Ultrasonic beam fluctuation and flaw signal variance in inhomogeneous media

Ahmed, S.; Roberts, R.; Margetan, F.
May 2000
AIP Conference Proceedings;2000, Vol. 509 Issue 1, p985
Academic Journal
This paper examines the effect of forward scattering on ultrasonic beam propagation and flaw signal amplitude in inhomogeneous material microstructures. A beam propagating through a weakly-scattering, randomly inhomogeneous medium will display random fluctuations in amplitude and phase, attributable to forward scattering. Correspondingly, the signal received from a given flaw at a given position in the beam volume will fluctuate as the beam and flaw are simultaneously scanned throughout the volume of an inhomogeneous host medium. These effects have been prominently observed in the inspection of titanium. For example, maps of beam amplitude profiles after transmission through titanium reveal severe distortion of beam amplitude and phase. Similarly, signals from “identical” flat bottom holes (FBH) at equal depths but different lateral positions in titanium display a random variation in amplitude. Interestingly, it has been noted that this FBH signal variance varies inversely to the beam diameter, that is, signal variance normalized to the mean signal amplitude is a minimum when the flaw is in the focal zone of a focused bearn. As this observation has great significance to the inspection of titanium, a model, prediction of this phenomenon is being sought. In the work reported here, beam propagation is formulated as a volumetric integral equation employing the Green function for the homogeneous spatial mean of the medium. The integral equation is solved using iterative methods. Preliminary work considering scalar two-dimensional propagation in inhomogeneous media has predicted a flaw signal variance that displays an inverse relation to beam diameter, thus reproducing the qualitative behavior seen in experimental data in titanium. Current work is extending the preliminary two-dimensional scalar result to three-dimensional elasticity, representing propagation in an actual titanium microstructure. Progress on this effort will be reported. © 200...


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