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At least 1,369 records · Page 76

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NASA NTRS1985

Properties of autoclaved Gr/PI composites made from improved tack PMR-15 prepreg

Autoclave processing studies were conducted, using improved tack PMR-15 prepreg, to determine the effect of tack enhancing PMR resin modifications on composite processability and mechanical properties. Improved tack graphite fiber reinforced PMR-15 prepregs were prepared and exposed to ambient conditions for various times and then autoclave molded into composites. Composite specimens were prepared and tested for flexural and interlaminar shear strengths at room temperature and 316 C. The retention of flexural and interlaminar shear strength as a function of exposure in air at 316 C was also determined. The results show that the modified PMR resin solutions provide prepreg with improved tack and drape retention characteristics without adversely affecting processability or mechanical properties of autoclave molded graphite fiber reinforced PMR-15 composites.

Vannucci, R. D.↗

NASA NTRS1985

Environmental stability graphite/PMR-15 composites

During the past few years the Boeing Company was screening graphite composites for use in hot areas of engine nacelle structure. Structural and thermal analyses have shown that there is the potential for a 25 to 30 percent weight savings by using a graphite polyimide (Gr/PI) composite material in this type of structure. Work conducted on the NASA CASTS program (Composites for Advanced Space Transportation Systems) amply demonstrated the capability of Graphite/PMR-15 for short term service (125 hours) at temperatures up to 589 K (600 F). In addition, the CASTS program demonstrated that large structures could be fabricated using Gr/PMR-15. In commercial applications, however, the requirement exists for long term service capability (tens of thousands of hours) at temperatures ranging upwards from 449 K (350 F). The results of Graphite/PMR-15 materials characterization efforts conducted at Boeing are presented with emphasis on materials properties after isothermal aging at temperatures of 449 K (350 F) and above.

Sheppard, C. H.↗

NASA NTRS1985

Abrasion behavior of aluminum and composite skin coupons, stiffened skins and stiffened panels representative of transport airplane structures

A three-phase investigation was conducted to compare the friction and wear response of aluminum and graphite-epoxy composite materials when subjected to loading conditions similar to those experienced by the skin panels on the underside of a transport airplane during an emergency belly landing on a runway surface. The first phase involved a laboratory test which used a standard belt sander to provide the sliding abrasive surface. Small skin-coupon test specimens were abraded over a range of pressures and velocities to determine the effects of these variables on the coefficient of friction and wear rate. The second phase involved abrading I-beam stiffened skins on actual runway surface over the same range of pressures and velocities used in the first phase. In the third phase, large stiffened panels which most closely resembled transport fuelage skin construction were abraded on a runway surface. This report presents results from each phase of the investigation and shows comparisons between the friction and wear behavior of the aluminum and graphite-epoxy composite materials.

Jackson, K. E.↗

NASA NTRS1986

High performance mixed bisimide resins and composites based thereon

Mixtures of bismaleimide/biscitraconirnide resins produces materials which have better handling, processing or mechanical and thermal properties, particularly in graphite composites, than materials made with the individual resins. The mechanical strength of cured graphite composites prepared from a 1:1 copolymer of such bisimide resins is excellent at both ambient and elevated temperatures. The copolymer mixture provides improved composites which are lighter than metals and replace metals in many aerospace applications.

Parker, J. A.↗

NASA NTRS1985

A fracture mechanics analysis of impact damage in a thick composite laminate

Graphite/epoxy filament-wound cases (FWC) for the solid rocket motors of the space shuttle are being made by NASA. The FWC cases are wound with AS4W graphite fiber impregnated with an epoxy resin and are about 1.4 inches or more thick. Graphite-epoxy composite laminates, unlike metals, can be damaged easily by low velocity impacts of objects like dropped tools. The residual tension strength of the FWC laminate, after impact, is being studied at Langley Research Center. The conditions that give minimum visual evidence of damage are being emphasized. A fracture mechanics analysis was developed to predict the residual strength, after impact, using radiographs to measure the size of the damage and an equivalent surface crack to represent the damage.

Poe, C. C., Jr.↗

NASA NTRS1986

Compression failure mechanisms of composite structures

An experimental and analytical study was conducted to delineate the compression failure mechanisms of composite structures. The present report summarizes further results on kink band formation in unidirectional composites. In order to assess the compressive strengths and failure modes of fibers them selves, a fiber bundle was embedded in epoxy casting and tested in compression. A total of six different fibers were used together with two resins of different stiffnesses. The failure of highly anisotropic fibers such as Kevlar 49 and P-75 graphite was due to kinking of fibrils. However, the remaining fibers--T300 and T700 graphite, E-glass, and alumina--failed by localized microbuckling. Compressive strengths of the latter group of fibers were not fully utilized in their respective composite. In addition, acoustic emission monitoring revealed that fiber-matrix debonding did not occur gradually but suddenly at final failure. The kink band formation in unidirectional composites under compression was studied analytically and through microscopy. The material combinations selected include seven graphite/epoxy composites, two graphite/thermoplastic resin composites, one Kevlar 49/epoxy composite and one S-glass/epoxy composite.

Hahn, H. T.↗

NASA NTRS1986

A study of Kapton degradation under simulated shuttle environment

A system was developed which employs a source of low energy oxygen ion to simulate the shuttle low Earth orbit environment. This source, together with diagnostic tools including surface analysis ans mass spectroscopic capability, was used to measure the dependence of ion energy of the oxygen induced CO signals from pyrolytic graphite and Kapton. For graphite the CO signal was examined at energies ranging form 4.5 to 465 eV and for Kapton from 4.5 to 188 eV. While the relative quantum yields inferred from the data are reasonably precise, there are large uncertainties in the absolute yields because of the assumptions necessary to covert the measured signal strengths to quantum yields. These assumptions are discussed in detail.

Eck, T. G.↗

NASA NTRS1986

Secondary-electron-emission losses in multistage depressed collectors and traveling-wave-tube efficiency improvements with carbon collector electrode surfaces

Secondary-electron-emission losses in multistage depressed collectors (MDC's) and their effects on overall traveling-wave-tube (TWT) efficiency were investigated. Two representative TWT's and several computer-modeled MDC's were used. The experimental techniques provide the measurement of both the TWT overall and the collector efficiencies. The TWT-MDC performance was optimized and measured over a wide range of operating conditions, with geometrically identical collectors, which utilized different electrode surface materials. Comparisons of the performance of copper electrodes to that of various forms of carbon, including pyrolytic and iisotropic graphites, were stressed. The results indicate that: (1) a significant improvement in the TWT overall efficiency was obtained in all cases by the use of carbon, rather than copper electrodes, and (2) that the extent of this efficiency enhancement depended on the characteristics of the TWT, the TWT operating point, the MDC design, and collector voltages. Ion textured graphite was found to be particularly effective in minimizing the secondary-electron-emission losses. Experimental and analytical results, however, indicate that it is at least as important to provide a maximum amount of electrostatic suppression of secondary electrons by proper MDC design. Such suppression, which is obtained by ensuring that a substantial suppressing electric field exists over the regions of the electrodes where most of the current is incident, was found to be very effective. Experimental results indicate that, with proper MDC design and the use of electrode surfaces with low secondary-electron yield, degradation of the collector efficiency can be limited to a few percent.

Ramins, P.↗

NASA NTRS1984

Acousto-ultrasonic measurements to monitor damage during fatigue of composites

An acousto-ultrasonic nondestructive testing method used to monitor damage during static and fatigue loading of thin graphite epoxy laminates is described. The experimental procedure, the signal analysis by the Fast Fourier Transform (FFT) algorithm, and the results of this analysis are discussed. Quasi-static tension tests showed a sharp decrease in the quantitative parameters when transverse cracks developed in the 90 degrees plies of a (0, 90/2/)s laminate. When internal micro-delaminations unite to form macro-delaminations, a sharp decrease in the parameters is also observed. The parameters are found to correlate well with other indications of damage development such as stiffness and degradation. The root mean square value of the moment is found to be more sensitive to damage than stiffness. Various signals and spectrums of graphite epoxy systems are presented.

Govada, A.↗

NASA NTRS1985

Effect of substituted phenylnadimides on processing and properties of PMR polyimide composites

Three nitrophenylnadimide cure initiators and two phenylnadimides (without nitros) were evaluated as additives to PMR-15 resins and Celion 6000 graphite fiber composites. The results of a resin screening study eliminated all of the additives except 3-nitrophenylnadimide (NO2PN) for use as a low temperature curing additive for PMR-15. Thus, NO2PN and the two control additives were investigated in PMR-15 formulations from which Celion 6000 graphite fiber/PMR-15 composites were processed both with low temperature (274 C) and normal (316 C) cure cycles. Comparisons of the two processing cycles, the resultant glass transition temperatures (Tg), the ambient, 274 and 316 C composite mechanical properties determined before and after 316 C postcure, the 316 C thermo-oxidative weight losses and the retention of 316 C composite mechanical properties are presented. Empirical correlations of the type and amount of nadimide additives with processing parameters, Tg, composite mechanical properties, composite thermo-oxidative stability and long term retention of 316 C composite mechanical properties are also presented.

Alston, W. B.↗

NASA NTRS1984

Biaxial fatigue properties of thin-walled composite tubes

A testing method for obtaining fatigue and fracture data under biaxial loading and for negative R-ratios is described. Thin-walled tubes of 1-in. outer nominal diameter with a tapered grip section were tested in an Axial-Torsion Servohydraulic Testing Machine with a 463 Data Control Processor at frequencies below 10 Hz up to the 1-million cycle range. Room temperature, uniaxial constant amplitude fatigue data for high temperature cure graphite-epoxy material are presented at R = -0.4 for a (+, - 5 deg) fiber orientation. The fatigue curve is almost horizontal. Tests on (+, - 5 deg) 'SP-328' resulted in failures which predominantly originated in the grip section. Presently, a new specimen design is being tested intended to eliminate cracking in the grip section. Biaxial in-phase loading of (0/+, - 45) tubes of Fiberite graphite-epoxy resulted in much steeper fatigue curves than observed under uniaxial loading.

Krempl, E.↗

NASA NTRS1985

Free edge strain concentrations in real composite laminates Experimental-theoretical correlation

The magnitude of the maximum shear strain at the free edge of axially loaded theta (2)/theta(2)(s) and (+ or - theta(2) (s) composite laminates was investigated experimentally and numerically to ascertain the actual value of strain concentration in resin matrix laminates and to determine the accuracy of finite element results. Experimental results using moire interferometry show large, but finite, shear strain concentrations at the free edge of graphite-epoxy and graphite-polyimide laminates. Comparison of the experimental results with those obtained using several different finite element representations showed that a four node isoparametric finite element provided the best and most trouble free numerical results. The results indicate that the ratio of maximum shear strain at the free edge to applied axial strain varies with fiber orientation and does not exceed nine for the most critical angle which is 15 deg.

Herakovich, C. T.↗

NASA NTRS1985

Orthotropic fracture using a singular isoparametric element

The six noded quarter point natural isoparametric triangular element is employed to obtain displacement and stress distributions in the vicinity of the crack tip in a center cracked tensile coupon of unidirectional graphite epoxy. The material is considered to be homogeneous, elastic and orthotropic. The finite element results are compared to the analytical solution of anisotropic elasticity. Convergence as a function of mesh parameters is studied for isotropic and orthotropic materials. It is shown that displacements converge faster than stresses and that meshes which are convergent for isotropic and orthotropic materials. It is shown that displacements converge faster than stresses and that meshes which are convergent for isotropic materials are not completely convergent for the highly orthotropic graphite epoxy.

Gregory, M. A.↗

NASA NTRS1985

Design aspects of the XV-15 advanced technology blade program

The paper discusses the design of a tiltrotor blade for application to the XV-15 research demonstration aircraft. The design features 43 deg nonlinear twist and nonuniform tapered planform. The structure is composite with extensive use of graphite in the primary structure. Instrumentation and wiring is encapsulated in the composite structure during the cure cycle. The tip shell is removable, providing access to the tracking and balance weights. This feature provides research facility of alternate tip configurations. The cuff is similarly removable. The graphite epoxy system used is high strain American Cyanamid's Celion 6000ST/Cycom 950. This is the first application of this material in rotor blades.

Smith, K. E.↗

NASA NTRS1985

Effect of fiber and matrix maximum strain on the energy absorption of composite materials

Static crushing tests were conducted on graphite composite tubes to examine the influence of fiber and matrix maximum strain at failure on the energy absorption capability of graphite reinforced composite material. Fiber and matrix maximum strain at failure were determined to significantly effect energy absorption. The higher strain at failure composite material system, AS-4/5245, exhibited superior energy absorption capability compared to AS-4/934, T300/5245 or T300/934 composite material. Results of this investigation suggest that to achieve maximum energy absorption from a composite material a matrix material that has a higher strain at failure than the fiber reinforcement should be used.

Farley, G. L.↗

NASA NTRS1985

Dual-ion-beam deposition of carbon films with diamond-like properties

A single and dual ion beam system was used to generate amorphous carbon films with diamond like properties. A methane/argon mixture at a molar ratio of 0.28 was ionized in the low pressure discharge chamber of a 30-cm-diameter ion source. A second ion source, 8 cm in diameter was used to direct a beam of 600 eV Argon ions on the substrates (fused silica or silicon) while the deposition from the 30-cm ion source was taking place. Nuclear reaction and combustion analysis indicate H/C ratios for the films to be 1.00. This high value of H/C, it is felt, allowed the films to have good transmittance. The films were impervious to reagents which dissolve graphitic and polymeric carbon structures. Although the measured density of the films was approximately 1.8 gm/cu cm, a value lower than diamond, the films exhibited other properties that were relatively close to diamond. These films were compared with diamond like films generated by sputtering a graphite target.

Mirtich, M. J.↗

NASA NTRS1986

Effects of thermal cycling on graphie-fiber-reinforced 6061 aluminum

Graphite-reinforced aluminum alloy metal-matrix composites are among materials being considered for structural components in dimensionally stable space structures. This application requires materials with low values of thermal expansions and high specific stiffnesses. They must remain stable during exposures to the space environment for periods extending to 20 years. The effects of thermal cycling on the thermal expansion behavior and mechanical properties of Thornel P100 graphite 6061 aluminum composites, as fabricated and after thermal processing to eliminate thermal strain hysteresis, have been investigated. Two groups of composites were studied: one was fabricated by hot roll bonding and the other by diffusion bonding. Processing significantly reduced strain hysteresis during thermal cycling in both groups and improved the ultimate tensile strength and modulus in the diffusion-bonded composites. Thermal cycling stabilized the as-fabricated composites by reducing the residual fabrication stress and increased the matrix strength by metallurgical aging. Thermal expansion behavior of both groups after processing was insensitive to thermal cycling. Data scatter was too large to determine effects of thermal cycling on the mechanical properties. The primary effects of processing and thermal cycling can be attributed to changes in the metallurgical condition and stress state of the matrix.

Dries, G. A.↗

NASA NTRS1986

Effect of hard particle impacts on the atomic oxygen survivability of reflector surfaces with transparent protective overcoats

Silver mirror samples with protective coatings were subjected to a stream of 27 microns alumina particles to induce pinhole defects. The protective coating consisted of a layer of aluminum dioxide over silver followed by a layer of silicon dioxide over the alumina. Samples were prepared on both graphite-epoxy composite and fused quartz substrates. After exposure to the hard particle stream, the samples were exposed to an oxygen plasma environment in a laboratory plasma asher. The effects of both the hard particles and the oxygen plasma were documented by both reflectance measurements and scanning electron microscopy. The results indicated that oxidative damage to the silver reflecting layer continues beyond that of the erosively exposed silver. Oxidative undercutting of the silver layer and graphite-epoxy substrate continues in undamaged areas through adjacent, particle damaged defect sites. This may have implications for the use of such mirrors in a space station solar dynamic power system.

Gulino, D. A.↗