Browsing by Subject "Finite element"
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Item Analysis of soil-structure system response with adjustments to soil properties by perturbation method(2014-05) Patta, Sang Putra Pasca Rante; Tassoulas, John LambrosThe research described in this dissertation undertakes a computational study of wave motion due to ground excitation in layered soil media. Adjustments of soil properties consistent with the level of deformation is applied using an equivalent linear approach. The finite element method provides the basis of the numerical procedure for soil-structure system response calculation in conjunction with a first-order perturbation scheme. Available experimental data are employed for shear-modulus and damping adjustments. The findings of the research are expected to lead to efficient calculation of structural response to earthquake ground motion.Item Analysis of the shear behavior of prestressed concrete spliced girders(2016-08) Al-Tarafany, Dhiaa Mustafa T.; Jirsa, J. O. (James Otis); Bayrak, Oguzhan; Tassoulas, John; Hrynyk, Trevor; Ghannoum, Wassim; Wheat, HarovelImplementation of the spliced girder technology in bridges has been growing in recent years. Increased girder lengths can now be realized by splicing shorter precast segments to produce a long span. The research conducted in this dissertation is focused on an evaluation of spliced girders using a three dimensional finite element analysis. The project consisted of a series of tests that were conducted in two phases. In Phase I, the effect of post-tensioning ducts on the shear behavior and strength of prestressed concrete girders was evaluated. In Phase II, the focus was on the behavior of cast-in-place splice regions between precast segments. Since a limited number of full scale beams could be tested, a three-dimensional advanced finite element program is an effective alternative to expensive tests. The parameters considered were grout to girder concrete strength ratio, splice to girder concrete strength ratio, concrete shear key detailing, coupler diameter, duct to web width ratio, shear span to depth ratio, and concrete shrinkage losses. The findings are described in detail. Using the experimental and analytical results, it was found that the grout to concrete strength ratio for grouted ducts should not to be less than 0.3. The effect of increasing the duct diameter to web width ratio from 0.43 to 0.57 was minimal. Splice to girder concrete strength ratio should be greater than 0.6. The addition of a shear key had no effect on the shear capacity of the girder. The coupler diameter in the splice region had no effect on the behavior of the spliced girder for coupler diameter to web width ratio up to 0.55. Including concrete shrinkage in the analysis slightly improved the correlation with observed response.Item Building a framework for predicting the settlements of shallow foundations on granular soils using dynamically measured soil properties(2014-05) Kacar, Onur; Stokoe, Kenneth H.In this dissertation, the framework is being developed for a new method to predict the settlements of shallow foundations on granular soil based on field seismic and laboratory dynamic tests. The new method combines small-strain seismic measurements in the field with nonlinear measurements in the field and/or in the laboratory. The small-strain shear modulus (Gmax ) of granular soil and the stress dependency of Gmax is determined from the shear wave velocity measurements in the field. Normalized shear modulus (G/Gmax ) versus log shear strain(log [gamma]) curves are determined from field or laboratory measurements or from empirical relationships. The G/Gmax -- log [gamma] curves and Gmax values are combined to determine the shear stress-shear strain response of granular soil starting from strains of 0.0001% up to 0.2-0.5%. The shear stress-shear strain responses at strains beyond 1.0-2.0 % are evaluated by adjusting the normalized shear modulus curves to larger-strain triaxial test data. A user defined soil model (MoDaMP) combines these relationships and incorporates the effect of increasing confining pressure during foundation loading. The MoDaMP is implemented in a finite element program, PLAXIS, via a subroutine. Measured settlements from load-settlement tests at three different sites where field seismic and laboratory dynamic measurements are available, are compared with the predicted settlements using MoDaMP. Predictions with MoDaMP are also compared with predictions with two commonly used methods based on Standard Penetration and Cone Penetration tests. The comparison of the predicted settlements with the measured settlements show that the new method developed in this research works well in working stress ranges. The capability of the new method has significant benefits in hard-to-sample soils such as in large-grained soils with cobbles and cemented soils where conventional penetration test methods fail to capture the behavior of the soil. The new method is an effective-stress analysis which has applicability to slower-draining soils such as plastic silts and clays.Item Finite element analysis of welds attaching short doubler plates in steel moment resisting frames(2014-12) Marquez, Alberto C.; Engelhardt, Michael D.A number of recent research studies have investigated the performance of panel zones in seismic-resistant steel Special Moment Resisting Frames (SMF). These recent studies investigated various options for attaching doubler plates to the column at beam-column joints in SMF for purpose of increasing the shear strength of the panel zone. This previous work was primarily focused on doubler plates that extend beyond the top and bottom of the attached beams, and considered cases both with and without continuity plates. As an extension to this previous research, this thesis explores the situation when a doubler plate is fitted between the continuity plates. The objective of this research was to evaluate various options for welding fitted doubler plates to the column and continuity plates through the use of finite element analysis, and to provide recommendations for design. The development and validation of the finite element model are described, along with the results of an extensive series of parametric studies on various panel zone configurations and attachment details for fitted doubler plates. Based on the results of these analyses, recommendations are provided for design of welds used for attaching fitted doubler plates in the panel zone of SMF systems.Item Frequency-dependent mechanical properties of geomaterials : laboratory experiments and digital rock physics(2021-05-11) Ikeda, Ken, 1993-; Tisato, Nicola; Spikes, Kyle Thomas; Hesse, Marc Andre; Lavier, Luc Louis; Sen, Mrinal Kanti; Quintal, BeatrizGeoscientists often model the subsurface by studying the propagation of seismic waves. As a seismic wave propagates through a medium, its amplitude and phase change according to the medium physical properties. Geoscientists often simplify the rheology of geomaterials to elastic media where elastic properties are frequency-independent. However, geomaterials possess frequency-dependent properties. Such oversimplification can create inaccurate subsurface models. Therefore, often geomaterials should be modeled as frequency-dependent materials using appropriate attenuation models. In this dissertation, I explore the elastic properties of rocks at low-frequencies (~0.1–100 Hz) and ultrasonic frequencies (10⁵-10⁶ Hz). I use laboratory measurements to estimate elastic properties of rocks. These measurements are then compared to Digital Rock Physics (DRP) results, where the same laboratory measurements are numerically simulated on Computed-Tomographic (CT) images. The first part of this dissertation explores elastic properties of a quartz-rich sandstone at ultrasonic frequencies. I demonstrate that the laboratory-measured elastic properties could be efficiently predicted using a new DRP based technique called Segmentation-Less withOut Targets (SLOT). The SLOT method uses the local variation of X-ray attenuation in CT-images to map the density distribution of the corresponding material. Numerical simulations of wave propagations are used to estimate the elastic properties of the sample, and show a small mismatch to the laboratory measurements. The second part of the dissertation focuses on estimating low-frequency elastic properties using the SLOT technique, which has been extended to accommodate the characteristics of a polymineralic carbonate. The modified-SLOT combines segmentation-based DRP with segmentation-less DRP to create elastic property distribution maps. The DRP predictions agree with the laboratory measurements. The last part of the dissertation shows the development of a new apparatus to measure low-frequency attenuation: the Low-Frequency Module (LFM). The mechanical and electronic design of the LFM is carefully chosen to accommodate a decametric sample that can be tested at reservoir pressure-temperature conditions. The design and the limitation of the apparatus are discussed. The newly developed DRP techniques and the state-of-the-art apparatus will help geoscientists exploring the elastic properties of rocks at different bandwidths. Improved estimations of elastic properties will help to better capture subsurface featuresItem A hybrid-stress finite element approach for stress and vibration analysis in linear anisotropic elasticity(1987) Mahadevan, L. (Lakshminarayanan); Oden, J. Tinsley (John Tinsley), 1936-A hybrid-stress finite element method is developed for accurate stress and vibration analysis of problems in linear anisotropic elasticity. A modified form of the Hellinger-Reissner principle is formulated for dynamic analysis and an algorithm for the determination of the anisotropic elastic and compliance constants from experimental data is developed. These schemes have been implemented in a finite element program for static and dynamic analysis of linear anisotropic two-dimensional elasticity problems. Specific numerical examples are considered to verify the accuracy of the hybrid-stress approach and compare it with that of the standard displacement method, especially for highly anisotropic materials. It is that the hybrid-stress approach gives significantly better results than the displacement method. Preliminary work on extensions of this method to three-dimensional elasticity is discussed, and the stress shape functions necessary for this extension are includedItem Mechanical and thermal properties of kenaf/polypropylene nonwoven composites(2013-05) Hao, Ayou; Chen, Jonathan Yan; Koo, Joseph H.; Kovar, Desiderio; Krifa , Mourad; Shi, Li; Xu, BugaoThe objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics.Item Nanofabrication via directed assembly: a computational study of dynamics, design & limits(2016-08) Arshad, Talha Ali; Bonnecaze, R. T. (Roger T.); Ellison, Christopher J.; Ganesan, Venkat; Sreenivasan, S. V.; Willson, Carlton G.Three early-stage techniques, for the fabrication of metallic nanostructures, creation of controlled topography in polymer films and precise deposition of nanowires are studied. Mathematical models and computational simulations clarify how interplay of multiple physical processes drives dynamics, provide a rational approach to selecting process parameters targeting specific structures efficiently and identify limits of throughput and resolution for each technique. A topographically patterned membrane resting on a film of nanoparticles suspended in a solvent promotes non-uniform evaporation, driving convection which accumulates particles in regions where the template is thin. Left behind is a deposit of particles the dimensions of which can be controlled through template thickness and topography as well as film thickness and concentration. Particle distribution is shown to be a competition between convection and diffusion represented by the Peclet number. Analytical models yield predictive expressions for bounds within which deposit dimensions and drying time lie. Ambient evaporation is shown to drive convection strong enough to accumulate particles 10 nm in diameter. Features up to 1 µm high with 10 nm residual layers can be deposited in < 3 minutes, making this a promising approach for continuous, single-step deposition of metallic nanostructures on flexible substrates. Selective exposure of a polystyrene film to UV radiation has been shown to result in non-uniform surface energy which drives convection on thermal annealing, forming topography. Film dynamics are shown to be a product of interplay between Marangoni convection, capillary dissipation and diffusion. At short times, secondary peaks form at double the pattern density of the mask, while at long times pattern periodicity follows the mask. Increased temperature, larger surface tension differentials and thick films result in faster dynamics and larger features. Electric fields in conjunction with fluid flow can be used to position semi-conducting nanowires or nanotubes at precise locations on a substrate. Nanowires are captured successfully if they arrive within a region next to the substrate where dielectrophoresis dominates hydrodynamics. Successful assembly is predicated upon a favorable balance of hydrodynamics, dielectrophoresis and diffusion, represented by two dimensionless groups. Nanowires down to 20 nm in length can be assembled successfully.Item Numerical modeling of thermal fracture development in unconventional reservoirs(2015-05) Enayatpour, Saeid; Patzek, Tadeusz W.; Olson, Jon; Tassoulas, John; Sepehrnoori, Kamy; Sharma, Mukul MRapid depletion of hydrocarbons in conventional reservoirs and the availability of abundant oil and gas resources in unconventional forms demand new technology to economically produce these energy resources. From exploration to consumption of hydrocarbons, a great number of complex physical, chemical, mechanical, electrical, and thermal phenomena occurs in reservoir from reservoir rock to surface facilities. In addition to good rock samples and cores which could represent the reservoir rock for laboratory experiments, numerical tools should always be used to provide predictive capability of reservoir rock and fluid behavior. A great number of numerical methods have been developed and used over the past decades for rock mechanics problems. In this research the main focus is on thermal fracturing, heat transfer in rock, flow in porous media and stress-deformation analysis. We use three numerical methods for thermal fracturing of reservoir rock and investigate their capabilities in providing a solution for fracture propagation in rock. We finally propose the best numerical method among the ones we used in this work. We finally show two applications of thermal rock fracturing for improvement of hydrocarbon recovery in tight formations.Item On the 3D contractile properties of the aortic heart valve interstitial cell in health and disease(2022-05-02) Khang, Alex, Ph. D.; Sacks, Michael S.; Anseth, Kristi S; Baker, Aaron B; Cosgriff-Hernandez, Elizabeth M; Ferrari, GiovanniAortic valve interstitial cells (AVICs) are fibrobast-like cells that reside within all layers of the aortic valve (AV) and maintain extracellular matrix (ECM) turnover and remodeling. In disease, AVICs can undergo activation and take on a myofibroblastic phenotype characterized by increases in ECM deposition, remodeling, and cellular contractility brought about through expression of alpha-smooth muscle actin stress fibers (SFs). AVIC contractility via stress fibers is a physical indicator that reflects both AVIC activation level as well as biophysical state and is known to be correlated with crucial processes such as collagen deposition and ECM remodeling. My dissertation focuses on investigating the 3D contractile properties of AVICs within tissue-mimicking, 3D peptide-modified poly (ethlyene glycol) (PEG) hydrogels that crucially allow for direct visualization and assessment of AVIC behaviors. First, I used a flexure setup to quantify the contractile states of AVICs embedded within PEG gels, which showed similarity to our earlier native tissue work and demonstrated that the PEG gel environment reproduces many of the same functional characteristics as soft tissue. Then, I investigated the 3D contractile properties of AVICs in greater detail using 3D traction force microscopy and found that AVIC shape orientation and principal contractile direction were correlated. Further analysis showed that AVIC protrusions were the main drivers of AVIC contractile behaviors and that they deformed in a uniform, piston-like manner, indicative of highly-aligned underlying SFs. To gain deeper insight into SF architecture and contractile forces, I developed a 3D computational model of the contracting AVIC within a PEG hydrogel medium. First, the model predicted that AVICs stiffen the local material likely due to nascent ECM deposition. The local variations in hydrogel moduli were then incorporated with a mechanical model of the contracting AVIC which predicted that the greatest SF alignment and contractile force levels were localized at the AVIC protrusions, showing consistency with experimentation. Finally, I extended this approach to investigate intrinsic differences between AVICs extracted from human bicuspid AVs (BAVs) and structurally normal tricuspid AVs (NAVs) and found that AVICs from BAVs showed lower levels of activation as evidenced by lesser SF alignment and contractility. These findings suggest that intrinsic differences among the AVICs likely contribute to the increased rate of valve disease experienced by many BAV patients. In addition, this work highlights the importance of investigating cellular and sub-cellular differences among the BAV and NAV toward identifying targets for novel, non-surgical therapies.Item Quantifying three dimensional effects in acoustic rough surface scattering(2011-05) Joshi, Sumedh Mohan; Hamilton, Mark F.; Isakson, Marcia J.Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional (2D) in nature; three dimensional (3D) modeling requires significantly more time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions 3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution.Item The seismic response to fracture clustering : a finite element wave propagation study(2014-05) Becker, Lauren Elizabeth; Spikes, KyleCharacterizing natural and man-made fracture networks is fundamental to predicting the storage capacity and pathways for flow of both carbonate and shale reservoirs. The goal of this study is to determine the seismic response specifically to networks of fractures clustered closely together through the analysis of seismic wavefield scatter, directional phase velocities, and amplitude attenuation. To achieve this goal, finite element modeling techniques are implemented to allow for the meshing of discontinuous fracture interfaces and, therefore, provide the most accurate calculation of seismic events from these irregular surfaces. The work presented here focuses on the center layer of an isotropic model that is populated with two main phases of fracture network alteration: a single large-scale cluster and multiple smaller-scale clusters. Phase 1 first confirms that the seismic response of a single idealized vertically fractured cluster is distinct crosscutting energy within a seismogram. Further investigation shows that, as fracture spacing within the cluster decreases, the depth at which crosscutting energy appears exponentially increases, placing it well below the true location of the cluster. This relationship holds until 28% of the fractures are moved from their uniformly spaced locations to random locations within the cluster. The vertical thickness of the cluster has little effect on the location or strength or the crosscutting signature. Phase 2 shows that, although clusters of more randomly spaced fractures mask crosscutting energy, a marked decrease in amplitude coinciding with a bend in the wavefront produces a heterogeneous anisotropic seismic response. This amplitude decay and heterogeneous anisotropy is visible until cluster spacing drops below one half of the wavelength or the ratio of fractured material to matrix material within a cluster drops below 37%. Therefore, the location of an individual fracture cluster can be determined from the location of amplitude decay, heterogeneous anisotropy, and crosscutting energy. Furthermore, the density of the cluster can be determined from the degree of amplitude decay, the angle of heterogeneous anisotropy, and the depth of cross-cutting energy. These relationships, constrained by limits on their detectability, can aid fracture network interpretation of real seismic data.Item Toward practical modeling of reinforced concrete flat plate structures(2017-12-11) Goh, Chong Yik Melvin; Hrynyk, Trevor; Bayrak, Oguzhan; Kyriakides, Stelios; Murcia-Delso, Juan; Williamson, Eric B.Most reinforced concrete (RC) design provisions have been developed on the basis of experimental data involving isolated and highly-idealized structural elements. As a result, many of the RC-related design provisions currently used are unsuitable for providing accurate assessments of RC structural systems. Thus, there is a need for practical modeling procedures that can be used to reliably investigate the performance of real-world RC structures. Such tools can aid in the assessment of structures that are exhibiting distress, that employ atypical or code-deficient design details, are in need of retrofit or modification, or simply are of a type that fall outside of traditional design provisions. This dissertation presents the development and application of nonlinear finite element analysis procedures employing cracked concrete material modeling based on the formulations of the Disturbed Stress Field Model (DSFM). These procedures are used to investigate the punching shear performance of RC flat plates, an area where design is typically done in a highly-idealized manner; yet, structure performance may be greatly impacted by system-level response. Nonlinear finite element modeling was done using two approaches. Three-dimensional solid continuum modeling was found to provide good response estimates for slab-column connection specimens under concentric shear loading and effectively captured the effects of different parameters on punching performance: reinforcement ratio, column size, and slab thickness. The validated solid modeling procedure was used to perform a parametric study on the influence of boundary conditions on the response of multi-bay flat plate structures. The findings indicated that RC slab systems are likely to have load capacities that are greater than that inferred from isolated connection elements. Alternatively, RC slab modeling was also done using computationally-efficient layered shell finite elements that accommodate through-thickness shearing effects. A simple sectional analysis modification procedure was used to incorporate strength enhancements within slab disturbed regions and the proposed procedure was shown to be capable of providing reasonable estimates for the punching shear strength of RC slab-column connections and slab subsystems. Predicted load-displacement curves and failure modes were in reasonable agreement with test data and provided estimates that were similar to those obtained using costlier solid modeling techniques.