FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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ULTRASONIC ANALYSIS OF VOIDS IN GLASS; THEORY AND PRACTICE
Aitkux J. Stockman, Jan van den Ande.1 and Patrick S. Hlcholion
McMaiie-t linivdtisitij
Hamilton, Ontaiio, Canada
ABSTRACT
The characterization of voids in glass enables the study of reflected ultrasonic signals off . known but
undisturbed defects which can also be characterized by optical means. The reflected ultrasonic
signals can be compared with theoretical signals transformed to a frequency spectrum by a computer
and a frequency spectrum can be transformed to a time signal (A-scanl.
Flaw information can now be read from such signals, and such will eventually help to characterize
flaws in opaque ceramics.
I INTRODUCTION
The characterization of defects in high performance ceramics is important when the components are
to be used in situations of high stress. Size, shape and composition of the target defects will determine
whether or not the ceramic will fail. Interest in ultrasonic testing to determine these factors has led
to many advances in the field of defect characterization by scattered signal analysis.' 1 '
On the link between theory and practice, it was found by the authors that a beam entry of exactly 90°
with the entrance surface of the component was important.' 2 ' The essence ofthat article was that the
frequency components of the ultrasonic beam pass the interface undistorted if a perfect 90° angle-is
maintained. A slight misalignment will cause enough distortion to make the frequency spectrum of
the reflected beam lop-sided and random misalignments from sample to sample means that the
analysis will no longer yield accurate results.
The importance of proper alignment is brought out even more in a study of reflections of previouslyfound
defects. 13 ' Each defect responds differently to a frequency component so their reflected signals
contain information about their type and shape. The change in frequency component amplitude can
also be calculated, measured and the resulting spectrum compared to the perfectly aligned spectrum,
i.e. the only distortion is caused by the defect rather than by off-normal set-up conditions
The technique was applied to ceramics at frequencies above 10 MHz in order to test for flaws
< 100 urn. A precision immersion scanner with a resolution of 20 urn was built especially for locating
these small defects. Beams were focussed to obtain higher power densities and even though these
beams do not provide a true plane wave, the wave theory as reported by Ying and Truell' 4 ' was
applied along with the "high frequency decomposition" part, reported by Tittman et al.' 5 '