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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

A three-node C deg element for analysis of laminated composite sandwich shells

A three-node flat shell element with C deg rotation fields has been developed for analysis of arbitrary composite shells. The element may consist of any number of orthotropic layers, each layer having different material properties and angular orientation. The formulation includes coupling between bending and extension, which is essential for analysis of unsymmetric laminates. Shearing deflections are included, since laminated and sandwich construction frequently results in shear stiffness much smaller than bending stiffness. Formulation of the element is straightforward, and calculation of its stiffness matrix is simple and fast. Convergence of solutions with mesh refinement is uniform for both thin and thick shells and is insensitive to element shape, although not as rapid as some other elements that lack one or more capabilities of the newly developed element. An experimental verification of the shall element is reported in the appendix.

Martin, C. Wayne↗
NASA NTRS1988

Stability of a spherical shell distribution of pickup ions

The stability of a spherical shell (complete or incomplete) is investigated. The study is motivated by the research of cometary pickup ions. The present discussion is restricted to low-frequency electromagnetic waves propagating along the ambient magnetic field. It is found that a complete shell is stable, whereas an incomplete shell can be unstable. The numerical results are reported and discussed.

Freund, H. P.↗
NASA NTRS1989

Axisymmetric shell analysis of the Space Shuttle solid rocket booster field joint

The Space Shuttle Challenger (STS 51-L) accident led to an intense investigation of the structural behavior of the solid rocket booster (SRB) tang and clevis field joints. The presence of structural deformations between the clevis inner leg and the tang, substantial enough to prevent the O-ring seals from eliminating hot gas flow through the joints, has emerged as a likely cause of the vehicle failure. This paper presents results of axisymmetric shell analyses that parametrically assess the structural behavior of SRB field joints subjected to quasi-steady-state internal pressure loading for both the original joint flown on mission STS 51-L and the redesigned joint recently flown on the Space Shuttle Discovery. Discussion of axisymmetric shell modeling issues and details is presented and a generic method for simulating contact between adjacent shells of revolution is described. Results are presented that identify the performance trends of the joints for a wide range of joint parameters.

Nemeth, Michael P.↗
NASA NTRS1989

Response of moderately thick laminated cross-ply composite shells subjected to random excitation

This study deals with the dynamic response of transverse shear deformable laminated shells subjected to random excitation. The analysis encompasses the following problems: (1) the dynamic response of circular cylindrical shells of finite length excited by an axisymmetric uniform ring loading, stationary in time, and (2) the response of spherical and cylindrical panels subjected to stationary random loadings with uniform spatial distribution. The associated equations governing the structural theory of shells are derived upon discarding the classical Love-Kirchhoff (L-K) assumptions. In this sense, the theory is formulated in the framework of the first-order transverse shear deformation theory (FSDT).

Elishakoff, Isaak↗
NASA NTRS1989

Sensitivity of buckling loads of anisotropic shells of revolution to geometric imperfections and design changes

Buckling load sensitivity calculations in the shell-of-revolution program FASOR are discussed. This development is based on Koiter's initial postbuckling theory, which has been generalized to include the effect of stiffness changes, as well as geometric imperfections. The implementation in FASOR is valid for anisotropic, as well as orthotropic, shells. Examples are presented for cylindrical panels under axial compression, complete cylindrical shells in torsion, and antisymmetric angle-ply cylindrical panels under edge shear.

Cohen, Gerald A.↗
NASA NTRS1989

Superclustering in the explosion scenario. II - Prolate spheroidal shells from superconducting cosmic strings

If galaxies form on shells, then clusters of galaxies should form at the vertices where three shells intersect. Weinberg, Ostriker, and Dekel (WOD, 1989) studied this picture quantitatively and found that an intersecting spherical shell model reproduces many of the properties of the observed distribution of galaxy clusters, but that too much superclustering is produced. In this paper, the WOD analysis is repeated with prolate spheroids that could be created by superconducting cosmic strings. It is found that most of the attractive features of the WOD model are maintained in the more general case and there is slight improvement in some aspects, but that the overall problem of excessive superclustering is not really alleviated.

Borden, David↗
NASA NTRS1989

Evolution of an unstable shell distribution of pickup cometary ions

Consideration is given to the shell distribution formed by pickup ions in the solar wind. Numerical simulations are used to study the linear and nonlinear development of the waves and the evolution of the initial shell distribution. The linear theory is generalized to include additional effects. It is shown that unstable hydromagnetic fluctuations give rise to diffusion in velocity space and thickening of the shell distribution.

Yoon, Peter H.↗
NASA NTRS1990

Spectrophotometry of the shell around AG Carinae

Spatially-resolved long-slit spectrophotometry are presented for two regions of the shell nebula around the P-Cygni variable star AG Carinae. The spectra cover the 3700-6800 A wavelength range. Emission-line diagnostics are used to derive extinction, electron temperatures, and densities for various positions in the nebula. The chemical abundances and ionization structure are calculated and compared with other types of planetary nebulae and shells around other luminous stars. It is found that the N/O and N/S ratios of Ag Car are high compared to solar neighborhood ISM values. The O/H depletion found for the AG Car shell approaches that found in the condensations of the Eta Car system.

Mitra, P. Mila↗
NASA NTRS1989

Shell nebulae around luminous evolved stars

Shell nebulae around luminous Population I Wolf-Rayet, Of, and P-Cygni stars are astrophysically interesting since they are indicators of pre-supernova mass loss and how such massive stars prepare their surrounding interstellar medium prior to explosion. Some twenty-odd such nebulae are known, for which detailed study of their morphological and spectroscopic characteristics have only begun in this decade. In this paper, some of these characteristics are reviewed in general, and new observations are reported. Emphasis has been placed on several 'prototype 'objects (NGC 7635, NGC 2359, NGC 6888, and the Eta Carinae condensations) to illustrate the varied massive-star mass-loss, the physics of their winds and shell ejecta, and related nucleosynthesis effects in the compositions of the winds and shells.

Dufour, Reginald J.↗
NASA NTRS1990

Kinetic hydromagnetic instabilities due to a spherical shell distribution of pickup ions

The stability of a spherical shell ion distribution function against low-frequency hydromagnetic waves is discussed by relaxing the assumption of parallel propagation which was imposed in the preceding discussions published in the literature. It is found that Alfven waves as well as magnetosonic waves with oblique angles of propagation can be excited by a shell distribution. In the present analysis, the emphasis is placed on the case in which the thickness of the spherical shell is small.

Wu, C. S.↗
NASA NTRS1990

The interpretation of solar system abundances at the N = 50 neutron shell

New data on CI chondrite abundances demonstrate a high degree of smoothness for the A = 75 - 100 mass range for odd A nuclei, except a single element peak at K ascribable to the s-process peak for the N = 50 neutron shell. Literature estimates of s-process abundances, Ns, permit a smooth Ns curve to be drawn; however the resultant 'non-s' abundance curve (nominally r-process) does not show a peak analogous to peaks associated with the N = 82 or 126 shells. Assuming the systematics of the r-process even and odd A abundance peaks at the N = 82 and 126 shells apply to N = 50, the odd A r-process peak for N = 50 can be obtained, which in turn permits a new calculation of Ns for odd A.

Burnett, D. S.↗
NASA NTRS1991

Axisymmetric shell analysis of the space shuttle solid rocket booster field joint

The Space Shuttle Challenger (STS 51-L) accident led to an intense investigation of the structural behavior of the solid rocket booster (SRB) tang and clevis field joints. Results are presented of axisymmetric shell analyses that parametrically assess the structural behavior of SRB field joints subjected to quasi-steady-state internal pressure loading for both the original joint flown on mission STS 51-L and the redesigned joint flown for the first time after the STS 51-L accident on the Space Shuttle Discovery. Discussion of axisymmetric shell modeling issues and details is presented and a generic method for simulating contact between adjacent shells of revolution is described. Results are presented that identify the performance trends of the joints for a wide range of joint parameters. An important finding is that the redesigned joint exhibits significantly smaller O-ring gap changes and much less sensitivity to joint clearances than the original joint. For a wide range of joint parameters, the result presented indicate that the redesigned joint provides a much better pressure seal than the original joint.

Nemeth, Michael P.↗
NASA NTRS1989

Using infrared spectral features to probe circumstellar dust shells around cool stars

IRAS observations of cool stars provide low resolution spectra in the mid-infrared and also give fluxes at four wavelength bands from which color-color diagrams are constructed. The later have been used to study the evolution of these stars: as an O-rich star evolves to become a C-rich star and its detached dust shell moves further away, its evolution can be tracked on a color-color diagram. A major factor in determining the position of either C-rich or O-rich stars on the 12-25-60 micron color-color diagram is the presence of spectral features in the mid-IR. O-rich stars show a 9.8 micron silicate feature, while C-rich stars have a SiC feature at 11.2 microns. IRAS observations indicate that the SiC feature is quite narrow and uniform in shape showing little variation from star to star. The full width at half maximum (FWHM) is 1.6 + or - 0.15 microns. On the other hand, the shape of the silicate feature varies widely among the O-rich stars, with a FWHM ranging from 2 to 3 microns. The characteristics of circumstellar dust shells should manifest themselves both in the flux spectrum and in the details of the spectral features. To provide a coherent interpretation for these IRAS observations, models were constructed (using a radiative transfer code) of dust shells around O-rich and C-rich stars. Realistic grain opacities were used which include spectral features of varying intrinsic widths (e.g., Gaussian features at 10 microns with half width at half maximum of 0.5 and 1.0 microns).

Egan, Michael P.↗
NASA NTRS1991

The optimum shape for a rigid rotating shell enclosing an isotropic spherical planetary mass

Analysis of the Dyson Sphere, an extremely advanced civilization's hypothetical construct entirely surrounding a star, shows that new stress inward to the star increases to maxima at the poles if the sphere is rotating. This is because the centrifugal force in the rotating frame of reference vanishes at zero rotational radius, which occurs at the poles. There is less of the centrifugal force at high latitudes than low to offset the star's gravity. A form is derived for a thin, rigid, rotating shell, surrounding a large pointlike mass and/or charge, which will experience the least possible net stress at every point upon it - a shape on which every point not on the shell's equator is as near as possible to being in orbit. In orbit, whose plane passes through the primary body's center of mass or of charge, F(grav), or Fg, is exactly opposite in direction to F(centrif), or Fc, and is equal in amount. At all points not on the equator, Fc will not entirely offset Fg, because of Fg's vector decomposition. However, both forces are always constrained to be equal in absolute amount everywhere on the shell, equator included. The derived shape, given by the figure of revolution around the x-axis of x = square root (y-1-72), will prove useful in large-scale space construction. Also, various engineering problems are discussed.

Covington, Tatiana↗
NASA NTRS1991

Fuselage shell and cavity response measurements on a DC-9 test section

A series of fuselage shell and cavity response measurements conducted on a DC-9 aircraft test section are described. The objectives of these measurements were to define the shell and cavity model characteristics of the fuselage, understand the structural-acoustic coupling characteristics of the fuselage, and measure the response of the fuselage to different types of acoustic and vibration excitation. The fuselage was excited with several combinations of acoustic and mechanical sources using interior and exterior loudspeakers and shakers, and the response to these inputs was measured with arrays of microphones and accelerometers. The data were analyzed to generate spatial plots of the shell acceleration and cabin acoustic pressure field, and corresponding acceleration and pressure wavenumber maps. Analysis and interpretation of the spatial plots and wavenumber maps provided the required information on modal characteristics, structural-acoustic coupling, and fuselage response.

Simpson, M. A.↗
NASA NTRS1991

Analysis of laminated composite shells under internal pressure

A closed form solution is presented that predicts the response of filament wound composite shells subjected to internal pressure. The material of the shell is assumed to be general cylindrically anisotropic. Based on the theory of cylindrical anisotropic elasticity coupled differential equations are developed using Lekhnitskii's stress function approach. Two composite systems, graphite/epoxy and glass/epoxy, are selected to demonstrate the influence of degree of material anisotropy and fiber orientations on the axial and induced twisting deformation. Detailed stress distributions of (45) off-axis unidirectional and (45/-45)s symmetric angle-ply fiber-reinforced laminated shells are shown to illustrate the effect of radius-to-thickness ratio and stacking sequence.

Yuan, F. G.↗
NASA NTRS1991

The circumstellar shell of SN 1987A

An hourglass-shaped circumstellar shell, with radius about 0.65 lt-yr at the equator and about 2.4 lt-yr at the poles, surrounds SN 1987A. The shell is a bubble blown by the wind of the blue giant progenitor of SN 1987A into an equatorially concentrated wind of a prior red giant stage. The bubble was ionized by the initial UV flash of the supernova, and the emission-line region is concentrated in a ring at the waist of the hourglass. In about 2002 AD the supernova envelope will strike the shell. The resulting young supernova remnant will become a luminous source of soft X-rays and broad optical and ultraviolet emission lines.

Luo, Ding↗
NASA NTRS1991

Nonlinear response of infinitely long circular cylindrical shells to subharmonic radial loads

The nonlinear response of infinitely long circular cylindrical shells (thin circular rings) in the presence of a two-to-one internal (autoparametric) resonance to a subharmonic excitation of order one-half of the higher mode is analyzed with the multiple-scale method. Four autonomous first-order ordinary differential equations are derived for the modulation of the amplitudes and phases of the interacting models. These modulation equations are used to determine the fixed points and their stability. The fixed points correspond to periodic oscillations of the shell, whereas the limit-cycle solutions of the modulation equations correspond to amplitude and phase-modulated oscillations of the shell. The force response curves exhibit saturation, jumps, and Hopf bifurcation. As excitation frequency changes, all limit cycles deform and lose stability through either pitchfork or cyclic-fold (saddle-node) bifurcations. Some of these saddle-node bifurcations cause a transition to chaos. The pitchfork bifurcations break the symmetry of the limit cycles.

Nayfeh, Ali H.↗