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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Efficient coupling of micro/macroscale analyses with stochastic variations of constituent properties

McWilliams, James Keith 28 April 2014 (has links)
<p>Full-domain multiscale analyses of unidirectional AS4/H3502 open-hole composite tensile specimens were performed to assess the effect of microscale progressive fiber failures in regions with large stress/strain gradients on macroscale composite strengths. The effect of model discretization at the microscale and macroscale on the calculated composite strengths and analysis times was investigated. Multiple sets of microscale analyses of repeating unit cells, each containing varying numbers of fibers with a distinct statistical distribution of fiber strengths and fiber volume fractions, were used to establish the microscale discretization for use in multiscale calculations. In order to improve computational times, multiscale analyses were performed over a reduced domain of the open-hole specimen. The calculated strengths obtained using reduced domain analyses were comparable to those for full-domain analyses, but at a fraction of the computational cost. Such reduced domain analyses likely are an integral part of efficient adaptive multiscale analyses of large all-composite air vehicles.</p>
2

Development of a long-term Earth-Moon trans-lunar libration point orbit for a communication relay satellite

Rowells, Michelle Tamasco 23 April 2013 (has links)
<p>Given the absence of an orbital system to communicate from the far side of the Moon, it is likely that such a system shall be needed in the near future with the continued interest in lunar exploration and utilization. A strategy for a long-term libration orbit at the Earth-Moon L2 point is developed. This strategy allows for uninterrupted two-way communication between the Earth and the far side of the Moon and vice versa while also maintaining low fuel costs, thus increasing the duration of possible missions. A scenario was created in STK that created such an orbit. The results of the scenario predict a total station keeping delta-V cost of only 3.23 m/s for a satellite to stay in the vicinity of the EML2 point for five years.</p>
3

Structural analysis and testing of a carbon-composite wing using fiber Bragg gratings

Nicolas, Matthew James 23 April 2013 (has links)
<p>The objective of this study was to determine the deflected wing shape and the out-of-plane loads of a large-scale carbon-composite wing of an ultralight aerial vehicle using Fiber Bragg Grating (FBG) technology. The composite wing was instrumented with an optical fiber on its top and bottom surfaces positioned over the main spar, resulting in approximately 780 strain sensors bonded to the wings. The strain data from the FBGs was compared to that obtained from four conventional strain gages, and was used to obtain the out-of-plane loads as well as the wing shape at various load levels using NASA-developed real-time load and displacement algorithms. The composite wing measured 5.5 meters and was fabricated from laminated carbon uniaxial and biaxial prepreg fabric with varying laminate ply patterns and wall thickness dimensions. A three-tier whiffletree system was used to load the wing in a manner consistent with an in-flight loading condition.</p>
4

Navigation system design with application to the Ares I Crew Launch Vehicle and Space Launch Systems

Oliver, Jr., Ted Emerson 23 April 2013 (has links)
<p>For a launch vehicle, the Navigation System is responsible for determining the vehicle state and providing state and state derived information for Guidance and Controls. The accuracy required of the Navigation System by the vehicle is dependent upon the vehicle, vehicle mission, and other consideration, such as impact foot print. NASAs Ares I launch vehicle and SLS are examples of launch vehicles with are/where to employ inertial navigation systems. For an inertial navigation system, the navigation system accuracy is defined by the inertial instrument errors to a degree determined by the method of estimating the initial navigation state. Utilization of GPS aiding greatly reduces the accuracy required in inertial hardware to meet the same accuracy at orbit insertion. For a launch vehicle with lunar bound payload, the navigation accuracy can have large implications on propellant required to correct for state errors during trans-lunar injection.</p>
5

Viscoelastic characterization of vapor-grown carbon nanofiber/vinyl ester nanocomposites using a response surface methodology

Drake, Daniel Adam 23 April 2013 (has links)
<p>The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic responses (creep strain, creep rate, and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). The nanocomposite test articles were fabricated by high shear mixing, casting, curing, and post-curing in an open face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at prescribed combinations of temperatures (23.8 69.2 C), applied stresses (30.2 49.8 MPa), and VGCNF weight fractions (0.00 1.00 parts of VGCNF per hundred parts of resin, phr) determined from the CCD. The response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction decreases the creep resistance of the VGCNF/VE nanocomposites at high temperatures (46.5 69.2 C).</p>
6

Statistical characterization of viscoelastic creep compliances of a vinyl ester polymer

Simsiriwong, Jutima 28 April 2014 (has links)
<p>The objective of this study was to develop a model to predict the viscoelastic material functions of a vinyl ester (VE) polymer (Derakane 441-400, Ashland Co.,) with variations in its material properties. Short-term tensile creep/creep recovery experiments were conducted at two stress levels and at four temperatures below the glass transition temperature of the VE polymer, with 10 replicates for each test configuration. Experimental strains in both the longitudinal and transverse directions were measured using a digital image correlation technique. The measured creep strain versus time responses were subsequently used to determine the creep compliances using the generalized viscoelastic constitutive equation with a Prony series representation.</p> <p>The variation in the creep compliances of Derakane 441-400 was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the creep compliances using the two-parameter Weibull and log-normal distributions. The maximum likelihood estimation technique was used to obtain the Weibull shape and its scale parameters and the log-normal location and its scale parameters. The goodness-of-fit of the distributions was determined by performing Kolmogorov-Smirnov (K-S) hypothesis tests. Based on the K-S test results, the Weibull distribution is a better representation of the creep compliances of Derakane 441-400 when compared to the log-normal distribution. Additionally, the Weibull scale and shape parameters of the creep compliance distributions were shown to be time and temperature dependent. Therefore, two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and subsequently the CDFs of the creep compliances for the complete design temperature range during steady state creep. At each test temperature, creep compliance curves were obtained for CDF values of 0.05, 0.50 and 0.95 and compared with the experimentally obtained lowest, mean and highest creep compliances, respectively. The predicted creep compliances of Derakane 441-400 in the design space are in good agreement with the experimental data.</p>
7

Affecting attitudes towards science, high school African American students

Anderton, Charles Brett 28 April 2014 (has links)
<p>Racial minorities, women, and people with disabilities are underrepresented in the fields of Science, Technology, Engineering, and Mathematics (STEM). Attitude towards science has been shown to be a reliable predictor of science achievement. Project-Based Learning (PBL) has been shown to improve attitude towards a topic. The sample selected consisted of 113 African American high school students (68% to 32% female to male ratio) from Alabama, Louisiana, and Mississippi. A quasi-experimental research design which consisted of pre and post intervention measures of participants attitudes towards science was utilized in this study. Overall, Phase 1, a week-long residential camp, saw greater increases with direct respect to time or gender due to the immersive nature of the camp, whereas Phase 2, an eight week long outreach, saw a more complex interaction of the two factors. PBL was shown to be an effective method of instruction to reach African American and women populations.</p>
8

Generalized polynomial chaos and markov chain monte carlo methods for nonlinear filtering

Cai, Sheng 25 July 2014 (has links)
<p>In science and engineering research, filtering or estimation of systems states is widely used and developed, so the structure of a new nonlinear filter is proposed. This thesis focuses on the procedures of propagation and update step of the new filter. The algorithms used in the filter, including generalized Polynomial Chaos Algorithms, Markov Chain Monte Carlo algorithms, and Gaussian Mixture Model algorithms, are introduced. Then, the propagation and update step of the proposed filter are applied in solving two nonlinear problems: Van der Pol Oscillator and Two Body System. The simulation shows that the results of the propagation and update step are reasonable and their designs are valuable for further tests. The propagation step has the same accuracy level compared with a Quasi Monte Carlo simulation while using a much smaller number of points. The update step can build a useful Gaussian Mixture Model as the posterior distribution.</p>
9

A validation study of OpenFOAM for Hybrid RANS-LES simulation of incompressible flow over a backward facing step and delta wing

Choudhury, Visrant 28 April 2014 (has links)
<p>The primary objective of this study is to validate and/or identify issues for available numerical methods and turbulence models in OpenFOAM 2.0.0. Such a study will provide a guideline for users, will aid acceptance of OpenFOAM as one of the research solvers at institutions and also guide future multidisciplinary research using OpenFOAM. In addition, a problem of aerospace interest such as the flow features and vortex breakdown around a VFE-II model is obtained for SA, SST RANS and SA-DDES models and compared with DLR experiment. The available numerical methods such as time schemes, convection schemes, P-V couplings and turbulence models are tested as available for a fundamental case of a backward facing step for RANS and Hybrid RANSLES prediction of fully turbulent flow at a Reynolds number of 32000 and the OpenFOAM predictions are validated against experimental data by Driver et.al and compared with Fluent predictions.</p>
10

The development of multi-range inertial measurement units

Kelly, James Paul 25 July 2014 (has links)
<p>There exist numerous commercial six-degree-of-freedom inertial measurement units capable of measuring low-range accelerations and rotation rates. A commercially available multi-range IMU capable of measuring low and high-range motions does not exist. An IMU with this capability was developed for measuring trajectory data of projectiles such as high-powered rockets. This data can be used to provide performance feedback to projectile designers and users.</p> <p>A small footprint printed circuit board was designed to minimize the overall size of the unit, compared to perf-board prototypes. Several PCB design guidelines were closely followed to reduce electrical interference in digital/analog components and traces. Embedded C code was developed to control the IMU. The unit features a wireless user interface, providing several control options, including an option to download data sampled at 1KHz per sweep of all twelve sensor channels. Preliminary testing reveals good consistency among the high and low-range sensors and acceptably low noise levels.</p>

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