Surface charging is a fundamental interaction process in space plasma engineering. A three-dimensional Immersed Finite Element Particle-In-Cell (IFE-PIC) method is developed to model surface charging involving complex boundary conditions. This method extends the previous IFE-PIC algorithm to explicitly include charge deposition on a dielectric surface for charging calculations. Three simulation studies are carried out using the new algorithm to model current collection and charging in both the orbital motion limited (OML) and space charge limited regime. The first one is a full particle simulation of the charging process of single small sphere and clusters of multiple small spheres in plasma. We find that while single sphere charging agrees well with the predictions of the OML theory, the charging of a sphere in a cluster is significantly, indicating that the often used OML charging model is not an accurate one to model charging in dusty plasma. The second one concerns a secondary electron emission experiment. The simulation includes detailed experimental setup in a vacuum chamber and the results are compared against experimental data. The simulation is used to determine the facility error in experiments. The third one is a full particle simulation of charging on lunar surface. The simulation concerns both flat and non-flat surface, and spacecraft on lunar surface, in the lunar polar region. The surface sees a mesothermal solar wind plasma flow and the emission of photoelectrons and secondary electrons. At a small sun elevation angle, the surface landscape generates a complex plasma flow field and local differential charging on surface. The results will be useful for further study of charging and levitation of lunar dust. / Ph. D.

Inflatable dams are flexible, cylindrical structures anchored to a foundation. They are used for a variety of purposes, e.g. diverting water for irrigation or groundwater recharging, impounding water for recreational purposes, and raising the height of existing dams or spillways. The vibration behavior of such dams is analyzed. Single-anchor inflatable dams with fins are considered. First, a static analysis is performed which yields the equilibrium shapes of the dam, both in the presence and absence of water. Then, a dynamic analysis is undertaken which analyzes the small vibrations of the inflatable dam about the equilibrium configuration, both in the presence of water (hydrostatic water as well as parallel flowing water) and absence of water. The dam is modeled as an elastic shell. It is assumed to be air-inflated and resting on a rigid foundation. The cross-sectional perimeter, material thickness, modulus of elasticity, and Poisson's ratio are given. The analysis is performed for different values of internal pressure and external water heads. Initially, the dam is assumed to lie flat. The internal pressure is then increased slowly until it reaches the desired value. Then the external water is applied and the equilibrium configuration is obtained. Small vibrations about this configuration are considered. The water is assumed to be inviscid and incompressible, and potential theory is used. The infinite-frequency limit is assumed on the free surface. A boundary element technique is utilized to determine the behavior of the water, and the finite element program ABAQUS is used to analyze the structural behavior. Both the cases of fluid at rest and flowing parallel to the dam are considered. The vibration frequencies and mode shapes are computed. The effect of the internal pressure of the dam is investigated, and the results are compared to those for the dam in the absence of external water. / Master of Science

Simulations of the welding process for butt and tee joints using finite element analyses are presented. The base metal is aluminum alloy 2519-T87 and the filler material is alloy 2319. The simulations are performed with the commercial software SYSWELD+®, which includes moving heat sources, material deposit, metallurgy of binary aluminum, temperature dependent material properties, metal plasticity and elasticity, transient heat transfer and mechanical analyses. One-way thermo-mechanical coupling is assumed, which means that the thermal analysis is completed first, followed by a separate mechanical analysis based on the thermal history. The residual stress state from a three-dimensional analysis of the butt joint is compared to previously published results. For the quasi-steady state analysis the maximum residual longitudinal normal stress was within 3.6% of published data, and for a fully transient analysis this maximum stress was within 13% of the published result. The tee section requires two weld passes, and both a fully three-dimensional (3-D) and a 3-D to 2-D solid-shell finite elements model were employed. Using the quasi-steady state procedure for the tee, the maximum residual stresses were found to be 90-100% of the room-temperature yield strength. However, the longitudinal normal stress in the first weld bead was compressive, while the stress component was tensile in the second weld bead. To investigate this effect a fully transient analysis of the tee joint was attempted, but the excessive computer times prevented a resolution of the longitudinal residual stress discrepancy found in the quasi-steady state analysis. To reduce computer times for the tee, a model containing both solid and shell elements was attempted. Unfortunately, the mechanical analysis did not converge, which appears to be due to the transition elements used in this coupled solid-shell model. Welding simulations to predict residual stress states require three-dimensional analysis in the vicinity of the joint and these analyses are computationally intensive and difficult. Although the state of the art in welding simulations using finite elements has advanced, it does not appear at this time that such simulations are effective for parametric studies, much less to include in an optimization algorithm. / Master of Science

The mechanical response tissue analyzer (MRTA) tests long bone quality through low frequency, low amplitude vibration in vivo. The MRTA measures complex stiffness over a range of low frequencies, offering a wealth of information on bone composition. Previous MRTA interpretation used lumped parameter algorithms focused on reliably estimating the bone's bending stiffness (EI). To interpret the stiffness response, the first finite element (FE) simulation of the MRTA test of a human tibia was developed to identify dominant parameters that will possibly make linear prediction algorithms more suitable for estimating bone quality. Five FE models were developed in stages by adding complexity. Starting with a solid mesh of the diaphysis, each model was created from its predecessor by sequentially adding: a medullary canal, linear elastic (LE) cancellous epiphyses, linear viscoelastic (LVE) cancellous and cortical bone, and a LVE skin layer. The models were simulated in vibration using a direct steady-state dynamics procedure in ABAQUS to calculate the complex stiffness response. Natural frequency analysis (ABAQUS) verified that the FE models accurately reproduced previous experimental and computational resonances for human tibiae. A solid, LE cortex roughly matched the dominant frequency from experimental MRTA raw data. Adding the medullary canal and LVE properties to bone did not greatly spread the peak or shift the resonant frequency. Adding the skin layer broadened the peak response to better match the MRTA experimental response. These results demonstrate a simulation of the MRTA response based upon published geometries and material data that captures the essence of the instrument. / Master of Science

The interest in the use of aluminum as a structural material in marine applications has increased greatly in recent years. This increase is primarily due to the low weight of aluminum compared to other structural materials as well as its ability to resist corrosion. However, a critical issue in the use of any structural material for naval applications is its response to fire. Past experience has shown that finite element programs can produce accurate predictions of failure of structural components. Parameter studies conducted within finite element programs are often easier to implement than corresponding studies conducted experimentally. In this work, the compression-controlled failures of aluminum plates subjected to an applied mechanical load and an applied heat flux (to simulate fire) were predicted through the use of finite element analysis. Numerous studies were completed on these finite element models. Thicknesses of the plates were varied as well as the applied heat flux and the applied compressive stresses. The effect of surface emissivity along with the effect of insulation on the exposed surface of the plate was also studied. The influence of the initial imperfection of the plates was also studied. Not only were the physical conditions of the model varied but the element type of both the solid and shell models as well as the mesh density were also varied. Two different creep laws were used to curve fit raw creep data to understand the effects of creep in the buckling failure of the aluminum plates. These predictions were compared with experiments (from a previous study) conducted on aluminum plates of approximately 800mm in length, 200mm in width, 6-9mm in thickness and clamped at both ends to create fixed boundary conditions. A hydraulic system and a heater were used to apply the compressive load and the heat flux respectively. Comparisons between predicted and experimental results reveal that finite element analysis can accurately predict the compression-controlled failure of aluminum plates subjected to simulated fire. However, under certain combinations of the applied heat flux and compressive stress, the mesh density as well as the choice of element may have a significant impact on the results. Also, it is undetermined which creep curve-fitting model produces the most accurate results due to the influence of other parameters such as the initial imperfection. / Master of Science

Finite Element Analysis (FEA) is proposed to simulate the connection response of practical in-service conditions and test the performance of Oil Country Tubular Goods (OCTG) premium threaded connections. A plug-in is developed in Abaqus/CAE for creating the 360-degree full 3D parametric finite element model with helical threads as an effective design and analysis tool. All size, position and material data of the model are parameterized. The premium connection plug-in accepts input from the Graphical User Interface (GUI) for further modification. Each premium connection component is programed as a collection of single-purpose independent functions organized as an independent module in order to allow users to modify specific function behavior conveniently. A main program is designed as an Abaqus kernel plug-in to achieve all the functions by calling these independent functions, making the plug-in flexible. Each single script file is not too long to jeopardize readability. The GUI of the plug-in is designed with proper layout arrangement and illustrations to make the plug-in user-friendly and easy to use. The premium connection FE model is used in a virtual test to validate the model against the ISO 13679 test protocol and is used to develop the seal metrics for points on the ISO 13679 sealability envelope. The plug-in can be used to develop and evaluate the design envelope of the premium connection. / Master of Science / Oil Country Tubular Goods (OCTG) refers to a specific kind of steel tube used in the oil and gas industry--following the specifications set by the American Petroleum Institute (API). As the drilling of the modern oil well goes deeper, the extremely high temperatures and pressures require better quality oil tubes and connections. The drill pipe, connected by Premium Connections, are designed and tested carefully in order to avoid any possible environmental pollution or financial loss resulting from technical failures. Physical testing of each design takes time and costs a lot. Finite Element Analysis (FEA) is proposed to simulate the connection response of practical in-service conditions and test the performance of OCTG premium threaded connections. Full 360-degree 3D finite element models are the most complete representation of premium threaded connections. A plug-in is developed in Abaqus/CAE for creating the finite element model with helical threads as an effective design and analysis tool. The plug-in can be used to develop and evaluate the design envelope of the premium connection.

The research focuses on developing a methodology for modeling a pneumatic bias-ply tire with the finite element method for vehicle dynamics simulation. The tire as a load-carrying member in a vehicle system deserves emphasized formulation especially for the contact patch because its representation of mechanics in the contact patch directly impacts the handling and ride performance of a vehicle. On the other hand, the load transfer from the contact patch to the wheel hub is necessary for determining the inputs to a chassis. A finite element (FE) tire model has strong capability to handle these two issues. However, the high cost of computing resources restrains its application mainly in the tire design domain. This research aims to investigate how to balance the complexity of a simplified FE tire model without diminishing its capability towards representing the load transmission for vehicle dynamics simulation. The traditional FE tire model developed by tire suppliers usually consists of an extremely large number of elements, which makes it impossible to be included in a full-vehicle dynamics simulation. The material properties required by tire companies' FE tire models are protected. The car companies have an increasing need for a physical-based tire model to understand more about the interaction between the tire and chassis. A gap between the two sides occurs because the model used for tire design cannot directly help car companies for their purpose. All of these reasons motivate the current research to provide a solution to narrow this gap. Other modern tire models for vehicle dynamics, e.g. FTire or TAME, require a series of full-tire tests to calibrate their model parameters, which is expensive and time-consuming. One great merit of the proposed simplified FE tire model is that determining model inputs only requires small-scale specimen tests instead of full-tire tests. Because much of the usability of a model hinges on whether its input parameters are easily determined, this feature makes the current model low cost and easily accessible in the absence of proprietary information from the tire supplier. A Hoosier LC0 racing tire was selected as a proof of modeling concept. All modeling work was carried out using the general purpose commercial software Abaqus. The developed model was validated through static load-deflection test data together with Digital Image Correlation (DIC) data. The finite element models were further evaluated by predicting the traction/braking and cornering tire forces against Tire Test Consortium (TTC) data from the Calspan flat-track test facility. The emphasis was put on modeling techniques for the transient response due to the lack of available test data. The in-plane and out-of-plane performance of the Hoosier tire on the full-tire test data is used for model validation, not for "calibrating" the model. The agreement between model prediction and physical tests demonstrate the effectiveness of the proposed methodology. / PHD / This research aims to develop a method to build a physically-based tire model less relying on the information of products from tire providers for the purpose of vehicle dynamics simulation. The tire model is a mathematical description of the behavior of tires under various operational conditions. The model is said to be ‘physically-based’ if it is derived from physical laws. In contrast, if the model is termed ‘semi-empirical,’ it means that the model is mainly based on tire measurement data. A physically-based model usually gives more insights to and a better understanding of tire mechanics than a semi-empirical tire model. The tire as a load-carrying member in a vehicle system deserves emphasized formulation especially for the tire-road contact patch because its representation of mechanics in the contact patch directly impacts the handling and ride performance of a vehicle. Therefore, a physically-based tire model is preferred. One kind of physically-based models are developed through the multi-body dynamics (MBD) approach. Various full tire tests are required to identify the parameters associated with the model. Since full tire tests should be conducted on professional tire test machines, the high-cost prevents many users to have a tire model of such kind. The other kind of physically-based models are developed through the finite-element method (FEM). The FEM has strong capability to describe the mechanism of tire-road contact and deformation of the tire body. Also, parameters needed by a finite element tire model are basic material properties of different components of the tire structure, which implies the possibility to acquire parameters through small-scale sample tests instead of full tire tests. However, most of FE tire models are developed for tire design with high complexity, not good for vehicle simulation. This research made efforts to degrade the complexity of the FE tire model and tailor the FE modeling technique suitable for the purpose of vehicle simulation. In addition, the process was designed and implemented for obtaining the necessary parameters associated with the model. A Hoosier LC0 racing tire was selected as a proof of modeling concept without any tire property data provided by tire producers. This research has a practical meaning on building tire models independent of tire companies and at low cost.

An important part of an aerodynamic design process is optimizing designs to maximize quantities such as lift and the lift-to-drag ratio, in a process known as shape optimization. It is the goal of this thesis to develop and apply understanding of mixed finite element method and sensitivity analysis in a way that sets the foundation for shape optimization. The open-source Incompressible Flow Iterative Solution Software (IFISS) mixed finite element method toolbox for MATLAB developed by Silvester, Elman, and Ramage is used. Meshes are produced for a backward-facing step problem, using built-in tools from IFISS as well as the mesh generation software Gmsh, and grid convergence studies are performed for both sets of meshes along a sampled data line to ensure that the simulations converge asymptotically with increasing mesh resolution. As a preliminary study of sensitivity analysis, analytic sensitivities of velocity components along the backward-facing step data line to inflow velocity parameters are determined and verified using finite difference and complex step sensitivity values. The method is then applied to pressure drag calculated by integrating the pressure over the surface of a circular cylinder in a freestream flow, and verified and validated using published simulation data and experimental data. The sensitivity analysis study is extended to shape optimization, wherein the shape of a circular cylinder is altered and the sensitivities of the pressure drag coefficient to the changes in the cylinder shape are determined and verified. / Master of Science / When looking at designing an aircraft, it is important to consider the forces air flow exerts on the wings. The primary forces of interest for aerodynamic analysis are lift, which generally acts upward perpendicular to the flow of air, and drag, which opposes the motion of the wing through the air. Optimization is the process of developing a design in such a way that a specific quantity, such as lift or drag, is either maximized or minimized. Many methods exist of predicting the behavior of air flow, and various methods of optimization exist which take already existing predictive software and progressively alter the design to try to meet the minimized or maximized objective. This thesis outlines a multi-step effort to modify an open source software such that it could be used for design optimization.

Lattice structures are widely used in nowadays industries in combination with additive manufacturing technology to obtain components with a limited weight and tuneable mechanical properties. However, industries still find challenging a complete implementation of these metamaterials in the product development due to the complexity given by an accurate prediction of the mechanical and fatigue properties. To overcome this limitation, analytical and numerical techniques are developed, to help designers to achieve the desired performances. Finite Element simulations are a common tool utilized in this sense, where solid models can provide accurate results. Nevertheless, the implementation of this technique requires high computational costs, often not compatible with an iterative design process where versions of the component are constantly updated considering the feedback provided by actors having different backgrounds and product interactions. Accurate and computationally efficient simulations strategies are thus required. The proposed thesis investigates three possible simulations ideas able to describe the mechanical properties of the lattice-based components. Two main properties are studied: the lattice structure elastic behaviour, which is important to determine the in-service behaviour of the designed component and the fatigue resistance, which defines the component service duration. Homogenization technique is the first numerical method analysed and it is pivoted on the idea of substituting the intricate lattice geometries with a solid fictitious material displaying the same elastic properties. In this framework, a case study is analysed, where the design process of a total hip replacement prosthetic device is developed. The workflow starts with a preliminary experimental campaign on lattice specimens with the aim of determining the printing quality, the mechanical properties, and the biological characteristics. In this phase, a verification of the homogenization predictions is performed. On this base, the best specimens’ configurations are selected to design and manufacture the prosthetic device. The second simulation technique leverages on the observation of the onedimensional nature of the strut-based lattice structures. Lattice structures’ behaviour can thus be simulated through the usage of truss and beam elements, depending on the stretching or bending dominated nature of the lattice topologies. Based on this observation, two different paths are followed, the first one aiming to improve the fatigue life of lattice components by acting of their orientation in the printing chamber. It is known that printing orientation influences the surface quality of the components and, in lattice struts this effect can be directly linked to a variation in the fatigue life. An optimization algorithm is thus developed, aiming to optimize the fatigue resistance of the manufactured components. Following this idea, a control and an optimized lattice batch are printed and an improvement in the fatigue resistance is found, even if not as large as expected by the simulations. Improvements in the predictions can be observed if the as-build geometry of the struts is considered. The second path is devoted to the computation of the corrective coefficients able to properly describe the elastic properties of bending dominated lattice structures. One-dimensional simulations are normally too severe for bending dominated lattice topologies, and a compensation has to be provided to match the elastic properties calculated trough computational efficient beam models and lattice ones. To address this problem, an optimization routine is developed, where the compensation factors are computed comparing the elastic properties of the beam models and a homogenised solid model taken as reference. A benchmark testing between the beam model, - built with the so computed compensation coefficients - a homogenised, and a solid model is developed. Compensated beam models are found to be able to improve the predictions of lattice structures elastic properties if compared to the homogenization techniques, showing a comparable computational time. Nevertheless, a reduced accuracy is found in presence of dense lattice structures, where the hypothesis of one-dimensional is weaker. The third analysed simulation method aims to obtain a precise fatigue life estimation at the expense of computational time. Starting from an as-build geometry reconstructed trough CT-scan analysis, a finite element simulation built with solid elements is performed. To reduce the computational cost, an innovative finite element theory is adopted, the Finite Cell Method. A two-step simulation is performed, and thanks to the usage of the average strain energy density method, the fatigue life estimation can be obtained. An excellent agreement is found; however, a complete validation is required for this method before its safe implementation in the design process.

Cases of brittle fractures in major bridges prompted AASHTO to publish its first fracture control plan in 1978. It focused on material and fabrication standards, and required periodic 24-month hands-on inspection of bridges with fracture critical members. The practical result of this plan was to significantly increase the life cycle cost of these bridges, rendering them uneconomical. Apart from the Point Pleasant Bridge that failed in 1967, no other bridge has collapsed in the USA following a fracture, even though large fractures have been observed in many other bridges. All these bridges showed some degree of redundancy and therefore could be reclassified as non-fracture critical if detailed analyses are carried out. The goal of this study is to provide guidance on redundancy evaluation of fracture critical bridges, specifically three girder bridges and twin box-girder bridges. The effect of various loading, analysis and geometric parameters on the post fracture response and the remaining load carrying capacity of the damaged bridge is evaluated through nonlinear finite element analysis of two well-documented structures: the Hoan Bridge and the twin box-girder bridge. Parameters such as damping definition, modelling of composite action, modelling of secondary elements, boundary conditions, and rate dependent material properties are found to be crucial in capturing the bridge response. A two-step methodology for system redundancy analysis of fracture critical bridges is proposed, leading to a reclassification of these elements as non-fracture critical for in-service inspection. The first step evaluates bridge capacity to withstand collapse following fracture based on whether the residual deformation is perceivable to people on or off the bridge. If the bridge satisfies the first step requirements, then the reserve load carrying capacity of the damaged bridge is evaluated in the second step. The Hoan Bridge failed to satisfy the proposed requirements in the first step and therefore its girders could not be reclassified as non-fracture critical. The twin box-girder bridge successfully resisted the collapse in two out three loading scenarios and displayed reserve load carrying capacity following full depth fracture in the exterior girder, and therefore can be reclassified as non-fracture critical for in-service inspection. / Ph. D.