, this thesis presents a fast and effective method to extract the dispersion curves: Finite element modal analysis, specific work is as follows:First, the research situation of the finite element modal analysis method at home and abroad is described. Introduced the calculation principle of modal dispersion characteristics analysis, the calculation method of phase velocity and group velocity, the calculation of cell size and model size, the selection of unit type and modeling consideration.Secondly, the finite element modal analysis for drawing the dispersion curves of Lamb wave is elaborated. In ABAQUS software "post-processing visualization module", identified the required modal vibration mode, extracted the corresponding modal natural frequency of the vibration mode, calculated the corresponding to the frequency of the corresponding phase velocity and the group velocity by the formula. The thin structure’s phase velocity and the group velocity dispersion curves are obtained by MATLAB software date processing, then compared to the analytical solution, determined the correctness of this method.Then, the finite element modal analysis for drawing the longitudinal guided wave dispersion curve of pipe structure is elaborated. Using the same method asthin structure, modal analysis, modal identification, extracting frequency, the data processing,then the pipe structure’s dispersion curves was plotted and compared with the analytical solutions. Determining the validity of the method, which is prepared for the later application of this method.Finally, this thesis expounds the application of finite element modal analysis. the circumferential waves and the structure of the composite’s dispersion curves were plotted by using the finite element modal analysis, the composite material’s modeling method was simply introduced, after analyzing the phase velocity and the group velocity dispersion curve of circumferential waves and composite structures. Thereby, the guiding role of the modal analysis method for research ultrasonic guided wave dispersion characteristic was determined, so this thesis provide a feasible method to study the structure that it is difficult to get dispersion curves with the analytical method.