Electromagnetic acoustic transducers: Physical principles and finite element modeling.

摘要

Electromagnetic acoustic transducers (EMATs) generate ultrasonic waves due to Lorentz and magnetostrictive forces. Conversely, an EMAT can also be applied as a receiver for acoustic waves, thus forming a complete transmitter/receiver pair. Unlike piezoelectric transducers, which have to be tightly coupled to the medium under inspection, EMATs do not require direct contact with the specimen. This contactless feature makes them particularly suitable for the nondestructive inspection of all conductive materials in the steel industry where elevated temperatures and rough surface conditions are of concern.; The theoretical basis of the electro-acoustic transduction phenomenon of an EMAT system requires solutions to both Maxwell's field equations and the elastodynamic equation of motion. In the past, only approximate solution approaches have been developed to investigate the complex coupling between the electrical and mechanical systems.; In this dissertation, a complete set of governing equations and their boundary conditions are systematically derived from momentum conservation laws, which provides a clear picture of the physical principles of an EMAT. Numerical analysis techniques are applied to solve the underlying electrodynamic equations for realistic magnetostatic and pulsed eddy current distributions in the specimen. Resulting field predictions from these elliptic and parabolic system models are then coupled into a hyperbolic description of the elastic wave equation to simulate transient ultrasonic waves in isotropic solids. Both the electrodynamic and acoustic wave equations are solved in two dimensions based on hybrid implicit and explicit finite element and finite difference time stepping algorithms.; In order to test the numerical simulations, analytical solutions of both parabolic and hyperbolic systems are derived for certain canonical examples which permit direct comparison between numerical and analytical predictions in the time domain. Having confirmed the correctness of the overall numerical model, it is subsequently extended to simulate a generic two-wire transient EMAT configuration for an isotropic half-space with electric and acoustic material parameters equivalent to those of aluminum. The predominate Lorentz forces have been successfully implemented leaving magnetostrictive forces and magnetization effects for future development.