An EMAT for improving the purity and amplitude of S₀ Lamb waves

Abstract

Different waves of the Lamb waves may be superimposed in practical tests due to its multimodal characteristics and dispersion characteristics, thus greatly increasing the difficulty of later signal analysis and creating a huge limitation to their application in non-destructive testing (NDT). Only a single magnet was used to generate a static magnetic field in the traditional S0 mode Lamb wave (S0 wave) electromagnetic acoustic transducer (EMAT), which resulted in a less pure S0 wave. Additionally, the accompanying A0 mode Lamb wave (A0 wave) will decrease the accuracy of the results in defect detection and increase the difficulty of the signal processing. In order to obtain a single-mode S0 wave, a periodic magnet structure EMAT structure was proposed in this study. Finite element models were established and the effect of single and periodic magnet structure on the static magnetic field was analyzed. The periodic magnet structure EMAT can substantially enhance the S0 mode Lamb wave amplitude by increasing the flux density of the local magnetic field using periodically arranged magnets. However, it also causes enhancement to the A0 wave and decreases the detection accuracy. An improved EMAT was further proposed to increase the S0 wave amplitude with no enhancement to the A0 wave amplitude. Experiments were conducted and two types of EMAT were comparatively investigated, which agreed with the simulated results. The S0 wave amplitude can be increased by 7 times with the improved EMAT compared to traditional EMAT, and the A0 wave is completely submerged in the noise. The improved EMAT proposed in this study can provide guidance for the single-mode S0 wave defect detection.

A Lorentz Force EMAT Design with Racetrack Coil and Periodic Permanent Magnets for Selective Enhancement of Ultrasonic Lamb Wave Generation

“…This results in tangential Lorentz forces whose directions alternately change with the polarities of the magnet units. This type of EMAT can achieve purer S0 mode waves as compared with the conventional MLC EMAT, as already reported in [ 35 ]. In WBG EMAT, the wires and eddy currents are laid beneath the gaps between the magnets, and the efficient magnetic fields are parallel to the sample surface.…”

“…Such parameters as the static magnetic fields, eddy currents, Lorentz forces, and displacements were comparatively studied with respect to their contribution to the generation of the symmetric and antisymmetric mode of ultrasonic Lamb waves. Each model consisted of a magnet domain, coil domain, sample domain, and air domain, as shown in [ 35 ]. As mentioned above, the widely used Nd-Fe-B magnets and aluminium plates are chosen in the simulations.…”

Debonding Detection in Aluminum/Rigid Polyurethane Foam Composite Plates Using A0 Mode LAMB Wave EMATs

“…However, compared with PZTs, EMATs have low transduction efficiency and, therefore, have a lower signal-to-noise ratio [ 19 , 20 , 21 ]. At the same time, when EMATs are used to generate Lamb waves, the multimode and dispersion characteristics of Lamb waves affect the detection [ 22 ]. Many scholars have conducted research on this issue.…”

Antisymmetric Lamb Wave Simulation Study Based on Electromagnetic Acoustic Transducer with Periodic Permanent Magnets

“…Wilcox et al [ 14 ] proposed an omnidirectional transducer that uses a spiral coil and cylindrical permanent magnet to excite the S0 mode. Yang et al [ 15 ] proposed a periodic magnet EMAT configuration, which can significantly enhance the amplitude of S0 mode by using periodically arranged magnets to increase the magnetic flux density of the local magnetic field. Sun et al [ 16 ] proposed an EMAT in which the magnet consisted of a cylindrical magnet and a ring magnet around the outside of the cylindrical one, which eliminated the multi-mode phenomenon and improved the purity of the A0 wave.…”