Degree Type

Dissertation

Date of Award

1992

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

William Lord

Abstract

Many of the innovations in modern materials testing technology make use of ultrasound. Therefore, the theory and application of ultrasound have become of extreme importance in nondestructive inspection of complete engineered systems. However, despite the fact that most of these ultrasound inspection techniques are based on well-established phenomena, two key problems pertaining to their application still remain unresolved. These problems can be identified as (1) the material being tested is assumed to be isotropic and homogeneous by nature, and (2) the scanning/data collection process, prior to the reconstruction scheme, is very time consuming. As a result, techniques for fast, accurate testing of anisotropic and nonhomogeneous media have been the focus of attention in modern non-destructive testing research;This dissertation first describes the development and implementation of a time domain synthetic aperture focusing technique (SAFT) to reconstruct flaws imbedded within Plexiglass[superscript] TM\/ and Graphite/Epoxy samples. A modification to the present SAFT algorithm is then proposed in order to improve the quality of the images produced by SAFT when applied to composites. In addition, since the finite element method (FEM) can be used to solve hyperbolic partial differential equations, which govern wave propagation, FEM solutions are used to mimic a SAFT measurement. That is, the FEM is used to simulate the action of a transducer array. This is done to study the sensitivity of parameters involved in the SAFT algorithm. Using the same FEM model as a test bed, the data independent beamformer, in its basic form, is studied to determine its performance in reducing data acquisition time. It is seen that this technique is capable of adjusting the weights of the interpolating filter (beamformer) to predict an incoming signal from a desired direction while discriminating against other signals from different directions;SAFT results indicate that the FEM model can be used as a Test Bed for SAFT sensitivity studies, and point out the possible use of lateral displacement data for SAFT reconstruction. Extensive beamforming test comparisons reveal that the resulting optimal filters are indeed able to predict not only A-scan signals from a set of data produced by a nonuniformly spaced transducer baseline, based on one material geometry, but also A-scan signals from a data set produced by a nonuniformly spaced transducer baseline based on a different material geometry. This has particular significance for fast testing and imaging of isotropic and anisotropic materials in ultrasonic nondestructive evaluation techniques.

DOI

https://doi.org/10.31274/rtd-180813-10932

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Sleiman Riad Ghorayeb

Language

en

Proquest ID

AAI9220969

File Format

application/pdf

File Size

203 pages

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