# Browsing by Subject "Finite element"

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Item A finite element test bed for development of feedback control laws for electrostatic MEMS(2005-12) Kawade, Balasaheb D.; Berg, Jordan M.; Dallas, Timothy E. J.; Idesman, Alexander V.Show more This project presents the ANSYS simulation techniques for an electrostatically-actuated MEMS device incorporating feedback control laws. The electrostatic MEMS device consists of a movable electrode, suspended on flexible, elastic structures, and one or more fixed drive electrodes. Nonlinear feedback control laws are simulated in ANSYS multi-physics solver and a transducer element. ANSYS multi-physics solver is limited for these types of simulations. ANSYS doesn’t support multiframe restart and the combined circuit and electrostatic analysis are incompatible. This work presents simulation techniques based on numerical methods to circumvent these limitations. The proposed technique eliminates the circuit elements from the model, and instead propagates the associated states in an APDL macro. ANSYS auto time stepping method is not applicable for closed-loop feedback control systems because loads are calculated at each step based on simulation output at the previous step. An adaptive step size Runge-Kutta integration routine is incorporated within APDL macro to develop an efficient simulation technique. The simulation efficiency of the static closed loop feedback control systems is increased by a factor more than 100. However, a dynamic closed loop feedback control systems exhibits only a brief initial transient, and then does not permit further step size increases. To increase the simulation efficiency of such systems, the adaptation logic is turned off once the step size stabilizes. Simulation results for representative MEMS devices including a one-DOF piston microactuator and a two-DOF rotating/translating microactuator demonstrate the efficiency of these simulation techniques.Show more Item A finite element test bed for development of feedback control laws for electrostatic MEMS(Texas Tech University, 2005-12) Kawade, Balasaheb D.; Berg, Jordan M.; Dallas, Timothy E. J.; Idesman, Alexander V.Show more This project presents the ANSYS simulation techniques for an electrostatically-actuated MEMS device incorporating feedback control laws. The electrostatic MEMS device consists of a movable electrode, suspended on flexible, elastic structures, and one or more fixed drive electrodes. Nonlinear feedback control laws are simulated in ANSYS multi-physics solver and a transducer element. ANSYS multi-physics solver is limited for these types of simulations. ANSYS doesn’t support multiframe restart and the combined circuit and electrostatic analysis are incompatible. This work presents simulation techniques based on numerical methods to circumvent these limitations. The proposed technique eliminates the circuit elements from the model, and instead propagates the associated states in an APDL macro. ANSYS auto time stepping method is not applicable for closed-loop feedback control systems because loads are calculated at each step based on simulation output at the previous step. An adaptive step size Runge-Kutta integration routine is incorporated within APDL macro to develop an efficient simulation technique. The simulation efficiency of the static closed loop feedback control systems is increased by a factor more than 100. However, a dynamic closed loop feedback control systems exhibits only a brief initial transient, and then does not permit further step size increases. To increase the simulation efficiency of such systems, the adaptation logic is turned off once the step size stabilizes. Simulation results for representative MEMS devices including a one-DOF piston microactuator and a two-DOF rotating/translating microactuator demonstrate the efficiency of these simulation techniques.Show more Item Analysis of soil-structure system response with adjustments to soil properties by perturbation method(2014-05) Patta, Sang Putra Pasca Rante; Tassoulas, John LambrosShow more The 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.Show more 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, HarovelShow more Implementation 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.Show more 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.Show more 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.Show more Item Finite element analysis of welds attaching short doubler plates in steel moment resisting frames(2014-12) Marquez, Alberto C.; Engelhardt, Michael D.Show more 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.Show more Item Finite element modeling of martensitic phase transformation(Texas Tech University, 2009-05) Cho, Joon-Yeoun; Idesman, Alexander V.; Levitas, Valery; Han, Seon; Aulisa, Eugenio; Bhattacharya, SukalyanShow more Two finite element approaches are suggested for the modeling of multi-variant martensitic phase transitions in elastic materials at different length scales. The first one is designed for the modeling of phase transformation at a large length scale including meso-scale. It is based on the thermomechanical phenomenological model for phase transformation that represents strain softening during phase transition. In contrast to the known publications on phase transformation, which apply the standard elasto-plastic models with strain softening, our model is related to multi-variant martensitic phase transformation. Rate dependent constitutive equations used in the model facilitate avoidance of the mesh sensitivity at the numerical implementation of the approach. Due to strain softening a microstructure containing pure martensitic and austenitic domains with the small transition zones can be obtained as the solution of the corresponding boundary value problem. A finite element algorithm for the first approach is developed and implemented into the software ABAQUS. Several two dimensional problems for martensitic phase transformation in single crystal and poly crystal elastic materials are solved and analyzed. The second approach is developed for the description of phase transition at nano-scale and based on the Ginzburg-Landau theory with a new thermodynamic potential that captures the main features of macroscopic stress-strain curves. Distributions of different martensitic variants are the solution of the coupled system of the evolution equations (time dependent Ginzburg- andau equation) for the order parameters, which represent different phases, and the elasto-dynamics equations. Due to similarity between the evolution equations and heat transfer equations, numerical simulations of evolving microstructure during phase transition can be obtained from the solution of the coupled heat transfer and elasticity equations with the replacement of temperature by different order parameters. In known approaches based on the Ginzburg-Landau theory, constant stresses are used, or stresses are calculated by solving elasto-static equations which are a particular case of elasto-dynamic equations. However, in our model, the general elasto-dynamic equations are used for the calculation of stresses. A numerical algorithm for the solution of the coupled system of equations is suggested and implemented into the finite element program 'FEAP'. Several numerical examples of the modeling of evolving microstructure during multi-variant martensitic phase transition such as phase transition in 2-D single and poly-crystal, and 3-D single crystal specimens are solved and analyzed. The results are compared with the solutions using the elasto-static equations and show the importance of inertial forces.Show more Item Least squares based finite element formulations and their applications in fluid mechanics(2009-05-15) Prabhakar, VivekShow more In this research, least-squares based finite element formulations and their applications in fluid mechanics are presented. Least-squares formulations offer several computational and theoretical advantages for Newtonian as well as non-Newtonian fluid flows. Most notably, these formulations circumvent the inf-sup condition of Ladyzhenskaya-Babuska- Brezzi (LBB) such that the choice of approximating space is not subject to any compatibility condition. Also, the resulting coefficient matrix is symmetric and positive-definite. It has been observed that pressure and velocities are not strongly coupled in traditional leastsquares based finite element formulations. Penalty based least-squares formulations that fix the pressure-velocity coupling problem are proposed, implemented in a computational scheme, and evaluated in this study. The continuity equation is treated as a constraint on the velocity field and the constraint is enforced using the penalty method. These penalty based formulations produce accurate results for even low penalty parameters (in the range of 10-50 penalty parameter). A stress based least-squares formulation is also being proposed to couple pressure and velocities. Stress components are introduced as independent variables to make the system first order. The continuity equation is eliminated from the system with suitable modifications. Least-squares formulations are also developed for viscoelastic flows and moving boundary flows. All the formulations developed in this study are tested using several benchmark problems. All of the finite element models developed in this study performed well in all cases. A method to exploit orthogonality of modal bases to avoid numerical integration and have a fast computation is also developed during this study. The entries of the coefficient matrix are calculated analytically. The properties of Jacobi polynomials are used and most of the entries of the coefficient matrix are recast so that they can be evaluated analytically.Show more Item 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, BugaoShow more The 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.Show more Item Mechanics of lithospheric delamination in extensional settings.(2015-03-23) Jex, Jeffrey A. 1988-; Dunbar, John A., 1955-Show more Delamination, the foundering of the lower crust and sub-crustal lithosphere, is one of the most important geodynamic processes that is still poorly understood. Geodynamic modeling has constrained conditions and likely outcomes of delamination in orogenically-thickened crust. In this study, I do the same for delamination in extensional settings by using finite element models of young passive margins. Delamination in these models may occur as melt beneath oceanic crust intrudes between the lower continental crust and sub-crustal lithosphere, driven by buoyancy. When sufficient melt is available and the lower crust is weak, the melt wedges between the lower crust and sub-crustal lithosphere, initiating delamination of the sub-crustal lithosphere. The speed of delamination is strongly dependent on weakness of the lower crust followed by the amount of melt present.Show more Item Modeling and simulation of a high pressure hydrogen storage tank with dynamic wall(Texas Tech University, 2005-12) Cumalioglu, Ilgaz; Ertas, Atila; Ekwaro-Osire, Stephen; Ma, Yanzhang; Maxwell, Timothy T.Show more Hydrogen storage is one of the divisions of hydrogen powered vehicles technology. To increase performances of high pressure hydrogen storage tanks, a multilayered design is proposed featuring the dynamic wall capable of absorbing hydrogen. Modeling and parametric study have been done to extract information on required mechanical and physical properties of the wall. Parameters and system constraints have been defined, relations are found and discussed.Show more Item Modeling and simulation of a high pressure hydrogen storage tank with Dynamic Wall(2005-12) Cumalioglu, Ilgaz; Ertas, Atila; Ekwaro-Osire, Stephen; Ma, Yanzhang; Maxwell, Timothy T.Show more Hydrogen storage is one of the divisions of hydrogen powered vehicles technology. To increase performances of high pressure hydrogen storage tanks, a multilayered design is proposed featuring the dynamic wall capable of absorbing hydrogen. Modeling and parametric study have been done to extract information on required mechanical and physical properties of the wall. Parameters and system constraints have been defined, relations are found and discussed.Show more 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.Show more 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.Show more Item Quantifying three dimensional effects in acoustic rough surface scattering(2011-05) Joshi, Sumedh Mohan; Hamilton, Mark F.; Isakson, Marcia J.Show more 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.Show more Item The seismic response to fracture clustering : a finite element wave propagation study(2014-05) Becker, Lauren Elizabeth; Spikes, KyleShow more Characterizing 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.Show more