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At least 469 records · Page 26

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NASA NTRS1975

Experimentally reproduced textures and mineral chemistries of high-titanium mare basalts

Many of the textures, morphologies, and mineral chemistries of the high-titanium mare basalts have been experimentally duplicated using single-stage cooling histories. Lunar high-titanium mare basalts are modeled in a 1 m thick gravitationally differentiating flow based on cooling rates, thermal models, and modal olivine contents. The low-pressure equilibrium phase relations of a synthetic high-titanium basalt composition were investigated as a function of oxygen fugacity, and petrographic criteria are developed for the recognition of phenocrysts which were present in the liquid at the time of eruption.

Usselman, T. M.↗

NASA NTRS1978

An analysis of delamination in angle-ply fiber reinforced composites

An analytical investigation of the mechanics and failure modes of delamination initiated from a service-induced surface flaw in angle-ply fiber reinforced composites is presented. The analysis employs a hybrid-stress finite element method including a crack-tip singular element with its field variables expressed by Muskhelishvili's complex stress functions. Solutions are obtained for delaminated composites with various laminate parameters. The results elucidate unique and important characteristics of delamination crack-tip response and interlaminar stress transfer mechanisms. Of particular interest are the mixed-mode stress intensity factors associated with the delamination crack. The influence of ply orientation on KI and KII and their effects on subsequent crack extension are discussed.

Wang, S. S.↗

NASA NTRS1979

Energy Spectra of Cosmic Ray Nuclei to Above 100 Gev/nucleon

The chemical composition cosmic rays as a function of energy in the range of a few GeV/nucleon to some hundreds of GeV/nucleon for boron through iron are presented. The experiment combined an ionization spectrometer and a gas Cherenkov counter, which was flown on a balloon, to perform two different and independent energy measurements. The experimental apparatus is described in detail. The energy dependence of the cosmic ray escape length for boron and iron is reported and predicted changes in the energy dependence of the ratios of primary nuclei 0/C and iron/C+0 are discussed.

Simon, M.↗

NASA NTRS1980

Influence of excess diamine on properties of PMR polyimide resins and composites

By varying the stoichiometry of the reactants in the preparation of PMR polyimide resin, changes occur in molecular weight distribution which influence the rheological properties and thus the processability of the resin, as well as the mechanical properties of the composite. The influence of 1-10 percent molar excess MDA on the molecular weight distribution and rheological properties of an imidized PMR system were exposed. Molecular weight distribution is characterized by gel permeation chromatography of the imidized molding compound; shear viscosity is related to changes in average molecular weight. The thermo-oxidative stability at 600 F, glass transition temperature, flexural and interlaminar shear properties of PMR polyimide/Celion 6000 graphite fiber composites are compared as a function of the percent excess MDA in the monomer reactant mixture.

Hurwitz, F. I.↗

NASA NTRS1981

Strain rate, temperature, and humidity on strength and moduli of a graphite/epoxy composite

Results of an experimental study of the influence of strain rate, temperature and humidity on the mechanical behavior of a graphite/epoxy fiber composite are presented. Three principal strengths (longitudinal, transverse and shear) and four basic moduli (E1, E2, G12 and U12) of a unidirectional graphite/epoxy composite were followed as a function of strain rate, temperature and humidity. Each test was performed at a constant tensile strain rate in an environmental chamber providing simultaneous temperature and humidity control. Prior to testing, specimens were given a moisture preconditioning treatment at 60 C. Values for the matrix dominated moduli and strength were significantly influenced by both environmental and rate parameters, whereas the fiber dominated moduli were not. However, the longitudinal strength was significantly influenced by temperature and moisture content. A qualitative explanation for these observations is presented.

Lifshitz, J. M.↗

NASA NTRS1980

Influence of excess diamine on properties of PMR polyimide resins and composites

This preliminary study explores the influence of 1-10 percent molar excess MDA on the molecular weight distribution and rheological properties of an imidized PMR system. Molecular weight distribution is characterized by gel permeation chromatography of the imidized molding compound; shear viscosity is related to changes in average molecular weight. The thermo-oxidative stability at 600 F glass transition temperature, flexural and interlaminar shear properties of PMR polyimide/Celion 6000 graphite fiber composites are compared as a function of the percent excess MDA in the monomer reactant mixture.

Hurwitz, F. I.↗

NASA NTRS1982

Structural tailoring of engine blades (STAEBL)

A mathematical optimization procedure was developed for the structural tailoring of engine blades and was used to structurally tailor two engine fan blades constructed of composite materials without midspan shrouds. The first was a solid blade made from superhybrid composites, and the second was a hollow blade with metal matrix composite inlays. Three major computerized functions were needed to complete the procedure: approximate analysis with the established input variables, optimization of an objective function, and refined analysis for design verification.

Platt, C. E.↗

NASA NTRS1982

PMR-15 polyimide modifications for improved prepreg tack

The use of mixed solvents and of modified monomeric ester reactants was investigated as a means of improving the tack and drape retention characteristics of PMR-15 polyimide prepreg. Methanol, ethanol, 1-propanol and 1-butanol were used to prepare the esters, prepreg solutions, and T-300 graphite fabric and Celion 6000 unidirectional fiber prepregs. The tack retention characteristics of the T-300 fabric prepreg after exposure to simulated use conditions were determined using a simple lap shear test. Drape was qualitatively assessed by visually monitoring the deformability of the prepreg. Thermo-oxidative stability and mechanical properties retention of the Celion 6000 grahite fiber composites were determined as a function of exposure time in air at 600 F.

Vannucci, R. D.↗

NASA NTRS1983

Ultrasonic wave propagation in two-phase media: Spherical inclusions

The scattering theory, recently developed via the extended method of equivalent inclusion, is used to study the propagation of time-harmonic waves in two-phase media of elastic matrix with randomly distributed elastic spherical inclusion materials. The elastic moduli and mass density of the composite medium are determined as functions of frequencies when given properties and concentration of the spheres and the matrix. Velocity and attenuation of ultrasonic waves in two-phase media are determined for cases of distributed spheres and localized damage. An averaging theorem that requires the equivalence of the strain energy and the kinetic energy between the effective medium and the original matrix with spherical inhomogeneities is employed to derive the effective moduli and mass density. The functional dependency of these quantities upon frequencies and concentration provides a method of data analysis in ultrasonic evaluation of material properties. Numerical results or moduli, velocity and/or attenuation as functions of concentration of inclusion material, or porosity, are graphically displayed.

Fu, L. S.↗

NASA NTRS1984

Ultrasonic wave propagation in two-phase media - Spherical inclusions

Fu, L. S.↗

NASA NTRS1985

The charge-energy-mass spectrometer for 0.3-300 keV/e ions on the AMPTE CCE

The charge-energy-mass (CHEM) spectrometer on the Charge Composition Explorer (CCE) has the function to measure the energy spectra, pitch-angle distributions, and ionization states of ions in the earth's magnetosphere and magnetosheath in the energy range from 0.3 to 300 keV/charge with a time resolution of less than 1 min. The obtained data will provide essential information on outstanding problems related to ion sources and dynamical processes of space plasmas and of suprathermal ions. A description of the CHEM experiment is given, taking into account the principle of operation, the sensor, the electronics, instrument characteristics, specifications, and requirements. Questions of postlaunch performance are also discussed.

Gloeckler, G.↗

NASA NTRS1985

The liquid-vapor interface of a binary liquid mixture near the consolute point

The present investigation is concerned with measurements of the reflectance of polarized light by the liquid-vapor interface above liquid mixtures of a fluorocarbon and an alcohol in the vicinity of the consolute point of the liquid mixture. The denser fluorocarbon is preferentially adsorbed at the interface. The denser fluorocarbon is preferentially adsorbed at the interface. The data are consistent with a simple model of the dielectric constant vs height profile in the interface region. Liquid mixtures of isopropanol (i-C3H7OH) and perfluoromethylcyclohexane (C7F14) near their consolute point (Tc = 363 K) are utilized in the conducted experiments. The primary quantity measured is the ellipticity as a function of temperature and liquid composition. Attention is given to details regarding the experimental techniques, model dielectric constant profiles, and the calculation of the ellipticity.

Schmidt, J. W.↗

NASA NTRS1989

Non-Planckian behaviour of burst spectra - Dependence of the blackbody radius on the duration of bursts

Exosat observations indicate a strong correlation between X-ray burst duration and the blackbody temperature measured at the moment when the flux has decayed to 10 percent of the Eddington flux. No correlation is observed between the persistent flux near the burst and this temperature. Burst spectra reveal deviations from a Planckian spectrum which depend upon the chemical composition of the bursting layer. It is noted that this composition is not a simple function of the accretion rate.

Damen, E.↗

NASA NTRS1989

The interlaminar fracture toughness of woven graphite/epoxy composites

The interlaminar fracture toughness of 2-D graphite/epoxy woven composites was determined as a function of stacking sequence, thickness, and weave pattern. Plain, oxford, 5-harness satin, and 8-harness satin weaves of T300/934 material were evaluated by the double cantilever beam test. The fabric material had a G (sub Ic) ranging from 2 to 8 times greater than 0 degrees unidirectional T300/934 tape material. The interlaminar fracture toughness of a particular weave style was dependent on whether the stacking sequence was symmetric or asymmetric and, in some cases, on the fabric orientation.

Funk, Joan G.↗

NASA NTRS1991

Grooved surfaces on InP

Formation of a textured or grooved front surface on a solar cell can increase the efficiency in several ways, including enhanced absorption and light trapping. In III-IV materials the (111) plane is chemically different form the (1'1'1') plane, and both etching and epitaxial deposition behave differently on these surfaces. The current state of profile etching in InP is summarized. Data are presented on novel geometries attainable as a function of etchant temperature and composition, substrate orientation and carrier concentration, and the oxide thickness between the substrate and the photoresist. Depending on dopant concentration, the same etchant can produce either anisotropic or isotropic grooves. V-grooved solar cells were manufactured on InP, and the improved optical absorption was demonstrated. Preferred parameters for various applications are listed and discussed.

Bailey, Sheila G.↗

NASA NTRS1992

Closed-form analysis of fiber-matrix interface stresses under thermo-mechanical loadings

Closed form techniques for calculating fiber matrix (FM) interface stresses, using repeating square and diamond regular arrays, were presented for a unidirectional composite under thermo-mechanical loadings. An Airy's stress function micromechanics approach from the literature, developed for calculating overall composite moduli, was extended in the present study to compute FM interface stresses for a unidirectional graphite/epoxy (AS4/3501-6) composite under thermal, longitudinal, transverse, transverse shear, and longitudinal shear loadings. Comparison with finite element results indicate excellent agreement of the FM interface stresses for the square array. Under thermal and longitudinal loading, the square array has the same FM peak stresses as the diamond array. The square array predicted higher stress concentrations under transverse normal and longitudinal shear loadings than the diamond array. Under transverse shear loading, the square array had a higher stress concentration while the diamond array had a higher radial stress concentration. Stress concentration factors under transverse shear and longitudinal shear loadings were very sensitive to fiber volume fraction. The present analysis provides a simple way to calculate accurate FM interface stresses for both the square and diamond array configurations.

Naik, Rajiv A.↗

NASA NTRS1992

Crushing characteristics of composite tubes with 'near-elliptical' cross sections

An experimental investigation was conducted to determine whether the energy-absorption capability of near-elliptical cross-section composite tubular specimens is a function of included angle. Each half of the near-elliptical cross-section tube is a segment of a circle. The included angle is the angle created by radial lines extending from the center of the circular segment to the ends of the circular segment. Graphite- and Kevlar-reinforced epoxy material was used to fabricate specimens. Tube internal diameters were 2.54, 3.81, and 7.62 cm, and included angles were 180, 160, 135, and 90 degrees. Based upon the test results from these tubes, energy-absorption capability increased between 10 and 30 percent as included angle decreased between 180 and 90 degrees for the materials evaluated. Energy-absorption capability was a decreasing nonlinear function of the ratio of tube internal diameter to wall thickness.

Farley, Gary L.↗

NASA NTRS1992

A new class of g-modes in neutron stars

Because a neutron star is born hot, its internal composition is close to chemical equilibrium. In the fluid core, this implies that the ratio of the number densities of charged particles (protons and electrons) to neutrons is an increasing function of the mass density. This composition gradient stably stratifies the matter giving rise to a Brunt-Vaisala frequency N of about 500/s. Consequently, a neutron star core provides a cavity that supports gravity modes (g-modes). These g-modes are distinct from those previously identified with the thermal stratification of the surface layers and the chemical stratification of the crust. We compute the lowest-order, quadrupolar, g-modes for cold, Newtonian, neutron star models with M/solar M = 0.581 and M/solar M = 1.405, and show that the crustal and core g-modes have similar periods. We also discuss damping mechanisms and estimate damping rates for the core g-modes. Particular attention is paid to damping due to the emission of gravitational radiation.

Reisenegger, Andreas↗