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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Probes of large-scale structure in the Corona Borealis region

The present redshift survey of the Corona Borealis region encompasses redshifts, magnitudes and positions for 83 galaxies covering a 16.4 sq deg region; a complementary survey covers 39.2 sq deg, is complete to m(B)0 of 15.5, and encompasses 37 galaxies. The combined survey furnishes further support for the 'bubble-like' geometry revealed by the shallower CfA survey. The redshift distribution obtained is noted to be very similar to that in the neighboring Bootes region. The structure of voids and surfaces in the galaxy distribution is insensitive to luminosity for M(B)0 of less than about -17.4.

Postman, M.↗
NASA NTRS1986

Guidance automation for nap-of-the-earth flight

Low-Altitude/Nap-of-the-Earth (NOE) rotorcraft flight in adverse weather has been identified as a major technology void by both military and civil agencies because of its direct relationship to the needs of the military's new scout-attack helicopter and to various civilian applications including Emergency Medical Services (EMS). As part of NASA's Aircraft Automation Program, Ames Research Center has identified as a major goal the automating and advancing of applicable terrain following/terrain avoidance/obstacle avoidance (TF/TA/OA) guidance technologies to a level where NOE rotorcraft flight can be demonstrated in an advanced simulator and, later, in fully automatic flight. Areas of emphasis to date for achieving automatic guidance include real-time sensor blending and the real-time computation of NOE guidance commands. The present paper reports on the guidance command generation. The applicability of TF/TA techniques to the NOE problem is described, and simulation results are presented that suggest that automatic NOE can indeed be accomplished.

Denton, Richard V.↗
NASA NTRS1986

An assessment of the small-crack effect for 2024-T3 aluminum alloy

Data on small-crack behavior were obtained for a single-edge-notched tensile specimen made of 2024-T3 Al alloy and used to evaluate the capability of a semiempirical crack-growth and closure model to predict the fatigue life of notched specimens. Fatigue tests were conducted under either constant-amplitude loading (with stress ratios of 0.5, 0, -1, and -2) or spectrum loading, using a replication technique to record growth. It was found that small cracks exhibited the 'small-crack' effect in that they grew faster than large cracks when subjected to the same stress intensity factor range. Experimental small-crack growth rates agreed well with the model predictions. For making predictions of fatigue life, an initial surface defect void size of 3 x 12 x 0.4 microns was used in all calculations; predicted fatigue lives agreed well with experimentally determined values obtained in all tests. The crack-closure model indicated that the 'small-crack' effect on fatigue life was greatest in tests involving significant compressive loads.

Newman, J. C., Jr.↗
NASA NTRS1986

Interfaces and failure mechanisms in Al-SiC composites

Aluminum alloys reinforced with silicon carbide whiskers exhibit significantly higher strength and modulus than the unreinforced alloys. However, the composites also exhibit low ductility and poor fracture toughness for reasons which are not well-understood. In this study, high resolution and conventional TEM techniques are used to analyze interface microstructure and failure mechanisms in P/M 2124 and 6061 aluminum alloys reinforced with silicon carbide whiskers. Interfacial oxides are typical of both composite materials, often distributed in clusters or in a discrete layer 2-5 nm thick along the whisker-matrix interface. Highly deformed regions beneath bulk tensile fracture surfaces reveal possible fracture nucleation centers as well as sites of stress concentration where intense plastic strain has occurred. Observations of such highly deformed regions include (1) void initiation at whisker ends, (2) interface decohesion involving the thin oxide layer, (3) transverse cracks in whiskers, and (4) cracks in large constituent particles. TEM results are presented and discussed in relation to mechanisms of composite failure.

Nutt, S. R.↗
NASA NTRS1987

Does lower Omega allow a resolution of the large-scale structure problem?

The intermediate angular scale anisotropy of the cosmic microwave background, peculiar velocities, density correlations, and mass fluctuations for both neutrino and baryon-dominated universes with Omega less than one are evaluated. The large coherence length associated with a low-Omega, hot dark matter-dominated universe provides substantial density fluctuations on scales up to 100 Mpc: there is a range of acceptable models that are capable of producing large voids and superclusters of galaxies and the clustering of galaxy clusters, with Omega roughly 0.3, without violating any observational constraint. Low-Omega, cold dark matter-dominated cosmologies are also examined. All of these models may be reconciled with the inflationary requirement of a flat universe by introducing a cosmological constant 1-Omega.

Silk, Joseph↗
NASA NTRS1986

Characterization of hydrogen embrittlement in nickel base superalloy single crystals

In order to study the role of CMSX2 single crystal microstructure on the combined stress-hydrogen environment effects, hydrogen was introduced by cathodic charging. Concentration measurements were carried out to investigate the dependence of hydrogen solubility and trapping on microstructure. Mechanical properties were measured at room temperature on smooth tensile specimens as a function of heat treatment, crystal orientation and H charging conditions. SEM and TEM allow to study H induced cracks initiation and propagation. A large amount of hydrogen can be dissolved and trapped in CMSX2 single crystals when exposed to a high hydrogen fugacity environment. The strong H trapping evidenced in voids explains the predominant role of these defects as crack initiation sites. The strong detrimental effect of hydrogen on the material tenacity is discussed.

Chene, J.↗
NASA NTRS1988

Detecting Faults In High-Voltage Transformers

Simple fixture quickly shows whether high-voltage transformer has excessive voids in dielectric materials and whether high-voltage lead wires too close to transformer case. Fixture is "go/no-go" indicator; corona appears if transformer contains such faults. Nests in wire mesh supported by cap of clear epoxy. If transformer has defects, blue glow of corona appears in mesh and is seen through cap.

Blow, Raymond K.↗
NASA NTRS1988

Reliability Of Inspection By SLAM

Scanning laser acoustic microscopy finds flaws in ceramic specimens. Laser light used to detect distortions, on angstrom scale, produced on surface of specimen by ultrasonic waves transmitted through specimen. From distortion, SLAM creates picture on video monitor of such defects as voids, inclusions, and cracks.

Roth, Don J.↗
NASA NTRS1988

Additives Improve Processing Of Polyimides

Melt-processable polyimides maintain desirable polymer properties prepared by adding small amounts of certain low-molecular-weight, thermally stable additives. Useful in preparing void-free polyimide composites from higher flow resins that wet out fibers at lower processing temperatures. Applied to preparing high-temperature polyimide adhesives allowing enhanced wetting of adherends and ultimately better adhesion. Improvement to processing polyimides and potential application for production of films, coatings, adhesives, and composites.

St. Clair, Terry L.↗
NASA NTRS1988

Fibers And Composites Derived From Silsesquioxanes

In new method, silsesquioxane powders blended to control ratio of carbon to silicon. Powders melted, and excess silanol groups condense with evolution of water. When melt attains suitable viscosity, extruded into fibers through die or drawn into fibers from melt at uniform rate. Fibers cured and heat treated. Enables easy fabrication of thermally stable fibers from inexpensive silsesquioxane precursors. Impregnation of fibers and preforms without solvent minimizes both shrinkage and formation of voids resulting from volatilization of trapped solvent.

Hurwitz, Frances I.↗
NASA NTRS1987

A life test of a 22-Newton (5-lbf) hydrazine rocket

Life tests were conducted on a 22-N (5-lb) hydrazine rocket thruster which incorporates the latest technology to obtain long life from the catalyst bed. A spring mechanism surrounding the catalyst bed continually applies compression to the catalyst bed to prevent the formation of any void channels. The research rocket thruster was tested over an operational cycle of both steady state and pulse firing which simulated a possible space station duty cycle. The thruster ran as expected for about 40 hours, or 3.2 times 10 to the 6th power N-sec (7.2 times 10 to the 5th power lb-sec) total impulse. Subsequently, some thrust chamber pressure decreases were noted during long steady state test periods. After 60.2 hours of run time, tests had to be terminated due to a blockage in the propellant injector tube which occurred during heating of the thruster by a heat lamp. A chemical analysis of the catalyst indicated that iron and nickel metals had poisoned some of the catalyst, thereby causing a degradation in performance. It was determined that a contaminated barrel of hydrazine was the source of the metal poisoning.

Meng, P. R.↗
NASA NTRS1987

Nondestructive evaluation of structural ceramics by photoacoustic microscopy

A photoacoustic microscopy (PAM) digital imaging system was developed and utilized to characterize silicon nitride material at the various stages of the ceramic fabrication process. Correlation studies revealed that photoacoustic microscopy detected failure initiating defects in substantially more specimens than microradiography and ultrasonic techniques. Photoacoustic microscopy detected 10 to 100 micron size surface and subsurface pores and inclusions, respectively, up to 80 microns below the interrogating surface in machined sintered silicon nitride. Microradiography detected 50 micron diameter fracture controlling pores and inclusions. Subsurface holes were detected up to a depth of 570 microns and 1.00 mm in sintered silicon nitride and silicon carbide, respectively. Seeded voids of 20 to 30 micron diameters at the surface and 50 microns below the interrogating surface were detected by photoacoustic microscopy and microradiography with 1 percent X-ray thickness sensitivity. Tight surface cracks of 96 micron length x 48 micron depth were detected by photoacoustic microscopy. PAM volatilized and removed material in the green state which resulted in linear shallow microcracks after sintering. This significantly limits the use of PAM as an in-process NDE technique.

Khandelwal, Pramod K.↗
NASA NTRS1988

Structural assessment of a space station solar dynamic heat receiver thermal energy storage canister

The structural performance of a space station thermal energy storage (TES) canister subject to orbital solar flux variation and engine cold start up operating conditions was assessed. The impact of working fluid temperature and salt-void distribution on the canister structure are assessed. Both analytical and experimental studies were conducted to determine the temperature distribution of the canister. Subsequent finite element structural analyses of the canister were performed using both analytically and experimentally obtained temperatures. The Arrhenius creep law was incorporated into the procedure, using secondary creep data for the canister material, Haynes 188 alloy. The predicted cyclic creep strain accumulations at the hot spot were used to assess the structural performance of the canister. In addition, the structural performance of the canister based on the analytically determined temperature was compared with that based on the experimentally measured temperature data.

Thompson, R. L.↗
NASA NTRS1988

Characterization of sintered SiC by using NDE

Capabilities of projection microfocus X-radiography and of ultrasonic velocity and attenuation for characterizing silicon carbide specimens were assessed. Silicon carbide batches covered a range of densities and different microstructural characteristics. Room-temperature, four-point flexural strength tests were conducted. Fractography was used to identify types, sizes, and locations of fracture origins. Fracture toughness values were calculated from fracture strength and flaw characterization data. Detection capabilities of radiography for fracture-causing flaws were evaluated. Applicability of ultrasonics for verifying material strength and toughness was examined. Radiography proved useful in detecting high-density inclusions and isolated voids, but failed in detecting surface and subsurface agglomerates and large grains as fracture origins. Ultrasonic velocity dependency on density was evident. Attenuation dependency on density and mean pore size was clearly demonstrated. Understanding attenuation as a function of toughness was limited by shortcomings in K sub IC determination.

Baaklini, George Y.↗
NASA NTRS1987

Investigation into the development of computer aided design software for space based sensors

The described effort is phase one of the development of a Computer Aided Design (CAD) software to be used to perform radiometric sensor design. The software package will be referred to as SCAD and is directed toward the preliminary phase of the design of space based sensor system. The approach being followed is to develop a modern, graphic intensive, user friendly software package using existing software as building blocks. The emphasis will be directed toward the development of a shell containing menus, smart defaults, and interfaces, which can accommodate a wide variety of existing application software packages. The shell will offer expected utilities such as graphics, tailored menus, and a variety of drivers for I/O devices. Following the development of the shell, the development of SCAD is planned as chiefly selection and integration of appropriate building blocks. The phase one development activities have included: the selection of hardware which will be used with SCAD; the determination of the scope of SCAD; the preliminary evaluation of a number of software packages for applicability to SCAD; determination of a method for achieving required capabilities where voids exist; and then establishing a strategy for binding the software modules into an easy to use tool kit.

Pender, C. W.↗
NASA NTRS1986

Strength optimization of alpha-SiC by improved processing

Silicon carbide is of great interest for structural use in aircraft and automobile engines. This ceramic combines high thermal conductivity and low coefficient of thermal expansion, and consequently has good thermal shock resistance. However, like other ceramics, silicon carbide shows strength variability due to processing flaws such as large voids, shrinkage cracks, and inclusions. Agglomerates in the starting powder seem to be the predominant cause of such defects. Improved processing techniques such as slurry pressing and hot isostatic pressing (HIP) were employed to minimize these defects and to improve strength and reliability in the fabricated material. For this purpose 2-inch diameter disks were fabricated by various consolidation techniques. These include: dry pressing and sintering, slurry pressing and sintering, and slurry pressing and HIPing. The results are discussed.

Dutta, Sunil↗
NASA NTRS1986

Teleoperator Human Factors Study

An investigation of the spectrum of space teleoperation activities likely in the 1985 to 1995 decade focused on the resolution of critical human engineering issues and characterization of the technology effect on performance of remote human operators. The study began with the identification and documentation of a set of representative reference teleoperator tasks. For each task, technology, development, and design options, issues, and alternatives that bear on human operator performance were defined and categorized. A literature survey identified existing studies of man/machine issues. For each teleoperations category, an assessment was made of the state of knowledge on a scale from adequate to void. The tests, experiments, and analyses necessary to provide the missing elements of knowledge were then defined. A limited set of tests were actually performed, including operator selection, baseline task definition, control mode study, lighting study, camera study, and preliminary time delay study.

Source record↗
NASA NTRS1988

Thermoplastic coating of carbon fibers

Now that quantities of prepreg were made on the thermoplastic coating line, they are being formed into both textile preform structures and directly into composite samples. The textile preforms include both woven and knitted structures which will be thermoformed into a finished part. In order to determine if the matrix resin is properly adhering to the fibers or if voids are being formed in the coating process, the tensile strength and modulus of these samples will be tested. The matrix uniformity of matrix distribution in these samples is also being determined using an image analyzer.

Edie, D. D.↗