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Flaw signals measured in ultrasonic testing include the effects of the measurements system and are corrupted by noise. The measurement system response is both bandlimited and frequency dependent within the bandwidth, resulting in measured signals which are blurred and distorted estimates of actual flaw signatures. The Wiener filter can be used to estimate the flaw's scattering amplitude by removing the effect of the measurement system in the presence of noise. A method is presented for implementing an optimal form of the Wiener filter that requires only estimates of the noise distribution parameters. The theoretical error for scattering amplitude estimation, assuming various levels of available prior information, is analyzed. Three estimation techniques, one a maximum-likelihood based method and the other two residual-sum-of-squares methods, are formulated and tested. The results demonstrate that any of the three approaches could be used to optimally implement the alternative form of the Wiener filter with limited prior information.

The Wiener filter is currently used in the ultrasonic scatterin~ amplitude estimation problem as a means of desensitization during deconvolution [1,2,3]. The work summarized here focuses on a Wiener filtering approach which incorporates ~ priori flaw and noise information. It will be shown that this approach leads to improved scattering amplitude estimates and improved radius estimates. BACKGROUND

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, "A prior knowledge based optimal Wiener filtering approach to ultrasonic scattering amplitude estimation " (1988). Retrospective Theses and Dissertations. 8877. http://lib.dr.iastate.edu/rtd/8877 INFORMATION TO USERSThe most advanced technology has been used to photo graph and reproduce this manuscript from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer.The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction.In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion.Oversize materials (e.g., maps, drawings, charts) are re produced by sectioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. These are also available as one exposure on a standard 35mm slide or as a 17" x 23" black and white photographic print for an additional charge.Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. Signature was redacted for privacy.Signature was redacted for privacy.Signature was redacted for privacy. The work reported here is associated with flaw characterization.This work is motivated from an engineering perspective by considering two fundamentally different approaches to life prediction that can be found in fatigue of materials and structures. In a simplistic sense, a "safe life"or "zero defects" fatigue life prediction approach (Coffin and Tiffany 1976) assumes that once a flaw is macroscopic, i.e., once a flaw has grown to a size which can be detected by NDE techniques, failure is imminent and the component must be taken out of service. With this approach, qualitative NDE indicating the presence or absence of a flaw provides sufficient information to determine if a component should be retired. An alternative approach, which is sometimes referred to as a "damage tolerance" approach, is based on a fracture mechanics methodology (Coffin and Tiffany 1976, Rolfe and Barsom 1977). With this approach, instead of assuming that all flawed components must be taken out of service, the 2 severity of the flaw is assessed and a more rational determination of the remaining life of the component is made. To implement this approach, quantitative flaw characterization is required. As a minimum...

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The starting point for solving inverse scattering problems in acoustics is an estimate of the target's scattering function. A typical procedure for estimating a scattering function is to apply an inverse filter to the Fourier transform of the backscattered signal from the target. The inverse filter is based on an estimate of the measurement system response function that includes electronic, transducer, and wave propagation related effects. In this paper, we address the problem of estimating the backscatter coefficient for scattering from a large number of randomly distributed scatterers. A method is presented for estimating the backscatter coefficient for weak scattering polycrystalline metals for the case in which the backscatter coefficient is independent of depth into the material. Measurement system response function estimates were based on volume integrations over velocity fields calculated using GaussHermite polynomials. Ultrasonic measurements were made for direct backscatter in an immersion mode at normal incidence using both planar and focused probes operated broadband. Results are presented that show close agreement between the frequency dependence of the backscatter coefficient estimates and the frequency dependence of the classically estimated attenuation coefficient. The backscatter coefficient estimates are shown to be reasonably independent of the system configuration and depth into the material.