EMAT (electromagnetic accoustic transmission)

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.

Potential Applications

-Thickness characterisation on aged boiler tubes[1]
-Gas pipeline inspection[2]
-Analysis of adhesive bonds[3]

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