Browsing by Author "Larry Silverberg, Committee Member"

Now showing 1 - 8 of 8

A Computational Intelligence Approach to the Mars Precision Landing Problem
(2008-04-21) Birge, Brian Kent III; Mark White, Committee Member; Paul Ro, Committee Member; Larry Silverberg, Committee Member; Gerald Walberg, Committee Chair
Grain Subdivision and Microstructural Interfacial Scale Effects in Polycrystalline Materials
(2006-01-06) Rezvanian, Omid; Murthy Guddati, Committee Member; Mohammed Zikry, Committee Chair; Kara Peters, Committee Member; Larry Silverberg, Committee Member
The major objective of this research is to develop a unified physically-based representation of the microstructure in f.c.c. crystalline materials to investigate finite inelastic deformation and failure modes and scenarios at different physical scales that occur due to a myriad of factors, such as texture, grain size and shape, grain subdivision, heterogeneous microstructures, and grain boundary misorientations and distributions. The microstructurally-based formulation for inelastic deformation is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations (SSDs), geometrically necessary dislocations (GNDs), and grain boundary dislocations (GBDs). This dislocation density based multiple-slip crystal plasticity formulation is then coupled to specialized finite-element methods to predict the scale-dependent microstructural behavior, the evolving heterogeneous microstructure, and the localized phenomena that may contribute to failure initiation for large inelastic strains. The SSD densities provide a representation of cell-type dislocation microstructures and their related processes. The GND densities provide an understanding of the scale-dependent deformation behavior of crystalline materials as a function of grain and aggregate sizes. The GBD densities are formulated to represent the misfit dislocations that arise due to lattice misorientations across GBs, and to provide a framework to investigate the phenomena associated with the grain boundary orientations and distributions. This provides a local criterion of how GB interfaces, such as triple junctions are potential sites for failure initiation and localized behavior. The evolution of the GNDs is used to predict and understand how crystallographic and non-crystallographic microstructures relate to intragranular and intergranular deformation patterns and behavior. Furthermore, a clear understanding of how GB strength changes due to microstructural evolution is obtained as a function of microstructural heterogeneities that occur at different physical scales.
Mechanical Engineering Capstone Senior Design Course Textbook
(2006-09-08) Barrett, Rolin Farrar; Joseph Hobbs, Committee Member; Larry Silverberg, Committee Member; Eric Klang, Committee Chair; Richard Johnson, Committee Member; Kara Peters, Committee Member
This textbook is intended to bridge the gap between mechanical engineering equations and mechanical engineering design. To that end, real-world examples are used throughout the book. Also, the material is presented in an order that follows the chronological sequence of coursework that must be performed by a student in the typical capstone senior design course in mechanical engineering. In the process of writing this book, the author surveyed the fifty largest engineering schools (as ranked by the American Society of Engineering Education, or ASEE) to determine what engineering instructors are looking for in a textbook. The survey results revealed a clear need for a textbook written expressly for the capstone senior design course as taught throughout the nation. This book is designed to meet that need. This text was written using an organizational method that the author calls the General Topics Format. The format gives the student reader rapid access to the information contained in the text. All manufacturing methods, and some other material presented in this text, have been presented using the General Topics Format. The text uses examples to explain the importance of understanding the environment in which the product will be used and to discuss product abuse. The safety content contained in this text is unique. The Safety chapter teaches engineering ethics and includes a step-by-step guide to resolving ethical conflicts. The chapter includes explanations of rules, recommendations, standards, consensus standards, key safety concepts, and the legal implications of product failure. Key design principles have been listed and explained. The text provides easy-to-follow design steps, helpful for both the student and new engineer. Prototyping is presented as consisting of three phases: organization, building, and refining. A chapter on common manufacturing methods is included for reference.
Microstructural Modeling of Heterogeneous Failure Modes in Martensitic Steels
(2009-03-18) Hatem, Tarek Moustafa; Larry Silverberg, Committee Member; Kara Peters, Committee Member; Ronald Scattergood, Committee Member; Mohammed Zikry, Committee Chair
A three-dimensional multiple-slip dislocation-density-based crystalline formulation, specialized finite-element formulations, predictive failure models, and infinity-power integrable function based Voronoi tessellations adapted to martensitic orientations, were used to investigate large strain inelastic deformation, dislocation-density evolution in martensitic transformation, and heterogeneous failure modes in martensitic microstructures. The formulation is based on accounting for variant morphologies and orientations, secondary phases, such as retained austenite and inclusions, and initial dislocations-densities that are uniquely inherent to martensitic microstructures. The computational framework and the constitutive formulation were validated with experimental results for 10% Ni high-strength steel alloy. Furthermore, the formulation was used to investigate microstructures mapped directly from SEM/EBSD images of martensitic steel alloys. The interrelated effects of microstructural characteristics, such as parent austenite orientation, variants distribution and arrangement, retained austenite, inclusions, initial dislocation-density, and defects, such as microcracks, and microvoids, were investigated for different failure modes such as rupture, transgranular and intergranular fracture, and shear localization over a broad spectrum of loading conditions that range from quasi-static to high strain-rate conditions. The computational predictions, consistent with experimental observations, indicated that variant morphology and orientations have a direct consequence on how shear-strain accumulation and failure evolves in martensitic microstructures subjected to quasi-static and high strain-rate loading conditions. The analysis shows that shear-strain localization occurs due to slip-system compatibilities corresponding to low-angle blocks boundaries, the loading direction and the long direction of laths, which result in shear-pipes. At specific triple junctions, rotation misalignments due to lattice and slip incompatibilities occur, and this further exacerbated by defects. The results underscore the inherent competition between shear localization, transgranular, and intergranular failure modes. For certain variant arrangements, which correspond to random low angle orientations, cracks can be blunted by dislocation-density activities along transgranular planes. The effects of strain rate and inclusions on the evolution of shear-strain localization were also investigated under both tensile and compressive loadings. Tensile hydrostatic pressure forms under dynamic loads, and combined with plastic shear-slip accumulation between inclusions and the martensitic matrix accelerate shear-strain localization.
Optimal Control for Spacecraft Large Angle Maneuvers Using H infinity Linear Varying Parameter Control Techniques
(2006-12-11) Hughes, Hunter; Larry Silverberg, Committee Member; Fen Wu, Committee Chair; M. K. Ramasubramanian, Committee Member
This study investigates the possibility of designing and implementing a Linear Parameter Varying controller with H infinity performance criteria integrated into the synthesis of the controller for a spacecraft undergoing a large angle maneuver about its principal axes. Towards this end, Cayley-Rodrigues parameters were used to model nonlinear spacecraft dynamics and to ensure up to 180 degerees of rotation about the principal axis without singularities in the system. Several linear parameter varying (LPV) controllers with different parameter ranges were designed and the closed-loop performances were compared with respect to the H infinity upper bound gamma. The optimal gamma value obtained is roughly 0026. Two resulting LPV controllers were then examined through a series of simulations in order to observe both fuel economy and disturbance rejection capabilities for these controllers. The two controllers demonstrated fast response times and good disturbance rejection. It was also found that the controller with the smaller performance level gamma did perform better. Both control systems seemed to show some positive signs of enhanced fuel economy. There was chatter involved with certain aspects of the controller input profiles, but the simulations showed evidence that this was not caused by external disturbance, and can be eliminated by proper selection of weighting functions in the control design process.
Output Feedback Receding Horizon Control for Constrained Linear Systems
(2008-08-08) Chen, Youhua; Winser E. Alexander, Committee Member; Fen Wu, Committee Chair; M. K. Ramasubramanian, Committee Member; Larry Silverberg, Committee Member
A Photometric Characterization Methodology for Daylighting Fixtures
(2006-08-06) Carrington, Kenneth Daryl; Larry Silverberg, Committee Member; Wayne Place, Committee Chair; Perver Baran, Committee Member; James Tomlinson, Committee Member
This dissertation conceptualizes and computationally demonstrates a photometric characterization methodology for daylighting fixtures. The methodology is based upon applying far-field photometric evaluation techniques. The photometric characterization methodologies use computer simulations to characterize visible light and thermal performance of daylighting fixtures. A daylighting fixture is defined to include the boundary and all the components that exist between the admittance plane and exit plane of a daylighting aperture. The fixture concept allows the characterization data to be applied to daylighting fixtures in any setting. The lack of comprehensive daylighting performance evaluation protocols and basic daylighting component research indicate the need to develop this characterization methodology. This dissertation articulates key weaknesses in current daylighting indicators and performance characterization research, which support the need for additional protocols to characterize daylight, including angular intensity distribution at the point it is admitted into a building and associated thermal gains. The characterization methodology is demonstrated using four lighting software programs to assess louver control components for toplighting. Visible light characterizations addressed in this dissertation study include angular intensity distribution, illuminance distribution and visual effect. Thermal characterizations addressed include irradiance transmittance through the aperture and heat gain associated with irradiance absorbed by the louver control component. Simulations are limited to using currently available computer tools. Where simulation tools do not exist to properly support the characterization, a description is given of the software capabilities that need to be developed. The characterization data provided by this methodology is fully compatible with electric lighting and glazing performance data currently used by designers. The proposed methodology will support architects and lighting designers in selecting daylighting components and/or systems on a performance basis comparable to the selection of other building products. Given the huge potential for daylighting, the lack of well daylighted buildings indicates the potential for this research. Daylighting in buildings has implications for renewable energy, and the health and well-being of human occupants. It is envisioned that the proposed photometric characterization data will be used by architects and lighting designers to develop daylighting strategies incorporating performance, experiential, and aesthetic criteria.
Predictive Microstructural Modeling of Grain-boundary Interactions and Their Effects on Overall Crystalline Behavior
(2009-02-26) Shi, Jibin; Larry Silverberg, Committee Member; Mohammed Zikry, Committee Chair; Ron Scattergood, Committee Member; Kara Peters, Committee Member
A dislocation-density grain boundary interaction scheme (DDGBI) has been developed to account for complex interrelated dislocation-density interactions of emission, absorption and transmission in grain-boundary (GB) regions for bicrystals and polycrystals with different random and coincident site lattice (CSL) GB arrangements. This scheme is coupled to a dislocation-density crystalline plasticity formulation and specialized finite-element scheme at different physical scales. The DDGBI scheme is based on slip-system compatibility, local resolved shear stresses, and immobile and mobile dislocation-density activities at GBs. A conservation law for dislocation-densities is used to balance dislocation-density absorption, transmission and emission in GB regions. It is shown that dislocation-density absorptions and pile-ups will increase immobile dislocation-densities in high angle CSL boundaries, such as Î£17b. Lower angle CSLs, such as Î£1, are characterized by high transmission rates and insignificant GB dislocation-density accumulations. The identification of how different material mechanisms dominate underscores that GB activities, such as dislocation-density absorption, transmission and emission are interrelated interactions. These GB processes can be potentially controlled for desired material behavior. This methodology, together with grain boundary sliding (GBS) scheme and a misorientation dependence on initial GB dislocation-densities, was extended to account for grain size effects on strength. The behavior of polycrystalline aggregates with random low angle and random high angle GBs was also investigated with different crack lengths. For aggregates with random low angle GBs, dislocation-density transmission dominates at the GBs, which indicates that the low angle GB will not significantly change crack growth orientations. For aggregates with random high angle GBs, extensive dislocation-density absorption and pile-ups occur. The high stresses along the GB regions can result in intergranular crack growth due to potential crack nucleation sites in the GB. It is also shown that GB sliding affects crack behavior by attenuating normal stresses and dislocation-density accumulation at critical GB interfaces.