Modelling and design of shear wave EMATs

Abstract

In this thesis two physical phenomena were investigated: the mechanical load generation and wavefield radiation of Electro-Magnetic Acoustic Transducers (EMATs). The conceptual results obtained in this work addressed two major research questions: how the transduction of EMATs and the properties of materials affect the EMAT's wavefield generation, and how orthogonal shear waves can be used to detect and characterise surface-breaking defects. The outcome of these two research questions were tested in the design of an orthogonal and co-located coils shear wave EMAT and in the design of a shear wave phased array EMAT.

EMATs, are transducers that by means of electromagnetic forces excite mechanical loads in metallic materials. EMATs are often utilised in the Non-Destructive Evaluation, NDE, of a material to detect and characterise the material anomalies. Compared to their counterpart, Piezoelectric transducers, EMATs have several properties that make them a desirable means of transduction for NDE applications. EMATs are contactless and special designs can tolerate high temperatures. Hence, EMATs do not require direct contact with the inspected material to perform the NDE inspection nor cooling system to keep them operational during a NDE procedure. Nevertheless, the use of EMATs in NDE inspections is not standard procedure in industry. This is because there are concerns over the strength and orientation of the mechanical loads that EMATs produce, and the lack of knowledge about the effects that the magnetic properties of the inspected material have on the wavefield generated with EMATs. To address these concerns, the excitation mechanisms of EMAT transduction were studied on different metallic materials. The three excitation mechanisms -the Lorentz force, the magnetisation force and the magnetostriction force- of a shear wave phased-array EMAT were estimated on paramagnetic, diamagnetic, ferromagnetic and magnetostrictive materials. The results obtained showed that by designing an EMAT to maximise the Lorentz Force mechanism, an EMAT can maximise the strength of the mechanical loads it excites and yield a consistent wavefield in the inspected material.

The interaction of orthogonal and co-located shear waves with surface-breaking defects was also studied in this thesis. The study focused on the detection and characterisation of surface-breaking defects in isotropic and anisotropic materials. To execute the mentioned analysis, the design of an EMAT that can simultaneously excite two co-located shear waves with orthogonal polarisation was proposed. Once the wave interaction with the defects was understood, a methodology that by means of the two shear waves can gauge the inspected material, detect the presence of a defect, characterise the principal shear direction of the inspected material and characterise the height of the surface-breaking defect was presented.