Ultrasonic non-destructive evaluation of thin specimens

Abstract

In the field of non-destructive evaluation of materials, conventional ultrasonic measurement techniques are limited to materials having a thickness which is relatively large compared to the wavelength of the ultrasonic signal used. The present technique enables the accurate ultrasonic non-destructive measurement of materials which are relatively small compared to the wavelength of the ultrasonic signal used. Ultrasonic signals received from a thin material are processed in the frequency-domain either directly or by use of a Fast Fourier Transform. Specifically, the frequency response of the ultrasonic transducers used in the measurement is removed from the frequency response of the signal received when measuring the material. This yields a frequency response which is indicative of the material alone. Then, the measured frequency response of the material is evaluated to determine unknown parameters of the material. For instance, the phase of the measured signal is determined over a predetermined frequency range (usually the bandwidth of the transducers), and the phase is used to determine the speed of ultrasonic waves travelling in the material for frequencies within the predetermined range. Likewise, the magnitude of the measured signal is determined over the predetermined frequency range, and the magnitude is used to determine the attenuation of ultrasonic waves travelling in the material for frequencies within the predetermined range.