Application of chirp pulse compression technique to a high-temperature EMAT with a large lift-off

Abstract

The interest in applying non-destructive testing to the examination of the flaw and measurement of the thickness in the metals at high temperatures is increasing as the metal processing becomes continuous. Based on this background, we designed a water-cooled shear wave Electromagnetic Acoustic Transducer (EMAT) with a surface cooling technique, and the conversion efficiency could be improved by surface cooling. We used the chirp pulse compression technique to process the raw ultrasonic wave signal without synchronous average. Besides, a large casting slab with coarse grains and a massive forging with high temperatures were chosen as the testing objects. Then, the effect of the lift-off distance on the Signal-to-Noise Ratio (SNR) of ultrasonic signals from the EMATs operated in both the pulse-echo mode and pitch-catch mode was investigated. Subsequently, the ultrasonic signals were de-noised respectively by synchronous averages and pulse compression, and the noise reduction ability was compared. Finally, the EMAT was tested with the forging at different temperatures from 92 °C to 730 °C. The performance of the EMAT with the excitation of a tone-burst signal was also compared with the performance of the EMAT using pulse compression technique. Results show that the lift-off, SNR, range resolution, and inspection efficiency can be largely enhanced with the signal processing of chirp pulse compression to the ultrasonic wave signal with no synchronous averaging. When the forging’s temperature is 730 °C, the SNR of the compressed signal is comparable to the SNR of the signal processed with 64 synchronous averages, and the wave-packet duration can be reduced by 80.5%. The proposed EMAT detection technique is particularly suited for real-time measurement and high-temperature testing.