Non-contact ultrasonic thickness gauging of alum inium sheet with submicron accuracy using Electromagnetic Acoustic Transducers (EMATs)

We have developed a non-contact ultrasonic system capable of measuring the thickness of aluminium sheet with sub-micron accuracy. The thickness of sheet has been calculated from ultrasonic data obtained using a send-receive, radially polarised shear wave Electromagnetic Acoustic Transducer (EMAT). Sheets in the thickness range between 0.1mm to 0.5mm have been measured using this non-contact approach at a stand-off of up to 1.5mm. The ultrasonic echo trains have been processed using Fourier analysis to extract transit time measurements for the two possible shear wave polarisations within the sheet. Two broadband EMAT systems have been used to perform the measurements with centre frequency of approximately 5MHz and frequency content up to 10MHz and 20MHz respectively. The most accurate measurements of thickness on thin sheets have been made using Fourier analysis rather than direct temporal measurements and have yielded thicknesses accurate to within 0.2%. This accuracy is potentially of significant commercial benefit to sheet manufacturers. There are also other potential applications of this approach, particularly for the measurement of very thin layers on aluminium substrates and these will be discussed in the presentation