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At least 703 records · Page 39

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NASA NTRS1979

Feasibility Study of Graphite Epoxy Antenna for a Microwave Limb Sounder Radiometer (MLSR)

Results are presented of a feasibility study to design graphite epoxy antenna reflectors for a jet propulsion laboratory microwave limb sounder instrument (MLSR). Two general configurations of the offset elliptic parabolic reflectors are presented that will meet the requirements on geometry and reflector accuracy. The designs consist of sandwich construction for the primary reflectors, secondary reflector support structure and cross-tie members between reflector pairs. Graphite epoxy materials of 3 and 6 plies are used in the facesheets of the sandwich. An aluminum honeycomb is used for the core. A built-in adjustment system is proposed to reduce surface distortions during assembly. The manufacturing and environmental effects are expected to result in surface distortions less than .0015 inch and pointing errors less than .002 degree.

Source record↗

NASA NTRS1979

Advanced risk assessment of the effects of graphite fibers on electronic and electric equipment, phase 1

A model was developed to generate quantitative estimates of the risk associated with the release of graphite fibers during fires involving commercial aircraft constructed with graphite fiber composite materials. The model was used to estimate the risk associated with accidents at several U.S. airports. These results were then combined to provide an estimate of the total risk to the nation.

Pocinki, L. S.↗

NASA NTRS1978

A comparison of the lubricating mechanisms of graphite fluoride and molybdenum disulfide films

A microscopic study of 440 C steel sliding surfaces lubricated by graphite fluoride or molybdenum disulfide solid lubricant rubbed films was conducted. The sliding surfaces, along with the friction, wear, and wear life were observed as a function of the number of sliding revolutions in three different atmospheres: moist air (10,000 ppm H2O), dry air (less than 20 ppm H2O), or dry argon (less than 20 ppm H2O). In general, the lubricating mechanisms of the two solid lubricants were found to be relatively similar; that is, a dynamic, thin, layer-like film (which sheared on relative motion) was formed between the two metallic surfaces. The mechanisms of failure were found to be somewhat different, however. Failure of MoS2 films was very dependent on atmospheric degradation, while that of graphite fluoride films was more dependent on flow of the lubricant film out of the contact zone

Fusaro, R. L.↗

NASA NTRS1978

Glass matrix composites. I - Graphite fiber reinforced glass

An experimental program is described in which graphite fibers of Hercules HMS and HTS, Thornel 300, and Celanese DG-12 were used to reinforce, both uniaxially and biaxially, borosilicate pyrex glass. Composite flexural strength distribution, strength as a function of test temperature, fracture toughness and oxidative stability were determined and shown to be primarily a function of fiber type and the quality of fiber-matrix bond formed during composite fabrication. It is demonstrated that the graphite fiber reinforced glass system offers unique possibilities as a high performance structural material.

Prewo, K. M.↗

NASA NTRS1979

Effects of graphite fiber stability on the properties of PMR polyimide composites

Studies were performed to investigate the effect of the stability of graphite fibers on composite properties after exposure in air at 600 F. Composites were fabricated from PMR-15 and PMR-II monomer solutions, using HTS-2 and Celion 6000 graphite fibers as the reinforcement. The effect of long-term exposure in air at 600 F on composite weight loss and mechanical properties was determined. These composites exhibited a significantly increased lifetime at 600 F compared to composites fabricated from HTS fiber sold prior to 1975. The effect of the PMR-15 and PMR-II resin compositions on long-term composite performance at 600 F is also discussed.

Delvigs, P.↗

NASA NTRS1979

Electrical hazards posed by graphite fibers

The direct electrical resistance and the electrical contact resistance of graphite fibers released from both burned and unburned Fiberite T300/1034 composite were measured. The voltage at which arcing occurs due to fibers settling on electrical conductors was also determined. On the basis of results obtained, an assessment was made of the hazards posed by graphite fibers to electrical equipment.

Springer, G. S.↗

NASA NTRS1980

Examining graphite reinforcement in composites

Structure of graphite layers in composite parts can be checked by pyrolizing epoxy portion of composite samples. After 2-3 hours in nitrogen atmosphere at 540 C, only graphite fibers remain. These can be separated and checked for proper number, thickness, and orientation.

Sanders, R. E.↗

NASA NTRS1979

Aging problems in graphite phenolic nozzle liners

Age related cracks in the graphite phenolic linear and/or ATJ insert in several nine year old Altair IIA solid rocket nozzles were investigated and determined to be limited to a single lot of nozzles. Test results and photomicrographs of fracture surfaces are presented which establish that in the discrepant nozzles the bond between the graphite fibers and the phenolic resin was tenacious, resulting in a nearly homogenous behavior. These materials are shown to have a relatively weak resin - fiber bond and behave more like a released fiber composite. Lot qualification fiber directional tensile strength and the chemical composition of the two materials were comparable, indicating that standard acceptance testing could not have differentiated between the desirable and undesirable material. Review of processing records failed to reveal any cause for the difference in the aging characteristics.

Mccorkle, G. S.↗

NASA NTRS1980

The 737 graphite composite flight spoiler flight service evaluation

The flight service experience of 111 graphite-epoxy spoilers on 737 transport aircraft and related ground based enviromental exposure of graphite-epoxy material specimens is reported. Spoilers were installed on 28 aircraft representing seven major airlines operating throughout the world. Over 1,188,367 spoiler flight hours and 1,786,837 spoiler landings were accumulated by this fleet. Tests of removed spoilers and ground-based exposure specimens after the fifth year of service indicate modest changes in composite strength properties. Two incidents of trailing edge delamination with subsequent core corrosion were observed. Based on visual, ultrasonic, and destructive testing, there has been no evidence of moisture migration into the honeycomb core and no core corrosion.

Hoffman, D. J.↗

NASA NTRS1980

Properties of PMR Polyimide composites made with improved high strength graphite fibers

High strength, intermediate modulus graphite fibers were obtained from various commercial suppliers, and were used to fabricate PMR-15 and PMR-2 polyimide composites. The effects of the improved high strength graphite fibers on composite properties after exposure in air at 600 F were investigated. Two of the improved fibers were found to have an adverse effect on the long term performance of PMR composites. The influence of various factors such as fiber physical properties, surface morphology and chemical composition were also examined.

Vannucci, R. D.↗

NASA NTRS1980

Graphite-epoxy panel compression strength reduction due to local impact

A review of results from on-going research to investigate the effects of low-velocity impact on the compressive strength of graphite-epoxy structures is presented. Extensive tests have been conducted on sandwich beams, laminated plates and stiffened panels. Conditions for failures were investigated by impact tests on statically loaded test specimens. The effects of compression load intensity were such that lightly loaded graphite structures (such as aircraft secondary structure) were insensitive to impact damage. In more heavily loaded structures, (such as wing panels), however, appreciable reductions in compressive strength occurred. The implications of the tests for structural design are discussed by comparing panel masses for designs where ultimate strains have been reduced due to impact considerations with the masses of designs with higher ultimate strains. Finally, preliminary test data is presented to show the possibility of improvements in damage tolerance achievable by using an alternate matrix material.

Card, M. P.↗

NASA NTRS1980

Hybrid composites that retain graphite fibers on burning

A laboratory scale program was conducted to determine fiber release tendencies of graphite reinforced/resinous matrix composites currently used or projected for use in civil aircraft. In the event of an aircraft crash and burn situation, there is concern that graphite fibers will be released from the composites once the resin matrix is thermally decomposed. Hybridizing concepts aimed at preventing fiber release on burning were postulated and their effectiveness evaluated under fire, impact, and air flow during an aircraft crash.

House, E. E.↗

NASA NTRS1980

Graphite composites with advanced resin matrices

The effect of processing variables on the flammability and mechanical properties for state-of-the-art and advanced resin matrices for graphite composites were studied. Resin matrices which were evaluated included state-of-the-art epoxy, phenolic-novolac, phenolic-xylok, two types of bismaleimides, benzyl, polyethersulfone, and poly(p-phenylene sulfone). Comparable flammability and thermochemical data on graphite-reinforced laminates prepared with these resin matrices are presented, and the relationship of some of these properties to the anaerobic char yield of the resins is described.

Kourtides, D. A.↗

NASA NTRS1979

Process for the manufacture of carbon or graphite fibers

Carbon or graphite fibers are manufactured by heating polyacrylonitrile fiber materials in various solutions and gases. They are characterized in that the materials are heated to temperatures from 150 to 300 C in a solution containing one or more acids from the group of carbonic acids, sulfonic acids, and/or phenols. The original molecular orientation of the fibers is preserved by the cyclization that occurs before interlacing, which gives very strong and stiff carbon or graphite fibers without additional high temperature stretching treatments.

Overhoff, D.↗

NASA NTRS1981

Electron reflection and secondary emission characteristics of sputter-textured pyrolytic graphite surfaces

Low secondary and reflected primary electron emission from the collector electrode surfaces is important for optimum collector efficiency and hence for high overall efficiency of microwave amplifier tubes used in communication satellites and in military systems. Ion sputter texturing of the surface effectively suppresses electron emission from pyrolytic graphite, which is a promising collector electrode material. Secondary and reflected primary electron emission characteristics of sputter textured pyrolytic graphite surfaces with microstructures of various sizes and densities are presented. The microstructure with the lowest electron emission levels, less than those of soot, consists of a dense array of tall, thin spires.

Wintucky, E. G.↗

NASA NTRS1981

Statistical distribution of mechanical properties for three graphite-epoxy material systems

Graphite-epoxy composites are playing an increasing role as viable alternative materials in structural applications necessitating thorough investigation into the predictability and reproducibility of their material strength properties. This investigation was concerned with tension, compression, and short beam shear coupon testing of large samples from three different material suppliers to determine their statistical strength behavior. Statistical results indicate that a two Parameter Weibull distribution model provides better overall characterization of material behavior for the graphite-epoxy systems tested than does the standard Normal distribution model that is employed for most design work. While either a Weibull or Normal distribution model provides adequate predictions for average strength values, the Weibull model provides better characterization in the lower tail region where the predictions are of maximum design interest. The two sets of the same material were found to have essentially the same material properties, and indicate that repeatability can be achieved.

Reese, C.↗

NASA NTRS1980

Environmental effects on graphite fiber reinforced PMR-15 polyimide

Studies were conducted to establish the effects of thermo-oxidative and hydrothermal exposure on the mechanical properties of T300 graphite fabric reinforced PMR-15 composites. The effects of hydrothermal exposure on the mechanical properties of HTS-2 continuous graphite fiber composites were also investigated. The thermo-oxidative stability characteristics of T300 fabric and T300 fabric/PMR-15 composites were determined. Flexural strengths of specimens were determined. The useful lifetime of T300 fabric/PMR-15 composites in air at 316 C was found to be about 100 hours. The useful lifetimes in air at 228 and 260 C were determined to be 500 and 1000 hours, respectively. Absorbed moisture was found to reduce the elevated temperature properties of both the T300 fabricate and HTS-2 continuous fiber composites. The moisture effect was found to be reversible.

Serafini, T. T.↗

NASA NTRS1981

Fatigue behavior of graphite-epoxy laminates at elevated temperatures

An examination and analysis is presented of the elevated temperature fatigue behavior of multidirectional graphite-epoxy laminates in terms of single lamina behavior. The laminate strength is predicted by considering the cyclic stress field in each lamina, the interlaminar stresses, and the experimentally determined temperature-effect 'shifting factors'. The initial failure of a lamina in a laminate is examined first in terms of stress redistribution, and then in terms of total failure and final laminate fracture; these analytical results are then compared with the actual fatigue behavior of T300/5208 graphite-epoxy composite laminates. In view of the agreement obtained, it is concluded that the temperature 'shifting factors' introduced here enable one to predict long-term behavior at a given temperature from short-time testing at elevated temperatures.

Rotem, A.↗