Enhancement of the Rayleigh wave signal at surface defects

We present details of a non-contacting technique that uses ultrasound to detect reliably surface defects with depths that are significantly smaller than the average ultrasonic wavelength to many times this depth. Two electro-magnetic acoustic transducers (EMATs) are used to generate and detect a broadband Rayleigh wave signal with a central frequency close to 200 kHz and extending to approximately 400 kHz. These transducers are moved with a fixed separation between them along a sample towards a defect. Enhancement of the ultrasonic signal is observed when the detector is very close to the defect, with the enhanced signal about 1.6 times the signal away from the defect. A similar enhancement is observed when using an EMAT for generation and a laser interferometer to measure the absolute surface displacement of the sample. We attribute the enhancement of the signal to straightforward interference between the ultrasonic signal that passes directly from the generating coil to the detector and that reflected from the defect. The enhancement can be used to pinpoint accurately a surface or subsurface defect, with the wide-band nature of the signal allowing us to gauge the depth of a wide range of surface-breaking defects.