FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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frequency of about 50 MHz is optimum for the detection of surface cracks as small
as 20 um in silicon nitride. This frequency is equivalent to an acoustic
wavelength of 112 um in silicon nitride. Beyond 50 MHz, the detectability tends
to level off due to the increase of attenuation losses. Thus, the crack
measurements in this investigation were carried out at 50 MHz.
The leaky Rayleigh waves propogating along the water-solid interface, lose their
energy rapidly and particularily at high frequencies. Thus, only those flaws
which are located near the point of incidence of the impinging beam are detected.
As a result, the resolution of this method is very good since reflections from
neighbouring flaws or specimen edges do not overlap with the signal from the flaw
of interest. The resolution is particularily good when a narrow Rayleigh beam is
employed by focusing the incident beam on the surface of the test piece using a
focused transducer.
Overall, the scanning simplicity, the high resolution and high sensitivity of
leaky Rayleigh waves make this technique a practical and reliable method of
detecting small surface flaws in ceramic materials.
2.4 Measurement techniques
The normal procedure for flaw size measurement by any ultrasonic technique
utilizes the acoustic reflection coefficient, S, of the flaw for size estimation.
S is defined as the ratio of the peak amplitudes of the flaw signal to input
signal at the transducer terminal. In the case of cracks, however, the acoustic
reflection is very much dependent on orientation of the crack with respect to the
incident beam. This is demonstrated in the experimental results of Fig. 5
obtained for a 100 ym Knoop indentation. Analytical solutions are given by
Auld^ ' for calculation of the reflection coefficient at oblique angles. Both
the analytical and experimental results indicate that as the direction of the
crack relative to the Rayleigh beam deviates from normal, a smaller portion of
the crack face reflections are received by the transducer. Thus, for complete
characterization of cracks from the reflection coefficient, it is necessary to
scan the test material in more than one direction.
Another important factor affecting the acoustic signal amplitude from a flaw is
the wavelength compared to the flaw size. Since at 50 MHz the Rayleigh
wavelength in silicon nitride is about 112 pm and this is in the flaw size range
of our interest (i.e. 10-200 um), therefore both the long wavelength (X^>a) and
the short wavelength (A^