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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Toughened composites; Proceedings of the Symposium, Houston, TX, Mar. 13-15, 1985

The papers presented in this volume provide an overview of current research aimed at the development of tougher high-performance continuous graphite fiber composites for aerospace applications. Topics discussed include toughened composites selection criteria, micromechanisms of fracture in toughened graphite/epoxy laminates, the characterization of edge delamination growth in laminates under fatigue loading, and rate effects on delamination fracture toughness of a toughened graphite/epoxy. Papers are also presented on the development of residual thermal stresses in amorphous and semicrystalline thermoplastic matrix composites, structural property improvements through hybridized composites, and screening of advanced matrix resin systems in high performance aircraft.

Johnston, Norman J.↗
NASA NTRS1988

Archean crustal evolution of the northern North China Craton

The Archean granultie facies rocks of the North China (Sino-Korean) Craton mostly occur inside the northern boundary forming a unique and spectacular granulite belt trending roughly E-W from eastern Hebei, North China in the east to Mt. Daqinchan, western Inner Mongolia in the west, ranging about 1,000 km long. Over the years in the middle portion of this Archean high-grade metamorphic belt a stratigraphic unconformity between the khondalite rock assemblage and the medium in composition granulite assemblage in Datong-Xinghe area is determined. The geological structural properties of the North China Craton are discussed.

Qian, Xianglin↗
NASA NTRS1988

Marine stratocumulus cloud fields off the coast of southern California observed using Landsat imagery. I - Structural characteristics

The structural characteristics of stratocumulus cloud fields off the coast of southern California are investigated using Landsat Multispectral Scanner imagery. Twelve scenes in this area are examined along with three other stratocumulus scenes near San Francisco, over central Oregon, and in the Gulf of Mexico. Results from this initial study of stratocumulus clouds indicate that cloud-background threshold selection techniques based upon edge detection gradient assumptions are not appropriate for cloud segmentation and classification algorithms, cloud size distributions obey a power law, and cell horizontal aspect ratio increases with cell diameter. It was also found that stratocumulus clouds are bifractal in nature with fractal dimension of d of about 1.2 for cells with diameter D smaller than 0.5 km and d of about 1.5 for cells with D greater than 0.5 km; stratocumulus cloud fields appear to be homogeneous over regions of about 100 km x 100 km, a much smaller region than the 2.5-deg x 2.5-deg boxes to be used in the ISCCP regional averaging algorithms; and that structural properties of stratocumulus clouds observed off the coast of southern California are similar to those observed for stratocumulus clouds at three other locations.

Welch, R. M.↗
NASA NTRS1988

A review of dynamic mechanical characterization of high temperature PMR polyimides and composites

This paper reviews the applications of dynamic mechanical characterization for high-temperature PMR polyimides and their graphite-fiber-reinforced composites. This characterization technique provides insights into the processability, performance, and structure property relationships of the polyimides and composites. The dynamic mechanical properties of various molding powders, commercially obtained prepregs, neat resins, and as-fabricated as well as aged composites are presented. Some applied aspects of the dynamic mechanical data are discussed.

Pater, Ruth H.↗
NASA NTRS1988

Fiber optic sensor technology - An opportunity for smart aerospace structures

Fiber optic sensors provide the opportunity for fabricating materials with internal sensors which can serve as lifetime health monitors, analogous to a central nervous system. The embedded fiber optic sensors can be interrogated by various techniques to measure internal strain, temperature, pressure, acoustic waves and other parameters indicative of structural integrity. Experiments have been conducted with composite samples with embedded sensors to measure strain using optical time domain reflectometry, modal interference and an optical phase locked loop. Fiber optic sensors have been developed to detect acoustic emission and impact damage and have been demonstrated for cure monitoring. These sensors have the potential for lifetime monitoring of structural properties, providing real time nondestructive evaluation.

Heyman, J. S.↗
NASA NTRS1988

Space environmental effects on polymeric materials

Two of the major environmental hazards in the Geosynchronous Earth Orbit (GEO) are energetic charged particles and ultraviolet radiation. The charged particles, electrons and protons, range in energy from 0.1 to 4 MeV and each have a flux of 10 to the 8th sq cm/sec. Over a 30 year lifetime, materials in the GEO will have an absorbed dose from this radiation of 10 to the 10th rads. The ultraviolet radiation comes uninhibited from the sun with an irradiance of 1.4 kw/sq m. Radiation is known to initiate chain sission and crosslinking in polymeric materials, both of which affect their structural properties. The 30-year dose level from the combined radiation in the GEO exceeds the threshold for measurable damage in most polymer systems studied. Of further concern is possible synergistic effects from the simultaneous irradiation with charged particles and ultraviolet radiation. Most studies on radiation effects on polymeric materials use either electrons or ultraviolet radiation alone, or in a sequential combination.

Kiefer, Richard L.↗
NASA NTRS1988

Isomeric oxydiphthalic anhydride polyimides

Much of the polyimide research at Langley Research Center has focused on isomeric modification of the diamine component; polyimides having considerably improved processability and adhesion have resulted. The present structure-property study was designed to investigate how isomeric attachment of the three oxydiphthalic anhydride (ODPA) polyimides affects their properties. Each dianhydride, 3,4,3',4'-oxydiphthalic anhydride (4,4'-OPDA,I), 2,3,2',3'-oxydiphthalic anhydride (3,3'-ODPA,II), and 2,3,3',4'-oxydiphthalic anhydride (3,4'-OPDA,III), was reacted with p-phenylenediamine, 4,4'-oxydianiline, 3,3'-diaminodiphenylsulfone, 3,3'-diaminobenzophenone, and 4,4'-bis(3-aminophenoxy)benzophenone in DMAc. The inherent viscosities of the resulting poly(amic acids) were determined. Thermally imidized films were studied for their creasability and solubility, as well as by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and wide angle X-ray scattering (WAXS). A comparison of these properties will be made.

Gerber, Margaret K.↗
NASA NTRS1989

An experimental and analytical investigation of the effect of spanwise curvature on wing flutter at Mach number of 0.7

An experimental and analytical study was conducted at Mach 0.7 to investigate the effects of spanwise curvature on flutter. Two series of rectangular planform wings of aspect ration 1.5 and curvature ranging from zero (uncurved) to 1.04/ft were flutter tested in the NASA Langley Transonic Dynamics Tunnel (TDT). One series consisted of models with a NACA 65 A010 airfoil section and the other of flat plate cross section models. Flutter analyses were conducted for correlation with the experimental results by using structural finite element methods to perform vibration analysis and two aerodynamic theories to obtain unsteady aerodynamic load calculations. The experimental results showed that for one series of models the flutter dynamic pressure increased significantly with curvature while for the other series of models the flutter dynamic pressure decreased with curvature. The flutter analyses, which generally predicted the experimental results, indicated that the difference in behavior of the two series of models was primarily due to differences in their structural properties.

Rivera, Jose A., Jr.↗
NASA NTRS1989

Structural behavior of composites with progressive fracture

The objective of the study is to unify several computational tools developed for the prediction of progressive damage and fracture with efforts for the prediction of the overall response of damaged composite structures. In particular, a computational finite element model for the damaged structure is developed using a computer program as a byproduct of the analysis of progressive damage and fracture. Thus, a single computational investigation can predict progressive fracture and the resulting variation in structural properties of angleplied composites.

Minnetyan, L.↗
NASA NTRS1989

Dwarf spheroidal galaxies and resonant orbital coupling

The structural properties of the dwarf spheroidal satellite galaxies of the Milky Way may be strongly affected by their time-dependent interactions with the 'tidal' field of the Milky Way. A low Q resonance of the tidal driving force with collective oscillation modes of the dwarf system can produce many of the observed properties of the Local Group dwarf spheroidal galaxies, including large velocity dispersions that would normally be interpreted as indicating large dynamical masses.

Kuhn, J. R.↗
NASA NTRS1989

Disk-instability model for soft-X-ray transients containing black holes

Consideration is given to the time-dependent behavior of the accretion disks in low-mass X-ray binaries as a model for soft X-ray transients. The thermal stability and vertical structure properties of the disk are studied in the range between 10 to the 7th and 10 to the 8th cm. An Eulerian time-dependent thin disk model is used to show that, for relevant accretion rates, the disk suffers a thermal instability leading to intermittent accretion onto the central compact object. The disk instability model reproduces the basic features of outbursts of soft X-ray transients. Also, the physical state of quiescence of the binary A0620-00 is examined, focusing on the possible existence of a black hole. It is suggested that the modulation of the accretion flow in the inner disk may be responsible for the bimodal behavior observed in the spectra of black hole candidates such as Cyg X-1, GX 339-4, and A0620-00.

Mineshige, Shin↗
NASA NTRS1989

Analysis of vibrations of the National Transonic Facility model support system using a 3-D aeroelastic code

Vibrations of the National Transonic Facility model support system were analyzed using Computational Aeroelasticity Program-Transonic Small Disturbance, a three-dimensional transonic small disturbance code designed for aeroelastic analysis of complete aircraft. The model support system was represented as a tunnel-spanning flexible wing whose structural properties were obtained from measured responses. Aeroelastic transients were calculated and analyzed to obtain modal stability characteristics. The results show that there is tendency to exhibit a hump mode instability in one of the modes, and that there are other weakly damped or unstable modes. Most of the motion is in the aft section of the model support system. The close proximity of the solid wind-tunnel diffuser side walls to the model support system is shown to have adverse effects on its aeroelastic stability characteristics. When the effects of the wind-tunnel walls are modeled, the analyses indicate the presence of wind-tunnel resonance modes which are not present when free-air boundary conditions are used.

Whitlow, Woodrow, Jr.↗
NASA NTRS1990

AMSAHTS 1990: Advances in Materials Science and Applications of High Temperature Superconductors

This publication is comprised of abstracts for oral and poster presentations scheduled for AMSAHTS '90. The conference will focus on understanding high-temperature superconductivity with special emphases on materials issues and applications. AMSAHTS '90, will highlight the state of the art in fundamental understanding of the nature of high-Tc superconductivity (HTSC) as well as the chemistry, structure, properties, processing and stability of HTSC oxides. As a special feature of the conference, space applications of HTSC will be discussed by NASA and Navy specialists.

Flom, Yury↗
NASA NTRS1990

Probabilistic low cycle fatigue failure analysis with application to liquid propellant rocket engines

A probabilistic Low Cycle Fatigue (LCF) failure analysis of a candidate turbine disk for use in a turbopump of a rocket engine of the Space Shuttle Main Engine class is described. A state-of-the-art LCF failure prediction method was used in a Monte Carlo simulation to generate a distribution of failure lives. A stochastic Stress/Life (S/N) model was used for materials characterization. The LCF failure model expresses fatigue life as a function of stochastic parameters including environmental parameters, loads, material properties, structural parameters, and model specification errors. The rationale for the particular characterization of each stochastic input parameter is described. The results and interpretation of the failure analysis are given.

Newlin, L.↗
NASA NTRS1989

Study of indium tin oxide films exposed to atomic axygen

A qualitative simulation of the effects of atomic oxygen has been conducted on indium tin oxide (ITO) films prepared by dc sputtering onto room-temperature substrates, by exposing them to an RF-excited oxygen plasma and characterizing the resulting changes in optical, electrical, and structural properties as functions of exposure time with ellipsometry, spectrophotometry, resistivity, and X-ray measurements. While the films thus exposed exhibit reduced resistivity and optical transmission; both of these effects, as well as partial crystallization of the films, may be due to sample heating by the plasma. Film resistivity is found to stabilize after a period of exposure.

Snyder, Paul G.↗
NASA NTRS1990

Combined design of structures and controllers for optimal maneuverability

Approaches to the combined design of structures and controllers for achieving optimal maneuverability are presented. A maneuverability index which directly reflects the minimum time required to perform a given set of maneuvers is introduced. By designing the flexible appendages, the maneuver time of the spacecraft is minimized under the constraints of structural properties, and post maneuver spillover is kept within a specified bound. The spillover reduction is achieved by making use of an appropriate reduced order model. The distributed parameter design problem is approached using assumed shape functions, and finite element analysis with dynamic reduction. Solution procedures have been investigated. Approximate design methods have been developed to overcome the computational difficulties. Some new constraints on the modal frequencies of the spacecraft are introduced in the original optimization problem to facilitate the solution process. It is shown that the global optimal design may be obtained by tuning the natural frequencies to satisfy specific constraints. Researchers quantify the difference between a lower bound to the solution for maneuver time associated with the original problem and the estimate obtained from the modified problem, for a specified application requirement. Numerical examples are presented to demonstrate the capability of this approach.

Ling, Jer↗
NASA NTRS1990

Plasma-assisted physical vapor deposition surface treatments for tribological control

In any mechanical or engineering system where contacting surfaces are in relative motion, adhesion, wear, and friction affect reliability and performance. With the advancement of space age transportation systems, the tribological requirements have dramatically increased. This is due to the optimized design, precision tolerance requirements, and high reliability expected for solid lubricating films in order to withstand hostile operating conditions (vacuum, high-low temperatures, high loads, and space radiation). For these problem areas the ion-assisted deposition/modification processes (plasma-based and ion beam techniques) offer the greatest potential for the synthesis of thin films and the tailoring of adherence and chemical and structural properties for optimized tribological performance. The present practices and new approaches of applying soft solid lubricant and hard wear resistant films to engineering substrates are reviewed. The ion bombardment treatments have increased film adherence, lowered friction coefficients, and enhanced wear life of the solid lubricating films such as the dichalcogenides (MoS2) and the soft metals (Au, Ag, Pb). Currently, sputtering is the preferred method of applying MoS2 films; and ion plating, the soft metallic films. Ultralow friction coefficients (less than 0.01) were achieved with sputtered MoS2. Further, new diamond-like carbon and BN lubricating films are being developed by using the ion assisted deposition techniques.

Spalvins, Talivaldis↗
NASA NTRS1990

Plastic deformation mechanisms in polyimide resins and their semi-interpenetrating networks

High-performance thermoset resins and composites are critical to the future growth of space, aircraft, and defense industries in the USA. However, the processing-structure-property relationships in these materials remain poorly understood. In the present ASEE/NASA Summer Research Program, the plastic deformation modes and toughening mechanisms in single-phase and multiphase thermoset resins were investigated. Both thermoplastic and thermoset polyimide resins and their interpenetrating networks (IPNs and semi-IPNs) were included. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers were evaluated. Other possible toughening mechanisms in multiphase thermoset resins were also examined. The topological features of network chain configuration/conformation and the multiplicity of phase morphology in INPs and semi-IPNs provide unprecedented opportunities for studying the toughening mechanisms in multiphase thermoset polymers and their fiber composites.

Jang, Bor Z.↗