Scale effects in buckling, postbuckling, and crippling of graphite-epoxy Z-section stiffeners
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Wieland, Todd M.↗
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Properties of hybrid CVD/PAN graphite fibers and their bromine intercalation compounds
A hybrid fiber with a PAN core surrounded by a vapor grown carbon fiber (VGCF) sheath was fabricated using a proprietary process. The density, ultimate tensile strength, Young's modulus, and resistivity of pristine and bromine intercalated fibers made by this technique having diameters varying from 5 to 50 microns were compared with the values predicted from the rule of mixtures model. For both the pristine and intercalated fibers, the density, ultimate tensile strength, and Young's modulus of the fibers were lower than predicted, but the resistivity was measured to be consistent with predictions. The lower than theoretical mechanical properties may be evidence of a low density disordered interface between the core and the sheath which would lower the density and degrade the mechanical properties, but would leave the resistivity nearly unaffected. Intercalation had little if any effect on the ultimate tensile strength and Young's modulus, but raised the density by about 11 pct., and lowered the resistivity by an order of magnitude. The diameter dependence of the resistivity showed evidence of a depletion layer of the type found in VGCF.
Gaier, James R.↗
Delamination behavior of quasi-isotropic graphite epoxy laminates subjected to tension and torsion loads
Sixteen and thirty-two ply quasi-isotropic laminates fabricated from AS4/3501-6 were subjected to pure tension, simultaneous tension and torsion, and torsion fatigue. Layups tested were (45 sub n/-45 sub n/0 sub n/90 sub n) sub s, with n = 2 or 4. A torsion damage pattern consisting of a localized matrix crack and delaminations was characterized, and the measured torsional stiffnesses were compared with calculated values. It was found that a combination of tension and torsion led to failure at smaller loads than either type of deformation acting alone. Further work is required to determine the exact form of the failure criterion.
Hinkley, J. A.↗
Dimensionally stable, graphite-fiber reinforced composite mirror technology
A research project intended to design and fabricate a dimensionally stable 2 mu spherical mirror 0.4 m in diameter, carried out by Composite Optics, Incorporated, is briefly reported. A laminate for the facesheet and the back surface that is intended to be exceptionally uniform in-plane has been designed using the P75/ERL 1999 material. The core is designed to be pseudoisotropic in all three directions.
Kulick, Shel↗
Chemical nonequilibrium Navier-Stokes solutions for hypersonic flow over an ablating graphite nosetip
The general boundary conditions including mass and energy balances of chemically equilibrated or nonequilibrated gas adjacent to ablating surfaces have been derived. A computer procedure based on these conditions was developed and interfaced with the Navier-Stokes solver for predictions of the flow field, surface temperature, and surface ablation rates over re-entry space vehicles with ablating Thermal Protection Systems (TPS). The Navier-Stokes solver with general surface thermochemistry boundary conditions can predict more realistic solutions and provide useful information for the design of TPS. A test case with a proposed hypersonic test vehicle configuration and associated free stream conditions was developed. Solutions with various surface boundary conditions were obtained, and the effect of nonequilibrium gas as well as surface chemistry on surface heating and ablation rate were examined. The solutions of the GASP code with complete ablating surface conditions were compared with those of the ASC code. The direction of future work is also discussed.
Chen, Y. K.↗
Indentation-flexure and low-velocity impact damage in graphite epoxy laminates
Static indentation-flexure and low-velocity impact tests were performed on quasi-isotropic and cross-ply graphite/epoxy composite laminates. The load-deflection relations in static tests and impact force history in impact tests were recorded. The damage was assessed by using ultrasonic C-scanning and photo-micrographic techniques. Some features of the static behavior were explained by simple analytical models. A good correlation existed between the load-deflection curves for static and impact loading. It was found that results from a few static indentation-flexure tests can be used to predict the impact force history and delamination radius in composite laminates due to low-velocity impact.
Kwon, Young S.↗
Preliminary Investigation of Graphite, Silicon Carbide, and Several Polymer--glass-cloth Laminates in a Mach Number 2 Air Jet at Stagnation Temperatures of 3,000 Degrees F and 4,000 Degrees F
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Casey, Francis W , Jr↗
Trade study plan for Graphite Composite Primary Structure (GCPS)
This TA 2 document (with support from TA 1) describes the trade study plan that will identify the most suitable structural configuration for an SSTO winged vehicle capable of delivering 25,000 lbs to a 220 nm circular orbit at 51.6 degree inclination For this most suitable configuration the structural attachment of the wing, and the most suitable GCPS composite materials for intertank, wing, tail and thrust structure are identified. This trade study analysis uses extensive information derived in the TA 1 trade study plan and is identified within the study plan. In view of this, for convenience, the TA 1 study plan is included as an appendix to this document.
Greenberg, H. S.↗
Milestone 4: Thrust structure concepts and IHM screening Graphite Composite Primary Structure (GCPS)
The first part of the task was to select up to three promising thrust structure constructions and to select materials for screening tests. Part of the nondestructive evaluation and inspection (NDE/I) and integrated health management (IHM) task is to acquire and develop NDE/I sensor technologies and to integrate those sensors into the full scale test articles which will be produced under the TA2 program. Review of the anticipated fault modes and the available sensor technology data indicates that three sensor technologies should be assessed for the in-situ monitoring of the composite primary structure elements. These are: ultrasonics (dry contact), acoustic emissions, and fiber optics (embedded or attached). In fact, a combination of sensor technologies will be needed to detect and evaluate the fault modes; not only do sensor technology have specific capabilities and applicability, but the three Gr/Ep primary structures being demonstrated under the TA2 effort have differing requirements based on their respective failure modes and designs.
Greenberg, H. S.↗
The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons
We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon grains which extends into the molecular domain. We have considered a wide range of physical conditions for the interstellar gas (1 less than G(sub 0) less than 10(exp 5), with G(sub 0) being the intensity of the incident far-UV field in units of the Habing interstellar radiation field; 2.5 x 10( exp -3) less than n(sub e) less than 75/cu cm, with n(sub e) being the electron density; 10 less than T less than 10,000 K, with T being the gas temperature). The results show that about half of the heating is due to grains less than 1500 C atoms (less than 15 A). The other half originates in somewhat larger grains (1500-4.5 x 10(exp 5) C atoms; 15 less than 100 A). While grains larger than this do absorb about half of the available far-UV photons, they do not contribute appreciably to the gas heating. This strong dependence of gas heating on size results from the decrease in yield and from the increased grain charge (hence larger Coulomb losses) with increasing grain size. We have determined the net photoelectric heating rate and evaluated a simple analytical expression for the heating efficiency, dependent only on G(sub 0), T, and n(sub e). This expression is accurate to 3% over the whole parameter range and is valid up to gas temperatures of 10(exp 4) K, at which point the dominant gas-dust heat exchange mechanism becomes the recombination of electrons with grains rather than photoelectric ejection. The calculated heating efficiency for neutral grains is in good agreement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the Far Infrared Absolute Spectrometer (FIRAS) observations on the Cosmic Background Explorer Satellite. Finally, our photoelectric heating efficiency is compared to previous studies.
Bakes, E. L. O.↗
Thermal and Mechanical Durability of Graphite-Fiber-Reinforced PMR-15 Composites
Earlier work, which reported relationships between compression properties and elevated temperature aging times and weight losses, also pointed out the apparent influence of surface layer formation and growth on the retention of compression properties during extended aging times. Since that time, studies have been directed toward evaluating the growth of the surface layer. This layer was found to change in its composition and features as the aging temperature changed. Microcracks and small voids initiated and advanced inward at all temperatures. Visible oxidation at the surface occurred only at temperatures above 260 C. Relationships between layer thickness and aging time and temperature were evaluated and empirically formulated. Then, the compression properties were graphically related to the surface layer thickness with excellent correlation. The surface layer was observed to influence the compression strength of thin samples only.
Bowles, Kenneth J.↗
Thermal and Mechanical Durability of Graphite-Fiber-Reinforced PMR-15 Composites
Earlier work, which reported relationships between compression properties and elevated temperature aging times and weight losses, also pointed out the apparent influence of surface layer formation and growth on the retention of compression properties during extended aging times. Since that time, studies have been directed toward evaluating the growth of the surface layer. This layer was found to change in its composition and features as the aging temperature changed. Microcracks and small voids initiated and advanced inward at all temperatures. Visible oxidation at the surface occurred only at temperatures above 260 deg. C. Relationships between layer thickness and aging time and temperature were evaluated and empirically formulated. Then, the compression properties were graphically related to the surface layer thickness with excellent correlation. The surface layer was observed to influence the compression strength of thin samples only.
Bowles, Kenneth J.↗
Raman Spectroscopic Detection of Graphitic Carbon of Biogenic Parentage in an Ancient South African Chert
The detection of reduced carbon in martian rocks and soils is important in the search for evidence of life. A Raman spectroscopic study of South Africa chert reveals that 50 ppm carbon or less can be determined by this technique. Additional information is contained in the original extended abstract.
Wang, Alian↗
Evaluation of Graphite Fiber/Polyimide PMCs from Hot Melt versus Solution Prepreg
Carbon fiber reinforced high temperature polymer matrix composites (PMC) have been extensively investigated as potential weight reduction replacements of various metallic components in next generation high performance propulsion rocket engines. The initial phase involves development of comprehensive composite material-process-structure-design-property in-service performance correlations and database, especially for a high stiffness facesheet of various sandwich structures. Overview of the program plan, technical approaches and current multi-team efforts will be presented. During composite fabrication, it was found that the two large volume commercial prepregging methods (hot-melt vs. solution) resulted in considerably different composite cure behavior. Details of the process-induced physical and chemical modifications in the prepregs, their effects on composite processing, and systematic cure cycle optimization studies will be discussed. The combined effects of prepregging method and cure cycle modification on composite properties and isothermal aging performance were also evaluated.
Shin, Eugene E.↗
Studies on Hot-Melt Prepregging of PMR-II-50 Polyimide Resin with Graphite Fibers
A Second generation PMR (in situ Polymerization of Monomer Reactants) polyimide resin, PMR-II-50, has been considered for high temperature and high stiffness space propulsion composites applications for its improved high temperature performance. As part of composite processing optimization, two commercial prepregging methods: solution vs. hot-melt processes were investigated with M40J fabrics from Toray. In a previous study a systematic chemical, physical, thermal and mechanical characterization of these composites indicated that poor resin-fiber interfacial wetting, especially for the hot-melt process, resulted in poor composite quality. In order to improve the interfacial wetting, optimization of the resin viscosity and process variables were attempted in a commercial hot-melt prepregging line. In addition to presenting the results from the prepreg quality optimization trials, the combined effects of the prepregging method and two different composite cure methods, i.e., hot press vs. autoclave on composite quality and properties are discussed.
Shin, E. Eugene↗
Studies on Hot-Melt Prepregging on PRM-II-50 Polyimide Resin with Graphite Fibers
A second generation PMR (in situ Polymerization of Monomer Reactants) polyimide resin PMR-II-50, has been considered for high temperature and high stiffness space propulsion composites applications for its improved high temperature performance. As part of composite processing optimization, two commercial prepregging methods: solution vs. hot-melt processes were investigated with M40J fabrics from Toray. In a previous study a systematic chemical, physical, thermal and mechanical characterization of these composites indicated the poor resin-fiber interfacial wetting, especially for the hot-melt process, resulted in poor composite quality. In order to improve the interfacial wetting, optimization of the resin viscosity and process variables were attempted in a commercial hot-melt prepregging line. In addition to presenting the results from the prepreg quality optimization trials, the combined effects of the prepregging method and two different composite cure methods, i.e. hot press vs. autoclave on composite quality and properties are discussed.
Shin, E. Eugene↗
Evaluation of Graphite Fiber/Polyimide PMCs from Hot Melt vs Solution Prepreg
Carbon fiber reinforced high temperature polymer matrix composites (PMC) have been extensively investigated as potential weight reduction replacements of various metallic components in next generation high performance propulsion rocket engines. The initial phase involves development of comprehensive composite material-process-structure-design-property-in-service performance correlations and database, especially for a high stiffness facesheet of various sandwich structures. Overview of the program plan, technical approaches and current multi-team efforts will be presented. During composite fabrication, it was found that the two large volume commercial prepregging methods (hot-melt vs. solution) resulted in considerably different composite cure behavior. Details of the process-induced physical and chemical modifications in the prepregs, their effects on composite processing, and systematic cure cycle optimization studies will be discussed. The combined effects of prepregging method and cure cycle modification on composite properties and isothermal aging performance were also evaluated.
Shin, E. Eugene↗
Electroformed Nickel-Graphite Composite
Future x-ray astronomy will demand larger optics than Chandra, currently in orbit. Ways must be devised to produce cheaper and lighter x-ray mirrors to save the cost of manufacturing and launching this future telescope. One technique, being developed at Marshall Space Flight Center and elsewhere, is electroformed nickel replication technique, wherein mirror shells are electroformed (using pure nickel or a nickel alloy) onto super-polished and figured aluminum mandrels and are subsequently released by cooling. This technique can produce relatively inexpensive mirrors, but is hampered by the high density of nickel (8.9 g / cm3). An alternative is to develop a composite, with lower mass density and compatible mechanical properties to the nickel cobalt alloy, as the mirror shell material.
Xiong-Skiba, Pei↗