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At least 343 records · Page 19

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NASA NTRS1972

Evaluation of solution procedures for material and/or geometrically nonlinear structural analysis by the direct stiffness method.

This paper presents an assessment of the solution procedures available for the analysis of inelastic and/or large deflection structural behavior. A literature survey is given which summarized the contribution of other researchers in the analysis of structural problems exhibiting material nonlinearities and combined geometric-material nonlinearities. Attention is focused at evaluating the available computation and solution techniques. Each of the solution techniques is developed from a common equation of equilibrium in terms of pseudo forces. The solution procedures are applied to circular plates and shells of revolution in an attempt to compare and evaluate each with respect to computational accuracy, economy, and efficiency. Based on the numerical studies, observations and comments are made with regard to the accuracy and economy of each solution technique.

Stricklin, J. A.↗

NASA NTRS1974

Dynamic transformation method

Eigenvalue problem associated with modal-synthesis vibration analysis of complex structures requires simplifying assumptions for solution. Computer program, Dynamic-transformation Adapted to Modal-synthesis Using Stiffness-coupling, improves computational economy for vibration analysis of complex structures while still considering substructure modes.

Admire, J. R.↗

NASA NTRS1975

Flutter analysis of two parallel elastically coupled flat plates

Flutter of two parallel elastically coupled flat plates was investigated analytically. A closed-form solution including both aerodynamic and structural damping is presented for flutter of flat orthotropic plates coupled by an elastic medium. Both plates are simply supported along the side edges but are supported by deflectional, rotational, and torsional springs of arbitrary stiffness at the leading and trailing edges. Two-dimensional quasi-steady aerodynamics was utilized in the solution. Since the large number of variables present in the problem precludes extensive parametric studies, results are presented to indicate the basic flutter characteristics of coupled two-plate systems and to assess the validity of previously published modal solutions for similar problems.

Shore, C. P.↗

NASA NTRS1984

A survey of payload integration methods

Several full-scale and short-cut methods for analyzing a booster/payload system are presented. Two full-scale techniques are considered: (1) a technique that uses a restrained payload together with a free-booster model, the latter being augmented with residual mass and stiffness correction and (2) a technique that uses a restrained payload and booster model. Both techniques determine the 'modal modes', which require the solution of a system eigenvalue problem; the loads usually are then determined via an acceleration approach. A brief description is given of a number of short-cut methods which are of special interest to Shuttle payload design: structural modification, base drive, and interface impedance methods. Directions for further research and development are suggested.

Engels, R. C.↗

NASA NTRS1988

Nondestructive construction error detection in large space structures

Continuum modeling of large space structures is extended to the problem of detecting construction errors in large space structures such as the proposed space station. First-order dynamic sensitivity equations for structures involving eigenfrequencies, modal masses, modal stiffnesses, and modal damping are presented. Matrix equations relating changes in element parameters to dynamic sensitivities are summarized. The sensitivity equations for the entire dynamical system are rearranged as a system of algebraic equations with unknowns of stiffness losses at selected locations. The feasibility of the formulation is numerically demonstrated on a simply-supported Euler-Bernouilli beam with simulated construction defects. The method is next extended to large space structures modelled as equivalent continua with simulated construction defects.

Stubbs, Norris↗

NASA NTRS1990

Design of a telescoping tube system for access and handling equipment

Spacecraft processing presents unique problems for the design of ground support equipment. A telescoping tube system consists of a number of nested structural tubes that can be extended and retracted (telescoped) while supporting a load. A typical telescoping tube system provides lateral, torsional, and vertical support for an access platform. Several concepts for improved telescoping tubes were developed with emphasis placed on reliability, ease of maintenance, and cost effectiveness. The most promising prototype design utilizes adjustable rollers running on tracks bolted to the tube sections. A wire rope deployment system ensures that all tube sections are controlled during extension and retraction. Track shimming and roller adjustment allow for fabrication of a high precision tube assembly that does not require extensive machining or unusually large shop equipment. The use of rolling contact eliminates the contamination problems encountered with sliding tubes in previous designs. The prototype design is suitable for indoor or outdoor use. A prototype tube assembly was fabricated and tested for strength, stiffness, maintainability, and endurance.

Littlefield, Alan C.↗

NASA NTRS1991

Radiative interactions in chemically reacting supersonic internal flows

The two-dimensional, elliptic Navier-Stokes equations are used to investigate supersonic flows with finite-rate chemistry and radiation for hydrogen-air systems. The chemistry source terms in the species equation is treated implicitly to alleviate the stiffness associated with fast reactions. The explicit, unsplit MacCormack finite-difference scheme is used to advance the governing equations in time, until convergence is achieved. The specific problem considered is the premixed flow in a channel with a ten-degree compression ramp. Three different chemistry models are used, accounting for increasing number of reactions and participating species. Two chemistry models assure nitrogen as inert, while the third model accounts for nitrogen reactions and NO(x) formation. The tangent slab approximation is used in the radiative flux formulation. A pseudo-gray model is used to represent the absorption-emission characteristics of the participating species. Results obtained for specific conditions indicate that the radiative interactions vary substantially, depending on reactions involving HO2 and NO species and that this can have a significant influence on the flowfield.

Tiwari, S. N.↗

NASA NTRS1991

Stacking-sequence optimization for buckling of laminated plates by integer programming

Integer-programming formulations for the design of symmetric and balanced laminated plates under biaxial compression are presented. Both maximization of buckling load for a given total thickness and the minimization of total thickness subject to a buckling constraint are formulated. The design variables that define the stacking sequence of the laminate are zero-one integers. It is shown that the formulation results in a linear optimization problem that can be solved on readily available software. This is in contrast to the continuous case, where the design variables are the thicknesses of layers with specified ply orientations, and the optimization problem is nonlinear. Constraints on the stacking sequence such as a limit on the number of contiguous plies of the same orientation and limits on in-plane stiffnesses are easily accommodated. Examples are presented for graphite-epoxy plates under uniaxial and biaxial compression using a commercial software package based on the branch-and-bound algorithm.

Haftka, Raphael T.↗

NASA NTRS1991

Radiative interactions in a hydrogen-fueled supersonic combustor

The two-dimensional, elliptic Navier-Stokes equations are used to investigate supersonic flows with finite-rate chemistry and radiation, for hydrogen-air systems. The chemistry source term in the species equation is treated implicitly to alleviate the stiffness associated with fast reactions. The explicit, unsplit MacCormack finite-difference scheme is used to advance the governing equations in time, until convergence is achieved. The specific problem considered is the premixed flow in a channel with a 10-deg compression ramp. Three different chemistry models are used, accounting for increasing number of reactions and participating species. Two chemistry models assume nitrogen as inert, while the third model accounts for nitrogen reactions and NO(x) formation. The tangent slab approximation is used in the radiative flux formulation. A pseudogray model is used to represent the absorption-emission characteristics of the participating species. Results otained for specific conditions indicate that the radiative interactions vary substantially, depending on reactions involving HO2 and NO species, and that this can have a significant influence on the flowfield.

Chandrasekhar, R.↗

NASA NTRS1992

Thermochemical nonequilibrium and radiative interactions in supersonic hydrogen-air combustion

The two-dimensional, elliptic Navier-Stokes equations are used to investigate supersonic flows with nonequilibrium chemistry and thermodynamics, coupled with radiation, for hydrogen-air systems. The chemistry source term in the species equation is treated implicitly to alleviate the stiffness associated with fast reactions. The explicit, unsplit MacCormack finite-difference scheme is used to advance the governing equations in time, until convergence is achieved. The specific problem considered is the premixed, expanding flow in a supersonic nozzle. The reacting flow consists of seven species, one of which is the inert N2 molecule. The thermal state of the gas is modeled with one translational-rotational temperature and five vibrational temperatures. The harmonic oscillator model is used in the formulation for vibrational relaxation. The tangent slab approximation is used in the radiative flux formulation. A pseudo-gray model is used to represent the absorption-emission characteristics of the participating species. Results obtained for specific conditions indicate the presence of nonequilibrium in the expansion region. This reduces the radiative interactions and can have a significant influence on the flowfield.

Chandrasekhar, R.↗

NASA NTRS1991

High temperature structural fibers: Status and needs

The key to high temperature structural composites is the selection and incorporation of continuous fiber reinforcement with optimum mechanical, physical, and chemical properties. Critical fiber property needs are high strength, high stiffness, and retention of these properties during composite fabrication and use. However, unlike polymeric composites where all three requirements are easily achieved with a variety of commercially available carbon-based fibers, structural fibers with sufficient stiffness and strength retention for high temperature metal, intermetallic, and ceramic composites are not available. The objective here is to discuss in a general manner the thermomechanical stability problem for current high performance fibers which are based on silicon and alumina compositions. This is accomplished by presenting relevant fiber property data with a brief discussion of potential underlying mechanisms. From this general overview, some possible materials engineering approaches are suggested which may lead to minimization and/or elimination of this critical stability problem for current high temperature fibers.

Dicarlo, James A.↗

NASA NTRS1990

Finite element analysis and optimization of composite structures

Linearly elastic fiber reinforced composite discs and laminates in plane stress with variable local orientation and concentration of one or two fiber fields embedded in the matrix material, are considered. The thicknesses and the domain of the discs or laminates are assumed to be given, together with prescribed boundary conditions and in-plane loading along the edge. The problem under study consists in determining throughout the structural domain the optimum orientations and concentrations of the fiber fields in such a way as to maximize the integral stiffness of the composite disc or laminate under the seven loading. Minimization of the integral stiffness can also be performed. The optimization is performed subject to a prescribed bound on the total cost or weight of the composite that for given unit cost factors or specific weights determines the amounts of fiber and matrix materials in the structure. Examples are presented.

Thomsen, Jan↗

NASA NTRS2021

Modeling and Analysis of Fluted-Core Composite Structures for Aerospace Applications

Fiber-reinforced composites are becoming more frequently used for aerospace applications, and because of mass and stiffness requirements, sandwich composites are often selected where shell-type structures are needed. However, traditional-core sandwich composites can exhibit certain manufacturing and in-service problems that have the potential to be alleviated through the use of other sandwich composite concepts. Fluted-core sandwich composite structures, which consist of integral angled web members with structural radius fillers, or noodles, spaced between laminate face sheets, is one such alternative and is considered herein. Because the noodles can consist of unidirectional fiber and can represent a significant amount of the total fluted-core cross section, accurate prediction of the structural response requires that the noodles be modeled with sufficient detail. Previous work showed that the structural response of fluted-core sandwich composite structures could be accurately represented with detailed finite element analysis using a combination of solid and shell elements. In these models, solid elements were used to represent the noodles and shell elements were used to represent the webs and faces. However, this previous solid-noodle modeling approach was also quite computationally intensive, and therefore not practical for large or complicated structures. In this document, a less computationally intensive shell-noodle approach, wherein the entire fluted-core sandwich composite construction is modeled with shell elements, is discussed and predicted structural responses are compared with those of the solid-noodle approach. It is found that by proper selection of certain geometric parameters (thickness and offset of the shell elements that represent the noodle), the shell-noodle modeling approach can be made “structurally equivalent” to the solid-noodle modelling approach.

Leonard Oremont↗

NASA NTRS2019

Modeling and simulation challenges for softgoods in space deployable structures

This paper examines modeling and simulation challenges for deployable spacecraft structures with softgood components. Previous studies highlighted the numerical challenges due to the mixture of high stiffness and low stiffness contact in this class of structures. This paper examines the use of the commercial LS-DYNA nonlinear finite element code on a set of relevant benchmark problems.

Mobrem, Mehran↗

NASA NTRS2006

Multimodal Platform Control for Robotic Planetary Exploration Missions

Planetary exploration missions pose unique problems for astronauts seeking to coordinate and control exploration vehicles. These include working in an environment filled with abrasive dust (e.g., regolith compositions), a desire to have effective hands-free communication, and a desire to have effective analog control of robotic platforms or end effectors. Requirements to operate in pressurized suits are particularly problematic due to the increased bulk and stiffness of gloves. As a result, researchers are considering alternative methods to perform fine movement control, for example capitalizing on higher-order voice actuation commands to perform control tasks. This paper presents current research at NASA s Neuro Engineering Laboratory that explores one method-direct bioelectric interpretation-for handling some of these problems. In this type of control system, electromyographic (EMG) signals are used both to facilitate understanding of acoustic speech in pressure-regulated suits 2nd to provide smooth analog control of a robotic platform, all without requiring fine-gained hand movement. This is accomplished through the use of non-invasive silver silver-chloride electrodes located on the forearm, throat, and lower chin, positioned so as to receive electrical activity originating from the muscles during contraction. For direct analog platform control, a small Personal Exploration Rover (PER) built by Carnegie Mellon University Robotics is controlled using forearm contraction duration and magnitudes, measured using several EMG channels. Signal processing is used to translate these signals into directional platform rotation rates and translational velocities. higher order commands were generated by differential contraction patterns called "clench codes."

Jorgensen, Charles↗

NASA NTRS1984

A simple rectangular element for 2-dimensional analysis of laminated composites

A simple rectangular finite element was developed for two dimensional analysis of laminated composite materials. The rectangular laminated composite element eliminates the need to add elements to a model simply to account for the material properties of various laminae. This is particularly advantageous for thick laminates with many lamina. Explicit integration in terms of generalized displacements minimizes the algebraic effort required to derive the element stiffness and the thermal load vector. A substitute shape function technique is used to improve the performance of the element in modeling bending type deformation. Results for several example problems are discussed.

Whitcomb, J. D.↗

NASA NTRS1986

A simple nonlinear joint model

Hertzian contact theory is applied to a butt joint with specially mismatched bearing surfaces to devise a simple mathematical model of nonlinear axial force-displacement behavior in jointed members. Normalized tangent stiffness-force plots, for several values of a joint imperfection parameter, are presented for the sample case of solid structural members of circular section. The results illustrate the potential problem of high joint compliance at low axial-force levels, as well as the generally desirable stiffening and linearizing effects of preload. A nonlinear oscillator problem based on the static model is also formulated and solved to illustrate the effect of amplitude on natural frequency. As expected, natural frequency is low when amplitude is small. The results call attention to the important roles that tight tolerances and preload are expected to play in the design and fabrication of deployable and erectable truss-type space structures.

Fichter, W. B.↗

NASA NTRS1986

A simple rectangular element for two-dimensional analysis of laminated composites

A simple rectangular finite element was developed for two-dimensional analysis of laminated composite materials. The rectangular laminated composite element eliminates the need to add elements to a model simply to account for the material properties of various laminae. This is particularly advantageous for thick laminates with many lamina. Explicit integration in terms of generalized displacements minimizes the algebraic effort required to derive the element stiffness and the thermal load vector. A substitute shape function technique is used to improve the performance of the element in modeling bending type deformation. Results for several example problems are discussed.

Whitcomb, J. D.↗