Structures and Materials Commons^™

Multiscale Modeling Of Impact On Heterogeneous Viscoelastic Solids With Evolving Microcracks, Flavio V. Souza

Department of Engineering Mechanics: Dissertations, Theses, and Student Research

Multiscale computational techniques play a major role in solving problems related to viscoelastic composite materials due to the complexities inherent to these materials. In the present work, a numerical procedure for multiscale modeling of impact on heterogeneous viscoelastic solids containing evolving microcracks is proposed in which the (global scale) homogenized viscoelastic incremental constitutive equations have the same form as the local scale viscoelastic incremental constitutive equations, but the homogenized tangent constitutive tensor and the homogenized incremental history dependent stress tensor depend on the amount of damage accumulated at the local scale. Furthermore, the developed technique allows the computation of the …

Effect Of Steam Environment On Creep Behavior Of Nextel720/Alumina-Mullite Ceramic Matrix Composite At Elevated Temperature, Tolga Kutsal

Theses and Dissertations

The tensile creep behavior of an oxide-oxide ceramic matrix composite (CMC) was investigated at 1000 and 1100° C in laboratory air and steam. The oxide-oxide CMC studied in this research was Nextel™ 720/alumina-mullite (N720/AM). The composite consists of N720/fibers with 0°/90° fiber orientation and a porous alumina-mullite matrix. Tensile-strain behavior was investigated and tensile properties measured at 900, 1000 and 1100° C. The effect of loading rate on tensile properties of N720/AM ceramic matrix composite at 1100° C in steam was also examined. Creep-rupture tests were performed at 1100° C in laboratory air and steam, and at 1000° C only …

Effects Of Prior Aging At Elevated Temperature In Air And In Argon Environments On Creep Response Of Pmr-15 Neat Resin, Joseph L. Broeckert

Theses and Dissertations

Creep behavior of PMR-15 neat resin, a polyimide thermoset polymer, aged in air or argon gaseous environments at 288 °C for up to 1000 h was evaluated. Creep tests were performed at 288 C at creep stress levels of 20 and 10 MPa. Creep periods of at least 25 h in duration were followed by recovery at zero stress. Weight loss and growth of the oxidation layer were also monitored and correlated with aging time. The aging time in both air and argon environments had a strong influence on the creep and recovery response of PMR-15 neat resin. Samples aged …

Structural Design And Analysis Of A Rigidizable Space Shuttle Experiment, Raymond G. Holstein Iii

Theses and Dissertations

AFIT is in the process of designing a Space Shuttle experiment designated as the Rigidized Inflatable Get-Away-Special Experiment (RIGEX) to study the effects of microgravity on the deployment of rigidizable composite structures. Once in space, the experiment will inflate and rigidize three composite structures and perform a vibration analysis on each by exciting the tubes using piezoelectric patches and collecting data via an accelerometer. This paper presents the structural and vibration analysis of the RIGEX assembly and inflatable composite tubes using ABAQUS Finite Element Analysis (FEA) software. Comparison of the analysis has been carried out with Eigenvalue/Eigenvector experimentation by means …

Effect Of A Variable Contact Load On Fretting Fatigue Behavior Of Ti-6al-4v, Andrew J. Jutte

Theses and Dissertations

Effects of a variable contact load on the high cycle fretting fatigue behavior of Ti-6AL-4V were investigated. Experimental tests were performed using a new test setup capable of applying a contact load varying in amplitude, frequency, and phase and independently measuring shear forces on opposite sides of a specimen. Finite element analysis (FEA) of experimental and idealized loading conditions was performed and local mechanistic parameters and the Modified Shear Stress Range (MSSR) fatigue parameter were determined. Correlations between contact width, slip amplitude, fatigue life and a variable contact load were established with variable contact loading shown to have a damaging …

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 …

On Multifunctional Collaborative Methods In Engineering Science, Jonathan B. Ransom Sr.

Mechanical & Aerospace Engineering Theses & Dissertations

Engineers are challenged to produce better designs in less time and for less cost. Hence, to investigate novel and revolutionary design concepts, accurate, high-fidelity data must be assimilated rapidly into the design, analysis and simulation process. This data assimilation should consider diverse mathematical modeling and multi-discipline interactions necessitated by concepts exploiting advanced materials and structures. Integrated high-fidelity methods with diverse engineering applications provide the enabling technologies to assimilate these high-fidelity, multi-disciplinary data rapidly at an early stage in the design. These integrated methods must be multifunctional, collaborative and applicable to the general field of engineering science and mechanics.

Multifunctional methodologies …

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 …

Large Amplitude Pitching Of Supermaneuver Delta Wings Including Flow Control, Yahia A. Abdelhamid

Mechanical & Aerospace Engineering Theses & Dissertations

The unsteady, three-dimensional Navier-Stokes equations are solved to simulate and study the aerodynamic response of a delta wing undergoing large amplitude pitching motion up to 90° angle of attack. The primary model under consideration consists of a 76° swept, sharp-edged delta wing of zero thickness, initially at zero angle of attack. The freestream Mach number and Reynolds number are 0.3 and 0.45 × 106, respectively. The governing equations are solved time-accurately using the implicit, upwind, Roe flux-difference splitting, finite-volume scheme. Both laminar and turbulent flow solutions are investigated. In the laminar flow solutions, validation of the computational results is carried …

Efficient Dynamic Unstructured Methods And Applications For Transonic Flows And Hypersonic Stage Separation, Xiaobing Luo

Mechanical & Aerospace Engineering Theses & Dissertations

Relative-moving boundary problems have a wide variety of applications. They appear in staging during a launch process, store separation from a military aircraft, rotor-stator interaction in turbomachinery, and dynamic aeroelasticity.

The dynamic unstructured technology (DUT) is potentially a strong approach to simulate unsteady flows around relative-moving bodies, by solving time-dependent governing equations. The dual-time stepping scheme is implemented to improve its efficiency while not compromising the accuracy of solutions. The validation of the implicit scheme is performed on a pitching NACA0012 airfoil and a rectangular wing with low reduced frequencies in transonic flows. All the matured accelerating techniques, including the …

Placement Of Piezoelectric Actuators For Active Control Of Vibration Using Modal Parameters, Xuegeng Zhu

Mechanical & Aerospace Engineering Theses & Dissertations

An equation is derived to model the piezoelectric actuators incorporation with flexible structures. This equation permits the comparison of the performance indices over the entire structure for a piezoelectric actuator with constant area, which is unachievable if the Finite Element Method is used for complicated structures.

An index has been developed for placement of piezoelectric actuator for control of vibration of a flexible structure. This index is derived from the definition of H2norm. Computation of the proposed index requires only the natural frequencies and corresponding mode shapes of the structures of interest. The method is well suited to large …

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 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

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

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

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

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

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 Frequency Domain Solution Of Nonlinear Panel Flutter With Temperature Effects And Fatigue Life Analysis, David Yongxiang Xue

Mechanical & Aerospace Engineering Theses & Dissertations

A frequency domain solution method for nonlinear panel flutter with thermal effects using a consistent finite element formulation has been developed. The von Karman nonlinear strain-displacement relation is used to account for large deflections, the quasi-steady first-order piston theory is employed for aerodynamic loading and the quasi-steady thermal stress theory is applied for the thermal stresses with a given change of the temperature distribution, ΔΤ (x, y, z). The equation of motion under a combined thermal-aerodynamic loading can be mathematically separated into two equations and then solved in sequence: (1) thermal-aerodynamic postbuckling and (2) limit-cycle oscillation. The Newton-Raphson iteration technique …

Linearized Dynamics Of Space Structures With Articulated Appendages, Gopichand Koganti

Mechanical & Aerospace Engineering Theses & Dissertations

The inherent difficulties of the conventional spacecraft design, which first considered the optimization with respect to structural variables and then with respect to control variables, have led to the concept of Controls-Structures Interactive Design. The success of the CSI design of Space structures without articulated appendages has prompted the extension of the CSI design to systems with articulated appendages. The presentation is of two parts, the first part deals with the derivations of the equations of motion and the second part deals with the eigenvalue/vector approximations.

Three approaches, namely: the multibody approach, the finite element based approach with hinge joints …

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 …

Structural Self-Shadowing And Its Effects On The Thermal-Structural Behavior Of Orbiting Trusses, Jack Mahaney

Mechanical & Aerospace Engineering Theses & Dissertations

The determination of structural shadowing is described. The computation of environmental heat loads for slender structural members in earth orbit is described, and two methods for finite element thermal analysis are described . The effects of self-shadowing on the thermal structural behavior of single truss members and two different trusses are presented. In each case the shadowing produces a noticeable effect on the temperature distributions on truss members, and on the deformations of the trusses.