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Ultrasonic Scattering Model Based on Digital Three-Dimensional Microstructures
Ultrasonic nondestructive testing has been increasingly used to characterize heterogeneities of polycrystalline materials. These techniques are highly sensitive to the scattered energy arising from the interactions of coherent ultrasonic waves with material heterogeneities such as flaws or grain boundaries. Because the scattered waves carry information regarding the physical properties of the scatterer, microstructural information can be obtained through quantification of the scattered response. Current ultrasonic models include several assumptions about the macroscopic and microscopic properties of the polycrystals. This dissertation attempts to address these assumptions and evaluate their contribution to the ultrasonic parameters including attenuation and backscatter coefficient. This objective is achieved by employing synthetic three-dimensional (3D) polycrystals simulated using DREAM.3D software. A methodology is developed here by which ultrasonic responses are evaluated for synthetic microstructures. The material information required to calculate these properties is directly obtained from the discrete volumes. Hence, the need for the typical assumptions is eliminated and a more realistic model is achieved. First, the effect of the finite number of grains on statistical anisotropy of polycrystals is studied using a large number of volumes comprised of equiaxed cubic grains. The results show that the statistical variation in phase velocities are inversely proportional to the square root of the number of grains. Master curves are then provided to predict the degree of elastic modulus and phase velocity variations with respect to the finite volume. Moreover, a comparison between the attenuation values obtained from the 3D volumes and those derived from the classical theories explores the validity of the assumptions. This approach is performed on microstructures with a range of volumes and grain size distributions. The results show that the exponential two-point spatial correlation function assumption is highly limited in its range of validity. In addition, the frequency relation of attenuation is found to vary with respect to the width of the grain size distribution. Finally, the techniques developed here are used to analyze diffuse ultrasonic backscatter data from a 1040 carbon steel sample for grain size characterization.
Reykandeh, Musa Norouzian, "Ultrasonic Scattering Model Based on Digital Three-Dimensional Microstructures" (2019). ETD collection for University of Nebraska - Lincoln. AAI13863377.