Preliminary tests, sponsored by the Association of American Railroads, were conducted in a laboratory setting using an off-the-shelf EMAT sensor. The sensor demonstrated excellent detection capability with a good signal-to-noise-ratio (SNR) for the wheel samples with internal defects.
An EMAT is an ultrasonic technique that generates sound using electromagnetic induction. Because the ultrasound is generated directly within the material adjacent to the transducer, this technique does not require any couplant to perform the inspection. EMAT ultrasound generation is based on the interaction between the magnetic field created by a magnet and the eddy currents induced in the test piece by a coil circuit. Together, the magnetic field and the currents create a Lorentz force or magnetostriction within the material, causing the vibration of the materialís lattice, and thereby generating ultrasonic waves. Figure 1 (above) shows the comparison of the EMAT principle with the conventional piezoelectric-based ultrasonic testing (UT) principle. Depending on the sensorís design, the wave type and wavelength can be varied. The same or a similar sensor receives a return signal and provides a very precise measurement of the surface acoustic wave velocity in the test piece.
Read the full article at Railway Age.