Numerical Model of Magnetostriction and Lorentz Force Mechanisms in Electromagnetic Acoustic Transducers

In this thesis, a successful development of a numerical model of magnetostriction and Lorentz force mechanisms in Electromagnetic Acoustic Transducers (EMATs) for nondestructive evaluation of steel components is presented. A Periodic Permanent Magnet Electromagnetic Acoustic Transducer (PPM-EMAT) scanner is designed and a prototype scanner is built for identifying corrosion defects that feature at least 20% wall-loss on ¼ "-thick mild steel plates. A sequence of experimental measurements is performed to evaluate the peak amplitude of the Shear Horizontal (SH) waves received from defect-free plates while the scanner moves in various directions on the surface of the sample plate. Scanner movement parallel to the direction of SH wave propagation led to an average of +/-20 % change in the received wave amplitude when the transmitter motion is towards/away from the receiver. This increase/decrease in amplitude remains intact even when scanner motion is stopped. Published models of PPM-EMAT transduction mechanisms are primarily focused on the Lorentz force coupling mechanism. These models ignore nonlinear magnetic behaviours of the steel samples and they cannot explain the source of asymmetrical coupling force distribution due to the scanner motion. A model is developed to evaluate the contribution of all transduction/reception mechanisms to the SH wave generation/reception of a stationary PPM-EMAT scanner operating on a mild steel plate. The PPM-EMAT transmitter, SH wave generation/propagation, and the PPM-EMAT receiver are modelled separately using a semi-analytical approach. The effects of the scanner motion are studied by including the magnetic hysteresis caused by the PPM array movement to the plate’s properties in the Finite Element Method (FEM) model of the magnetic fields. Model results demonstrated that the magnetostriction contribution to SH wave generation is approximately 55 % of the Lorentz mechanism for our EMAT configuration. The Lorentz mechanism is found to be insensitive to the scanner motion, while magnetostriction experiences a spatial displacement with respect to the Lorentz coupling mechanism in the transmitter EMAT. The new model results are consistent with the general behaviour of the experimental system.Ph.D