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Articles 91 - 120 of 120
Full-Text Articles in Structures and Materials
Experimental And Computational Failure Analysis Of Graphite/Bismaleimide Laminated Composite And Carbon Foam In Sandwich Construction, Troy C. Welker
Experimental And Computational Failure Analysis Of Graphite/Bismaleimide Laminated Composite And Carbon Foam In Sandwich Construction, Troy C. Welker
Theses and Dissertations
Sandwich beams consisting of a carbon foam core and graphite/bismaleimide face sheets were constructed and tested. Nine specimens were fabricated using three distinct cross-ply symmetric face sheet layups with a constant core thickness. Four-point bend testing controlled by a constant rate of midspan vertical displacement was used to load the specimens to failure. Displacements and strains from the experiment were compared to analytical sandwich beam theory and displacements and failure loads were compared to a layerwise finite element solution. A phenomenological failure criterion was developed that compares favorably with experimental failure data. The finite element solution gives failure within an …
Structurally Integrated Antennas On A Joined-Wing Aircraft, Ben P. Smallwood
Structurally Integrated Antennas On A Joined-Wing Aircraft, Ben P. Smallwood
Theses and Dissertations
This research is a foundational study of conformal, load-bearing antenna arrays embedded into the wing structure of a joined-wing aircraft. It is a multidisciplinary effort that touches on the aerodynamic structural and electromagnetic design considerations that stem from this unique type of sensor integration. The antenna performance Finite Element Model (FEM) and control surface effectiveness are investigated. The theory describing an ensemble of dipole antenna elements that conform to the shape of a section of the joined wing is developed. The far held flee space radiation pattern of the sensor is then analyzed for a wing that is deflected due …
Electromagnetic Scattering From A Gap In A Magneto-Dielectric Coating On An Infinite Ground Plane, George R. Simpson
Electromagnetic Scattering From A Gap In A Magneto-Dielectric Coating On An Infinite Ground Plane, George R. Simpson
Theses and Dissertations
The electromagnetic scattering from a gap in a magneto-dielectric coating on an infinite ground plane is analyzed. In this context, the gap forms a break only in the magneto-dielectric slab coating while the ground plane is continuous and unbroken. Volume equivalence is used to convert the gap region to one containing unknown volumetric equivalent electric and magnetic currents. The equivalent problem then is one of these currents radiating in the presence of an unbroken grounded magneto-dielectric slab. A Green's function for this geometry is developed consisting of two terms: a direct coupling term and correction term to account for the …
Experimental Vibration Analysis Of Inflatable Beams For An Afit Space Shuttle Experiment, Thomas G. Single
Experimental Vibration Analysis Of Inflatable Beams For An Afit Space Shuttle Experiment, Thomas G. Single
Theses and Dissertations
This study investigated the use of MEMS based devices to control the boundary layer separation from a circular cylinder in cross flow. Velocity profiles were measured experimentally in a low speed wind tunnel. Momentum thickness was measured as the primary way to determine a change in the flow field. The goal was to determine the angular location and frequency of operation that would provide an effect on the boundary layer of the cylinder. A 25% reduction of momentum loss in the cylinder wake was detected with an optimal angle of 69 degrees and an optimal frequency of operation equal to …
Modeling Axisymmetric Optical Precision Piezoelectric Membranes, James W. Rogers Jr.
Modeling Axisymmetric Optical Precision Piezoelectric Membranes, James W. Rogers Jr.
Theses and Dissertations
The US Department of Defense (DOD), as well as the National Aeronautics and Astronautics Administration (NASA) and the Jet Propulsion Laboratory (JPL) are interested in developing and deploying precise, compliant, light-weight, space-based structures. More specifically, the Air Force’s core competencies ‘Aerospace Superiority’ and ‘Information Superiority’ demand ever-increasing depth and breadth of capability. Whether used for energy transmission or optical reconnaissance, current launch restraints limit rigid space-based optical reflector size. To support this requirement, the Air Force Research Laboratory (AFRL) is developing a large space-based optical membrane telescope. Inflatable reflectors can conceptually break this barrier, but controlling such a compliant structure …
The Development Of A Finite Element Program To Model High Cycle Fatigue In Isotropic Plates, William C. Shipman
The Development Of A Finite Element Program To Model High Cycle Fatigue In Isotropic Plates, William C. Shipman
Theses and Dissertations
As part of a joint AFRL/DAGSI turbine blade research effort, a computer program has been developed that uses a von Karman large-deflection two-dimensional finite element approximation to determine stress levels and patterns in isotropic thin plates. The dynamic loading of various plates has been carried out in order to model a high cycle fatigue situation. The research considered the various effects of mode shapes, resident frequency, non-linear cyclic effect, endurance limits, and stress variations within a high cycle fatigue environment. Two main initiatives were taken. First, a transient analysis tool was developed that calculates stress and displacement patterns over a …
Monotonic And Fatigue Loading Behavior Of An Oxide/Oxide Ceramic Matrix Composite, Steven G. Steel
Monotonic And Fatigue Loading Behavior Of An Oxide/Oxide Ceramic Matrix Composite, Steven G. Steel
Theses and Dissertations
This thesis evaluates an oxide/oxide ceramic matrix composite, N720/A, for high temperature aerospace applications. The composite consists of a porous alumina matrix reinforced by ten plies of Nextel 720 fibers in a balanced 8 harness satin weave. Monotonic tensile and fatigue tests were performed at room temperature and 1200 °C. The effect of moisture on 1200 °C fatigue life was also examined. Modulus, maximum and minimum strain, and stress-strain hysteresis were monitored to characterized fatigue damage. Retained strength of all specimens that survived 100000 fatigue cycles was tested. Microscopy and fractography were used to examine damage and fracture surfaces. N720/A …
Electro-Mechanical Fatigue Behavior Of A Quasi-Isotropic Laminate With An Embedded Piezoelectric Actuator, Tse Lin Hsu
Electro-Mechanical Fatigue Behavior Of A Quasi-Isotropic Laminate With An Embedded Piezoelectric Actuator, Tse Lin Hsu
Theses and Dissertations
This study primarily investigated the electro-mechanical fatigue behavior of the embedded piezoelectric actuators in graphite/epoxy laminate with a lay-up of 0 | ± 45 | 90s. A secondary focus was the investigation of the mechanical fatigue effects of the 0 | 0 | ± 45 | 0 | 0 | 90s laminate with embedded PZT under tensile loading. All the fatigue tests were conducted with a triangular loading waveform which had a frequency of 10 Hz and with R=0. 1. In the electro-mechanical testing, the embedded actuator was excited by a -10 V to -100 V or a 10 V …
Fiber Volume Fraction Effects On Fatigue Response Of A Scs-6/Ti-15-3 Metal Matrix Composite At Elevated Temperature, Sean C. Coghlan
Fiber Volume Fraction Effects On Fatigue Response Of A Scs-6/Ti-15-3 Metal Matrix Composite At Elevated Temperature, Sean C. Coghlan
Theses and Dissertations
The purpose of this study was to determine the effects of fiber volume fraction on the fatigue behavior of Silicon Carbide fiber-reinforced Titanium alloy, SCS-6/Ti-15-3. Three fiber volume fractions were investigated; 15%, 25%, and 42%. The tests were performed under fully-reversed, strain-controlled conditions at 427 °C. The primary objectives of this study were to develop a fatigue life diagram and to document the damage and failure mechanisms. Compressive loads on the slender specimens were kept from buckling the specimens through the use of a buckling guide. This device allows unrestricted axial movement of the composite, while preventing any out-of-plane motion. …
Residual Strength After Fatigue Of A Cross-Ply Metal Matrix Composite At Elevated Temperature, Alvin N. Ruiz
Residual Strength After Fatigue Of A Cross-Ply Metal Matrix Composite At Elevated Temperature, Alvin N. Ruiz
Theses and Dissertations
The objective of this study was to investigate the residual strength of a cross-ply, SCS-6/Ti-15-3, metal matrix composite (MMC) at elevated temperature (427 °C) when exposed to fatigue loading. Several specimens were subjected to a strain-controlled, fully reversed loading (R=-1), at various strain levels. The specimens were fatigued up to a portion of their life expectancy and then loaded to failure. Stress and strain data obtained during the test provided useful information for the macro-mechanical behavior of the material. It was determined that the residual strength was directly related to the amount of damage present in the matrix, meanwhile, the …
Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup
Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup
Theses and Dissertations
The objective of the research is to analytically model the response of composite sandwich plates to low velocity impact. A displacement based, plane stress, finite element code is modified for this purpose. Major new algorithms include 5th order Hermitian interpolation, three dimensional equilibrium integration for transverse stress calculations, sandwich core modeling as an elastic plastic foundation, loading by simulated contact with a spherical indentor, adaptive mesh, damage prediction, damage progression via stiffness reduction, and local-global analysis for displacement. An experimental effort is also included in which composite sandwich plates with graphite epoxy facesheets and Nomex honeycomb core are subjected to …
Residual Strength After Fatigue Of Unidirectional And Cross-Ply Metal Matrix Composites At Elevated Temperature, Sen-Tzer Chiou
Residual Strength After Fatigue Of Unidirectional And Cross-Ply Metal Matrix Composites At Elevated Temperature, Sen-Tzer Chiou
Theses and Dissertations
This study investigated the residual strength of the unidirectional and cross-ply laminates of SCS-6 / Ti-15-3, metal matrix composite at elevated temperature 427°C (800°F) after under tension-tension load controlled mode. For this purpose, several specimens were fatigued to various fractions of the fatigue life and then loaded monotonically to failure. The purpose of this study was to determine the effects of different levels of fatigue damage on the composite's strength. The unidirectional specimens were cycled at a 900 MPa maximum stress at a frequency of 10 Hz, while, the cross-ply specimens were tested at both 300 MPa and 450 MPa …
Modeling Of Progressive Damage In Fiber-Reinforced Ceramic Matrix Composites, James P. Solti
Modeling Of Progressive Damage In Fiber-Reinforced Ceramic Matrix Composites, James P. Solti
Theses and Dissertations
An analytic methodology is developed to model the response of fiber-reinforced ceramic matrix composites (CMOs) when subjected to monotonic and fatigue loadings. The analysis requires the formulation of (1) a micromechanics model which defines the laminate's geometry and constitutive relationship; (2) failure criteria which estimate the extent of microstructural damage, and, finally, (3) a means of analyzing frictional slip, fiber pull-out, interfacial wear and laminate failure. For the present study, the behavior of unidirectional and crossply CMOs is investigated using modified shear-lag theory in conjunction with a set of failure criteria with a minimum reliance on empirical data. The damage …
Investigation Of Residual Strength And Fatigue Life Of Unstiffened Aluminum Panels With Multiple Site Damage, Mark C. Cherry
Investigation Of Residual Strength And Fatigue Life Of Unstiffened Aluminum Panels With Multiple Site Damage, Mark C. Cherry
Theses and Dissertations
Multiple Site Damage MSD is the occurrence of small fatigue cracks at several sites within aircraft structures. This is important since it may lower the residual strength and fatigue life of the structure beyond what can be predicted using the damage tolerance technique based on a single crack, currently in use to design aircraft structures. This study investigated the effects of MSD on unstiffined panels. MSD usually occurs at rivet holes, or other stress concentration locations within an aircraft structure. This study simulated rivet holes with MSD, by using holes of constant diameter with small cracks, evenly spaced across the …
Thermo-Mechanical Fatigue Behavior Of Cross-Ply Ceramic Matrix Composite Under Tension-Tension Loading, Dana G. Allen
Thermo-Mechanical Fatigue Behavior Of Cross-Ply Ceramic Matrix Composite Under Tension-Tension Loading, Dana G. Allen
Theses and Dissertations
The purpose of this study was to investigate the combined effect of cyclic temperature and loading on the fatigue life of a ceramic matrix composite with a cross ply lay-up. The material used in this study was a potassium borosilicate glass (BSG) doped magnesium aluminosilicate (MAS) cordierite matrix reinforced with Nicalon (silicon carbide, SiC) fibers in a [0/90]4s lay-up. Thermomechanical fatigue (TMF) tests were performed with a period of 180 seconds/cycle, or 0.00556 Hz, and a triangular wave-form. The temperature was cycled between 566°C and l093°C, and the stress levels varied between 60 MPa and 145 MPa. All tests …
Fatigue Response Of Cracked Aluminum Panel With Partially Bonded Composite Patch, Jason J. Denney
Fatigue Response Of Cracked Aluminum Panel With Partially Bonded Composite Patch, Jason J. Denney
Theses and Dissertations
More and more aircraft, both commercial and military, are being called upon to fly well beyond their economic and structural service lives. Budget cuts and dwindling new aircraft development has forced the United States Air Force (USAF) to look toward more reliable structural repairs. One of these repair techniques, which was the subject of this study, is the repair of metallic aircraft structures using high strength composite materials. This study investigated the fatigue response of a precracked, 508x1 52x1 mm, 2024-T3 aluminum panel repaired with a partially bonded, unidirectional, three-ply boron/epoxy composite reinforcement with ply lengths of 68, 56 and …
Nonlinear Geometric And Material Behavior Of Composite Shells With Large Strains, Scott A. Schimmels
Nonlinear Geometric And Material Behavior Of Composite Shells With Large Strains, Scott A. Schimmels
Theses and Dissertations
A two-dimensional, geometrically and materially nonlinear shell theory applicable to arbitrary geometries described by orthogonal curvilinear coordinates and encompassing large displacements, moderate rotations for large strain situations has been developed. Additionally, the theory includes Jacobian transformation matrices, based upon displacement parameters, for the Cauchy - 2nd Piola-Kirchhoff stress-state and the Cauchy (Almansi) - Green strain-state transformations, and a layered material approach is included for the elastoplastic analysis to allow for variation of plasticity through-the-thickness. Doubly curved 20, 28, and 36 degree-of-freedom finite elements are defined based on specialization of the theory to spherical coordinates. The computer program includes algorithms for …
Finite Element Methods For Nonlinear Static Analysis Of Sandwich Plates, Damin J. Siler
Finite Element Methods For Nonlinear Static Analysis Of Sandwich Plates, Damin J. Siler
Theses and Dissertations
A finite element method, developed for static analysis of composite plates, was enhanced to be used with sandwich plates. The theory considers geometric nonlinearity and transverse shear. Furthermore, a new postprocessor was written to check for initial failure using the maximum stress criteria. It also includes a procedure for evaluating transverse normal stresses by enforcing equilibrium. The code modifications for sandwiches were verified by comparing finite element solutions to closed-form sandwich theories. Both methods showed good correlation. In addition, comparisons between one type of composite plate and a similar sandwich plate found that the sandwich had better specific stiffness for …
The Collapse Of Composite Cylindrical Panels With Various Thickness Using Finite Element Analysis, Christopher P. Chaplin
The Collapse Of Composite Cylindrical Panels With Various Thickness Using Finite Element Analysis, Christopher P. Chaplin
Theses and Dissertations
This study compared numerical collapse results of graphite-epoxy cylindrical panels with free vertical edges undergoing axial compression for three different shell theories. Symmetric quasi-isotropic laminates were investigated using solid panels and panels with five different centralized cutouts with three thicknesses. The theories compared in the study were the Simplified Large displacement moderate Rotation (SLR) theory, the Donnell Cylindrical Shell (Modified Donnell) theory, and the Classical Donnell theory. The purpose of the study was to determine when large rotations and through-the-thickness shear become important in the numerical collapse analysis of cylindrical composite shells. By observing cases where the numerical collapse results …
An Experimental Investigation Of Sandwich Flat Panels Under Low Velocity Impact, Timberlyn M. Harrington
An Experimental Investigation Of Sandwich Flat Panels Under Low Velocity Impact, Timberlyn M. Harrington
Theses and Dissertations
This study evaluated the failure modes and mechanisms associated with increasing face sheet thickness of flat sandwich panels under low velocity impact. The sandwich panels were fabricated using 1.27 cm thick, 145 kg-m3 (9 lb-ft3), 3.175 mm (1-8 in.) cell size Nomex honeycomb core, FM 300-2 film adhesive and AS4-3501-6 graphite-epoxy face sheets. The thickness of the core remained 1.27 cm, and the thickness of the adhesive remained 0.25 mm. The thickness of the face sheets varied using the following stacking sequences: [0-90]s, [0-90]2s, [0-90]4s, [0-90]8s, and [0-90]12s …
Prototype Space Fabrication Platform, James A. Bessel, James M. Ceney, David M. Crean, Edward A. Ingham, David J. Pabst
Prototype Space Fabrication Platform, James A. Bessel, James M. Ceney, David M. Crean, Edward A. Ingham, David J. Pabst
Theses and Dissertations
Current plans for constructing large structures in space entail fabricating the primary components, such as truss segments, on the ground and assembling them in space. This process requires an exorbitant number of support missions, and methods to minimize the number must be considered. Whenever the space shuttle is launched, its external tank is jettisoned and destroyed prior to reaching orbit. This aerospace grade aluminum structure can be carried into orbit and utilized extrusively. The Prototype Space Fabrication Platform SFP fabricates aluminum materials, reduced from external tanks, into functional trusses. The trusses are strong and can be used as the primary …
An Experimental Determination Of The Effects Of Blending Collocated And Non-Collocated Sensor Measurements To Control A Flexible Structure, Richard G. Cobb
An Experimental Determination Of The Effects Of Blending Collocated And Non-Collocated Sensor Measurements To Control A Flexible Structure, Richard G. Cobb
Theses and Dissertations
An experimental investigation was performed controlling a cantilevered beam in bending using proportional feedback on a blend of collocated and non-collocated sensor measurements using a single actuator. Exact transfer functions between the control input and the measurements were developed and compared to the finite element method. By analyzing the open loop pole-zero locations as a function of the measurement blending, insight into the closed loop behavior is obtained. Minimum phase behavior can be maintained for a range of the blending ratio. Results using blended control were compared to both proportional collocated feedback and compensated collocated feedback using LQG methods. The …
The Influence Of Structural Optimization On The Aeroelastic Properties Of A Vertical Tail, David G. Miller
The Influence Of Structural Optimization On The Aeroelastic Properties Of A Vertical Tail, David G. Miller
Theses and Dissertations
The purpose of this thesis is to investigate the influence of structural optimization parameters on the structural and aeroelastic behavior of a vertical tail. The outside geometry of the tail is assumed fixed. The objective of the optimization is to reduce the structural member sizes so that the overall weight of the tail is minimized. The influence that design loads, design constraints, design variable linking has on the optimal design is observed examining several design cases. The static aeroelastic properties a flutter speeds of the various design cases are then compared. The doublet lattice method is used to calculate the …
A Modal Analysis And Modelling Of A Lightly Damped Large Space Structure, Scott E. George
A Modal Analysis And Modelling Of A Lightly Damped Large Space Structure, Scott E. George
Theses and Dissertations
This study involves the modification to the PACOSS Dynamic Test Article to create a lightly damped representative large space structure with closely spaced, low frequency modes. An experimental modal analysis is performed to recharacterize the low frequency flexible vibration modes. A finite element model is created using MSC/NASTRAN to predict low frequency behavior using both real eigenvalue analysis and frequency response analysis. Then a reduced order model is created and fine-tuned to represent the dynamics of the structure for the identified low frequency modes.
Development Of A Stratified Plate Model For Composite Panels, Alan L. Lesmerises
Development Of A Stratified Plate Model For Composite Panels, Alan L. Lesmerises
Theses and Dissertations
A new model for composites represents a composite panel as an alternating stack of isotropic matrix and orthotropic fiber-dominated layers. An energy approach results in a set of partial differential equations and boundary conditions where the mechanical properties of the matrix and fibers appear separately. Two solution methods are developed. The first, called Strata Theory is developed for general applications, while the second, called the Exact Stratified Plate, is used as a benchmark for comparison to Strata Theory. Both solutions assume zero gradients of displacement through the thickness of a fiber layer. They are compared for an isotropic material and …
Sensitivity Analysis For The Selection Of Linear Quadratic Regulator Weighing Parameters Q And R For Active Vibration Suppression Of A Cantilevered Beam, Steven L. Story
Sensitivity Analysis For The Selection Of Linear Quadratic Regulator Weighing Parameters Q And R For Active Vibration Suppression Of A Cantilevered Beam, Steven L. Story
Theses and Dissertations
There has been much interest during the past decade to develop and launch large space structures. The high cost of launching material into orbit will require that these structures be assembled in space using light weight elements which are vulnerable to dynamic excitations. Active control may be necessary to rapidly attenuate large amplitude vibrations. The active vibration control system is usually designed after the structure has been optimized. The integrated design of the control system and the structure may provide additional weight savings. This thesis presents a sensitivity analysis of the structure/ control optimization problem. The structure used is an …
Finite Element Investigation Into The Dynamic Instability Characteristics Of Laminated Composite Panels, Walter W. Taylor Jr.
Finite Element Investigation Into The Dynamic Instability Characteristics Of Laminated Composite Panels, Walter W. Taylor Jr.
Theses and Dissertations
Dynamic instability of a laminated composite panel subjected to a transverse load is studied. Up to cubic variations in the thickness coordinate are included in the inplane displacement field, and only the constant component is kept in the transverse displacement. The transverse shear strains retain only linear displacement terms and vary parabolically through the thickness. The complete quadratic displacement functions are included in the inplane strains. A 36 degree of freedom shell element is used to obtain numerical results. The static snap through load vs displacement curve, as well as the critical collapse load, is examined by invoking the Riks …
Material Properties Of Silicon Carbide Fibers With Continuously Applied Sol-Gel Alumina Coatings, Christian C. Daehnick
Material Properties Of Silicon Carbide Fibers With Continuously Applied Sol-Gel Alumina Coatings, Christian C. Daehnick
Theses and Dissertations
An investigation was conducted to determine the effect of non-alpha- alumina coatings applied under varying conditions on some of the properties of silicon carbide fibers. Limited characterization of the coating by optical, scanning electron, and transmitted electron microscopy as well as energy- dispersive x-ray analysis was performed. Tensile tests were used to determine changes in elastic modulus, ultimate strength, and strain to failure of the coated fibers relative to uncoated fibers. The coatings were found to lower the mean value of the measured properties as their thickness increased, but the measurements were not accurate enough to determine if this corresponded …
Investigation Of Direct And Indirect Optimization Algorithms For Aerospace Structures, Harry Hopkins Iii
Investigation Of Direct And Indirect Optimization Algorithms For Aerospace Structures, Harry Hopkins Iii
Theses and Dissertations
This thesis investigated the performance of two direct and two indirect methods of structural optimization. A gradient calculator and overall design system was created with the finite element code FRAME. Three of the methods were employed using this system with the fourth being a commercial available code. The algorithms were applied to two different sized trusses using static constraints and were measured for accuracy, reliability, and efficiency. Results for the problems tested showed the Direct methods were sensitive to starting designs and their performance depended on the proper selection of internal parameters. They were also shown to have a high …
Solution To Eigenvalue Problems Of Antisymmetric Cross-Ply And Antisymmetric Angle-Ply Laminated Plates Using Affine Transformations, Zaffir A. Chaudry
Solution To Eigenvalue Problems Of Antisymmetric Cross-Ply And Antisymmetric Angle-Ply Laminated Plates Using Affine Transformations, Zaffir A. Chaudry
Theses and Dissertations
Using affine transformations and suitably recasting the buckling vibration differential equations, the eigenvalue problem of anti-symmetric cross-ply and antisymmetric angle-ply laminated rectangular plates has been reduced to a function of two strong material constants, the generalized rigidity ratio, whose range is in the closed interval from 0 to 1, and the ratio of principal lamina stiffness. With the reduction in number of constants an exhaustive parameter study of buckling and vibration solution trends, is possible. The buckling coefficients decrease with decreasing value of generalized rigidity ratio for both antisymmetric cross-ply and antisymmetric angle-ply laminates. For a given aspect ratio, and …