Browsing by Subject "X-ray CT"
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Item 3-D fracture tracing for X-ray computed tomography data(2016-12) Hildebrandt, Jordan; Ketcham, Richard Alan, 1965-; Cardenas, Meinhard B; Sharp, John MX-ray computed tomography (CT) is a nondestructive imaging method that shows differences in X-ray attenuation, which is a proxy for density. Sensitivity to density differences makes CT a good choice for imaging open natural fractures because of the significant density contrast between air and rock. The image data, however, are prone to artifacts and blurring, which makes taking accurate measurements challenging. As a first step to addressing this problem, a tool was created to quantify the spatial resolution of CT data with a point-spread function (PSF). The PSF tool permits accurate measurement of fine-scale features in CT data – critical for measuring fractures, which are often thinner in one dimension than the PSF size, in turn influencing measurement of aperture. The difference between the measurements given by the PSF method and a simple threshold value pick is shown to demonstrate a non-trivial improvement in accuracy. In addition, a 3-D fracture network tracing algorithm was developed, for which the PSF is a necessary input, to characterize accurately the network’s attributes, such as fracture orientations, apertures, and roughnesses. To date, only single, isolated, relatively flat fractures have been characterized thoroughly in 3-D for CT data, and most numerical modeling has been conducted on 2D subsets. Additionally, research involving fracture networks is currently limited to simplified numerical models and simulations. This new work extends CT fracture characterization to the majority of fractured materials with 3-D networks, thus providing a source of real data for studying the difference between fracture networks and single fractures. A sample of Packsaddle schist, with a large number of thin fractures and complex, anastomosing bifurcations, was selected as a testing ground for the network tracing algorithms being developed. Preliminary analysis verifies the method’s efficacy.Item Analysis of HMA permeability through microstructure characterization and simulation of fluid flow in X-ray CT images(Texas A&M University, 2005-02-17) Al Omari, Aslam Ali MuflehThe infiltration of water in asphalt pavements promotes moisture damage primarily through damaging the binder cohesive bond and the adhesive bond between aggregates and binder. Moisture damage is associated with excessive deflection, cracking, and rutting. The first step in addressing the problems caused by the presence of water within pavement systems is quantifying the permeability of hot mix asphalt (HMA) mixes. This dissertation deals with the development of empirical-analytical and numerical approaches for predicting the permeability of HMA. Both approaches rely on the analysis of air void distribution within the HMA microstructure. The empirical-analytical approach relies on the development of modified forms of the Kozeny-Carman equation and determining the material properties involved in this equation through three dimensional microstructure analyses of X-ray Computed Tomography (CT) images. These properties include connected percent air voids (effective porosity), tortuosity, and air void specific surface area. A database of materials and permeability measurements was used to verify the developed predicting equation. The numerical approach, which is the main focus of this study, includes the development of a finite difference numerical simulation model to simulate the steady incompressible fluid flow in HMA. The model uses the non-staggered system that utilizes only one cell to solve for all governing equations, and it is applicable for cell Reynolds number (Rec) values that are not restricted by |Rec|≤2. The validity of the numerical model is verified through comparisons with closed-form solutions for idealized microstructure. The numerical model was used to find the components of the three-dimensional (3-D) permeability tensor and permeability anisotropy values for different types of HMA mixes. It was found that the principal permeability directions values are almost in the horizontal and vertical directions with the maximum permeability being in the horizontal direction.Item Microstructural Characterization of Material Properties and Damage in Asphalt Composites(2013-05-03) Mohammad Khorasani, SaraAsphalt composites are used to construct 90% of roads in the United States. These composites consist of asphalt binder, which is a product of the refinery process of oil, aggregates, and air voids. Fatigue cracking is one of the most important distresses that causes damage in asphalt pavements. However, there is still a gap in the understanding of the fatigue process of asphalt composites, such as the influence of material properties on this phenomenon and how the material microstructure changes as a result of fatigue damage. This study focuses on the results of two experiments that were performed on asphalt composites to better understand phenomena related to fatigue cracking: nano-mechanical characterization of the properties of the asphalt composite material and X-ray Computed Tomography nondestructive imaging of damage in the microstructure. These experimental measurements were performed on specimens that are first damaged in the Dynamic Mechanical Analyzer (DMA). The DMA is a tool commonly used for the characterization of fatigue cracking. This test method applies cyclic loads on asphalt composites, damaging them, and in the process determines the viscoelastic properties of the composite throughout the test. The nano-mechanical characterization experiment gives valuable results of the elastic modulus and hardness of the aggregate, binder, and the aggregate-binder interface that can be used to characterize different binder and aggregate combinations. The nanoindentation experiment successfully measured interface properties in the mix. The interface has elastic modulus and hardness values greater than the binder but smaller than the aggregate. This demonstrates that an interaction between these two phases creates a dissimilar phase between the two. The second experiment using X-ray CT gives measurements that are indicative of the influences of fatigue damage on micro-level changes in the material microstructure. The results of this experiment revealed important changes regarding the nature of fatigue damage and its relationship to changes in the geometry of air voids and cracks in asphalt composites. The X-ray CT experiment measured size and shape parameters of air voids at 20 microns/pixel resolution at different damage levels. These results illustrated that reduction in bonding strength in the binder is involved in failure in the mix and thus fatigue cracking is not solely responsible for failure. This conclusion is made based on the results not showing a statistically significant change in air void shape and size parameters with increased damage. This is illustrated by viewing changes in the air void structure within the mix, there is no evidence of crack propagation, or drastic changes in the shape, size, or volume of air voids within the mix.Item Quantitative characterization of microstructure of asphalt mixtures to evaluate fatigue crack growth(2012-05) Izadi, Anoosha; Bhasin, Amit; Smit, AndreStudies show that the microstructure of the fine aggregate matrix has a significant influence on the mechanical properties and evolution of damage in an asphalt mixture. However, very little work has been done to quantitatively characterize the microstructure of the asphalt binder within the fine aggregate matrix of asphalt mixtures. The first objective of this study was to quantitatively characterize the three dimensional microstructure of the asphalt binder within the fine aggregate matrix (FAM) of an asphalt mixture and compare the influence of binder content, coarse aggregate gradation, and fine aggregate gradation on this microstructure. Studies indicate that gradation of the fine aggregate has the most influence of the degree of anisotropy whereas gradation of the coarse aggregate has the most influence on the direction anisotropy of the asphalt mastic within the fine aggregate matrix. Addition of asphalt binder or adjustments to the fine aggregate gradation also resulted in a more uniform distribution of the asphalt mastic within the fine aggregate matrix. The second objective of this study was to compare the internal microstructure of the mortar within a full-scale asphalt mixture to the internal microstructure of the FAM specimen and also conduct a limited evaluation of the influence of mixture properties and methods of compaction on the engineering properties of the FAM specimens. Fatigue cracking is a significant form of pavement distress in flexible pavements. The properties of the sand-asphalt mortars or FAM can be used to characterize the evolution of fatigue crack growth and self-healing in full-scale asphalt mixtures. The results from this study, although limited in number, indicate that in most cases the SGC (Superpave Gyratory Compactor) compacted FAM specimen had a microstructure that most closely resembled the microstructure of the mortar within a full-scale asphalt mixture. Another finding from this study was that, at a given level of damage, the healing characteristic of the three different types of FAM mixes evaluated was not significantly different. This indicates that the healing rate is mostly dictated by the type of binder and not significantly influenced by the gradation or binder content, as long as the volumetric distribution of the mastic was the same.