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Abstract
Diffuse ultrasonic backscatter describes the scattering of elastic waves from interfaces within heterogeneous materials. As one of several quantitative nondestructive evaluation techniques, diffuse ultrasonic backscatter requires accurate models to interpret experimental data for both microstructural characterization and flaw detection. A longitudinal-to-longitudinal wave scattering model was previously derived for polycrystalline materials under the assumption of a single scattering event in the time between excitation and detection. In this dissertation, three extensions of that model are made and the results are compared with experiments. First, a mode-conversion scattering model based on the previous formalism is presented in the single-scattering limit by utilizing Wigner transforms to represent the transducer beam patterns of the source and receiver transducers. The mode-conversion scattering model is used to quantify the component of an incident longitudinal wave that scatters and is converted to a transverse (shear) wave within the sample. Second, two transverse-to-transverse wave scattering models are developed to describe the mode conversion scattering of a transverse wave to a transverse wave. Finally, a doubly-scattered model for a normally incident pulse-echo configuration is presented based on the assumption that two scattering events occur between excitation and detection. This solution allows the range of validity of the single scattering solution to be examined. In order to validate all these three theoretical models, experimental measurements, i.e., a pitch-catch measurement with normal and oblique incidence transducers, an oblique incidence pulse-echo configuration, and a normal incidence pulse-echo configuration, are performed, respectively, to extract the correlation length of the sample. These results are then compared with values determined from the previous longitudinal-to-longitudinal single scattering measurements and the grain size determined from optical micrographs.
The models used in this dissertation are constructed within a multiple scattering frame work such that they are expected to be of broad use. It is anticipated that such models will allow additional microstructural properties to be determined and will expand the use of ultrasonic microstructural evaluation, especially in terms of probability of detection estimates for polycrystalline and other heterogeneous media.
