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At least 505 records · Page 28

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NASA NTRS1974

Stress, stability and vibration of complex, branched shells of revolution

A comprehensive computer program, designated BOSOR4, for analysis of the stress, stability, and vibration of segmented, ring-stiffened, branched shells of revolution and prismatic shells and panels is described. The program performs large-deflection axisymmetric stress analysis, small-deflection nonsymmetric stress analysis, modal vibration analysis with axisymmetric nonlinear prestress included, and buckling analysis with axisymmetric or nonsymmetric prestress. One of the main advantages of the code is the provision for realistic engineering details such as eccentric load paths, internal supports, arbitrary branching conditions, and a 'library' of wall constructions. The program is based on the finite-difference energy method, which is very rapidly convergent with increasing numbers of mesh points. The organization of the program is briefly described with the flow of calculations charted for each of the types of analysis. Overlay charts and core storage requirements are given for the CDC 6600, IBM 370/165, and UNIVAC 1108 versions of BOSOR4.

Bushnell, D.↗

NASA NTRS1994

Nonlinear analysis of damaged stiffened fuselage shells subjected to combined loads

The results of an analytical study of the nonlinear response of stiffened fuselage shells with long cracks are presented. The shells are modeled with a hierarchical modeling strategy that accounts for global and local response phenomena accurately. Results are presented for internal pressure and mechanical bending loads. The effects of crack location and orientation on shell response are described. The effects of mechanical fasteners on the response of a lap joint and the effects of elastic and elastic-plastic material properties on the buckling response of tension-loaded flat panels with cracks are also addressed.

Starnes, James H., Jr.↗

NASA NTRS2001

Processing and Testing of Thermoplastic Composite Cylindrical Shells Fabricated by Automated Fiber Placement

Two 61-cm-diameter eight-ply quasi-isotropic IM7/PEEK cylindrical shells were fabricated by automated fiber placement the NASA Langley Research Center using only infrared radiant heat to preheat the substrate and incoming composite uni-tape. The shells were characterized by ultrasonic c-scans for overall consolidation quality, and by optical microscopy and acid digestion for void content. Compression tests were also performed. Although the material used in the study was of generally poor quality due to numerous splits and dry fiber regions, the process was able to achieve a net reduction in void content in the as-placed component. Microscopy of the composite shells revealed well-consolidated, void-free interfaces. The two cylinders were then tested in uni-axial compression in a 1334 kN-capacity hydraulic test machine until buckling occurred. A geometrically nonlinear finite element analysis was conducted, and the differences between the predicted and measured values were 18.0 and 25.8%, respectively. Inclusion of measured imperfections of the cylinder into the analysis is expected to reduce these differences.

Hulcher, Anthony Bruce↗

NASA NTRS1992

The effect of initial imperfections on shell stability: An updated review

The development of the Delft Interactive Shell Design Code (DISDECO) is described. The purpose is to make the accumulated theoretical, numerical, and practical knowledge of the last years readily accessible to users interested in the analysis of buckling sensitive structures. With this open ended, hierarchical, interactive computer code, the user can access workstation programs of increasing complexity. As a first update of DISDECO, the computational modules of the initial level were modified so as to handle combined loads consisting of axial compression, internal and external pressure, and clockwise or counter clockwise torque. The applied load is assumed to have a uniform spatial distribution and is divided into a fixed and a variable part. The magnitude of the variable part is allowed to vary in proportion to a load parameter Lambda. This leads to an eigenvalue problem for the critical lambda sub C.

Arbocz, J.↗

NASA NTRS1998

Compression Strength of Composite Primary Structural Components

Research conducted under NASA Grant NAG-1-537 focussed on the response and failure of advanced composite material structures for application to aircraft. Both experimental and analytical methods were utilized to study the fundamental mechanics of the response and failure of selected structural components subjected to quasi-static loads. Most of the structural components studied were thin-walled elements subject to compression, such that they exhibited buckling and postbuckling responses prior to catastrophic failure. Consequently, the analyses were geometrically nonlinear. Structural components studied were dropped-ply laminated plates, stiffener crippling, pressure pillowing of orthogonally stiffened cylindrical shells, axisymmetric response of pressure domes, and the static crush of semi-circular frames. Failure of these components motivated analytical studies on an interlaminar stress postprocessor for plate and shell finite element computer codes, and global/local modeling strategies in finite element modeling. These activities are summarized in the following section. References to literature published under the grant are listed on pages 5 to 10 by a letter followed by a number under the categories of journal publications, conference publications, presentations, and reports. These references are indicated in the text by their letter and number as a superscript.

Johnson, Eric R.↗

NASA NTRS1991

A finite element program for postbuckling calculations (PSTBKL)

The object of the research reported herein was to develop a general mathematical model and solution methodologies for analyzing the structural response of thin, metallic shell structures under large transient, cyclic, or static thermochemical loads. This report describes the computer program resulting from the research. Among the system responses associated with these loads and conditions are thermal buckling, creep buckling, and ratcheting. Thus geometric and material nonlinearities (of high order) have been anticipated and are considered in developing the mathematical model. The methodology is demonstrated through different problems of extension, shear, and of planar curved beams. Moreover, importance of the inclusion of large strains is clearly demonstrated, through the chosen applications.

Simitses, G. T.↗

NASA NTRS2006

Analysis of Composite Panel-Stiffener Debonding Using a Shell/3D Modeling Technique

Interlaminar fracture mechanics has proven useful for characterizing the onset of delaminations in composites and has been used with limited success primarily to investigate onset in fracture toughness specimens and laboratory size coupon type specimens. Future acceptance of the methodology by industry and certification authorities however, requires the successful demonstration of the methodology on structural level. For this purpose a panel was selected that was reinforced with stringers. Shear loading cases the panel to buckle and the resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect. For finite element analysis, the panel and surrounding load fixture were modeled with shell element. A small section of the stringer foot and the panel in the vicinity of the embedded defect were modeled with a local 3D solid model. A failure index was calculated by correlating computed mixed-mode failure criterion of the graphite/epoxy material.

Krueger, Ronald↗

NASA NTRS1971

An experimental study of the strength and stability of thin monocoque shells with reinforced and unreinforced rectangular cutouts

Axial compression tests were run on eleven thin-walled aluminum cylinders having rectangular cutouts. Various types of reinforcement were used around the cutouts, and some tests were run with no reinforcement. The test results are compared with the cylinder buckling loads prior to installation of the cutouts (obtained without damaging the cylinder by using a buckle-capture technique), and correlated with computer-predicted failure loads. The latter were based on the use of the STAGS computer program. For thin cylinders such as these, the test and computer-based analysis shows that for small to moderate size cutouts, reinforcement of the cutout is of no benefit unless the cylinder is of extremely high (geometrical) quality. For cylinder quality and cutout size where reinforcement is beneficial, the relative merits of the various reinforcement configurations are discussed, and an empirical basis for design is proposed.

Holmes, A. M. C.↗

NASA NTRS1973

Computer analysis of ring stiffened shells of revolution

The equations and method of solution for a series of five compatible computer programs for structural analysis of axisymmetric shell structures are presented. These programs, designated as the SRA programs, apply to a common structural model but analyze different modes of structural response. They are: (1) linear asymmetric static response (SRA 100), (2) buckling of linearized asymmetric equilibrium states (SRA 101), (3) nonlinear axisymmetric static response (SRA 200), (4) buckling of nonlinear axisymmetric equilibrium states(SRA 201), and (5) vibrations about nonlinear axisymmetric equilibrium state (SRA 300).

Cohen, G. A.↗

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 NTRS1977

Stability analysis of a reinforced carbon carbon shell

This paper presents the development of a stability analysis for the nose cap of the NASA Space Shuttle Orbiter. Stability is evaluated by the differential stiffness analysis of the NASTRAN finite-element computer code, addressing those nonstandard characteristics in the nose cap such as nonuniform curvature, asymmetrical and nonuniform loads, support fixity, and various combinations of membrane and bending stresses. A full-sized nose cap, thinner than production, was statically tested and stability analyzed. The failing load level correlated to within 30%. The region and mode of buckling that occurred during test was accurately predicted by analysis. The criterion for predicting instability is based on the behavior of the nonlinear deflections. The deflections are nonlinear elastic in that the stresses are well within the elastic range of the material, but the geometry-load relationship produces nonlinear deflections. The load-deflection relationship is well defined by differential stiffness analysis up to the zero-slope portion of the curve, the point of neutral stability or where the shell 'snaps through' just prior to general instability.

Agan, W. E.↗

NASA NTRS2011

Compression Behavior of Fluted-Core Composite Panels

In recent years, fiber-reinforced composites have become more accepted for aerospace applications. Specifically, during NASA s recent efforts to develop new launch vehicles, composite materials were considered and baselined for a number of structures. Because of mass and stiffness requirements, sandwich composites are often selected for many applications. However, there are a number of manufacturing and in-service concerns associated with traditional honeycomb-core sandwich composites that in certain instances may be alleviated through the use of other core materials or construction methods. Fluted-core, which consists of integral angled web members with structural radius fillers spaced between laminate face sheets, is one such construction alternative and is considered herein. Two different fluted-core designs were considered: a subscale design and a full-scale design sized for a heavy-lift-launch-vehicle interstage. In particular, axial compression of fluted-core composites was evaluated with experiments and finite-element analyses (FEA); axial compression is the primary loading condition in dry launch-vehicle barrel sections. Detailed finite-element models were developed to represent all components of the fluted-core construction, and geometrically nonlinear analyses were conducted to predict both buckling and material failures. Good agreement was obtained between test data and analyses, for both local buckling and ultimate material failure. Though the local buckling events are not catastrophic, the resulting deformations contribute to material failures. Consequently, an important observation is that the material failure loads and modes would not be captured by either linear analyses or nonlinear smeared-shell analyses. Compression-after-impact (CAI) performance of fluted core composites was also investigated by experimentally testing samples impacted with 6 ft.-lb. impact energies. It was found that such impacts reduced the ultimate load carrying capability by approximately 40% on the subscale test articles and by less than 20% on the full-scale test articles. Nondestructive inspection of the damage zones indicated that the detectable damage was limited to no more than one flute on either side of any given impact. More study is needed, but this may indicate that an inherent damage-arrest capability of fluted core could provide benefits over traditional sandwich designs in certain weight-critical applications.

Schultz, Marc R.↗

NASA NTRSDate not supplied

Characterization and Modeling of Cell Wall Imperfections in Aluminum Honeycomb Cores using X-ray CT Imaging

Honeycomb cell imperfections may significantly affect the failure of sandwich composites in core crushing and shear modes. Identification and quantification of the different types of imperfections and evaluation of their effects on the mechanical response of honeycomb cores in flatwise compression is presented. X-Ray computed tomography (CT) scans of honeycomb cores in co-cured composite sandwich panels were performed. Image analysis methods are applied to characterize and quantify core cell geometry and cell wall waviness. An approach is developed for extracting cell wall mid surface planes and waviness for constructing finite element models using shell elements. Analysis results from models with the as-manufactured imperfections are compared to results from models with idealized geometries to evaluate the effect of the as-manufactured imperfections on the initiation of cell wall buckling and subsequent core crushing.

aluminum honeycomb↗

NASA NTRS1998

Collapse of Composite Cylinders in Bending

This paper summarizes the results of a numerical and experimental study of the collapse behavior of small-scale graphite-epoxy cylindrical shells subjected to overall bending loads, and in one case, an initial internal pressure. Shells with quasi-isotropic and orthotropic inplane stiffness properties are studied. Numerical results from geometrically nonlinear finite element analyses and results from experiments using a specially-built apparatus indicate that extensive stable postbuckling responses occur. Orthotropy influences the buckling values and the extent to which the bending moment decreases after buckling. Material damage is observed to initiate in the vicinity of the nodal lines of the postbuckled deflection patterns. Numerical results indicate that the magnitudes of the shear stress resultants are greatest in these nodal regions. Failure of the internally pressurized cylinder is catastrophic.

Fuchs, Hannes P.↗

NASA NTRS1972

NERVA nozzle design status report

The results of the design analyses are presented along with the status of the attained design maturity of the structural elements of the nozzle jacket and various aspects of the coolant passages. The design analyses relating to the nozzle shell were based on design allowables as supported by cursory values obtained from ARMCO 22-13-5 nozzle forgings. The major aspects of the coolant passages considered include: low cycle thermal fatigue, ability to operate at 4500 R gas temperature, tube buckling, and susceptibility to erosion. The scope of the analysis is limited to processes leading to reliability assessments of failure mechanisms.

Williams, J. J.↗

NASA NTRS1993

Probabilistic assessment of composite structures

A methodology and attendant computer code were developed and are used to computationally simulate the uncertain behavior of composite structures. The uncertain behavior includes buckling loads, stress concentration factors, displacements, stress/strain, etc., which are the consequences of the inherent uncertainties (scatter) in the primitive (independent random) variables (constituent, ply, laminate, and structural) that describe the composite structures. The computer code is IPACS (Integrated Probabilistic Assessment of Composite Structures). IPACS can simulate both composite mechanics and composite structural behavior. Application to probabilistic composite mechanics is illustrated by its use to evaluate the uncertainties in the major Poisson's ratio and in laminate stiffness and strength. IPACS' application to probabilistic structural analysis is illustrated by its used to evaluate the uncertainties in the buckling of a composite plate, the stress concentration factor in a composite panel, and the vertical displacement and ply stress in a composite aircraft wing segment. IPACS' application to probabilistic design is illustrated by its use to assess the thin composite shell (pipe).

Chamis, C. C.↗

NASA NTRS1972

Delta launch vehicle isogrid structure NASTRAN analysis

Structural analysis applications of NASTRAN to the isogrid structure of the Delta launch vehicle are discussed. Isogrid is a method employed to stiffen the booster cylindrical shell surface by integrally machining ribs and skin from one piece of metal. The ribs are arranged in a repetitive equilateral triangular pattern; this results in a structural surface whose stiffness is orthogonally isotopic. For that structure, both static and general instability buckling analyses are described and the theoretical results are compared to test data. One of the parameters included in the analysis is the effect of pressure loadings on the buckling allowable load. In addition, a description of a supplementary computer program is presented.

Knighton, D. J.↗

NASA NTRS1983

Buckling of imperfect periodic lattice structures

A simplified buckling analysis is presented for a family of periodic lattice structures such as those proposed for large space structures. A transcendental 6 x 6 matrix of eigenvalues is shown to be sufficient for modeling buckling behavior because member stiffnesses are based on an exact solution of the beam-column equation. Exact stiffnesses are derived for a curved member, thus allowing modeling of imperfect lattice structures. Comparisons of predictions of the lattice model with those available from shell and beam theory underscore the inaccuracies introduced by treating the lattice structure as a continuum. Sample calculations are provided for an isogrid cylinder and a three element double-laced truss.

Anderson, M. S.↗