Study On The Method Of Synthetic Aperture Nondestructive Defects Detection Based On The Lamb Waves

In recent years,the application of the Lamb waves in the Nondestructive Testing in the thin aluminum plates has become more and more extensive with the development of domestic industrial level and the extensive use of the thin aluminum plates.Electromagnetic acoustic transducers are often used to generate Lamb waves due to its non-contact,no need for couplant,flexibility and adaptability to special detection environments.The main challenge in electromagnetic ultrasonic Lamb waves inspection on the plate-like structures is the exact defect characterization and sizing.However,the dispersion and multi-mode characteristics of Lamb waves greatly reduce the resolution of damage imaging.Therefore,synthetic aperture focused imaging techniques are commonly used to improve the resolution of Lamb wave damage imaging.This paper grasps the propagation model of Lamb waves in the thin aluminum plate,and designs an EMAT that can excite the single A0 mode Lamb wave according to the excitation principle of the Lamb waves based on the Lorentz force.Combining the propagation characteristics of Lamb waves and the structural characteristics of the tested plate,a new synthetic aperture focusing imaging algorithm based on the Fourier domain SAFT is proposed,and the damage imaging experiment of the square through hole in the aluminum plate is completed using this new algorithm.The experimental results show that: the single A0 mode Lamb wave required by the study can be excited in the 1mm thin aluminum plate when the spacing of the meander coils in the EMATs is 3mm and the excitation center frequency is 132 kHz.Then,the generated A0 Lamb wave is used to complete the synthetic aperture scanning experiment,and the original scan data is applied to the traditional synthetic aperture algorithm and the improved synthetic aperture algorithm respectively.The damage imaging results show that: the size of the square slot detected by the improved algorithm is about 15 mm,while the size of the square through hole detected by the traditional algorithm is about 20 mm.The improvement of the lateral damage imaging resolution is approximately 20%.The entire experimental results show that: the dedicated EMATs designed in this paper can excite the single mode Lamb wave,and the improved SAFT algorithm proposed in this paper can significantly improve the resolution of damage imaging in the plate-like structure.This provides a theoretical basis for the excitation of single mode Lamb waves and the application of Lamb wave damage imaging in the platelike structures.