This technology offers an opportunity for non-contact generation of different types of acoustic waves with different polarization in the monitored object. Modern element base allows production of non-destructive testing devices and thickness meters that can operate with working gap up to 10mm (i.e. there can be paint, plastic, dirt, air and other non-conductive material up to 10mm thick between the surface of the monitored object and the meter surface) using EMAT. In addition, the acoustic wave is generated directly in the monitored object and it’s not distorted with coupling media. The mechanism of electric oscillation forming mechanical oscillation in EMAT can be divided into three components: magnetostriction, the Lorentz force interaction and magnetic interaction. Generally, for monitored objects made of steel the Lorentz force EMAT is applied.
EMAT operating principle
The transducer schematic structure is shown in Picture 1. It comprises permanent magnet and AC conductor. The AC flows through conductor and generates varying magnetic field that penetrates the monitored object and generates eddy current in it. Direction of charged particles that generate eddy current is opposite to the direction of current in the conductor. Permanent magnet generates normally directed to the surface of monitored object continuous magnetic field. The Lorentz force is applied to the charged particles flowing in the magnetic field and it’s directed along the surface region of the monitored object. The Lorentz force conduces some mechanical displacement of the eddy current region which forms the acoustic wave.
Main advantages of EMAT technology in comparison with traditional ultrasonic testing technology
- No couplant is needed compared to piezoelectric transducers
- No advance surface preparation needed (like stripping and polishing)
- EMAT are not sensible to the tilting angle of transducer relative to the surface of acoustic wave input. Transducer tilting only changes the level of signal, direction of wave, therefore, timed position of echo signals doesn’t depend on transducer tilting angle
- Gap between the meter and the surface of the monitored object can be up to 10mm
- Acoustic wave starts distribution directly in the surface of the monitored object passing material between transducer and the monitored object. This way, there is no wave distortion in this material
- EMAT technology allows generation of different types of waves with different polarization. This includes Lamb waves, Rayleigh waves, transverse waves with horizontal, vertical and radial polarization, Longitudinal waves