Modeling And Research Of EMAT Based On Lorentz Mechanism

Electromagnetic acoustic technique doesn't need couplant and can generate many kinds of ultrasonic waves, so it is widely applied in ultrasonic nondestructive detection. Modeling and simulating of Electromagnetic Acoustic Transducer (EMAT) based on Lorentz mechanism has been studied. The sound field characteristic of EMAT has been calculated by the combination of numerical calculations and analytical solutions, building theoretical foundation for optimization of EMAT.First, Lorentz mechanism has been deduced according to electromagnetic theory. Based on elastodynamic theory, the analytical equations of surface wave displacements and body wave displacements, generated by point sound source, have been deduced. And these equations have built theoretical foundation for calculating the sound fields of surface wave and body wave generated by EMAT.The model for surface wave EMAT with a meander-line coil has been established by 3-dimentional finite element method. The Lorentz force distribution and surface wave distribution have been simulated and analyzed. The Lorentz force consists of two components generated by static magnetic field and dynamic magnetic field respectively. The surface wave amplitude due to static magnetic Lorentz force is proportional to excitation current, while that due to dynamic magnetic field is a quadratic function of excitation current. When excitation current is 528.9 A, surface wave amplitudes due to static and dynamic magnetic fields are equal. When excitation current is large enough, the effect of dynamic magnetic Lorentz force can't be ignored.The model for body wave EMAT with a spiral coil has been established. The sound fields of shear and longitudinal waves in aluminum plate are simulated and analyzed. Radial Lorentz force mainly generates shear wave propagating along depth, while vertical Lorentz force mainly generates longitudinal wave propagating along depth. Shear wave is larger than longitudinal wave generated by spiral coil EMAT. Shear wave can propagate far while longitudinal wave decreases very quickly.To verify the validity of established model, EMAT probe has been used to measure sound characteristics generated by surface wave EMAT. Mechanical device is also applied to make sure the receiving EMAT probe moves accurately. The measured results agree well with the calculated ones verifying the accuracy and validity of the model.