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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Effects of various consolidation techniques on microstructure, strength, and reliability of alpha-SiC

The effects of slurry pressing, HIP, and sinter-HIP on the strength, microstructure, and critical processing flaws in alpha-SiC are examined. Also, the feasibility of glass encapsulation is determined. Baseline strength was improved by about 25 percent by using slurry pressing instead of dry pressing. HIP further improved average strength to as high as 655 MPa. Sinter-HIP did not noticeably improve average strength. Although process-related flaws such as shrinkage cracks and large voids were not seen in HIPed alpha-SiC, surface-related flaws were observed. It is suggested that surface finishing by lapping, polishing, and heat treating might reduce the effect of surface-related flaws, improving the Weibull modulus in HIPed alpha-SiC.

Dutta, Sunil↗
NASA NTRS1989

The age of the LMC globular cluster NGC 1783

The age of the LMC red globular cluster NGC 1783 is estimated as 0.9 + or - 0.4 billion yr by photometry of the main-sequence turnoff. The accuracy of the estimate is limited chiefly by the uncertainty in the distance modulus of the cluster. At (m - M)0 = 18.2 the cluster is aged 1.1 + or - 0.2 Gyr; at (m - M)0 = 18.7 it is 0.7 + or - 0.2 Gyr. NGC 1783 is a sufficiently rich cluster that one can see the full development of red giants on the asymptotic giant branch from the M type, through S, to carbon-rich atmospheres.

Mould, Jeremy↗
NASA NTRS1989

Screening of metal matrix composites using ultrasonic C-scans

Ultrasonic C-scans can be used to find some types of defects in continuous fiber-reinforced metal matrix composites such as boron/aluminum composites. These defects are related to the fatigue behavior and fracture location of each inspected specimen. The C-scan technique determined the relative amount of defects in boron/aluminum composites. The defects were primarily identified as gaps in the fiber spacing. Those specimens with higher defect densities had shorter fatigue lives, lower fatigue endurance limits, and greater reductions in the elastic unloading modulus (that is, stiffness) because of fatigue cycling. This type of data could be used to set accept/reject levels for a composite panel based on C-scan indications.

Johnson, W. S.↗
NASA NTRS1989

Photographic, near-infrared, and CCD photometry of the distant globular cluster AM-1

New photographic, CCD, and near-infrared photometry of stars in the distant globular cluster AM-1 are presented. Based on the photographic and CCD photometry down to the horizontal branch, a distance modulus (m-M)v = 20.4 mag is derived, with E(B-V) = 0.0 mag, corresponding to a distance of 120 kpc from the sun. Using the (V-K) colors of two giants in AM-1, a metallicity of Fe/H = -2.0 + or - 0.2 is deduced, in good agreement with earlier determinations. Using complete photometry and star counts, a core radius of 10 arcsec (6 pc) and a tidal radius of 100 arcsec (58 pc) are derived. Despite the low intrinsic luminosity (Mv = -5.0 mag) and the large galactocentric distance of this globular cluster, the apparent magnitude distribution of the giant branch stars of AM-1 is not found to be unusual.

Madore, Barry F.↗
NASA NTRS1989

Predictions of Poisson's ratio in cross-ply laminates containing matrix cracks and delaminations

A damage-dependent constitutive model for laminated composites has been developed for the combined damage modes of matrix cracks and delaminations. The model is based on the concept of continuum damage mechanics and uses second-order tensor valued internal state variables to represent each mode of damage. The internal state variables are defined as the local volume average of the relative crack face displacements. Since the local volume for delaminations is specified at the laminate level, the constitutive model takes the form of laminate analysis equations modified by the internal state variables. Model implementation is demonstrated for the laminate engineering modulus E(x) and Poisson's ratio nu(xy) of quasi-isotropic and cross-ply laminates. The model predictions are in close agreement to experimental results obtained for graphite/epoxy laminates.

Harris, Charles E.↗
NASA NTRS1989

Analysis of wind profile temporal variability

The modulus of the maximum wind change is used here to assess wind change at the Shuttle launch site at Kennedy Space Center. Empirical and theoretical distributions of the maximum wind change are presented, and the effect of wind profile smoothing on wind change distributions is analyzed. The capability of wind profile systems to measure extreme wind change is described.

Adelfang, Stanlay I.↗
NASA NTRS1960

The Dependency of Penetration on the Momentum Per Unit Area of the Impacting Projectile and the Resistance of Materials to Penetration

The results of this investigation indicate that the penetration of projectiles into quasi-infinite targets can be correlated as a function of the maximum momentum per unit area possessed by the projectiles. The penetration of projectiles into aluminum, copper, and steel targets was found to be a linear function while the penetration into lead targets was a nonlinear function of the momentum per unit area of the impacting projectiles. Penetration varied inversely as the projectile density and the elastic modulus of the target material for a given projectile momentum per unit area. Crater volumes were found to be a linear function of the kinetic energy of the projectile, the greater volumes being obtained in the target materials which had the lowest yield strength and the lowest speed of sound.

Collins, Rufus D., Jr.↗
NASA NTRS1987

Improved silicon carbide for advanced heat engines

This is the second annual technical report entitled, Improved Silicon Carbide for Advanced Heat Engines, and includes work performed during the period February 16, 1986 to February 15, 1987. The program is conducted for NASA under contract NAS3-24384. The objective is the development of high strength, high reliability silicon carbide parts with complex shapes suitable for use in advanced heat engines. The fabrication methods used are to be adaptable for mass production of such parts on an economically sound basis. Injection molding is the forming method selected. This objective is to be accomplished in a two-phase program: (1) to achieve a 20 percent improvement in strength and a 100 percent increase in Weibull modulus of the baseline material; and (2) to produce a complex shaped part, a gas turbine rotor, for example, with the improved mechanical properties attained in the first phase. Eight tasks are included in the first phase covering the characterization of the properties of a baseline material, the improvement of those properties and the fabrication of complex shaped parts. Activities during the first contract year concentrated on two of these areas: fabrication and characterization of the baseline material (Task 1) and improvement of material and processes (Task 7). Activities during the second contract year included an MOR bar matrix study to improve mechanical properties (Task 2), materials and process improvements (Task 7), and a Ford-funded task to mold a turbocharger rotor with an improved material (Task 8).

Whalen, Thomas J.↗
NASA NTRS1989

Fluorinated graphite fibers as a new engineering material: Promises and challenges

Pitch based graphitized carbon fibers with electrical resistivity of 300 micro-Ohm/cm were brominated and partially debrominated to 18 percent bromine at room temperature, and then fluorinated at 300 to 450 C, either continuously or intermittently for several cycles. In addition, on fluorine and titanium fluoride intercalated fiber sample was fluorinated at 270 C from the same fiber source. The mass and conductivity of the brominated-debrominated then fluorinated fibers (with fluorine-to-carbon atom ratio of 0.54 or higher) stabilized at room temperature air in a few days. However, at 200 C, these values decreased rapidly and then more slowly, throughout a 2-week test period. The electrically insulative or semiconductive fibers were found to be compatible with epoxy and have the fluorine-to-carbon atom ratio of 0.65 to 0.68, thermal conductivity of 5 to 24 W/m-K, electrical resistivity of 10(exp 4) to 10(exp 11) Ohm/cm, tensile strength of 70 to 150 ksi, Young's modulus of 20 to 30 msi, and CTE (coefficient of thermal expansion) values of 7 ppm/deg C. Data of these physical property values are preliminary. However, it is concluded that these physical properties can be tailor-made. They depend largely on the fluorine content of the final products and the intercalant in the fibers before fluorination, and, to a smaller extent, on the fluorination temperature histogram.

Hung, Ching-Cheh↗
NASA NTRS1989

Tensile film clamps and mounting block for the rheovibron and autovibron viscoelastometer

A set of film clamps and a mounting block for use in the determination of tensile modulus and damping properties of films in a manually operated or automated Rheovibron is diagrammed. These clamps and mounting block provide uniformity of sample gripping and alignment in the instrument. Operator dependence and data variability are greatly reduced.

Stoakley, Diane M.↗
NASA NTRS1989

Computer simulation of the mathematical modeling involved in constitutive equation development: Via symbolic computations

Development of new material models for describing the high temperature constitutive behavior of real materials represents an important area of research in engineering disciplines. Derivation of mathematical expressions (constitutive equations) which describe this high temperature material behavior can be quite time consuming, involved and error prone; thus intelligent application of symbolic systems to facilitate this tedious process can be of significant benefit. A computerized procedure (SDICE) capable of efficiently deriving potential based constitutive models, in analytical form is presented. This package, running under MACSYMA, has the following features: partial differentiation, tensor computations, automatic grouping and labeling of common factors, expression substitution and simplification, back substitution of invariant and tensorial relations and a relational data base. Also limited aspects of invariant theory were incorporated into SDICE due to the utilization of potentials as a starting point and the desire for these potentials to be frame invariant (objective). Finally not only calculation of flow and/or evolutionary laws were accomplished but also the determination of history independent nonphysical coefficients in terms of physically measurable parameters, e.g., Young's modulus, was achieved. The uniqueness of SDICE resides in its ability to manipulate expressions in a general yet predefined order and simplify expressions so as to limit expression growth. Results are displayed when applicable utilizing index notation.

Arnold, S. M.↗
NASA NTRS1990

Fiber pushout test: A three-dimensional finite element computational simulation

A fiber pushthrough process was computationally simulated using three-dimensional finite element method. The interface material is replaced by an anisotropic material with greatly reduced shear modulus in order to simulate the fiber pushthrough process using a linear analysis. Such a procedure is easily implemented and is computationally very effective. It can be used to predict fiber pushthrough load for a composite system at any temperature. The average interface shear strength obtained from pushthrough load can easily be separated into its two components: one that comes from frictional stresses and the other that comes from chemical adhesion between fiber and the matrix and mechanical interlocking that develops due to shrinkage of the composite because of phase change during the processing. Step-by-step procedures are described to perform the computational simulation, to establish bounds on interfacial bond strength and to interpret interfacial bond quality.

Mital, Subodh K.↗
NASA NTRS1990

Nicalon/siliconoxycarbide ceramic composites

A series of silsesquioxane copolymers was synthesized by acid hydrolysis and condensation of trimethoxysilanes of the form RSi(OCH3)3, where R = methyl or phenyl. By varying pH, water/methoxy and methyl/phenyl ratios, the molecular structure, polymer rheology and ceramic composition can be controlled. The polymers form an amorphous siliconoxycarbide on pyrolysis. Composites of Nicalon/siliconoxycarbide were fractured in four-point flexure and in tension to evaluate the influence of matrix composition, final fabrication temperature and use of filler on composite mode of failure, modulus, strain capability and strength. Incorporation of filler was found to increase matrix compressive strength. Employment of processing temperatures of 1375 to 1400 C enhanced strain to failure and reduced the tendency toward brittle fracture. Mixed mode (compression/shear and tension/shear) failures were observed in flexural samples processed to the higher temperatures, giving rise to nonlinear stress-strain curves. Tensile samples pyrolyzed to 1400 C showed linear-elastic behavior and failed by fracture of fiber bundles. Matrix material was found to be adherent to the fiber surface after failure. These results demonstrate the need for tensile testing to establish composite behavior.

Hurwitz, Frances I.↗
NASA NTRS1990

Optimal fabrication processes for unidirectional metal-matrix composites: A computational simulation

A method is proposed for optimizing the fabrication process of unidirectional metal matrix composites. The temperature and pressure histories are optimized such that the residual microstresses of the composite at the end of the fabrication process are minimized and the material integrity throughout the process is ensured. The response of the composite during the fabrication is simulated based on a nonlinear micromechanics theory. The optimal fabrication problem is formulated and solved with non-linear programming. Application cases regarding the optimization of the fabrication cool-down phases of unidirectional ultra-high modulus graphite/copper and silicon carbide/titanium composites are presented.

Saravanos, D. A.↗
NASA NTRS1990

Elevated temperature crack growth in advanced powder metallurgy aluminum alloys

Rapidly solidified Al-Fe-V-Si powder metallurgy alloy FVS0812 is among the most promising of the elevated temperature aluminum alloys developed in recent years. The ultra fine grain size and high volume fraction of thermally stable dispersoids enable the alloy to maintain tensile properties at elevated temperatures. In contrast, this alloy displays complex and potentially deleterious damage tolerant and time dependent fracture behavior that varies with temperature. J-Integral fracture mechanics were used to determine fracture toughness (K sub IC) and crack growth resistance (tearing modulus, T) of extruded FVS0812 as a function of temperature. The alloy exhibits high fracture properties at room temperature when tested in the LT orientation, due to extensive delamination of prior ribbon particle boundaries perpendicular to the crack front. Delamination results in a loss of through thickness constraint along the crack front, raising the critical stress intensity necessary for precrack initiation. The fracture toughness and tensile ductility of this alloy decrease with increasing temperature, with minima observed at 200 C. This behavior results from minima in the intrinsic toughness of the material, due to dynamic strain aging, and in the extent of prior particle boundary delaminations. At 200 C FVS0812 fails at K levels that are insufficient to cause through thickness delamination. As temperature increases beyond the minimum, strain aging is reduced and delamination returns. For the TL orientation, K (sub IC) decreased and T increased slightly with increasing temperature from 25 to 316 C. Fracture in the TL orientation is governed by prior particle boundary toughness; increased strain localization at these boundaries may result in lower toughness with increasing temperature. Preliminary results demonstrate a complex effect of loading rate on K (sub IC) and T at 175 C, and indicate that the combined effects of time dependent deformation, environment, and strain aging may play a role. Fractography showed that microvoid coalescence was the microscopic mode of fracture in FVS0812 under all testing conditions. However, the nature of the microvoids varied with test temperature and loading rate, and is complex for the fine grain and dipersoid sizes of FVS0812.

Porr, William C., Jr.↗
NASA NTRS1990

The assessment of metal fiber reinforced polymeric composites

Because of their low cost, excellent electrical conductivity, high specific strength (strength/density), and high specific modulus (modulus/density) short metal fiber reinforced composites have enjoyed a widespread use in many critical applications such as automotive industry, aircraft manufacturing, national defense, and space technology. However, little data has been found in the study of short metal fibrous composites. Optimum fiber concentration in a resin matrix and fiber aspect ratio (length-to-diameter ratio) are often not available to a user. Stress concentration at short fiber ends is the other concern when the composite is applied to a load-bearing application. Fracture in such composites where the damage will be initiated or accumulated is usually difficult to be determined. An experimental investigation is therefore carefully designed and undertaken to systematically evaluate the mechanical properties as well as electrical properties. Inconel 601 (nickel based) metal fiber with a diameter of eight microns is used to reinforce commercially available thermoset polyester resin. Mechanical testing such as tensile, impact, and flexure tests along with electrical conductivity measurements is conducted to study the feasibility of using such composites. The advantages and limitations of applying chopped metal fiber reinforced polymeric composites are also discussed.

Chung, Wenchiang R.↗
NASA NTRS1990

Influence of interfacial shear strength on the mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composites

The influence of fiber/matrix interface microstructure and interfacial shear strength on the mechanical properties of a fiber-reinforced ceramic composite was evaluated. The composite consisted of approximately 30 vol percent uniaxially aligned 142 microns diameter SiC fibers (Textron SCS-6) in a reaction-bonded Si3N4 matrix (SiC/RBSN). The interface microstructure was varied by controlling the composite fabrication conditions and by heat treating the composite in an oxidizing environment. Interfacial shear strength was determined by the matrix crack spacing method. The results of microstructural examination indicate that the carbon-rich coating provided with the as-produced SiC fibers was stable in composites fabricated at 1200 C in a nitrogen or in a nitrogen plus 4 percent hydrogen mixture for 40 hr. However this coating degraded in composites fabricated at 1350 C in N2 + 4 percent H2 for 40 and 72 hr and also in composites heat treated in an oxidizing environment at 600 C for 100 hr after fabrication at 1200 C in a nitrogen. It was determined that degradation occurred by carbon removal which in turn had a strong influence on interfacial shear strength and other mechanical properties. Specifically, as the carbon coating was removed, the composite interfacial shear strength, primary elastic modulus, first matrix cracking stress, and ultimate tensile strength decreased, but the first matrix cracking strain remained nearly the same.

Bhatt, Ramakrishna T.↗
NASA NTRS1990

Evaluation of EA-934NA with 2.5 percent Cab-O-Sil

Currently, Hysol adhesive EA-934NA is used to bond the Field Joint Protection System on the Shuttle rocket motors at Kennedy Space Center. However, due to processing problems, an adhesive with a higher viscosity is needed to alleviate these difficulties. One possible solution is to add Cab-O-Sil to the current adhesive. The adhesive strength and bond strengths that can be obtained when 2.5 percent Cab-O-Sil is added to adhesive EA-934NA are examined and tested over a range of test temperatures from -20 to 300 F. Tensile adhesion button and lap shear specimens were bonded to D6AC steel and uniaxial tensile specimens (testing for strength, initial tangent modulus, elongation and Poisson's ratio) were prepared using Hysol adhesive EA-934NA with 2.5 percent Cab-O-Sil added. These specimens were tested at -20, 20, 75, 100, 125, 150, 200, 250, and 300 F, respectively. Additional tensile adhesion button specimens bonding Rust-Oleum primed and painted D6AC steel to itself and to cork using adhesive EA-934NA with 2.5 percent Cab-O-Sil added were tested at 20, 75, 125, 200, and 300 F, respectively. Results generally show decreasing strength values with increasing test temperatures. The bond strengths obtained using cork as a substrate were totally dependent on the cohesive strength of the cork.

Caldwell, Gordon A.↗