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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Stability analysis of bridged cracks in brittle matrix composites

The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced brittle matrix composites. This paper presents the results of a nonlinear finite element analysis of the Mode-I propagation of a bridged matrix crack in a finite size specimen. The composite is modeled as an orthotropic continuum and the bridging due to the fibers is modeled as a distribution of tractions which resist crack opening. A critical stress intensity factor criterion is employed for matrix crack propagation while a critical crack opening condition is used for fiber failure. The structural response of the specimen (load-deflection curves) as well as the stress intensity factor of the propagating crack are calculated for various constituent properties and specimen configurations for both tensile and bending loading. By controlling the length of the bridged crack results are obtained which highlight the transition from stable to unstable behavior of the propagating crack.

Ballarini, Roberto↗
NASA NTRS1992

EMTP based stability analysis of space station electric power system in a test bed environment

The Space Station Freedom Electric Power System (EPS) will convert solar energy into electric energy and distribute the same using an 'all dc', Power Management and Distribution (PMAD) System. Power conditioning devices (dc to dc converters) are needed to interconnect parts of this system operating at different nominal voltage levels. Operation of such devices could generate under damped oscillations (instability) under certain conditions. Criteria for instability are examined and verified for a single device. Suggested extension of the criteria to a system operation is examined by using the EMTP model of the PMAD DC test bed. Wherever possible, data from the test bed is compared with the modeling results.

Dravid, Narayan V.↗
NASA NTRS1992

EMTP based stability analysis of Space Station Electric Power System in a test bed environment

The Space Station Freedom Electric Power System (EPS) will convert solar energy into electric energy and distribute the same using an 'all dc', Power Management and Distribution (PMAD) System. Power conditioning devices (dc to dc converters) are needed to interconnect parts of this system operating at different nominal voltage levels. Operation of such devices could generate under damped oscillations (instability) under certain conditions. Criteria for instability are examined and verified for a single device. Suggested extension of the criteria to a system operation is examined by using the EMTP model of the PMAD dc test bed. Wherever possible, data from the test bed is compared with the modeling results.

Dravid, Narayan V.↗
NASA NTRS1993

Shell stability analysis in a computer aided engineering (CAE) environment

The development of 'DISDECO', the Delft Interactive Shell DEsign COde is described. The purpose of this project is to make the accumulated theoretical, numerical and practical knowledge of the last 25 years or so readily accessible to users interested in the analysis of buckling sensitive structures. With this open ended, hierarchical, interactive computer code the user can access from his workstation successively programs of increasing complexity. The computational modules currently operational in DISDECO provide the prospective user with facilities to calculate the critical buckling loads of stiffened anisotropic shells under combined loading, to investigate the effects the various types of boundary conditions will have on the critical load, and to get a complete picture of the degrading effects the different shapes of possible initial imperfections might cause, all in one interactive session. Once a design is finalized, its collapse load can be verified by running a large refined model remotely from behind the workstation with one of the current generation 2-dimensional codes, with advanced capabilities to handle both geometric and material nonlinearities.

Arbocz, J.↗
NASA NTRS1993

A detailed study of mean-flow solutions for stability analysis of transitional flows

A finite-volume upwind-difference parabolized Navier-Stokes code is utilized to obtain laminar mean-flow solutions at Mach 3.5 on a half-angle cone of 5 deg at an angle-of-attack of 2 deg. A detailed study is conducted on this configuration; the main focus is the velocity profiles in the leeward and windward symmetry planes at various axial locations. Comparisons of the solution profiles are made with both a central-difference code that incorporates scalar and matrix dissipation models and another state-of-the-art upwind-difference finitevolume code. The results obtained emphasize the importance of using matrix dissipation models for schemes that require explicit artificial dissipation. These results also illustrate the accuracy and efficiency of the planeby-plane marching procedure for computing mean-flow solutions for predicting the onset of transition with linear instability.

Ramakrishnan, R.↗
NASA NTRS1993

A mesoscale gravity-wave event observed during CCOPE. IV - Stability analysis and Doppler-derived wave vertical structure

A summary of the results of a detailed study of the vertical structure of mesoscale gravity waves conducted during the Cooperative Convective Precipitation Experiment (CCOPE) is presented. Pressure perturbation fields derived from the Doppler wind fields are compared with the vertical structure of eigenfunctions resulting from a solution to the Taylor-Goldstein linear wave equation for an atmosphere whose mean state is described by vertical profiles obtained from a representative CCOPE sounding. An analysis of the potential for shear instability is also performed on all of the soundings taken on this day to assess the representativeness of the one chosen for the linear theoretical analysis.

Koch, Steven E.↗
NASA NTRS1947

A simplified method of elastic-stability analysis for thin cylindrical shells I : Donnell's equation

The equation for the equilibrium of cylindrical shells introduced by Donnell in NACA Technical Report no. 479 to find the critical stresses of cylinders in torsion is applied to find critical stresses for cylinders with simply supported edges under other loading conditions. It is shown that by this method solutions may be obtained very easily and the results in each case may be expressed in terms of two nondimensional parameters, one dependent on the critical stress and the other essentially determined by the geometry of the cylinder. The influence of boundary conditions related to edge displacements in the shell median surface is discussed. The accuracy of the solutions found is established by comparing them with previous theoretical solutions and with test results. The solutions to a number of problems concerned with the buckling of cylinders with simply supported edges on the basis of a unified viewpoint are presented in a convenient form for practical use. (author)

Batdorf, S B↗
NASA NTRS1947

A Simplified Method of Elastic-Stability Analysis for Thin Cylindrical Shells

This paper develops a new method for determining the buckling stresses of cylindrical shells under various loading conditions. In part I, the equation for the equilibrium of cylindrical shells introduced by Donnell in NACA report no. 479 to find the critical stresses of cylinders in torsion is applied to find critical stresses for cylinders with simply supported edges under other loading conditions. In part II, a modified form of Donnell's equation for the equilibrium of thin cylindrical shells is derived which is equivalent to Donnell's equation but has certain advantages in physical interpretation and in ease of solution, particularly in the case of shells having clamped edges. The question of implicit boundary conditions is also considered.

Batdorf, S B↗
NASA NTRS1995

Combustion-acoustic stability analysis for premixed gas turbine combustors

Lean, prevaporized, premixed combustors are susceptible to combustion-acoustic instabilities. A model was developed to predict eigenvalues of axial modes for combustion-acoustic interactions in a premixed combustor. This work extends previous work by including variable area and detailed chemical kinetics mechanisms, using the code LSENS. Thus the acoustic equations could be integrated through the flame zone. Linear perturbations were made of the continuity, momentum, energy, chemical species, and state equations. The qualitative accuracy of our approach was checked by examining its predictions for various unsteady heat release rate models. Perturbations in fuel flow rate are currently being added to the model.

Darling, Douglas↗
NASA NTRS1996

CLMNANAL: A C++ program for application of the Coleman stability analysis to rotorcraft

This program is an adaptation of the theory of Robert P. Coleman and Arnold M. Feingold as presented in NACA Report 1351, 1958. This theory provided a method for the analysis of multiple-bladed rotor systems to determine the system susceptibility to ground resonance. Their treatment also provided a simple means for determining the required product of rotor and chassis damping factors to suppress the resonance. This C++ program is based on a FORTRAN 77 version of a similar code.

Lance, Michael B.↗
NASA NTRS1998

Stability Analysis of Plates and Shells

This special publication contains the papers presented at the special sessions honoring Dr. Manuel Stein during the 38th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference held in Kissimmee, Florida, Apdl 7-10, 1997. This volume, and the SDM special sessions, are dedicated to the memory of Dr. Manuel Stein, a major pioneer in structural mechanics, plate and shell buckling, and composite structures. Many of the papers presented are the work of Manny's colleagues and co-workers and are a result, directly or indirectly, of his influence. Dr. Stein earned his Ph.D. in Engineering Mechanics from Virginia Polytechnic Institute and State University in 1958. He worked in the Structural Mechanics Branch at the NASA Langley Research Center from 1943 until 1989. Following his retirement, Dr. Stein continued his involvement with NASA as a Distinguished Research Associate.

Knight, Norman F., Jr.↗
NASA NTRS1998

Future Directions and Challenges in Shell Stability Analysis

An answer is sought to the question of today, in 1997, after so many years of concentrated research effort in designing buckling critical thin walled shells, why one cannot do any better than using the rather conservative Lower Bound Design Philosophy of the sixties. It will be shown that with the establishment of Initial Imperfection Data Banks and the introduction of Probabilistic Design Procedures one has a viable alternative, that when used judiciously, may lead to improved shell design recommendations.

Arbocz, Johann↗
NASA NTRS1997

Future Directions and Challenges in Shell Stability Analysis

An answer is sought to the question how comes that today, in 1997, after so many years of concentrated research effort, when it comes to designing buckling critical thin walled shells, one cannot do any better than using the rather conservative Lower Bound Design Philosophy of the sixties. It will be shown that with the establishment of Initial Imperfection Data Banks and the introduction of Probabilistic Design Procedures one has, what appears to be, a viable alternative that when used judiciously may lead step by step to improved shell design recommendations.

Arbocz, Johann↗