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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Stability and natural vibration analysis of laminated plates by using a mixed element based on a refined plate theory

A mixed shear flexible finite element, with relaxed continuity, is developed for the geometrically linear and nonlinear analysis of layered anisotropic plates. The element formulation is based on a refined higher order theory which satisfies the zero transverse shear stress boundary conditions on the top and bottom faces of the plate and requires no shear correction coefficients. The mixed finite element developed herein consists of eleven degrees of freedom per node which include three displacements, two rotations and six moment resultants. The element is evaluated for its accuracy in the analysis of the stability and vibration of anisotropic rectangular plates with different lamination schemes and boundary conditions. The mixed finite element described here for the higher order theory gives very accurate results for buckling loads and natural frequencies.

Putcha, N. S.↗
NASA NTRS1987

Application of a comprehensive analytical model of rotorcraft aerodynamics and dynamics (CAMRAD) to the McDonnell Douglas AH-64A helicopter

CAMRAD results on main rotor blade natural frequency and structural loads predictions are compared to flight test data and predictions of the Dynamic Analysis Research Tool (DART) and the Rotor/Airframe Comprehensive Aeroelastic Program (RACAP). Consideration is also given to the effects of various analytical options available in CAMRAD including dynamic stall, wake modeling, and the number of bending and torsion degrees of freedom included in the structural analysis. It is concluded that, while several modeling limitations are identified in the CAMRAD structural analyses, the structural loads predictions are as accurate as analysis codes which provide more representative models of the complex geometries and structures of the AH-64A rotor system. However, it is noted that the CAMRAD analysis cannot be considered fully reliable for dynamic predictions for more advanced bearingless rotor designs being proposed for future AH-64 configurations.

Callahan, C.↗
NASA NTRS1989

Calculating Buckling And Vibrations Of Lattice Structures

BUNVIS-RG computer program designed to calculate vibration frequencies or buckling loads of prestressed lattice structures used in outer space. For buckling and vibration problems, BUNVIS-RG calculates deadload axial forces caused in members by any combination of externally-applied static point forces and moments at nodes, axial preload or prestrain in members, and such acceleration loads as those due to gravity. BUNVIS-RG is FORTRAN 77 computer program implemented on CDC CYBER and VAX computer.

Anderson, M. S.↗
NASA NTRS1990

Structural behavior of composites with progressive fracture

Structural characteristics such as natural frequencies and buckling loads with corresponding mode shapes were investigated during progressive fracture of multilayer, angle-plied polymer matrix composites. A computer program was used to generate the numerical results for overall mechanical response of damaged composites. Variations in structural characteristics as a function of the previously applied loading were studied. Results indicate that most of the overall structural properties were preserved throughout a significant proportion of the ultimate fracture load. For the cases studied, changes in structural behavior began to occur after 70 percent of the ultimate fracture load was applied. However, the individual nature of the structural change was rather varied depending on the laminate configuration, fiber orientation, and the boundary conditions.

Minnetyan, Levon↗
NASA NTRS1991

Numerical prediction of the unsteady flowfield around the F-18 aircraft at large incidence

This paper describes a numerical method capable of solving the steady and unsteady viscous flow around complete aircraft configurations at high angles of attack. This method is used to simulate the external flow around the F-18 aircraft, including deflected control surfaces. The current technique employs a generalized overset zonal grid scheme to decompose the computational space around the aircraft. The grid around various components of the aircraft are created numerically using a three-dimensional hyperbolic grid generation procedure. The Reynolds-averaged Navier-Stokes equations are integrated using a time-accurate, implicit procedure. Results for the turbulent flow around the F-18 aircraft at 30 degrees angle of attack show the details of the flowfield structure, including the unsteadiness created by the vortex burst and the resulting fluctuating airloads exerted on the vertical tail. The computed results agree fairly well with flight data for surface pressure, surface flow pattern, vortex burst location, and the dominant frequency for tail load fluctuations.

Rizk, Yehia M.↗
NASA NTRS1991

An evaluation of HEMT potential for millimeter-wave signal sources using interpolation and harmonic balance techniques

A large-signal analysis method based on an harmonic balance technique and a 2-D cubic spline interpolation function has been developed and applied to the prediction of InP-based HEMT oscillator performance for frequencies extending up to the submillimeter-wave range. The large-signal analysis method uses a limited number of DC and small-signal S-parameter data and allows the accurate characterization of HEMT large-signal behavior. The method has been validated experimentally using load-pull measurement. Oscillation frequency, power performance, and load requirements are discussed, with an operation capability of 300 GHz predicted using state-of-the-art devices (fmax is approximately equal to 450 GHz).

Kwon, Youngwoo↗
NASA NTRS1992

Structural verification for GAS experiments

The purpose of this paper is to assist the Get Away Special (GAS) experimenter in conducting a thorough structural verification of its experiment structural configuration, thus expediting the structural review/approval process and the safety process in general. Material selection for structural subsystems will be covered with an emphasis on fasteners (GSFC fastener integrity requirements) and primary support structures (Stress Corrosion Cracking requirements and National Space Transportation System (NSTS) requirements). Different approaches to structural verifications (tests and analyses) will be outlined especially those stemming from lessons learned on load and fundamental frequency verification. In addition, fracture control will be covered for those payloads that utilize a door assembly or modify the containment provided by the standard GAS Experiment Mounting Plate (EMP). Structural hazard assessment and the preparation of structural hazard reports will be reviewed to form a summation of structural safety issues for inclusion in the safety data package.

Peden, Mark Daniel↗
NASA NTRS1992

EnviroNET

EnviroNET is a service/facility that provides users with on-line, dial-up technical information concerning environmental conditions likely to be encountered by instruments and experimental arrangements carried aboard spacecraft. EnviroNET incorporates at present a combination of expository text and numerical tables amounting to about two million characters (bytes), plus FORTRAN programs that model the space environment. The topics covered are presented in viewgraph form and include the following: advantages of EnviroNET; thermal and humidity; vibration and acoustics; electromagnetic interference; loads and low frequency dynamics; microbial and toxic contaminants; molecular contamination; natural environment; orbiter motion; particulate environment; surface interactions; spacecraft anomalies; and interactive graphics facility.

Vansant, Tim↗
NASA NTRS1993

Quantification of uncertainties in the performance of smart composite structures

A composite wing with spars, bulkheads, and built-in control devices is evaluated using a method for the probabilistic assessment of smart composite structures. Structural responses (such as change in angle of attack, vertical displacements, and stresses in regular plies with traditional materials and in control plies with mixed traditional and actuation materials) are probabilistically assessed to quantify their respective scatter. Probabilistic sensitivity factors are computed to identify those parameters that have a significant influence on a specific structural response. Results show that the uncertainties in the responses of smart composite structures can be quantified. Responses such as structural deformation, ply stresses, frequencies, and buckling loads in the presence of defects can be reliably controlled to satisfy specified design requirements.

Shiao, Michael C.↗
NASA NTRS1994

The effects of pitting on fatigue crack nucleation in 7075-T6 aluminum alloy

A high-strength aluminum alloy, 7075-T6, was studied to quantitatively evaluate chemical pitting effects of its corrosion fatigue life. The study focused on pit nucleation, pit growth, and fatigue crack nucleation. Pitting corrosion fatigue experiments were conducted in 3.5 percent NaCl aqueous solution under constant amplitude sinusoidal loading at two frequencies, 5 and 20 Hz. Smooth and unnotched specimens were used in this investigation. A video recording system was developed to allow in situ observation of the surface changes of the specimens during testing. The results indicated that pitting corrosion considerably reduces the fatigue strength by accelerating fatigue crack nucleation. A metallographic examination was conducted on the specimens to evaluate the nature of corrosion pits. First, the actual shapes of the corrosion pits were evaluated by cross-sectioning the pits. Secondly, the relation between corrosion pits and microstructure was also investigated. Finally, the possibility of another corrosion mechanism that might be involved in pitting was explored in this investigation. The fractography of the tested specimens showed that corner corrosion pits were responsible for fatigue crack nucleation in the material due to the associated stress concentration. The pits exhibited variance of morphology. Fatigue life for the experimental conditions appeared to be strongly dependent on pitting kinetics and the crack nucleation stage.

Ma, LI↗
NASA NTRS1994

Buckling and vibration analysis of laminated panels using VICONOPT

The analysis aspects of the 23,000-line FORTRAN program VIPASA with constraints and optimization (VICONOPT) are described. Overall stiffness matrices assembled from the earlier exact vibration and instability of plate assemblies including shear and anisotropy (VIPASA) flat plate stiffnesses are optionally coupled by Langrangian multipliers to find critical buckling loads, or natural frequencies of undamped vibration, of prismatic assemblies of anisotropic flat plates with arbitrarily located point supports or simple transverse supporting frames. The longitudunal continuity of typical wing and fuselage panels is closely approximated because the solutions are for the infinitely long structure obtained by repeating a bay and its supports longitudinally. Any longitudinally invariant in-plane plate stresses are permitted, and very rapid solutions are guaranteed by numerous refinements, including multilevel substructuring and a method for repetitive cross sections that is exact for regular polygons used to represent cylinders. Modal displacements and stresses in or between plies of laminated plates are calculated and plotted, with values being recovered at all nodes of substructures. Comparison with usual approximate finite-element methods confirms that, for comparably converged solutions, VICONOPT is typically between 100 and 10(exp 4) times faster.

Kennedy, David↗
NASA NTRS1998

Summary of Research Report

Ten papers, published in various publications, on buckling, and the effects of imperfections on various structures are presented. These papers are: (1) Buckling mode localization in elastic plates due to misplacement in the stiffner location; (2) On vibrational imperfection sensitivity on Augusti's model structure in the vicinity of a non-linear static state; (3) Imperfection sensitivity due to elastic moduli in the Roorda Koiter frame; (4) Buckling mode localization in a multi-span periodic structure with a disorder in a single span; (5) Prediction of natural frequency and buckling load variability due to uncertainty in material properties by convex modeling; (6) Derivation of multi-dimensional ellipsoidal convex model for experimental data; (7) Passive control of buckling deformation via Anderson localization phenomenon; (8)Effect of the thickness and initial im perfection on buckling on composite cylindrical shells: asymptotic analysis and numerical results by BOSOR4 and PANDA2; (9) Worst case estimation of homology design by convex analysis; (10) Buckling of structures with uncertain imperfections - Personal perspective.

Elishakoff, Isaac↗
NASA NTRS2000

Coupling Light from a High-Q Microsphere Resonator Using a UV-induced Surface Grating

High-Q microspheres with whispering-gallery modes have very narrow resonances that can be used for fiber-optic filters, ultra-compact narrow-linewidth lasers and optical/microwave oscillators. Whispering-gallery modes were previously excited in microspheres using evanescent optical fields. The necessary phase synchronism was obtained by adjusting the incident angle of input light beam (prism coupler) or adjustment of the waveguide propagation constant (fiber taper coupler). For many applications, however, bulky near-field couplers are undesirable. They compromise the symmetry and generate stray fields. Also, the control of coupling is crucial for the performance of microsphere resonators: in analogy with radio frequency circuits, the loading Q-factor should be less than the intrinsic Q-factor, Q(sub L) less than or equal to Q(sub O). Ideally one should combine a stable coupling element and a resonator into a single microsphere component.

Ilchenko, V. S.↗
NASA NTRS2004

Parametric Studies of Square Solar Sails Using Finite Element Analysis

Parametric studies are performed on two generic square solar sail designs to identify parameters of interest. The studies are performed on systems-level models of full-scale solar sails, and include geometric nonlinearity and inertia relief, and use a Newton-Raphson scheme to apply sail pre-tensioning and solar pressure. Computational strategies and difficulties encountered during the analyses are also addressed. The purpose of this paper is not to compare the benefits of one sail design over the other. Instead, the results of the parametric studies may be used to identify general response trends, and areas of potential nonlinear structural interactions for future studies. The effects of sail size, sail membrane pre-stress, sail membrane thickness, and boom stiffness on the sail membrane and boom deformations, boom loads, and vibration frequencies are studied. Over the range of parameters studied, the maximum sail deflection and boom deformations are a nonlinear function of the sail properties. In general, the vibration frequencies and modes are closely spaced. For some vibration mode shapes, local deformation patterns that dominate the response are identified. These localized patterns are attributed to the presence of negative stresses in the sail membrane that are artifacts of the assumption of ignoring the effects of wrinkling in the modeling process, and are not believed to be physically meaningful. Over the range of parameters studied, several regions of potential nonlinear modal interaction are identified.

Sleight, David W.↗
NASA NTRS2009

Miniature Piezoelectric Shaker Mechanism for Autonomous Distribution of Unconsolidated Sample to Instrument Cells

To perform in-situ measurements on Mars or other planetary bodies many instruments require powder produced using some sampling technique (drilling/coring) or sample processing technique (core crushing) to be placed in measurement cells. This usually requires filling a small sample cell using an inlet funnel. In order to minimize cross contamination with future samples and ensure the sample is transferred from the funnel to the test cell with minimal residual powder the funnel is shaken. The shaking assists gravity by fluidizing the powder and restoring flow of the material. In order to counter cross contamination or potential clogging due to settling during autonomous handling a piezoelectric shaking mechanism was designed for the deposition of sample fines in instrument inlet funnels. This device was designed to be lightweight, consume low power and demonstrated to be a resilient solid state actuator that can be mechanically and electrically tuned to shake the inlet funnel. In the final design configuration tested under nominal Mars Ambient conditions the funnel mechanism is driven by three symmetrically mounted piezoelectric flexure actuators that are out of the funnel support load path. The frequency of the actuation can be electrically controlled and monitored and mechanically tuned by the addition of tuning mass on the free end of the actuator. Unlike conventional electromagnetic motors these devices are solid state and can be designed with no macroscopically moving parts. This paper will discuss the design and testing results of these shaking mechanisms.

powder movement↗
NASA NTRS2016

Simple Test Functions in Meshless Local Petrov-Galerkin Methods

Two meshless local Petrov-Galerkin (MLPG) methods based on two different trial functions but that use a simple linear test function were developed for beam and column problems. These methods used generalized moving least squares (GMLS) and radial basis (RB) interpolation functions as trial functions. These two methods were tested on various patch test problems. Both methods passed the patch tests successfully. Then the methods were applied to various beam vibration problems and problems involving Euler and Beck's columns. Both methods yielded accurate solutions for all problems studied. The simple linear test function offers considerable savings in computing efforts as the domain integrals involved in the weak form are avoided. The two methods based on this simple linear test function method produced accurate results for frequencies and buckling loads. Of the two methods studied, the method with radial basis trial functions is very attractive as the method is simple, accurate, and robust.

Raju, Ivatury S.↗
NASA NTRS1992

Probabilistic structural analysis of space truss structures for nonuniform thermal environmental effects

A three-bay, space, cantilever truss is probabilistically evaluated to quantify the range of uncertainties of vibration frequencies and truss free end vertical displacements due to nonuniform thermal loads, applied loads and moments (mechanical loads), and combination of both. The truss members are assumed to be made from (1) aluminum tubes or (2) high modulus graphite-fiber/intermediate modulus epoxy-matrix composite tubes. Cumulative distribution function results show that certain combination of thermal loads with mechanical loads reduce the probabilistic vibration frequencies and increase the magnitude of the truss free end vertical displacements for the aluminum truss. However, for the composite truss, the thermal effects on the probabilistic vibration frequencies are not as substantial as that for an aluminum truss. This can be attributed to the large differences in the values of coefficient of thermal expansion. Finally, the sensitivities associated with the uncertainties in the structural, material and load variables (primative variables) are investigated. Results indicate that vibration frequencies and truss free end vertical displacements are sensitive to the uncertainties in spacial (geometry) variables as well as member modulus.

Pai, Shantaram S.↗
NASA NTRS1974

The prediction of rotor rotational noise using measured fluctuating blade loads

In tests conducted at the NASA Langley Research Center Helicopter Rotor Test Facility, simultaneous measurements of the high-frequency fluctuating aerodynamic blade loads and far-field radiated noise were made on a full-scale, nontranslating rotor system. After their characteristics were determined, the measured blade loads were used in an existing theory to predict the far-field rotational noise. A comparison of the calculated and measured rotational noise is presented with specific attention given to the effect of blade loading coefficients, chordwise loading distributions, blade loading phases, and observer azimuthal position on the predictions.

Hosier, R. N.↗