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At least 235 records · Page 13

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NASA NTRS1981

Control of large space structures using Annual Momentum Control Devices (AMCD's)

A controller design approach for large space structures, which proposes the use of several Annular Momentum Control Devices (AMCD's) for structural damping enhancement, and either torque actuators of AMCD's for primary attitude control, was investigated. The damping enhancement controller makes the system asymptotically stable under certain relatively simple conditions. The closed-loop stability of the system with the primary attitude controller as well as the overall controller was established. It is shown that the same AMCD's can be used for the actuation of the damping enhancement controller and the primary attitude controller. Numerical results were obtained for a finite-element model of a large, thin, completely free, flat aluminum plate.

Joshi, S. M.↗

NASA NTRS1981

Buffer thermal energy storage for an air Brayton solar engine

The application of latent-heat buffer thermal energy storage to a point-focusing solar receiver equipped with an air Brayton engine was studied. To demonstrate the effect of buffer thermal energy storage on engine operation, a computer program was written which models the recuperator, receiver, and thermal storage device as finite-element thermal masses. Actual operating or predicted performance data are used for all components, including the rotating equipment. Based on insolation input and a specified control scheme, the program predicts the Brayton engine operation, including flows, temperatures, and pressures for the various components, along with the engine output power. An economic parametric study indicates that the economic viability of buffer thermal energy storage is largely a function of the achievable engine life.

Strumpf, H. J.↗

NASA NTRS1982

The SPAR thermal analyzer: Present and future

The SPAR thermal analyzer, a system of finite-element processors for performing steady-state and transient thermal analyses, is described. The processors communicate with each other through the SPAR random access data base. As each processor is executed, all pertinent source data is extracted from the data base and results are stored in the data base. Steady state temperature distributions are determined by a direct solution method for linear problems and a modified Newton-Raphson method for nonlinear problems. An explicit and several implicit methods are available for the solution of transient heat transfer problems. Finite element plotting capability is available for model checkout and verification.

Marlowe, M. B.↗

NASA NTRS1982

Integrated thermal-structural analysis of large space structures

Optimum performance of large space antennas requires very fine control of the shape of the antenna surface since the shape affects both frequency control and pointing accuracy. A significant factor affecting the antenna shape is the temperature of the structure and the resulting deformation. To accurately predict the temperature of the structure, it is necessary first to accurately predict thermal loads. As the structure orbits the Earth, the thermal loads change constantly so that the thermal-structural response varies continuously throughout the orbit. The results from recent applications of integrated finite element methodology to heat load determination and thermal-structural analysis of large space structures are given. Four areas are concentrated on: (1) the characteristics of the integrated finite element methodology, (2) fundamentals of orbital heat load calculation, (3) description and comparison of some radiation finite elements, and (4) application of the integrated finite-element approach to the thermal-structural analysis of an orbiting truss structure.

Mahaney, J.↗

NASA NTRS1982

Guidelines for developing vectorizable computer programs

Some fundamental principles for developing computer programs which are compatible with array-oriented computers are presented. The emphasis is on basic techniques for structuring computer codes which are applicable in FORTRAN and do not require a special programming language or exact a significant penalty on a scalar computer. Researchers who are using numerical techniques to solve problems in engineering can apply these basic principles and thus develop transportable computer programs (in FORTRAN) which contain much vectorizable code. The vector architecture of the ASC is discussed so that the requirements of array processing can be better appreciated. The "vectorization" of a finite-difference viscous shock-layer code is used as an example to illustrate the benefits and some of the difficulties involved. Increases in computing speed with vectorization are illustrated with results from the viscous shock-layer code and from a finite-element shock tube code. The applicability of these principles was substantiated through running programs on other computers with array-associated computing characteristics, such as the Hewlett-Packard (H-P) 1000-F.

Miner, E. W.↗

NASA NTRS1981

Dynamic Analysis of Darrieus Vertical Axis Wind Turbine Rotors

The dynamic response characteristics of the vertical axis wind turbine (VAWT) rotor are important factors governing the safety and fatigue life of VAWT systems. The principal problems are the determination of critical rotor speeds (resonances) and the assessment of forced vibration response amplitudes. The solution to these problems is complicated by centrifugal and Coriolis effects which can have substantial influence on rotor resonant frequencies and mode shapes. The primary tools now in use for rotor analysis are described and discussed. These tools include a lumped spring mass model (VAWTDYN) and also finite-element based approaches. The accuracy and completeness of current capabilities are also discussed.

Lobitz, D. W.↗

NASA NTRS1982

A finite element stress analysis of spur gears including fillet radii and rim thickness effects

Spur gear stress analysis results are presented for a variety of loading conditions, support conditions, fillet radii, and rim thickness. These results are obtained using the SAP IV finite-element code. The maximum stresses, occurring at the root surface, substantially increase with decreasing rim thickness for partially supported rims (that is, with loose-fitting hubs). For fully supported rims (that is, with tight-fitting hubs), the root surface stresses slightly decrease with decreasing rim thickness. The fillet radius is found to have a significant effect upon the maximum stresses at the root surface. These stresses increase with increasing fillet radius. The fillet radius has little effect upon the internal root section stresses.

Chang, S. H.↗

NASA NTRS1982

Singularity embedding method in potential flow calculations

The so-called H-type mesh is used in a finite-element (or finite-volume) calculation of the potential flow past an airfoil. Due to coordinate singularity at the leading edge, a special singular trial function is used for the elements neighboring the leading edge. The results using the special singular elements are compared to those using the regular elements. It is found that the unreasonable pressure distribution obtained by the latter is removed by the embedding of the singular element. Suggestions to extend the present method to transonic cases are given.

Jou, W. H.↗

NASA NTRS1981

Interlaminar stress singularities at a straight free edge in composite laminates

A quasi-three-dimensional finite-element analysis was used to analyze the edge-stress problem in four-ply, composite laminates. The seven laminates that were considered belong to the laminate family where the outer ply angle is between 0 and 90 deg. Systematic convergence studies were made to explore the existence of stress singularities near the free edge. The present analysis appears to confirm the existence of stress singularities at the intersection of the interface and the free edge. The power of the stress singularity was the same for all seven laminates considered.

Raju, I. S.↗

NASA NTRS1981

An empirical stress-intensity factor equation for the surface crack

This paper presents an empirical stress-intensity factor equation for a surface crack as a function of parametric angle, crack depth, crack length, plate thickness and plate width for tension and bending loads. The stress-intensity factors used to develop the equation were obtained from a previous three-dimensional, finite-element analysis of semielliptical surface cracks in finite elastic plates subjected to tension or bending loads. A wide range of configuration parameters was included in the equation. The ratios of crack length to plate thickness and the ratios of crack depth to crack length ranged from 0 to 1.0. The effects of plate width on stress-intensity variations along the crack front were also included. The equation was used to predict patterns of surface-crack growth under tension or bending fatigue loads. The equation was also used to correlate surface-crack fracture data for a brittle epoxy material within + or - 10 percent for a wide range of crack shapes and crack sizes.

Newman, J. C., Jr.↗

NASA NTRS1981

Finite element analysis of instability related delamination growth

Instability-related growth of through-width delaminations in laminated coupons was investigated analytically and experimentally. In the analytical effort, a geometrically nonlinear finite-element stress analysis was used, and the numerical results were compared with exact solutions. In addition, measured lateral deflections of postbuckled through-width delaminations were compared with predicted deflections. Lateral deflections, stress distributions, and strain energy release rates were calculated for various delamination lengths, delamination depths, and applied loads. Delamination growth rates observed for unidirectional graphite/epoxy bonded to aluminum were qualitatively correlated with calculated strain-energy release rates and it was found that the Mode I component of energy release dominates the growth process.

Whitcomb, J. D.↗

NASA NTRS1982

Minimum mass sizing of a large low-aspect ratio airframe for flutter-free performance

A procedure for sizing an airframe for flutter-free performance is demonstrated on a large, flexible supersonic transport aircraft. The procedure is based on using a two-level reduced basis or modal technique for reducing the computational cost of performing the repetitive flutter analyses. The supersonic transport aircraft exhibits complex dynamic behavior, has a well-known flutter problem and requires a large finite-element model to predict the vibratory and flutter response. Flutter-free designs are produced with small mass increases relative to the wing structural weight and aircraft payload. In view of the ability of the resizing procedure to handle this supersonic transport configuration, it seems likely that the method could be used for many other aircraft.

Greene, W. H.↗

NASA NTRS1981

Some aspects of algorithm performance and modeling in transient thermal analysis of structures

The status of an effort to increase the efficiency of calculating transient temperature fields in complex aerospace vehicle structures is described. The advantages and disadvantages of explicit and implicit algorithms are discussed. A promising set of implicit algorithms with variable time steps, known as the GEAR package is described. Four test problems, used for evaluating and comparing various algorithms, have been selected and finite-element models of the configurations are described. These problems include a Space Shuttle frame component, an insulated cylinder, a metallic panel for a thermal protection system, and a model of the Space Shuttle Orbiter wing. Results generally indicate a preference for implicit over explicit algorithms for solution of transient structural heat transfer problems when the governing equations are 'stiff' (typical of many practical problems such as insulated metal structures).

Adelman, H. M.↗

NASA NTRS1982

Three dimensional finite element analysis of damage accumulation in composite laminate

A three dimensional finite-element computer program has been developed to analyze layered fiber-reinforced composite laminate. This program is capable of: (1) calculating the detailed stress distribution, (2) identifying the damage zone and mode of failure, (3) analyzing the damage accumulation, and (4) determining the ultimate strength of the composite laminate.

Lee, J. D.↗

NASA NTRS1981

The algebraic theory of latent projectors in lambda matrices

Multivariable systems such as a finite-element model of vibrating structures, control systems, and large-scale systems are often formulated in terms of differential equations which give rise to lambda matrices. The present investigation is concerned with the formulation of the algebraic theory of lambda matrices and the relationship of latent roots, latent vectors, and latent projectors to the eigenvalues, eigenvectors, and eigenprojectors of the companion form. The chain rule for latent projectors and eigenprojectors for the repeated latent root or eigenvalues is given.

Denman, E. D.↗

NASA NTRS1982

Interfacial fracture of Space-Shuttle thermal-protection system

Stable crack growth and fracture at the interface of undensified LI-900 reusable surface insulation (RSI) tile and the Nomex strain isolation pad (SIP) of the Space-Shuttle thermal-protection system (TPS) were modeled by double-edged notch-tension specimens. These specimens were loaded under uniaxial tension or 50-Hz cyclic loading and the resultant stable crack growth leading to eventual fracture was monitored by a videocamera. These tests showed that successive local tear-outs due to local tensile overload in the RSI tile resulted in the interfacial fracture where the crack-tip opening angle, CTOA, of the SIP was related to initiation and intermittent stable crack propagation. Fractures in similar static and dynamic test specimens using densified LI-900 RSI tiles occurred in the undensified regions of the RSI tiles. These failures were consistent with the above failure mechanism based on the local tensile strength of the undensified LI-900 RSI tile. The intermittent stable crack growth of undensified LI-900 RSI tile was reproduced by a deterministic, two-dimensional finite-element model with SIP of variable elastic moduli.

Komine, A.↗

NASA NTRS1982

Superplastic forming/weld-brazing of titanium skin-stiffened compression panels

A study was conducted to exploit the processing advantages of superplastic forming and weld-brazing for the fabrication of titanium skin-stiffened structural components. Small titanium compression panels were fabricated and tested at room temperature. Stiffeners having configurations of a conventional hat shape, beaded shaped web, ribbed shaped web, or stepped shaped web were investigated. The data from the panel tests included load-shortening curves, local buckling strengths, and failure loads. Experimental buckling loads were compared with buckling loads calculated using a finite-element analysis. The superplastic formed/weld-brazed panels having complex shaped stiffeners developed from 20 to 58 percent higher local buckling strengths than panels with conventionally shaped stiffeners.

Royster, D. M.↗

NASA NTRS1976

The Effects of Variable Mass and Geometry, Pretwist, Shear Deformation and Rotatory Inertia on the Resonant Frequencies of Intact Long Bones: A Finite Element Model Analysis

The influence of pretwist, nonuniformities in mass and flexural stiffness, rotatory inertia and shear deformation on the natural frequencies of intact bones is evaluated by means of a linear elastic, finite-element model which has been programmed for solution on the digital computer. Theoretical results are compared to the results on the forced vibration of intact canine radii obtained experimentally by Thompson. Surprisingly, inclusion of fairly large pretwist angles (from -14 to 12 deg for one specimen) had little affect on the first three frequencies of transverse vibration in either the cranial or lateral directions. Inclusion of shear deformation reduced the third-mode frequency in the stiffest (lateral) direction by about six percent, otherwise shear deformation played a minor role in determining natural frequencies. Similarly. rotatory inertia had negligible influence up to the third natural frequency. The predominant influence on the first three natural frequencies of transverse vibration could be attributed to the variations in mass and flexural stiffness along the length of the test specimens. Different effective moduli of elasticity are required to yield correct absolute values for the frequencies which correspond to experimental findings. thus implying the presence of some inhomogeneities in material properties around the bone cross-section and/or along its length.

Young, Donald R.↗