Lift-off compensation for improved accuracy in ultrasonic lamb wave velocity measurements using electromagnetic acoustic transducers (EMATs)

The crystalline texture of a sheet metal strongly affects its formability, so having knowledge of this texture is of great industrial relevance. The texture of rolled sheet metals, such as aluminium and steel, may be determined by ultrasonic measurement of the velocity of the zero order symmetric (S(0)) Lamb wave as a function of angle to the rolling direction. Electromagnetic acoustic transducers (EMATs) may perform this measurement without contacting the sample, therefore reducing perturbation to the plate wave system, as they are electromagnetically coupled to the sheet. The EMAT system measurements are non-destructive and may be made in real time, therefore offering advantages over the conventional techniques such as X-ray and neutron diffraction. It has been noticed that in the two EMAT pitch-catch system, the apparent arrival times of the ultrasonic waves change with variation in lift-off (distance between sample and transducer) due to impedance and aperture effects. For precise and accurate texture parameters to be obtained, accurate absolute ultrasonic velocity measurement is required and hence lift-off must be compensated for. This is of particular importance to online inspection systems where constant lift-off may be difficult to maintain. The impedance behaviour of various coil geometries has been investigated as a function of lift-off and frequency and compared to the received ultrasonic signal and the drive current pulse profile. Theoretical models have been used to explain the observed behaviour, and hence a scheme has been proposed for the compensation of lift-off effects in real time.