Advantages of EMAT-based flaw detectors and thickness gauges

The EMAT(Electro Magnetic Acoustic Transducer)-based flaw detectors and thickness gauges carry out measurements by exciting acoustic waves in the test object without direct contact of the sensor with the surface. The acoustic wave is generated due to electromagnetic effects in a surface layer of the object, while the surface can be dirty or covered with a paint, plastic, insulation or any other non-conducting coating up to 10 mm in thickness. In addition, the sensor may be skewed up to 25 degrees to the surface normal which virtually eliminates influence of the human factor on the measurement results.

EMAT technology overview

The EMA transducer consists of a permanent magnet and an induction coil. A high-frequency current pulse flows through the induction coil leading to generation of eddy currents in the surface layer of the test object. Charged particles in the surface layer of the test object create a current opposite to the current in the induction coil, and Lorentz force, directed along the surface of the test object, acts on these particles in a constant magnetic field. As a result, mechanical displacements occur in the areas with eddy current and the acoustic wave is generated. Cracks, corrosion, and other defects disrupt the guided ultrasonic wave, causing a reflected echo signal. Based on the analysis of the reflected echo signal, we can judge about the state of the test object.

Advantages of EMAT technology (as compared with the usual ultrasonic testing)

No need in contact fluid No need in surface grinding or polishing
No distortions of acoustic wave in the external environment (the wave is generated directly on the surface of the test object)
Wide range of frequencies and various types of waves (Lamb waves, Rayleigh waves, longitudinal waves, transverse waves with horizontal, vertical and radial polarization) enabling increase in the measured range and measurement accuracy and operation with complex geometric items
Not sensitive to inclination angle of the transducer relative to the surface of the test object
No need in sensor contact with the surface (a gap can be up to 10 mm)