Generation EMATs

The generation EMATs are separated into two main categories depending on whether the EMAT was designed to generate in conjunction with a broadband pulse, or via a narrow-band tone burst. The two broadband EMATs suitable for generation have been discussed briey in section 2.3.3, but the exact construction of the linear and spiral coils used is elaborated upon here.

For the generation EMAT in the initial experiments a linear coil was used (shown in gure 2.11 (a)), which consisted of 4 turns of 0.315 mm wire, wrapped in a loop around two 12 mm cubic magnets arranged with the north-south axis parallel to the surface of the sample. The relatively thick wire gauge served to protect the EMAT from damage due to ohmic heating as a result of the large current passed through the coil [126]. The front face of the EMATs were protected by a lip on the edge of the transducer housing, which kept the coil recessed from the sample. Although this simple design was sucient for laboratory based experiments, more complex steps must be taken in an industrial environment, such as adding a ceramic wear face to the EMAT. Unfortunately, due to the relatively small number of turns of wire in proximity to the surface of the sample the induced eddy currents, and thus the Lamb wave amplitudes, were relatively small. In addition, the small size of the cubic magnets caused a number of edge eects to be introduced into the generated Lamb waves. Although the use of larger magnets would improve the signal to noise of the generated ultrasound, as well as reduce the signicance of any edge eects, a dierent strategy was pursued in order to limit the size of the magnets used, as these can often attract swarf and other metal debris in an industrial environment.

In order to increase the amplitude of the Lamb waves generated, an alternative coil design was used which increased the eddy current density in the surface of the sample. This was achieved using a spiral coil consisting of 20 turns of 0.315 mm wire, with a ferrite backing plate designed to enhance the self eld of the coil [127]. An additional benet of using the spiral coil was that the impact of edge eects was greatly reduced, where the edge eects are attributed to the interaction of the eddy currents in the surface of the sample with the static bias eld at the edges of the linear

coil transducer. These interactions are undesirable, as the bias eld is not uniform, particularly towards the edges of the magnet, and can lead to unintended in-plane generation forces [118].

The narrow-band EMATs were designed to have a periodicity of 9 mm, which satises the criterion that for the combined approximation that the wavelength of the ultrasound be much larger than the thickness of the sample. The rst EMAT designed in with this periodicity was a meander coil, which had no bias eld and relied entirely on the self eld of the coil in order to generate an out-of-plane displacement. The self eld was enhanced using a ferrite backing material. The coil is only capable of producing a repulsive force, as discussed in section 2.3.1.1, as the Lorentz force of the dynamic magnetic eld produced by the coil and the eddy currents induced in the surface of a conductive sample will always act in the same direction [119]. Although the eect of the dynamic eld on the generation of acoustic waves using meander coils as a complement to the static bias elds has been discussed [147], it is believed that the reliance solely upon the dynamic eld, enhanced by a ferrite, is a novel method of generating periodic out-of-plane forces in order to excite Lamb waves. The interdigital spacing of the meander coil is equal to the wavelength, due to the solely repulsive nature of the Lorentz force, as shown in gure 3.2 (a). The meander coil initially used consisted of 5 turns of 0.15 mm diameter wire, where the decrease in wire gauge used in comparison to the broadband coils is due to the decrease in power of the generation electronics used for the narrow-band signals. The number of turns in the coil is a separate characteristic from the number of wavelengths of the meander coil, and eectively is the number of times the coil shown in gure 3.2 (a) is superposed.

Due to the periodicity of the EMAT being equal to the wavelength, the size of this meander coil was quite large, for the coil pictured a length of 45 mm, and an alternative design was explored. This novel design uses a racetrack coil in conjunction with a periodic permanent magnet (PPM) array, and is similar to those used to produce SH waves [111]. A schematic diagram showing the alternating eld of the PPM array EMAT is shown in gure 3.2 (b). The magnets are arranged to ensure that the static eld is parallel to the surface of the sample, which interacts with the induced current created by a coil, in order to provide an out-of-plane displacement which alternates with the same periodicity as the array. The EMAT used in this research was constructed using 4 mm cubic magnets, with 0.5 mm alumina spacers, the kerf k in the diagram, between the magnets. The sum of the magnet size and the kerf, 4.5 mm, is shown as d in the diagram, and due to the alternating polarity of the magnets this causes the periodicity of the PPM EMAT to be 9 mm. A racetrack coil, named for its elliptic shape, is used in the EMAT and consists of 20 turns of 0.15 mm wire. In addition to reducing the footprint of the coil from 5λto 3λthis coil generated

(a) λ I F (b) B I F d _k λ sample magnet array coil (c)

Figure 3.2: A schematic diagram of the EMATs used to generate out-of-plane displacements with narrow-band signal showing (a) a top down view of a self eld meander coil, (b) a side view of a PPM EMAT with a racetrack coil, and (c) a picture of the PPM EMAT without a protective housing.

(a) 0 20 40 60 80 100 −50 −40 −30 −20 −10 0 10 Time (µs) Amplitude (V) (b) 0 200 400 600 800 1000 0 0.2 0.4 0.6 0.8 1 Frequency (kHz)

Magnitude (Arb. Units)

Figure 3.3: Characteristics of the broadband generation pulse in (a) the time domain, and (b) the frequency domain.

length would have produced considerably less energy than the PPM array EMAT.