Abstract
This thesis presents a frequency-domain ultrasonic technique for a simultaneous determination of the thickness (h) and wavespeed (c) of the individual layers comprising a multi- layered medium. Each of the layers may be "thin"; by thin it is meant that the successive reflections from the two faces of a layer are non-separable in the time domain. Plane longitudinal waves which are normally incident upon the medium are considered, and the through-transmission transfer function is used. A systematic analysis of the sensitivity of the transfer function to the acoustical parameters of each of the layers has been carried out. An inverse algorithm (which utilizes either the Newton-Raphson or the Simplex method) has been developed to simultaneously reconstruct the thickness and phase velocity of each of the layers by minimizing the difference between the theory and the experiment in the mean-sum-square sense; the amplitude spectrum alone, as well as the entire complex spectrum (i.e., the amplitude and the phase spectrum) were used. The technique is fully automated and computer-controlled and can be readily used for in-situ NDE applications. Using this technique, the thickness and the wavespeed of the individual layers can be extractedfrom the same measurement, without knowing any of them. Results are presented for several single plates and three-layer specimens: aluminum, titanium, stainless steel, tungsten, Plexiglas, aluminum/water/aluminum, aluminum/water/titanium, and titanium/water/titanium. Successful inversion was obtained for the following cases: (1) simultaneous determination of h and c of any one of the three layers, given h and c of the remaining two layers; (2) simultaneous measurement of the three thicknesses, given the three wavespeeds; (3) simultaneous measurement of the three wavespeeds, given the three thicknesses; (4) simultaneous determination of all three thicknesses and one wavespeed, given the remaining two wavespeeds. The precision of our measurements was found to be extremely good: typically, ︢I gm in h and + one part per thousand in c. The accuracy was found to be about one order of magnitude worse than the precision: typically, ︢10 /Am in h and +I% in c.
Jaminet, Paul Thomas (1993). Ultrasonic quantitative NDE of layered media: the inverse problem. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1993 -THESIS -J32.