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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Optimal design of geodesically stiffened composite cylindrical shells

An optimization system based on the finite element code Computations Structural Mechanics (CSM) Testbed and the optimization program, Automated Design Synthesis (ADS), is described. The optimization system can be used to obtain minimum-weight designs of composite stiffened structures. Ply thickness, ply orientations, and stiffener heights can be used as design variables. Buckling, displacement, and material failure constraints can be imposed on the design. The system is used to conduct a design study of geodesically stiffened shells. For comparison purposes, optimal designs of unstiffened shells and shells stiffened by rings and stingers are also obtained. Trends in the design of geodesically stiffened shells are identified. An approach to include local stress concentrations during the design optimization process is then presented. The method is based on a global/local analysis technique. It employs spline interpolation functions to determine displacements and rotations from a global model which are used as 'boundary conditions' for the local model. The organization of the strategy in the context of an optimization process is described. The method is validated with an example.

Gendron, G.↗
NASA NTRS1990

Nonlinear shell analysis of the Space Shuttle Solid Rocket Boosters

A variety of structural analyses have been performed on the Solid Rocket Boosters (SRB's) to provide information that would contribute to the understanding of the failure which destroyed the Space Shuttle Challenger. This paper describes nonlinear shell analyses that were performed to characterize the behavior of an overall SRB structure and a segment of the SRB in the vicinity of the External Tank Attachment (ETA) ring. Shell finite element models were used that would accurately reflect the global load transfer in an SRB in a manner such that nonlinear shell collapse and ovalization could be assessed. The purpose of these analyses was to calculate the overall deflection and stress distributions for these SRB models when subjected to mechanical loads corresponding to critical times during the launch sequence. Static analyses of these SRB models were performed using a 'snapshot picture' of the loads. Analytical results obtained using these models show no evidence of nonlinear shell collapse for the pre-liftoff loading cases considered.

Knight, N. F., Jr.↗
NASA NTRS1992

Detection of Ca II absorption in the QSO 0958 + 559 due to a Heiles shell in NGC 3079

Optical observations of the QSO 0958 + 559 and the nearby, edge-on spiral galaxy NGC 3079 are presented. Spectra of the QSO show Ca II absorption lines due to extended gas in NGC 3079; the Na absorption lines due to NGC 3079 are not detected. An unrelated higher redshift absorption system at 1.1327 is also present in the QSO spectrum. The QSO sightline shines directly through a Heiles shell. The coincidence in position coupled with the agreement between the Ca II absorption and shell emission velocities demonstrates that the QSO absorption system is due to gas in this shell. This system is discussed in comparison with the six other low-redshift QSO-galaxy pair absorption systems, and the influence of Heiles shells on the interpretation of higher redshift QSO absorption systems is addressed.

Womble, Donna S.↗
NASA NTRS1992

Exact solutions for laminated composite cylindrical shells in cylindrical bending

Analytic elasticity solutions for laminated composite cylindrical shells under cylindrical bending are presented. The material of the shell is assumed to be general cylindrically anisotropic. Based on the theory of cylindrical anisotropic elasticity, coupled governing partial differential equations are developed. The general expressions for the stresses and displacements in the laminated composite cylinders are discussed. The closed form solutions based on Classical Shell Theory (CST) and Donnell's (1933) theory are also derived for comparison purposes. Three examples illustrate the effect of radius-to-thickness ratio, coupling and stacking sequence. The results show that, in general, CST yields poor stress and displacement distributions for thick-section composite shells, but converges to the exact elasticity solution as the radius-to-thickness ratio increases. It is also shown that Donnell's theory significantly underestimates the stress and displacement response.

Yuan, F. G.↗
NASA NTRS1993

The structure of circumstellar shells

This document provides a report on research activities carried out with the support of NASA grant NAG 5-1174, the Structure of Circumstellar Shells, funded under the Astrophysics Data Program. The research carried out with the support of this grant is a study of the properties of circumstellar dust shells for which spectra are available through IRAS low resolution spectrometry (LRS). This research consisted of the development and application of models of axisymmetric circumstellar shells and a preliminary survey of the applicability of neural nets for analysis of the IRAS LRS spectra of circumstellar dust shells.

Fix, John D.↗
NASA NTRS1947

A Simplified Method of Elastic-Stability Analysis for Thin Cylindrical Shells

This paper develops a new method for determining the buckling stresses of cylindrical shells under various loading conditions. In part I, the equation for the equilibrium of cylindrical shells introduced by Donnell in NACA 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. In part II, a modified form of Donnell's equation for the equilibrium of thin cylindrical shells is derived which is equivalent to Donnell's equation but has certain advantages in physical interpretation and in ease of solution, particularly in the case of shells having clamped edges. The question of implicit boundary conditions is also considered.

Batdorf, S B↗
NASA NTRS1995

Sensitivity analysis for the dynamic response of thermoviscoplastic shells of revolution

A computational procedure is presented for evaluating the sensitivity coefficients of the dynamic axisymmetric, fully-coupled, thermoviscoplastic response of shells of revolution. The analytical formulation is based on Reissner's large deformation shell theory with the effects of large-strain, transverse shear deformation, rotatory inertia and moments turning around the normal to the middle surface included. The material model is chosen to be viscoplasticity with strain hardening and thermal hardening, and an associated flow rule is used with a von Mises effective stress. A mixed formulation is used for the shell equations with the fundamental unknowns consisting of six stress resultants, three generalized displacements and three velocity components. The energy-balance equation is solved using a Galerkin procedure, with the temperature as the fundamental unknown. Spatial discretization is performed in one dimension (meridional direction) for the momentum and constitutive equations of the shell, and in two dimensions (meridional and thickness directions) for the energy-balance equation. The temporal integration is performed by using an explicit central difference scheme (leap-frog method) for the momentum equation; a predictor-corrector version of the trapezoidal rule is used for the energy-balance equation; and an explicit scheme consistent with the central difference method is used to integrate the constitutive equations. The sensitivity coefficients are evaluated by using a direct differentiation approach. Numerical results are presented for a spherical cap subjected to step loading.

Kulkarni, Makarand↗
NASA NTRS1995

A rule-based shell to hierarchically organize HST observations

An observing program on the Hubble Space Telescope (HST) is described in terms of exposures that are obtained by one or more of the instruments onboard the HST. These exposures are organized into a hierarchy of structures for purposes of efficient scheduling of observations. The process by which exposures get organized into the higher-level structures is called merging. This process relies on rules to determine which observations can be 'merged' into the same higher level structure, and which cannot. The TRANSformation expert system converts proposals for astronomical observations with HST into detailed observing plans. The conversion process includes the task of merging. Within TRANS, we have implemented a declarative shell to facilitate merging. This shell offers the following features: (1) an easy way of specifying rules on when to merge and when not to merge, (2) a straightforward priority mechanism for resolving conflicts among rules, (3) an explanation facility for recording the merging history, (4) a report generating mechanism to help users understand the reasons for merging, and (5) a self-documenting mechanism that documents all the merging rules that have been defined in the shell, ordered by priority. The merging shell is implemented using an object-oriented paradigm in CLOS. It has been a part of operational TRANS (after extensive testing) since July 1993. It has fulfilled all performance expectations, and has considerably simplified the process of implementing new or changed requirements for merging. The users are pleased with its report-generating and self-documenting features.

Bose, Ashim↗
NASA NTRS1994

Theoretical spectra of circumstellar dust shells around carbon-rich asymptotic giant branch stars

Realistic modeling of circumstellar dust shells around evolved stars has to include a physical treatment of the interaction among hydrodynamics, thermodynamics, radiative transfer, chemistry and dust formation and -growth. A self-consistent solution of this problem is presented in the case of stationary, spherical symmetric dust-driven winds. The resulting shell structure and the mass-loss rate are completely determined by the three fundamental stellar parameters stellar mass M(stellar), stellar luminosity L(stellar) and effective temperature T(sub eff) and by the element abundances epsilon(sub i). A detailed calculation of the transport coefficients of the dust component by means of the particle size distribution function and the solution of the non-grey radiative transfer problem provide realistic synthetic spectra of the dust shell models. We discuss the dependence of the resulting spectra on the stellar parameters in terms of infrared two color diagrams. Application of these model calculations to the prominent infrared object IRC +10216 yields best agreement with the observed spectrum and the visibility data at maximum light for the stellar parameters M(stellar) = 0.7 solar mass, L(stellar) = 2.4 x 10(exp 4) solar luminosity, T(stellar) = 2010K and a carbon to oxygen ratio of epsilon(sub c)/epsilon(sub o) = 1.40, which corresponds to a mass-loss rate of M-dot = 8 x 10(exp -5) solar mass/yr. In this model only amorphous carbon grains are considered as the main opacity source. From this model a distance to IRC +10216 of d = 170pc is deduced. The total mass contained in the circumstellar dust shell implies and initial main sequence mass of M(sub ZAMS) greater than or = 1.3 solar mass for IRC +10216.

Winters, J. M.↗
NASA NTRS1996

Coupled mixed-field laminate theory and finite element for smart piezoelectric composite shell structures

Mechanics for the analysis of laminated composite shells with piezoelectric actuators and sensors are presented. A new mixed-field laminate theory for piezoelectric shells is formulated in curvilinear coordinates which combines single-layer assumptions for the displacements and a layerwise representation for the electric potential. The resultant coupled governing equations for curvilinear piezoelectric laminates are described. Structural mechanics are subsequently developed and an 8-node finite-element is formulated for the static and dynamic analysis of adaptive composite structures of general laminations containing piezoelectric layers. Evaluations of the method and comparisons with reported results are presented for laminated piezoelectric-composite plates, a closed cylindrical shell with a continuous piezoceramic layer and a laminated composite semi-circular cantilever shell with discrete cylindrical piezoelectric actuators and/or sensors.

Saravanos, Dimitris A.↗
NASA NTRS1995

Effects of Stiffening and Mechanical Load on Thermal Buckling of Stiffened Cylindrical Shells

A study of thermal buckling of stiffened cylindrical shells with the proportions of a preliminary supersonic transport fuselage design (1970) is presented. The buckling analysis is performed using an axisymmetric shell-of-revolution code, BOSOR4. The effects of combined mechanical (axial loading) and thermal loading (heated skins) are investigated. Results indicate that the location of longitudinal eccentric stiffening has a very large effect on the thermal buckling strength of longitudinally stiffened shells, and on longitudinally stiffened shells with rings.

Johnson, Theodore F.↗
NASA NTRS1996

The Beta Pictoris Phenomenon in A-Shell Stars: Detection of Accreting Gas

We present the results of an expanded survey of A-shell stars using IUE high-dispersion spectra and find accreting, circumstellar gas in the line of sight to nine stars, in addition to the previously identified beta Pic, HR 10, and 131 Tau, which can be followed to between +70 and 100 km/s relative to the star. Two of the program stars, HD 88195 and HD 148283, show variable high-velocity gas. Given the small number of IUE spectra for our program stars, detection of high-velocity, accreting gas in 2/3 of the A-shell stars sampled indicates that accretion is an intrinsic part of the A-shell phenomenon and that beta Pic is not unique among main-sequence A stars in exhibiting such activity. Our program stars, as a group, have smaller column densities of high-velocity gas and smaller near-IR excesses compared with beta Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with beta Pic, and suggest that the majority of field A-shell stars are older than beta Pic.

Grady, C. A.↗
NASA NTRS1998

Development of Paper Products from Dried Sweetpotato Stems and Peanut Shells

One of the goals of NASA's Advanced Life Support Program (ALS) for sustaining human life in space is to achieve a closed system in plant production and usage. That all inedible plant parts should be recycled or used in some way. A Tuskegee University team researching sweetpotato and peanut for ALS has developed paper products from dried sweet-potato stems and peanut shells. In this study, the sweet-potato stems and peanut shells were soaked separately in water for 48 hours. After 48 hours, researchers manually separated the pulp and the unusable parts. To form the paper, 160 g of pulp and water mixture was poured through a 15.1 cm (diameter) filtration funnel and the pulp was trapped on 15 cm (diameter) filter paper. The filter paper and pulp were dried in an air oven, and the filter paper was removed, An examination under a scanning electron microscope showed that the sweet-potato paper was composed of "fibers", whereas the peanut shell paper was composed of "blocks". Results of physical testing showed that the sweet-potato stem paper was stronger than the peanut shell paper. It is anticipated that there may be other uses of these products such as writing paper, bags and packaging material. Because of its biodegradability, it can be incorporated into the resource recycling system at the end of its use.

R McConnell↗
NASA NTRS2001

The Nonlinear Response of Cracked Aluminum Shells Subjected to Combined Loads

The results of a numerical study of the nonlinear response of thin unstiffened aluminum cylindrical shells with a longitudinal crack are presented. The shells are analyzed with a nonlinear shelf analysis code that accurately accounts for global and structural response phenomena. The effects of initial crack length on the prebuckling, buckling and postbuckling responses of a typical shell subjected to axial compression loads, and subjected to combined internal pressure and axial compression loads are described. Both elastic and elastic-plastic analyses are conducted. Numerical results for a fixed initial crack length indicate that the buckling load decreases as the crack length increases for a given pressure load, and that the buckling load increases as the internal pressure load increases for a given crack length. Furthermore, results indicate that predictions from an elastic analysis for the initial buckling load of a cracked shell subjected to combined axial compression and internal pressure loads can be unconservative. In addition, the effect of crack extension on the initial buckling load is presented.

Rose, Cheryl A.↗
NASA NTRS2002

Applicability of the Continuum-Shell Theories to the Mechanics of Carbon Nanotubes

Validity of the assumptions relating the applicability of continuum shell theories to the global mechanical behavior of carbon nanotubes is examined. The present study focuses on providing a basis that can be used to qualitatively assess the appropriateness of continuum-shell models for nanotubes. To address the effect of nanotube structure on their deformation, all nanotube geometries are divided into four major classes that require distinct models. Criteria for the applicability of continuum models are presented. The key parameters that control the buckling strains and deformation modes of these classes of nanotubes are determined. In an analogy with continuum mechanics, mechanical laws of geometric similitude are presented. A parametric map is constructed for a variety of nanotube geometries as a guide for the applicability of different models. The continuum assumptions made in representing a nanotube as a homogeneous thin shell are analyzed to identify possible limitations of applying shell theories and using their bifurcation-buckling equations at the nano-scale.

Harik, V. M.↗
NASA NTRS1958

On the Flutter of Cylindrical Shells and Panels Moving in a Flow of Gas

The equations of shells are taken in the form of the general technical theory of shallow shells and shells of medium length. The aerodynamic forces acting on a shell are taken into account only as forces of excess pressure according to the formula proposed by A.A. Iliushin in reference 3.

Stepanov, R. D.↗
NASA NTRS2004

Brine Pockets in the Icy Shell on Europa: Distribution, Chemistry, and Habitability

On Earth, sea ice is rich in brine, salt, and gas inclusions that form through capturing of seawater during ice formation. Cooling of the ice over time leads to sequential freezing of captured sea-water, precipitation of salts, exsolution of gases, and formation of brine channels and pockets. Distribution and composition of brines in sea ice depend on the rate of ice formation, vertical temperature gradient, and the age of the ice. With aging, the abundance of brine pockets decreases through downward migration. De- spite low temperatures and elevated salinities, brines in sea ice provide a habitat for photosynthetic and chemosynthetic organisms. On Europa, brine pockets and channels could exist in the icy shell that may be from a few km to a few tens of km thick and is probably underlain by a water ocean. If the icy shell is relatively thick, convection could develop, affecting the temperature pattern in the ice. To predict the distribution and chemistry of brine pockets in the icy shell we have combined numerical models of the temperature distribution within a convecting shell, a model for oceanic chemistry, and a model for freezing of Europan oceanic water. Possible effects of brine and gas inclusions on ice rheology and tectonics are discussed.

Zolotov, M. Yu↗
NASA NTRS1989

Theoretical studies of the infrared emission from circumstellar dust shells: the infrared characteristics of circumstellar silicates and the mass-loss rate of oxygen-rich late-type giants

We have modeled the infrared emission of spherically symmetric, circumstellar dust shells with the aim of deriving the infrared absorption properties of circumstellar silicate grains and the mass-loss rates of the central stars. As a basis for our numerical studies, a simple semianalytical formula has been derived that illustrates the essential characteristics of the infrared emission of such dust shells. A numerical radiative transfer program has been developed and applied to dust shells around oxygen-rich late-type giants. Free parameters in such models include the absorption properties and density distribution of the dust. An approximate, analytical expression is derived for the density distribution of circumstellar dust driven outward by radiation pressure from a central source. A large grid of models has been calculated to study the influence of the free parameters on the emergent spectrum. These results form the basis for a comparison with near-infrared observations. Observational studies have revealed a correlation between the near-infrared color temperature, Tc, and the strength of the 10 micrometers emission or absorption feature, A10. This relationship, which essentially measures the near-infrared optical depth in terms of the 10 micrometers optical depth, is discussed. Theoretical A10-Tc relations have been calculated and compared to the observations. The results show that this relation is a sensitive way to determine the ratio of the near-infrared to 10 micrometers absorption efficiency of circumstellar silicates. These results as well as previous studies show that the near-infrared absorption efficiency of circumstellar silicate grains is much higher than expected from terrestrial minerals. We suggest that this enhanced absorption is due to the presence of ferrous iron (Fe2+) color centers dissolved in the circumstellar silicates. By using the derived value for the ratio of the near-infrared to 10 micrometers absorption efficiency, the observed A10-Tc relation can be calibrated in terms of the total dust column density of the circumstellar shell and thus the mass-loss rate of late-type giants can easily be derived. Detailed models have been made of the infrared emission of three well-studied Miras: R Cas, IRC 10011, and OH 26.5+0.6, with the emphasis on the shape of the 10 micrometers emission or absorption feature. The results show that the intrinsic shape of the 10 micrometers resonance varies from a very broad feature in R Cas to a relatively narrower feature in OH 26.5+0.6, with IRC 10011 somewhere in between. Possible origins of this variation are discussed. The mass-loss rates from these objects are calculated to be 3 x 10(-7), 2 x 10(-5), and 2 x 10(-4) M Sun yr-1 for R Cas, IRC 10011, and OH 26.5+0.6, respectively. These results are compared to other determinations in the literature.

NASA Discipline Exobiology↗