Correlations between Microstructure and Backscattered Ultrasonic Energy
Scattering of ultrasonic energy by discrete and random discontinuities has been studied on a number of scales in geophysics, submarine warfare, mine-hunting, weld inspection, medicine, and materials characterization, to name but a few application areas. This study belongs to the latter category, materials characterization, and is specifically directed towards establishing correlations between metallurgical microstructure and the observable backscatter that is produced when high frequency ultrasonic energy interacts with crystalline grain structures in an immersion test. Wave propagation near liquid-solid interfaces has been described in considerable mathematical detail [2–4], as has propagation in crystalline solids [5–7], but the scattering of sound from grain structure into a liquid half-space is complex, and has received little theoretical treatment. Adler and Bolland  measured backscattering from isotropic and anisotropic materials, but with beam diameters and wavelengths far greater than common metallic grain sizes. In studies of backscattering from annealed aluminum samples Bridge and Bin Saffiey  presented theoretical and empirical results relating attenuation to microstructure for relatively low frequencies. They noted that leaky waves, propagating both forward and backward, were generated at all angles of incidence; this finding complements the observation by Diachok and Mayer  that leaky waves propagate in a conical pattern (not just forward and backwards) when a liquid-solid interface is excited by a longitudinal wave incident at the Rayleigh angle. Because backscattered energy can exist in an acoustic environment which is free from specularly reflected signals, relatively high signal-to-noise ratios can be easily achieved, even from scatterers of microscopic dimensions. A wide range of signal processing tools are available to extract from the backscattered signals features which may correlate with microstructure [11–12]. Saniie and Bilgutay  applied several of these tools, including homomorphic deconvolution, to backscattered signals produced in normal incidence contact testing of stainless steel samples with a variety of grain sizes, with some success. The homomorphic deconvolution technique was also used by Kechter and Achenbach  to extract single-scatterer characteristics from the complex sound field produced by multiple scatterers.
KeywordsSurface Wave Frequency Index OFHC Copper Leaky Wave Quantitative Nondestructive Evaluation
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- 1.J. Mittleman and D.W. Mohr, “Correlating Microstructure with Backscattered Ultrasonic Energy”, Proc. MRS Symposium held 28 November-3 December, 1988, Boston, MA.Google Scholar
- 2.Markus Bath, “Mathematical Aspects of Seismology”, Elsevier Publishing Co., 1968.Google Scholar
- 3.J. E. White, “Seismic Waves: Radiation, Transmission, and Attenuation”, McGraw Hill, 1965.Google Scholar
- 4.Leonid M.Brekhovskikh, “Waves in Layered Media”, Academic Press, NY, 1960.Google Scholar
- 5.Robert E. Green, “Ultrasonic Investigation of Mechanical Properties”, Academic Press, NY, 1973.Google Scholar
- 6.Jack Blitz, “Fundamentals of Ultrasonics”, Plenum Press, 1967.Google Scholar
- 7.H. F.Pollard, “Sound Waves in Solids”, Pion Ltd., London, 1977.Google Scholar
- 8.Laszlo Adler and Ken Bolland, “Backscattering of Ultrasonic Leaky Waves from Liquid-Solid Interfaces”, Proc. Ninth Review in Quantitative Nondestructive Evaluation, held August 1–6 1982, Univ. Of California, San Diego, CA.Google Scholar
- 9.B. Bridge and H. J. Bin Saffiey, “Monitoring of the Annealing Temperature and Hardness of Aluminum Alloys by Ultrasonic Back-Scatter and Critical Angle Reflectivity”, British Journal of NDT V 30 pp 392 (1988).Google Scholar
- 11.Alan V. Oppenheim and Ronald W. Schafer, “Digital Signal Processing”, Prentice Hall, 1975.Google Scholar
- 12.C. H. Chen, “Nonlinear Maximum Entropy Spectral Analysis Methods for Signal Recognition”, Research Studies Press (John Wiley & Sons, Ltd), 1982.Google Scholar
- 16.Richard J. Wasley, “Stress Wave Propagation in Solids”, Marcel Dekker Inc., NY 1973.Google Scholar