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At least 577 records · Page 32

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NASA NTRS1989

Compressible flow calculations employing the Galerkin/least-squares method

A multielement group, domain decomposition algorithm is presented for solving linear nonsymmetric systems arising in the finite-element analysis of compressible flows employing the Galerkin/least-squares method. The iterative strategy employed is based on the generalized minimum residual (GMRES) procedure originally proposed by Saad and Shultz. Two levels of preconditioning are investigated. Applications to problems of high-speed compressible flow illustrate the effectiveness of the scheme.

Shakib, F.↗

NASA NTRS1989

A locally implicit finite element analysis algorithm for 3-D compressible flows

The paper outlines the research results on the development of a finite-element analysis procedure for three-dimensional compressible viscous flows. Some new algorithmic features of the computational model, such as a locally implicit solution scheme which avoids the necessity of computation and assembly of global matrices. The new algorithm makes it applicable to large-scale three-dimensional flow problems of practical interest, for example, the analysis of flows in Space Shuttle Main Engine (SSME).

Reddy, K. C.↗

NASA NTRS1989

Treatment of coupled fluid-structure interaction problems by a mixed variational principle

Felippa, Carlos A.↗

NASA NTRS1989

Finite element solvers for incompressible fluid flows and heat transfer

Two different finite-element solvers for incompressible viscous flow, i.e., the mixed interpolation method and the SIMPLE-type iterative method, are compared and tested with some benchmark problems. The advantages of the SIMPLE-type iterative method are the decoupling of the governing equations and the use of equal-order interpolation functions for both velocity and pressure. Even though there is a significant difference between the two methods in terms of the pressure field, similar solutions are obtained for the velocity field.

Sohn, Jeong L.↗

NASA NTRS1989

The Stokes problem with various well-posed boundary conditions - Symmetric formulations that converge for all velocity/pressure spaces

Symmetric finite-element formulations are presented for the primitive-variables form of the Stokes equations. These formulations are convergent for any combination of pressure and velocity interpolations. Various boundary conditions, such as pressure, are accommodated.

Hughes, Thomas J. R.↗

NASA NTRS1990

On the nature of the interaction between structures and proof-mass actuators

This paper presents an analysis of the interaction between a structure, an actuator used to control the vibration of the structure, and the control law to be implemented by the actuator. The control hardware used is a proof-mass actuator with experimentally verified dynamics capable of being used in a space structure configuration. A local rate-feedback control law is used. The control of two different structures is presented. The first structure is a cantilevered beam constructed of a quasi-isotropic composite material that is controlled by a single actuator forming the experimental component of the investigation. The second structure is a finite-element model of a truss system controlled by a single actuator. Models of both structures predict the presence of potential instabilities in system performance if proper consideration is not given to interactions between the control law, the structure, and the actuator.

Zimmerman, David C.↗

NASA NTRS1990

Dynamic characteristics of a Space Station Freedom Mars evolution reference configuration

One concept for a manned mission to Mars uses as a transportation mode an evolutionary version of Space Station Freedom (SSF), termed the Mars Evolution Reference Configuration (MERC). The MERC is configured by adding to SSF dual keels, an upper and lower boom, additional laboratory and habitation modules, increased power, and an assembly platform. A finite-element model of the MERC was formulated to investigate the expected low frequency modes, and its variations with the addition of large payload. A basic reboost procedure using near-continuous firing of reaction control system jets is described with the closed-loop attitude control system. The elastic dynamic behavior at critical points during a reboost of the MERC, both with and without the Mars piloted vehicle installed, is examined.

Ayers, J. Kirk↗

NASA NTRS1989

Sublaminate analysis of interlaminar fracture in composites. I - Analytical model

A simple analysis method based on a transverse shear-deformation theory and a sublaminate approach is utilized to analyze a mixed-mode edge delamination specimen. The analysis provides closed-form expressions for the interlaminar shear stresses ahead of the crack, the total strain-energy release rate, and the strain-energy release-rate components. The parameters controlling the behavior are identified. The effect of specimen stacking sequence and delamination interface on the strain energy release rate components is investigated. Results are compared with a finite-element simulation for reference. The simple nature of the method makes it suitable for preliminary design analyses which require a large number of configurations to be evaluated quickly and economically.

Armanios, Erian A.↗

NASA NTRS1989

Sublaminate analysis of interlaminar fracture in composites. II - Applications

A validation of the sublaminate analysis method of Armanios and Rehfield (1989) is provided by comparison of the predictions with a quasi-three-dimensional finite-element analysis. A comparison of the total strain-energy release rate with a classical plate model is also provided. Closed-form expressions for the total strain-energy rate and the strain-energy release-rate components are developed. The sublaminate analysis developed is simple, and the results are generated using a desktop computer.

Armanios, Erian A.↗

NASA NTRS1990

Finite element two-dimensional panel flutter at high supersonic speeds and elevated temperature

A consistent finite element formulation for flutter of two-dimensional panels including effects of temperature variation along panel length is presented. The von Karman nonlinear strain-displacement relation is used to account for the large-deflections, and the quasi-steady first-order piston theory is employed for aerodynamic loading in the finite-element formulation. The panel flutter under combined thermal/aerodynamic loading can be separated into solving two problems in sequence. They are: (1) postbuckling deflection, stresses and buckling stability boundary, and (2) critical dynamic pressure and linear flutter boundary. The solution procedures in solving these problems are presented in detail. Finite element results of two-dimensional panels with uniform temperature change are compared with classic analytical solutions. Influence of nonuniform temperature distributions on critical buckling temperature, flutter deflection and stresses, and flutter boundary is also given.

Xue, David Y.↗

NASA NTRS1990

Iosipescu shear characterization of polymeric and metal matrix composites

The paper outlines the results of an experimental/analytical investigation of deformation and stress fields in the test section of graphite/epoxy and boron/aluminum Iosipescu specimens using high-resolution moire interferometry and finite-element analysis. Very good correlation between experiment and analysis is demonstrated for the graphite/epoxy specimens. This, in turn, justifies the use of correction factors employed in the previous investigations to eliminate differences in the initial shear moduli obtained from 0-deg and 90-deg Iosipescu specimens. The significant discrepancies observed in the case of boron/aluminum specimens are attributed to localized yielding at the notch tips caused by the clamping action, the associated in-plane bending effects and changing boundary conditions due to slippage in the grips.

Pindera, M.-J.↗

NASA NTRS1989

Analysis of notched metal matrix composites under tensile loading

This paper presents techniques based on a three-dimensional finite-element analysis for the analysis of continuous fiber reinforced metal matrix composite. Examples are shown for specific metal matrix composites such as boron/aluminum and silicon-carbide/aluminum. Specimen stress-strain behavior and stress at first fiber failure were predicted for boron/aluminum laminates containing circular holes and crack-like slits. The predictions compared very well with test data for laminates containing 0 deg fibers and reasonably well for (+/-45)2s laminates. Mesh configuration was shown to have an effect on the calculation of stresses local to the notch. The presence of thin interface layers of matrix material had a significant influence on the slit tip stress state, causing sharper stress gradients near the notch. Interface layers reduced the slit-tip fibers stress in a (+/-45)2s silicon-carbide/aluminum laminate but increased them in a (0/90)2s laminate.

Bigelow, C. A.↗

NASA NTRS1990

Mode I delamination of a three-dimensional fabric composite

Finite-element analysis was conducted on a double-cantilever-beam (DCB) specimen of carbon-epoxy (T300/3501-6) orthogonal interlocked fabric composites. The mode I strain-energy release rate G(I) was evaluated to investigate the influence of through-the-thickness (z-axis) fibers on crack driving force as a function of crack length. The analysis showed good agreement with experimental results for both compliance and G(I). The presence of z-axis fibers sharply reduced G(I) in comparison to the traditional two-dimensional laminated material. A parametric study revealed that increasing the z-axis fiber stiffness reduces G(I). Progressive debonding of the z-axis reinforcement was also modeled. G(I) increased as the fiber debonded but was significantly less than in the two-dimensional laminated material in all cases.

Byun, Joon-Hyung↗

NASA NTRS1990

Mixed variational formulation of finite element analysis of acoustoelastic/slosh fluid-structure interaction

A general three-field variational principle is obtained for the motion of an acoustic fluid enclosed in a rigid or flexible container by the method of canonical decomposition applied to a modified form of the wave equation in the displacement potential. The general principle is specialized to a mixed two-field principle that contains the fluid displacement potential and pressure as independent fields. This principle contains a free parameter alpha. Semidiscrete finite-element equations of motion based on this principle are displayed and applied to the transient response and free-vibrations of the coupled fluid-structure problem. It is shown that a particular setting of alpha yields a rich set of formulations that can be customized to fit physical and computational requirements. The variational principle is then extended to handle slosh motions in a uniform gravity field, and used to derived semidiscrete equations of motion that account for such effects.

Felippa, C. A.↗

NASA NTRS1990

Surface-crack growth: Models, experiments, and structures; Proceedings of the Symposium, Sparks, NV, Apr. 25, 1988

The present volume on surface-crack growth modeling, experimental methods, and structures, discusses elastoplastic behavior, the fracture analysis of three-dimensional bodies with surface cracks, optical measurements of free-surface effects on natural surfaces and through cracks, an optical and finite-element investigation of a plastically deformed surface flaw under tension, fracture behavior prediction for rapidly loaded surface-cracked specimens, and surface cracks in thick laminated fiber composite plates. Also discussed are a novel study procedure for crack initiation and growth in thermal fatigue testing, the growth of surface cracks under fatigue and monotonically increasing load, the subcritical growth of a surface flaw, surface crack propagation in notched and unnotched rods, and theoretical and experimental analyses of surface cracks in weldments.

Reuter, Walter G.↗

NASA NTRS1989

On the accuracy of shape sensitivity derivatives

Six methods for calculating shape sensitivity derivatives are compared for a two-material beam problem with a moving interface. It is found that as the finite-element mesh is refined, displacement sensitivity derivatives converge more slowly than the displacement themselves. Five of the methods agree fairly well, but the adjoint variational surface method provides substantially different results. However, the difference is found to reflect convergence from another direction to the same answer, rather than reduced accuracy.

Haftka, Raphael T.↗

NASA NTRS1990

Ply-level failure analysis of a graphite/epoxy laminate under bearing-bypass loading

A combined experimental and analytical study was conducted to investigate and predict the failure modes of a graphite/epoxy laminate subjected to combined bearing and bypass loading. Tests were conducted in a test machine that allowed the bearing-bypass load ratio to be controlled while a single-fastener coupon was loaded to failure in either tension or compression. Onset and ultimate failure modes and strengths were determined for each test case. The damage-onset modes were studied in detail by sectioning and micrographing the damaged specimens. A two-dimensional, finite-element analysis was conducted to determine lamina strains around the bolt hole. Damage onset consisted of matrix cracks, delamination, and fiber failures. Stiffness loss appeared to be caused by fiber failures rather than by matrix cracking and delamination. An unusual offset-compression mode was observed for compressive bearing-bypass loading in which the specimen failed across its width along a line offset from the hole. The computed lamina strains in the fiber direction were used in a combined analytical and experimental approach to predict bearing-bypass diagrams for damage onset from a few simple tests.

Naik, R. A.↗

NASA NTRS1989

Vibration damping and robust control of the JPL/AFAL experiment using mu-synthesis

The technology for controlling elastic deformations of flexible structures is one of the key considerations for future space initiatives. A vital area needed to achieve this objective is the development of a control design methodology applicable to future structures. The mu-synthesis technique is employed to design a high-performance vibration attenuation controller for the JPL/AFAL experimental flexible antenna structure. The results presented deal primarily with the control of first two global flexible modes using only two hub actuators and two hub sensors. Implementation of the multivariable control laws based on a finite-element model is presented. All results are from actual implementation on the JPL/AFAL flexible structure testbed.

Balas, Gary J.↗