Electro Magnetic Acoustic Transducer

An Electro Magnetic Acoustic Transducer (EMAT), is a non-contact inspection device that generates an ultrasonic pulse in the part or sample inspected, instead of the transducer. The waves reflected by the sample induce a varying electric current in the receiver (which can be the same EMAT used to generate the ultrasound, or a separate receiver). This current signal is interpreted by software to provide clues about the internal structure of the sample.

Any faults or cracks in a sample constitute a boundary, which results in partial reflection of the incident ultrasonic pulse. Knowing the speed of ultrasound in the sample means that the depth of each crack can be calculated. This is done by halving the time taken between the generation of the pulse and the reception of the reflected signal, and multiplying by the speed of ultrasound in the sample. Thus, using an EMAT, it is possible to build up a profile of the interior of a sample without having to damage or deform it in any way.

As well as cracks in the interior, ultrasound will be reflected off the exterior boundaries of samples, meaning that the technique can also be used to calculate the thickness of samples. This is particularly useful when calculating the thickness of metal pipes, as the pipe does not have to be opened up or even empty for it to be tested. This is especially useful when dealing with pipes that are operational 24 hours a day - blockages, corrosion and other problems can be tested for and located without stopping the flow.

Generation

An EMAT induces ultrasonic waves into a test object with two interacting magnetic fields. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequency or static field generated by magnets to create the wave in the surface of the test material. Various types of waves can be generated using different RF coil designs and orientation to the low frequency field.

Transduction Process

1. Alternating current is passed through the coil, at megahertz frequency, for a short period of time.

2. The coil generates a magnetic field as it is a solenoid, which passes through the sample. The field changes direction at the same frequency as the alternating current.

3. The changing magnetic flux induces an EMF (electromotive force) in the sample. The direction of this EMF oscillates at the AC frequency. This means that the EMF provides eddy currents at the surface of the material. The induced EMF is a consequence of Faraday's law: ε = − dΦ_B/dt

4. A static magnetic field through the material interacts with the eddy currents: the Lorentz force acts upon the electrons in the eddies. The direction of this force is determined by the directions of the eddy current and static magnetic field, and so can controlled by the setup and directions of the coil and magnet.

Lorentz force equation:

F = J ∧ B₀ 5. As the eddies are changing direction, the Lorentz force changes direction too. So a Lorentz force is set up in the sample, oscillating at the AC frequency.

6. As the electrons are moving as a result of the Lorentz force, the whole atomic lattice oscillates at the megahertz frequency of the AC. This produces the ultrasonic wave.

Potential Applications

EMATs have a range of potential industrial applications, such as:

*Thickness characterisation on aged boiler tubes [M Gori, S Giamboni, E D’Alessio, S Ghia and F Cernuschi, ‘EMAT transducers and thickness characterization on aged boiler tubes’, Ultrasonics 34 (1996) 339-342.]
*Gas pipeline inspection [M Hirao and H Ogi, ‘An SH-wave EMAT technique for gas pipeline inspection’, NDT&E International 32 (1999) 127-132]
*Analysis of adhesive bonds [S Dixon, C Edwards and S B Palmer, ‘The analysis of adhesive bonds using electromagnetic acoustic transducers’, Ultrasonics Vol. 32 No. 6, 1994.]

Advantages

Because the sound is generated in the part inspected, EMATs have the following advantages over more conventional piezoelectric transducers:

*Nondestructive Testing Method (NDT).
*Dry inspection. EMATs do not require couplant for transmitting sound making them very well suited for inspection of hot parts or automated environments.
*Impervious to surface conditions. EMATs can inspect through coatings and are not affected by pollutants, oxidation or roughness
*Easier probe deployment. Since the sound is generated in the part, the angle of the probe does not affect the direction of sound making them easier to control and deploy in automated environments
*Ability to generate unique wave modes. EMATs are the only practical means for generating shear waves having a horizontal polarization (SH waves), which do not travel through low-density couplants. Shear Horizontal waves do not mode convert when striking surfaces that are parallel to the direction of polarization.

Disadvantages

*Low efficiency compared to piezoelectric transducers
*Larger transducer size
*Only available for conductive materials

References

More Information

American Society for Testing & Materials [http://www.astm.org (ASTM)] .

ASTM E1774-96 Standard Guide for Electromagnetic Acoustic Transducers (EMATs)

ASTM E1816-96 Standard Practice for Ultrasonic Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology

Innerspec Technologies: [http://www.innerspec.com (Commercial Applications of EMAT)] .

ASTM E1962-98 Standard Test Methods for Ultrasonic Surface Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology