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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Portable Microleak-Detection System

The figure schematically depicts a portable microleak-detection system that has been built especially for use in testing hydrogen tanks made of polymer-matrix composite materials. (As used here, microleak signifies a leak that is too small to be detectable by the simple soap-bubble technique.) The system can also be used to test for microleaks in tanks that are made of other materials and that contain gases other than hydrogen. Results of calibration tests have shown that measurement errors are less than 10 percent for leak rates ranging from 0.3 to 200 cm3/min. Like some other microleak-detection systems, this system includes a vacuum pump and associated plumbing for sampling the leaking gas, and a mass spectrometer for analyzing the molecular constituents of the gas. The system includes a flexible vacuum chamber that can be attached to the outer surface of a tank or other object of interest that is to be tested for leakage (hereafter denoted, simply, the test object). The gas used in a test can be the gas or vapor (e.g., hydrogen in the original application) to be contained by the test object. Alternatively, following common practice in leak testing, helium can be used as a test gas. In either case, the mass spectrometer can be used to verify that the gas measured by the system is the test gas rather than a different gas and, hence, that the leak is indeed from the test object.

Rivers, H. Kevin↗
NASA NTRS1992

Properties of Graphite Fiber Reinforced Copper Matrix Composites for Space Power Applications

The thermal and mechanical properties of pitch-based graphite fiber reinforced copper matrix (Gr/Cu) composites usable for space applications such as radiator fins were investigated. Thermal conductivity was measured as a function of fiber volume fraction and architecture. Results showed for unidirectional P-100 Gr/Cu composites, the longitudinal thermal conductivity was nearly independent of fiber volume fraction. Transverse thermal conductivities (perpendicular to the fibers) were strongly affected by the fiber volume fraction with higher volume fractions resulting in lower thermal conductivities. The effect of architecture on thermal conductivity followed the cosine squared law for simple architectures. Insufficient data are available currently to model more complex architectures, but adding fibers in the direction of the heat flow increases the thermal conductivity as low conductivity plies are supplemented by high conductivity plies. Thermal expansion tests were conducted on the Gr fibers and Gr/Cu composites. The results show a considerable thermal expansion mismatch between the fibers and the Cu matrix. The longitudinal thermal expansion showed a strong dependence on the architecture of the Gr/Cu composites. The composites also show a thermal expansion hysteresis. The hysteresis was eliminated by an engineered interface. Mechanical testing concentrated on the dynamic modulus and strength of the composites. The dynamic modulus of the Gr/Cu composites was 305 GPa up to 400 C, a value equivalent to Be. The strengths of the composites were less than expected, but this is attributed to the poor bond across the interface between the Gr fibers and Cu matrix. Testing of composites with an engineered interface is expected to yield strengths nearer the values predicted by the rule of mixtures.

Ellis, David L.↗
NASA NTRS1989

Mechanical properties of several neat polymer matrix materials and unidirectional carbon fiber-reinforced composites

The mechanical and physical properties of three neat matrix materials, i.e., PEEK (polyetheretherketone) thermoplastic, Hexcel F155 rubber-toughened epoxy and Hercules 8551-7 rubber-toughened epoxy, were experimentally determined. Twelve unidirectional carbon fiber composites, incorporating matrix materials characterized in this or earlier studies (with one exception; the PISO(sub 2)-TPI matrix itself was not characterized), were also tested. These composite systems included AS4/2220-1, AS4/2220-3, T500/R914, IM6/HX1504, T300/4901A (MDA), T700/4901A (MDA), T300/4901B (MPDA), T700/4901B (MPDA), APC2 (AS4/PEEK, ICI), APC2 (AS4/PEEK, Langley Research Center), AS4/8551-7, and AS4/PISO(sub 2)-TPI. For the neat matrix materials, the tensile, shear, fracture toughness, coefficient of thermal expansion, and coefficient of moisture expansion properties were measured as a function of both temperature and moisture content. For the unidirectional composites, axial and transverse tensile, longitudinal shear, coefficient of thermal expansion, and coefficient of moisture expansion properties were determined, at room temperature and 100 C.

Coguill, Scott L.↗
NASA NTRS1997

Strong and Tough Hi-Nicalon Fiber-Reinforced Celsian Matrix Composites

Strong, tough and almost fully dense Hi-Nicalon/BN/SiC fiber reinforced celsian matrix composites have been fabricated by impregnation of the fiber tows with the matrix slurry, winding on a drum, stacking the prepreg tapes in the desired orientation, and hot pressing. The monoclinic celsian phase in the matrix was produced in situ, during hot pressing, from a mixed oxide precursor. The unidirectional composites having approx. 42 volume percent of fibers exhibited graceful failure with extensive fiber pullout in three-point bend tests at room temperature. Values of first matrix cracking stress and strain were 435 +/- 35 MPa and 0.27 +/- 0.01 %, respectively, and ultimate strengths of 900 +/- 60 MPa were observed. The Young's modulus of the composites was 165 +/- 5 GPa.

Bansal, Narottam P.↗
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 NTRS2008

Post Test Evaluation of HSCT Nozzle Acoustic Liner Subcomponents Subjected to a Hot Acoustic Durability Test

The acoustic liner system designed for use in the High Speed Civil Transport (HSCT) was tested in a thermal-acoustic environment. Five ceramic matrix composite (CMC) acoustic tile configurations, five bulk acoustic absorbers, and one thermal protection system design were tested. The CMC acoustic tiles were subjected to two 2 3/4 hr ambient temperature acoustic exposures to measure their dynamic response. One exposure was conducted on the tiles alone and the second exposure included the tiles and the T-foam bulk absorber. The measured tile RMS strains were small. With or without the T-foam absorber, the dynamic strains were below strain levels that would cause damage during fatigue loading. After the ambient exposure, a 75-hr durability test of the entire acoustic liner system was conducted using a thermal-acoustic cycle that approximated the anticipated service cycle. Acoustic loads up to 139 dB/Hz and temperatures up to 1670 F (910 C) were employed during this 60 cycle test. During the durability test, the CMC tiles were exposed to temperatures up to 1780 F and a transient through thickness gradient up to 490 F. The TPS peak temperatures on the hot side of the panels ranged from 750 to 1000 F during the 60 cycles. The through thickness delta T ranged from 450 to 650 F, varying with TPS location and cycle number. No damage, such as cracks or chipping, was observed in the CMC tiles after completion of the testing. However, on tile warped during the durability test and was replaced after 43 or 60 cycles. No externally observed damage was found in this tile. No failure of the CMC fasteners occurred, but damage was observed. Cracks and missing material occurred, only in the fastener head region. No indication of damage was observed in the T-foam acoustic absorbers. The SiC foam acoustic absorber experienced damage after about 43 cycles. Cracking in the TPS occurred around the attachment holes and under a vent. In spite of the development of damage, the TPS maintained its insulative capability throughout the durability test. The durability test results demonstrate damage-tolerant CMC tile, CMC fastener, TPS, and T-foam absorber designs for the combined thermal and acoustic engine nozzle environment.

Verrilli, Michael J.↗
NASA NTRS1991

A curved beam test specimen for determining the interlaminar tensile strength of a laminated composite

A curved beam type of test specimen is evaluated for use in determining the through-the-thickness strength of laminated composites. Two variations of a curved beam specimen configuration (semicircular and elliptical) were tested to failure using static and fatigue loads. The static failure load for the semicircular specimens was found to be highly sensitive to flaw content, with the specimens falling into two distinct groups. This result supports the use of proof testing for structural validation. Static design allowables are derived based on the Weibull distribution. Fatigue data indicates no measured increase in specimen compliance prior to final fracture. All static and fatigue failures at room temperature dry conditions occurred catastrophically. The elliptical specimens demonstrated unusually high failure strengths indicating the presence of phenomena requiring further study. Results are also included for specimens exposed to a wet environment showing a matrix strength degradation due to moisture content. Further testing is underway to evaluate a fatigue methodology for matrix dominated failures based on residual static strength (wearout).

Hiel, Clement C.↗
NASA NTRS1978

Development of advanced fuel cell system

An experimental program was conducted continuing the development effort to improve the weight, life, and performance characteristics of hydrogen-oxygen alkaline fuel cells for advanced power systems. These advanced technology cells operate with passive water removal which contributes to a lower system weight and extended operating life. Endurance evaluation of two single cells and two, two-cell plaques was continued. Three new test articles were fabricated and tested. A single cell completed 7038 hours of endurance testing. This cell incorporated a Fybex matrix, hybrid-frame, PPF anode, and a 90 Au/10 Pt cathode. This configuration was developed to extend cell life. Two cell plaques with dedicated flow fields and manifolds for all fluids did not exhibit the cell-to-cell electrolyte transfer that limited the operating life of earlier multicell plaques.

Gitlow, B.↗
NASA NTRS1978

Thermal-vacuum facility with in-situ mechanical loading

The paper describes a thermal-vacuum space simulator used to assess property changes of fiber-reinforced polymer composite systems. The facility can achieve a vacuum of approximately .0000001 torr with temperatures ranging from -200 to +300 F. Some preliminary experimental results are presented for materials subjected to thermal loading up to 200 F. The tests conducted include the evaluation of matrix modulus and strength, coefficients of thermal expansion, and fracture toughness. Though the experimental program is at an early stage, the data appear to indicate that these parameters are influenced by hard vacuum.

Tennyson, R. C.↗
NASA NTRS1979

Tungsten wire-reinforced superalloys for 1093 C (2000 F) turbine blade applications

Various combinations of fiber and matrix materials were fabricated and evaluated for the purpose of selecting a specific combination that exhibited the best overall properties for a turbine blade application. A total of seven matrix alloys, including Hastelloy X, Nimonic 80A, Inconel 600, Inconel 625, IN-102, FeCrA1Y, were investigated reinforced with either 218CS tungsten, or W-Hf-C fibers. Based on preliminary screening studies, FeCrA1Y, Inconel 600 and Inconel 625 matrix composites systems were selected for extended thermal cycle tests and for property evaluations which included stress rupture, impact, and oxidation resistance. Of those investigated, the FeCrA1Y matrix composite system exhibited the best overall properties required for a turbine blade application. The W-Hf-C/FeCrA1Y system was selected for further property evaluation. Tensile strength values of up to 724 MPa (105,000 psi) were obtained for this material at 982 C and 607 MPa at 1093 C.

Friedman, G. I.↗
NASA NTRS1982

High temperature composites. Status and future directions

A summary of research investigations of manufacturing methods, fabrication methods, and testing of high temperature composites for use in gas turbine engines is presented. Ceramic/ceramic, ceramic/metal, and metal/metal composites are considered. Directional solidification of superalloys and eutectic alloys, fiber reinforced metal and ceramic composites, ceramic fibers and whiskers, refractory coatings, metal fiber/metal composites, matrix metal selection, and the preparation of test specimens are discussed.

Signorelli, R. A.↗
NASA NTRS1982

High-temperature composites - Status and future directions

A summary of research investigations of manufacturing methods, fabrication methods, and testing of high temperature composites for use in gas turbine engines is presented. Ceramic/ceramic, ceramic/metal, and metal/metal composites are considered. Directional solidification of superalloys and eutectic alloys, fiber reinforced metal and ceramic composites, ceramic fibers and whiskers, refractory coatings, metal fiber/metal composites, matrix metal selection, and the preparation of test specimens are discussed. Previously announced in STAR as N82-30336

Signorelli, R. A.↗
NASA NTRS1983

High-speed wide band 20 x 20 microwave switch matrix

The use of a dynamic switching matrix for future communication satellites will significantly increase the communication channel capacity and improve the system capability and flexibility. This paper describes the design and development of a unique coupler crossbar 20 x 20 microwave switch matrix. This paper also presents the test results of the proof-of-concept model that meets the requirements for a high-speed satellite switched, time division multiple access (SS-TDMA) system.

Coban, E.↗
NASA NTRS1986

An efficient method for solving the steady Euler equations

An efficient numerical procedure for solving a set of nonlinear partial differential equations is given, specifically for the steady Euler equations. Solutions of the equations were obtained by Newton's linearization procedure, commonly used to solve the roots of nonlinear algebraic equations. In application of the same procedure for solving a set of differential equations we give a theorem showing that a quadratic convergence rate can be achieved. While the domain of quadratic convergence depends on the problems studied and is unknown a priori, we show that firstand second-order derivatives of flux vectors determine whether the condition for quadratic convergence is satisfied. The first derivatives enter as an implicit operator for yielding new iterates and the second derivatives indicates smoothness of the flows considered. Consequently flows involving shocks are expected to require larger number of iterations. First-order upwind discretization in conjunction with the Steger-Warming flux-vector splitting is employed on the implicit operator and a diagonal dominant matrix results. However the explicit operator is represented by first- and seond-order upwind differencings, using both Steger-Warming's and van Leer's splittings. We discuss treatment of boundary conditions and solution procedures for solving the resulting block matrix system. With a set of test problems for one- and two-dimensional flows, we show detailed study as to the efficiency, accuracy, and convergence of the present method.

Liou, M. S.↗
NASA NTRS1986

Stability of semidiscrete approximations for hyperbolic initial-boundary-value problems: An eigenvalue analysis

A hyperbolic initial-boundary-value problem can be approximated by a system of ordinary differential equations (ODEs) by replacing the spatial derivatives by finite-difference approximations. The resulting system of ODEs is called a semidiscrete approximation. A complication is the fact that more boundary conditions are required for the spatially discrete approximation than are specified for the partial differential equation. Consequently, additional numerical boundary conditions are required and improper treatment of these additional conditions can lead to instability. For a linear initial-boundary-value problem (IBVP) with homogeneous analytical boundary conditions, the semidiscrete approximation results in a system of ODEs of the form du/dt = Au whose solution can be written as u(t) = exp(At)u(O). Lax-Richtmyer stability requires that the matrix norm of exp(At) be uniformly bounded for O less than or = t less than or = T independent of the spatial mesh size. Although the classical Lax-Richtmyer stability definition involves a conventional vector norm, there is no known algebraic test for the uniform boundedness of the matrix norm of exp(At) for hyperbolic IBVPs. An alternative but more complicated stability definition is used in the theory developed by Gustafsson, Kreiss, and Sundstrom (GKS). The two methods are compared.

Warming, Robert F.↗