CONOCIMIENTO DE LA EMPRESA RESPECTO A LA OFERTA DE PROVEEDORES

The Electromagnetic Acoustic Transducer (EMAT) provides an excellent option for NDT applications on metallic samples. They are coupled to the sample by electromagnetic waves, and therefore provide a non-contact solution to generating ultrasound [7]. EMATs can work at lift offs of the order of several millimetres above a sample surface, though efficiency falls with increasing separation between the conductive sample and transducer. Experiments utilising EMATs can be completed at elevated temperatures and in hostile environments [8]. The main disadvantages include they are restricted to only samples with good electrical conductivity, or with high magnetostriction such as the magnetite oxide scale often found on high temperature boiler tubes [9]. The transduction efficiency of EMATs is usually orders of magnitude lower than PETs, meaning that there needs to be a clear justification for using EMATs.

The ultrasonic measurements described in this thesis were done exclusively using EMATs. Although there are other EMAT designs available, such as ferrite-enhanced EMATs [10] or using EMATs that employ electromagnets [11], the EMATs used in this study are all consistent with the following described configuration.

The EMAT comprises of two main components: a coil and a permanent magnet [12]. The inductive coil is placed adjacent to the sample surface and positioned below a permanent NdFeB magnet arrangement. The magnetic flux density at the surface of the NdFeB magnets is typically between 0.5 T to 1 T. Changes in the geometry and arrangement of the coil and also the magnet configuration, allows for the generation of different wave modes and polarisations, depending on the desired application. The magnets, the coil and the electronics need to be housed in a suitable case and brass is used often, because it is easily machinable, provides good electrical screening and is non-magnetic. Figure 3.1 shows a photograph of an EMAT typically used in the work for this thesis, with figure 3.2 giving a simple schematic diagram displaying the components of an EMAT.

Figure 3.1: Photograph of a typical EMAT used for this study. The brass casing houses the permanent magnet and coil. The brass casing is 50 mm tall with a diameter of 45 mm.

Generating ultrasound using an EMAT is dependent on coupling electromagnetically to the sample. This is achieved by the induction of eddy currents in the sample skin-depth as a consequence of a time-varying current passing through the generation coil, and also the introduction of the static and dynamic magnetic fields due to the permanent magnet and coil.

As a result of this electromagnetic coupling, two mechanisms contribute to the force that is required to generate the subsequent ultrasonic wave. These are the Lorentzian and magnetoelastic mechanisms. The magnetoelastic mechanism is made up of contributions due to a magnetization force and a magnetostriction force. In this work, the Lorentzian

mechanism is the dominant contributor in both the aluminium and steel samples studied. Both mechanisms are discussed in detail in sections 3.2.1.3 - 3.2.1.5 after the electromagnetic coupling is formally introduced.

Figure 3.2: Schematic diagram of an EMAT and its typical components. BS is the static magnetic

field contribution from the permanent NdFeB magnet. There will also be a dynamic magnetic field contribution (BD) from the coil, which is not shown here as this is dependent on the coil geometry.

Before describing the electrical coupling mechanism and ultrasound generation in more detail, the electromagnetic framework is needed to fully describe and understand the electromagnetic interactions.

3.2.1 – EMAT operation theory

3.2.1.1 - Maxwell’s equations

In describing the electromagnetic interactions that produce the coupling mechanism between the EMAT and an electrically conductive metallic sample, the electromagnetic framework defined by Maxwell‟s equations is required [13], as listed in equations 3.1-3.6.



(3.1)

0

.



B

(3.2)

Inductive spiral coil

BS

N

S

t       E B

(3.3)

(3.4)

E

D



(3.5)

H

B



(3.6)

Here D is the electric displacement, B is the magnetic flux density, E is the electric field and

H is the magnetic field intensity. The parameter ε is the electric permittivity and μ the magnetic permeability. The subscripts 0 and r refer to the free space and the relative values

respectively. These equations link the electric and magnetic fields to the charge and current densities ρ and j respectively.

The following sections specifically detail the process of how an EMAT produces an ultrasonic response. To couple electromagnetically the sample to the EMAT, an image or eddy current needs to be generated in the sample.

3.2.1.2 – Eddy currents, J and H

When passing an alternating current through the inductive coil of an EMAT placed close to (within a few millimetres) a conducting surface, an image current, or eddy current, is induced within the sample [14]. Figure 3.3 shows a diagram of the induction of an eddy current. These currents are concentrated in the near-surface of the test sample, predominantly within the electromagnetic skin-depth δ. The magnitude and frequency of the eddy currents are dependent on the magnitude and rate of change in magnetic flux from the time-varying current in the generation coil. The induction of eddy currents is therefore insensitive to the presence of the static magnetic field produced by the permanent NdFeB magnet [15].