FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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It is also possible to do the reverse and transform the complex frequency spectra into time spectra;
Figure 8 is an example o[ this for a 90 pm void. Transformations for void sizes from 28 urn to 132 urn
diameter show that the fargest deflection from zero was negative-going. In Figures 9a-d, peak
deflections versus time have been plotted with the largest negative-going deflection normalized to -
100%. From this figure, it is evident that as the void diameter increases, the largest positive-going
deflection changes from the one following the largest negative-going deflection to the positive-going
deflection preceding it. This behaviour has been noted previously in the experimental results.
Plotting ratios of these two positive going peaks against the void dimensions (fig. 10) shows that there
exists a reasonable correlation with the minor dimension of the void. Although a similar relation
exists with the major void dimensions it is less apparent, especially for the larger dimensions which
approach or exceed the wavelength, \c, of the main frequency component..
V. CONCLUSIONS
Calculations have shown that trends apparent in-signals reflected from voids in the focal zone of
pulsed focused transducer can be reproduced by a model based on scattering of monochromatic plane
waves convoluted with the frequency distribution of the incoming wavepacket. Although direct
signal amplitude comparison of theory and experiment is not possible due to lack of information about
the focal zone pressure, comparison of normalized signals show strong correspondences in signal
shape. The calculated changes of the frequency distribution as a function of void diameter as
measured by the frequency at maximum amplitude and frequency full width at half maximum have
been followed closely by the measured values.
The ratio of the positive going signal peaks in the time spectra have proven to be related to the size of
the reflector and the wavelengths used. It can be used as a practical tool for absolute sizing without
the use of artificial reference.
REFERENCES
1. Proceedings of the DARPA/AFWAL Review of Progress in Quantitative NDE (1981).
2. Van den Andel, J Stockman, A., and Nicholson, P.S., "The Importance of Correct Alignment
in Ultrasonic Testing", Advanced NDE Technology, Bussiere, J.F. ed., NRC Canada, 1982, p.
21.
3. Nicholson, Stockman and Van den Andel, "Ultrasonic Detection of Defects in High
Performance Ceramics", CSNDT Journal, Vol. 4, No. 2, Nov. 1982.
4. Ying, CF. and Truell, R., "Scattering of a Plane Longitudinal Wave by a Spherical Obstacle
in an Isotropically Elastic Solid", J. Appl Phys.36, 1086(1956).
5 Tittmann, B.R., Cohen, R.E. and Richardson. J.M., "Scattering of Longitudinal Waves
Incident on a Spherical Cavity in a Solid", J. Acoust. Soc. Am, 63, 68(1978).