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Description
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This thesis discusses ultrasonic testing by means of numerical modeling and image reconstruction techniques using elastic and acoustic wave fields. Numerical modeling of elastic waves (part one of the thesis) is used to understand the elastic wave scattering due to material defects and the propagation of surface waves in inhomogeneous isotropic and anisotropic media, with special emphasis on transversely isotropic and orthotropic media. Different imaging techniques (part two of the thesis) are investigated to develop a software, implemented in Matlab, which can give imaging results immediately after the measurement almost in real time as it can read and process the data obtained directly from the measurement.
Acoustic wave scattering using analytical techniques and imaging techniques based on Radon transform are investigated. The data obtained from the Radon transform are subjected for imaging utilizing the filtered back projection algorithm and the Fourier slice theorem.
The fundamentals of elastic wave propagation in solids are extensively elaborated. The point source synthesis to compute the Green's functions for anisotropic media and the plane wave synthesis to compute slowness, phase and group velocity surfaces are studied. The elastic integral equations for the so called stretched coordinate system are derived. Based on these equations the numerical tool 'Three-dimensional Elastodynamic Finite Integration Technique' (3D-EFIT) has been enhanced to treat not only isotropic media but also anisotropic media. For fast computation, the 3D-EFIT code using the Message Passing Interface (MPI) is used by which processing on massive parallel computers is made possible. In 3D-EFIT the Convolutional Perfectly Matched Layers (CPML) can also be applied to absorb the elastic body waves as well as the surface and evanescent waves. 3D-EFIT for homogeneous anisotropic media is validated by comparing computed Green's functions with an analytical solution. After the validation, the applications of EFIT such as elastic wave modeling in inhomogeneous austenitic steel welds and inhomogeneous orthotropic wooden structures are presented. The results of the 2D-EFIT and 3D-EFIT modeling are compared against measurements performed at Federal Institute for Materials Research and Testing (BAM).
After the modeling part of the thesis, inverse scattering techniques for fast imaging of inhomogeneities are studied. For three-dimensional imaging of defects in concrete, the Synthetic Aperture and Focusing Technique (SAFT) and Fourier Transformed Synthetic aperture Focusing Technique (FT-SAFT) are applied to data measured using a transducer array. The seismic Dip-Moveout (DMO) method has been utilized to convert measured bistatic data into monostatic data. A special treatment of SAFT as a technique for back propagation of the wave fields using time reversal, utilizing the knowledge of the geometry, is presented. Finally, time domain anisotropic SAFT (AnSAFT) is studied for image reconstruction of defects in inhomogeneous geometry with orthotropic crystal structure of the embedding medium.
