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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Compression strength failure mechanisms in unidirectional composite laminates containing a hole

Experiments on graphite-epoxy laminated plates containing unloaded small holes show that these laminates are notch insensitive. That is, the uniaxial strength of these laminates with small holes exceeds the strength predicted by a point stress criterion using the stress concentration factor for the in-plane stress field. Laminates containing large holes exhibit notch sensitive behavior and consequently their strength is reasonably well predicted by the stress concentration effect. This hole size effect is manifested both in tension and in compression. Apparently, some mechanism must cause in-plane stress relief for laminates containing small holes. The purpose of this research was to study the influence of geometric nonlinearity on the micromechanical response of a filamentary composite material in the presence of a strain gradient caused by a discontinuity such as a hole. A mathematical model was developed at the micromechanical level to investigate this geometrically nonlinear effect.

Johnson, Eric R.↗
NASA NTRS1995

EMI Shields made from intercalated graphite composites

Electromagnetic interference (EMI) shielding typically makes up about twenty percent of the mass of a spacecraft power system. Graphite fiber/polymer composites have significantly lower densities and higher strengths than aluminum, the present material of choice for EMI shields, but they lack the electrical conductivity that enables acceptable shielding effectiveness. Bromine intercalated pitch-based graphite/epoxy composites have conductivities fifty times higher than conventional structural graphite fibers. Calculations are presented which indicate that EMI shields made from such composites can have sufficient shielding at less than 20% of the mass of conventional aluminum shields. EMI shields provide many functions other than EMI shielding including physical protection, thermal management, and shielding from ionizing radiation. Intercalated graphite composites perform well in these areas also. Mechanically, they have much higher specific strength and modulus than aluminum. They also have shorter half thicknesses for x-rays and gamma radiation than aluminum. Thermally, they distribute infra-red radiation by absorbing and re-radiating it rather than concentrating it by reflection as aluminum does. The prospects for intercalated graphite fiber/polymer composites for EMI shielding are encouraging.

Gaier, James R.↗
NASA NTRS1978

Research on graphite reinforced glass matrix composites

A composite that can be used at temperatures up to 875 K with mechanical properties equal or superior to graphite fiber reinforced epoxy composites is presented. The composite system consist of graphite fiber, uniaxially or biaxially, reinforced borosilicate glass. The mechanical and thermal properties of such a graphite fiber reinforced glass composite are described, and the system is shown to offer promise as a high performance structural material. Specific properties that were measured were: a modified borosilicate glass uniaxially reinforced by Hercules HMS graphite fiber has a three-point flexural strength of 1030 MPa, a four-point flexural strength of 964 MPa, an elastic modulus of 199 GPa and a failure strain of 0.0052. The preparation and properties of similar composites with Hercules HTS, Celanese DG-102, Thornel 300 and Thornel Pitch graphite fibers are also described.

Bacon, J. F.↗
NASA NTRS2011

Graphite Composite Panel Polishing Fixture

The use of high-strength, lightweight composites for the fixture is the novel feature of this innovation. The main advantage is the light weight and high stiffness-to-mass ratio relative to aluminum. Meter-class optics require support during the grinding/polishing process with large tools. The use of aluminum as a polishing fixture is standard, with pitch providing a compliant layer to allow support without deformation. Unfortunately, with meter-scale optics, a meter-scale fixture weighs over 120 lb (.55 kg) and may distort the optics being fabricated by loading the mirror and/or tool used in fabrication. The use of composite structures that are lightweight yet stiff allows standard techniques to be used while providing for a decrease in fixture weight by almost 70 percent. Mounts classically used to support large mirrors during fabrication are especially heavy and difficult to handle. The mount must be especially stiff to avoid deformation during the optical fabrication process, where a very large and heavy lap often can distort the mount and optic being fabricated. If the optic is placed on top of the lapping tool, the weight of the optic and the fixture can distort the lap. Fixtures to support the mirror during fabrication are often very large plates of aluminum, often 2 in. (.5 cm) or more in thickness and weight upwards of 150 lb (68 kg). With the addition of a backing material such as pitch and the mirror itself, the assembly can often weigh over 250 lb (.113 kg) for a meter-class optic. This innovation is the use of a lightweight graphite panel with an aluminum honeycomb core for use as the polishing fixture. These materials have been used in the aerospace industry as structural members due to their light weight and high stiffness. The grinding polishing fixture consists of the graphite composite panel, fittings, and fixtures to allow interface to the polishing machine, and introduction of pitch buttons to support the optic under fabrication. In its operation, the grinding polishing fixture acts as a reaction structure to the polishing tool. It must be stiff enough to avoid imparting a distorted shape to the optic under fabrication and light enough to avoid self-deflection. The fixture must also withstand significant tangential loads from the polishing machine during operations.

Hagopian, John↗
NASA NTRS1996

Durability of Intercalated Graphite Epoxy Composites in Low Earth Orbit

The electrical conductivity of graphite epoxy composites can be substantially increased by intercalating (inserting guest atoms or molecules between the graphene planes) the graphite fibers before composite formation. The resulting high strength, low density, electrically conducting composites have been proposed for EMI shielding in spacecraft. Questions have been raised, however, about their durability in the space environment, especially with respect to outgassing of the intercalates, which are corrosive species such as bromine. To answer those concerns, six samples of bromine intercalated graphite epoxy composites were included in the third Evaluation of Oxygen Interaction with Materials (EOIM-3) experiment flown on the Space Shuttle Discovery (STS-46). Changes in electrical conductivity, optical reflectance, surface texture, and mass loss for SiO2 protected and unprotected samples were measured after being exposed to the LEO environment for 42 hours. SiO2 protected samples showed no degradation, verifying conventional protection strategies are applicable to bromine intercalated composites. The unprotected samples showed that bromine intercalation does not alter the degradation of graphite-epoxy composites. No bromine was detected to have been released by the fibers allaying fears that outgassing could be disruptive to the sensitive electronics the EMI shield is meant to protect.

Gaier, James R.↗
NASA NTRS1983

Identification and management of filament-wound case stiffness parameters

The high specific strength and the high specific modules made graphite epoxy laminate an expedient material substitute for the Shuttle Solid Rocket Motor steel case to substantially increase the payload performance without increasing the composite case axial growth during thrust build up which was constrained to minimize liftoff excitation effects on existing structural elements and interfaces. Parameters associated with axial growth were identified for quality and manufacturing controls. Included is an innovative method for experimentally verifying extensional elastic properties on a laminate pressurized test bottle.

Verderaime, V.↗
NASA NTRS1984

Rheological characterization of addition polyimide matrix resins and prepregs

Although graphite-reinforced polyimide matrix composites offer outstanding specific strength and stiffness, together with high thermal oxidative stability, processing problems connected with their rheological behavior remain to be addressed. The present rheological studies on neat polyimide resin systems encountered outgassing during cure. A staging technique has been developed which can successfully handle polyimide samples, and novel methods were applied to generate rheological curves for graphite-reinforced prepregs. The commercial graphite/polyimide systems studied were PRM 15, LARC 160, and V378A.

Maximovich, M. G.↗
NASA NTRS1992

Carborane Dopant Strengthens Pitch Char

Addition of small amount of soluble, organic boron compound to matrix precursor of carbon-fiber/carbon-matrix (carbon/carbon) composite increases strength and toughness of composite. Compound catalyzes graphitization of matrix, giving rise to greater degree of graphitization at lower processing temperature. Technique used to advantage in carbon/carbon materials requiring lower temperature processing, such as those with inhibited matrices or materials sensitive to changes in fiber properties.

D Kyle Brown↗
NASA NTRS1992

Space environmental effects on LDEF low Earth orbit exposed graphite reinforced polymer matrix composites

The Long Duration Exposure Facility (LDEF) was deployed on April 7, 1984 in low earth orbit (LEO) at an altitude of 482 kilometers. On board experiments experienced the harsh LEO environment including atomic oxygen (AO), ultraviolet radiation (UV), and thermal cycling. During the 5.8 year mission, the LDEF orbit decayed to 340 kilometers where significantly higher AO concentrations exist. LDEF was retrieved on January 12, 1990 from this orbit. One experiment on board LDEF was M0003, Space Effects on Spacecraft Materials. As a subset of M0003 nearly 500 samples of polymer, metal, and glass matrix composites were flown as the Advanced Composites Experiment M0003-10. The Advanced Composites Experiment is a joint effort between government and industry with the Aerospace Corporation serving as the experiment integrator. A portion of the graphite reinforced polymer matrix composites were furnished by the Boeing Defense and Space Group, Seattle, Washington. Test results and discussions for the Boeing portion of M0003-10 are presented. Experiment and specimen location on the LDEF are presented along with a quantitative summary of the pertinent exposure conditions. Matrix materials selected for the test were epoxy, polysulfone, and polyimide. These composite materials were selected due to their suitability for high performance structural capability in spacecraft applications. Graphite reinforced polymer matrix composites offer higher strength to weight ratios along with excellent dimensional stability. The Boeing space exposed and corresponding ground control composite specimens were subjected to post flight mechanical, chemical, and physical testing in order to determine any changes in critical properties and performance characteristics. Among the more significant findings are the erosive effect of atomic oxygen on leading edge exposed specimens and microcracking in non-unidirectionally reinforced flight specimens.

George, Pete↗
NASA NTRS1995

Construction material processed using lunar simulant in various environments

The manufacture of construction materials from locally available resources in space is an important first step in the establishment of lunar and planetary bases. The objective of the CoMPULSIVE (Construction Material Processed Using Lunar Simulant In Various Environments) experiment is to develop a procedure to produce construction materials by sintering or melting Johnson Space Center Simulant 1 (JSC-1) lunar soil simulant in both earth-based (1-g) and microgravity (approximately 0-g) environments. The characteristics of the resultant materials will be tested to determine its physical and mechanical properties. The physical characteristics include: crystalline, thermal, and electrical properties. The mechanical properties include: compressive tensile, and flexural strengths. The simulant, placed in a sealed graphite crucible, will be heated using a high temperature furnace. The crucible will then be cooled by radiative and forced convective means. The core furnace element consists of space qualified quartz-halogen incandescent lamps with focusing mirrors. Sample temperatures of up to 2200 C are attainable using this heating method.

Chase, Stan↗
NASA NTRS1978

Correlation of fiber composite tensile strength with the ultrasonic stress wave factor

An ultrasonic-acoustic technique was used to indicate the strength variations of tensile specimens of a graphite-epoxy composite. A 'stress wave factor' was determined and its value was found to depend on variations of the fiber-resin bonding as well as fiber orientation. The fiber orientations studied were 0 deg (longitudinal), 10 deg (off-axis), 90 deg (transverse), (0 deg/+ or - 45 deg/0) symmetrical, and (+ or - 45 deg) symmetrical. The stress wave factor can indicate variations of the tensile and shear strengths of composite materials. The stress wave factor was also found to be sensitive to strength variations associated with microporosity and differences in fiber-resin ratio.

Vary, A.↗
NASA NTRS1978

Correlation of Fiber Composite Tensile Strength with the Ultrasonic Stress Wave Factor

An ultrasonic-acoustic technique was used to indicate the strength variations of tensile specimens of a graphite-epoxy composite. A stress wave factor was determined and its value was found to depend on variations of the fiber-resin bonding as well as fiber orientation. The fiber orientations studied were 0 deg (longitudinal), 10 deg (off-axis), 90 deg (transverse), 0 deg + or - 45 deg/0 deg symmetrical, and + or - 45 deg] symmetrical. The stress wave factor can indicate variations of the tensile and shear strengths of composite materials. The stress wave factor was also found to be sensitive to strength variations associated with microporosity and differences in fiber-resin ratio.

Vary, A.↗
NASA NTRS1986

Evaluation of residual strength of composite laminates damaged by low velocity impacts

This work presents the study of Low Velocity Transverse Impact damage of graphite-epoxy T300/5208 composite material. An energy dissipation model was developed to predict the residual strength from fracture mechanics concepts. The specimens, 100 mm diameter clamped plates, were impact damaged by a cantilever-type instrumented 1-inch diameter steel ball. This study was limited to impact velocity 6 m/sec. Rectangular strips, 50 mm x 125 mm, were cut from the impact-damaged specimens so that the impact damage zone was in the center of the strips. These strips were tested to obtain their residual strength. Predictions were compared with the test results.

Lal, K. M.↗
NASA NTRS1991

Evaluation of some scale effects in the response and failure of composite beams

The feasibility of using scale model testing for predicting full-scale behavior of composite beams loaded in tension and flexure was investigated. Classical laws of similitude were applied to fabricate and test replica model beams to identify scaling effects in the load response, strength, and mode of failure. Experiments were conducted using graphite-epoxy composite beams having different laminate stacking sequences and a range of scaled sizes. Results indicated that the elastic response of scaled composite beams was independent of specimen size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled beams of certain laminate stacking sequences. Weibull statistical and fracture mechanics based models were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size in failure.

Jackson, Karen E.↗
NASA NTRS1992

Scale effects in the response and failure of fiber reinforced composite laminates loaded in tension and in flexure

The feasibility of using scale model testing for predicting the full-scale behavior of flat composite coupons loaded in tension and beam-columns loaded in flexure is examined. Classical laws of similitude are applied to fabricate and test replica model specimens to identify scaling effects in the load response, strength, and mode of failure. Experiments were performed on graphite-epoxy composite specimens having different laminate stacking sequences and a range of scaled sizes. From the experiments it was deduced that the elastic response of scaled composite specimens was independent of size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled specimens of certain laminate stacking sequences. A Weibull statistical model and a fracture mechanics based model were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size on strength.

Jackson, Karen E.↗
NASA NTRS1972

Heat resistant composite structure for shuttle applications (Ryton-B)

A program was undertaken to characterize Ryton-B resin, develop graphite filament prepregs, undirectional laminates and determine the strength and heat resistance of the composite system. Through the use of a water soluble resin binder, high quality prepreg tape, three inches wide with 4 tows of HM-S were produced. The tape laminated to 0.00175 inch per ply. A wide range of properties in the cured resin and laminate were found using different curing conditions. The thermal stability and strength of molded laminates appears to be very dependent upon the cure cycle used for polymerization.

Source record↗
NASA NTRS1977

Use of the short-beam shear test for quality control of graphite-polyimide laminates

The properties of laminated composite materials are strongly affected by many fabrication variables including temperature, time, and molding pressure. It is therefore necessary to have reliable quality control tests to check the properties of manufactured materials. The short-beam shear test is used widely by both manufacturers and researchers as a quality control test in the production of materials and development of new material systems. There are, however, several limitations to the standard test method. This paper presents the results of short-beam shear tests on graphite-polyimide laminates and reports on stiffness-strength relationships and nondestructive evaluation methods which aid in the interpretation of the test data.

Stinchcomb, W. W.↗