Browsing by Subject "Elastic modulus"
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Item An analytical model for quantification of geosynthetic benefits in roadway base stabilization(2018-08) Liu, Ziye, M.S. in Engineering; Zornberg, Jorge G.Stabilization of base aggregate using geosynthetics may provide improved performance in flexible pavements. Through the aggregate-geosynthetic interaction, the induced shear stress in the base aggregates is transferred to the geosynthetic resulting in the development of tensile stresses. The tensioned geosynthetic applies in turn additional lateral confinement to the aggregates to restrain their movement under repeated traffic loading. The use of geosynthetics in base stabilization has been reported to result in reduced rutting depths in roadways. Design guidelines, either empirical or mechanistic-empirical, have been established for the design of non-stabilized roadways. These guidelines, however, do not take the geosynthetic effect into account. Accordingly, the geosynthetic benefits in base stabilization need to be quantified in a way that can easily be incorporated into the existing design procedures. This study proposed an analytical model to achieve this goal. In this study, the additional lateral confinement provided by the geosynthetic was modeled as a uniform additional confining stress within the geosynthetic influence zone. The pavement section was considered as an infinitely long elastic solid under plane strain conditions, which allowed the model framework to be established using the theory of elasticity. The aggregate-geosynthetic interaction, on the other hand, was modeled using the soil-geosynthetic composite (SGC) model. As a result, an additional confining stress could be defined from the stress-displacement relationships in the elastic model framework. An increased elastic modulus could then be predicted from the additional confining stress with a specific criterion for equivalency of the original base course with additional confinement and an alternative base course with enhanced elastic modulus but without additional confinement. The increased modulus can be used as an updated property for the geosynthetic-stabilized base aggregate in mechanistic-empirical pavement design procedures. The predicted equivalent increased moduli were validated using the results from repeated loading triaxial tests of two published studies. Overall, reasonably good agreement was found between model predictions and the test results. Predictions from this model were also compared against those of another analytical model, and close results were observed. A sensitivity analysis conducted using the proposed model indicated that model predictions are particularly sensitive to the original base modulus, geosynthetic properties (including confined geosynthetic stiffness and the stiffness of the soil-geosynthetic composite) and the thickness of the geosynthetic influence zone.Item Elasticity across the post-stishovite transition in subducted basalt(2022-11-18) Zhang, Yanyao; Lin, Jung-Fu; Grand, Stephen P.; Gardner, James E.; Sun, Chenguang; Zhou, JianshiRegional seismic studies have found many small-scale scatterers with shear wave velocity (Vₛ) anomalies near subducting slabs in the lower mantle, indicating chemical anomalies at depth. Subducting slabs transport surface hydrated mid-ocean ridge basalt (MORB) into deep Earth, which is chemically and physically distinct from the surrounding mantle. Physical properties of MORB materials are thus important to understand observed seismic Vₛ anomalies. In this dissertation, physical properties of stishovite and CaCl₂-type post-stishovite, which are abundant MORB components, have been determined at high pressure in order to study geophysical consequences across the post-stishovite transition in MORB. Specifically, I measured sound velocities using Brillouin light scattering and impulsive stimulated light scattering, Raman shifts of optic modes using Raman spectroscopy, and atomistic properties using synchrotron single-crystal X-ray diffraction in diamond anvil cells at high pressure. Sound velocity results show that the elastic modulus C₁₂ of pure-endmember stishovite converges with C₁₁ at 55 GPa where the shear wave Vₛ₁ propagating along [110] direction becomes zero and aggregate shear wave Vₛ drops by ~26%. Microscopically, this abnormal elastic property can be correlated with a crossover of the apical and equatorial Si-O bond lengths that results in the occurrence of the symmetry-breaking spontaneous strain in the post-stishovite phase. Furthermore, the Al and H incorporation into stishovite lattice structure can significantly reduce transition pressure and slightly enhance Vₛ reduction. For example, stishovite with 1.3-2.1 mol % Al undergoes the ferroelastic transition at ~16-21 GPa where Vₛ drops by ~29%. This Al,H-dependent post-stishovite transition in subducted MORB materials has been used to explain depth-dependent small-scale seismic Vₛ anomalies beneath the Tonga subduction region. The coupled H and Al substitution, together with literature element partitioning data, indicates that stishovite with 1.3 mol% Al in the upper part of the lower mantle could accommodate ~0.3 wt% H₂O.Item Plastic flow and microstructure evolution in niobium at elevated temperatures(2021-12-09) Brady, Emily Ann Dukes; Taleff, Eric M.; Kovar, Desiderio; Mangolini, Filippo; Seepersad, CarolynPlastic flow and microstructure evolution are investigated at elevated temperatures in two unalloyed niobium sheet materials, Type 1 and Type 2 as designated in ASTM B393-18. Tensile tests are conducted at temperatures from 1473 to 1823 K (1200 to 1550°C) at constant true strain rates of 10⁻³ and 10⁻⁴ s⁻¹. Deformation microstructures are characterized using backscatter electron (BSE) imaging, electron backscatter diffraction (EBSD), and high-resolution EBSD (HR-EBSD). The mechanical behaviors of the Type 1 and Type 2 niobium materials are compared to relevant data from the literature. Elevated temperature deformation in both niobium materials is dominated by the five-power creep mechanism and the associated development of subgrains. The higher impurity content of the Type 2 niobium led to: 1. greater strength, 2. delayed recrystallization, 3. slower grain growth, 4. inhomogeneous microstructures, and 5. slower recovery which resulted in finer and less distinct subgrains compared to the Type 1 niobium. The smaller subgrain size of the Type 2 niobium produces, through the five-power creep mechanism, a higher strength at elevated temperature compared to the Type 1 niobium. This is the first mechanistic explanation supported by direct microstructural data for how impurity content affects strength in refractory metals. HR-EBSD analysis is performed on the deformed Type 2 niobium material by developing new techniques to: 1. utilize data from a new EBSD instrument, 2. expand the capabilities of the OpenXY open-source cross-correlation software, and 3. enable cross correlation calculations spanning the breadth of heavily deformed grains. This is the first successful implementation of these techniques.