The main difference of this technology is that the ultrasonic wave is generated directly in the tested object.
The principle of the action of EMA transducers is based on the excitation and reception of ultrasonic waves by induction on the surface of an electrically conductive object of alternating and constant magnetic fields. The EMAT inductor, through which a high-frequency current pulse flows, creates an eddy current in the surface layer of the metal. As a result of the combined effect of the eddy current and the polarizing field, Lorentz forces arise, which ensure the excitation of ultrasonic oscillations of the corresponding type. Depending on the design parameters of the inductor and the orientation of the polarizing field, shear SH waves of radial or linear polarization (an oval or rectangular inductor) are excited during the monitoring process.
With the help of EMAT, it is easy to excite, for example, transverse waves propagating along the normal and at an angle to the surface, as well as transverse normal waves. EMATs do not load the surface of the monitoring object, which allows to eliminate problems associated with reverberation processes – phenomena on the interface between media in the contact or immersion liquid layer. The acoustic path of the EMAT of the flaw detector is very simple, since ultrasonic waves are excited and propagate, as a rule, only in the object of control. Only the object of control and the heterogeneities contained in it are capable of changing the picture of the wave field.
The electromagnetic contact of the EMAT with the surface of the monitoring object is, in most cases, much more stable than the acoustic contact for piezoelectric transducers. The oscillations of the reference “bottom signal on the defect-free sections, as a rule, do not exceed 4-6 dB. The acoustic axis does not deviate when the position of the EMAT is changed relative to the surface of the monitoring object.
Contact fluid is one of the weakness points of traditional ultrasonic methods as well as of the most systems that Implement them. The need for the use of liquid increasingly comes into conflict with modern metallurgical technologies. It is increasingly difficult to provide the necessary requirements for the speed of movement, temperature and condition of the surface of the objects of control. The use of water, even with the addition of additives, often leads to corrosion and, as a consequence, to a deterioration in the presentation of the product. The use of electromagnetic-acoustic transducers does not require the use of contact liquid and therefore is devoid of all the drawbacks associated with it.
Non-contact methods of excitation of acoustic waves by means of EMAT significantly expand the possibilities of ultrasonic testing at high and low temperatures, rough and contaminated surface of objects, as well as in cases when using the applied technology, contact fluids are unacceptable.
The main advantages of control technology with the use of EMAT in comparison with traditional ultrasound control
- EMAT can excite transverse waves in a wide frequency range, which allows to increase the measured range and increase the accuracy of measurements in comparison with the standard PEP
- The absence of direct contact between the EMAT and the metal significantly reduces the requirements for surface preparation and does not require the use of contact fluid
- Ultrasonic vibrations excited by the electromagnetic field are not sensitive to distortions when installing the EMAT on the product, eliminating measurement errors caused by the refraction of sound at the boundary “converter-object of control”
- The working surface can have a wide temperature range without the risk of damage to the sensor
- The control is not affected by the angle of the pulse input, so the transducers do not divide into inclined and direct
- The contactless converter is not subject to friction and as a result does not wear out
- Instruments using electromagnetic-acoustic transducers can work on objects of complex shape and pipes of small diameter
The disadvantages of EMAT include their relatively low sensitivity and noise protection. One of the possible methods for increasing the sensitivity of an EMA-signal is the use of coherent processing of received signals.