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At least 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

A preliminary mechanical property and stress corrosion evaluation of VIM-VAR work strengthened and direct aged Inconel 718 bar material

This report presents a preliminary mechanical property and stress corrosion evaluation of double melted (vacuum induction melted (VIM), and vacuum arc remelted (VAR)), solution treated, work strengthened and direct aged Inconel 718 alloy bar (5.50 in. (13.97 cm) diameter). Two sets of tensile specimens, one direct single aged and the other direct double aged, were tested at ambient temperature in both the longitudinal and transverse directions. Longitudinal tensile and yield strengths in excess of 200 ksi (1378.96 MPa) and 168 ksi (1158.33 MPa), respectively, were realized at ambient temperature, for the direct double aged specimen. No failures occurred in the single or double edged longitudinal and transverse tensile specimens stressed to 75 and 100 percent of their respective yield strengths and exposed to a salt fog environment for 180 days. Tensile tests performed after the stress corrosion test showed no mechanical property degradation.

Montano, J. W.↗
NASA NTRS1987

Altimetry data and the elastic stress tensor of subduction zones

The maximum shear stress (mss) field due to mass anomalies is estimated in the Apennines, the Kermadec-Tonga Trench, and the Rio Grande Rift areas and the results for each area are compared to observed seismicity. A maximum mss of 420 bar was calculated in the Kermadec-Tonga Trench region at a depth of 28 km. Two additional zones with more than 300 bar mss were also observed in the Kermadec-Tonga Trench study. Comparison of the calculated mss field with the observed seismicity in the Kermadec-Tonga showed two zones of well correlated activity. The Rio Grande Rift results showed a maximum mss of 700 bar occurring east of the rift and at a depth of 6 km. Recorded seismicity in the region was primarily constrained to a depth of approximately 5 km, correlating well to the results of the stress calculations. Two areas of high mss are found in the Apennine region: 120 bar at a depth of 55 km, and 149 bar at the surface. Seismic events observed in the Apennine area compare favorably with the mss field calculated, exhibiting two zones of activity. The case of loading by seamounts and icecaps are also simulated. Results for this study show that the mss reaches a maximum of about 1/3 that of the applied surface stress for both cases, and is located at a depth related to the diameter of the surface mass anomaly.

Caputo, Michele↗
NASA NTRS1987

Thermoplasticity of coupled bodies in the case of stress-dependent heat transfer

The problem of the thermal stresses in coupled deformable bodies is formulated for the case where the heat-transfer coefficient at the common boundary depends on the stress-strain state of the bodies (e.g., is a function of the normal pressure at the common boundary). Several one-dimensional problems are solved in this formulation. Among these problems is the determination of the thermal stresses in an n-layer plate and in a two-layer cylinder.

Kilikovskaya, O. A.↗
NASA NTRS1985

Observed changes in a-Si:H cell characteristics due to long term temperature stress

Determination of thermally induced degradation mechanisms affecting thin film amorphous silicon solar cells through the use of laboratory accelerated stress testing requires acquisition of specially constructed, individually addressable test cells and the development of highly repeatable instrumentation which is not influenced by short-term reversible optical effects. This paper illustrates these considerations by describing accelerated temperature stress procedures and test results for one type of a-Si:H cell. Observed electrical characteristic changes indicate that two types of stress induced mechanisms are operating - one of which is self limiting and can be observed at lower temperatures and one which continues to complete degradation. The self limiting mechanism can result in a dramatic improvement to cells having initially low Voc values.

Lathrop, Jay W.↗
NASA NTRS1986

Stresses and deformations in composite tubes due to a circumferential temperature gradient

A linear elasticity solution for determining the response of composite tubes subjected to a circumferential temperature gradient is presented. Numerical examples are used to show that, in a single layer tube, fiber orientation strongly influences response. When the fibers are aligned axially, all stress components in the tube are small. When the fibers are aligned circumferentially, the hoop stress becomes large. This difference in behavior is due to the large difference between the radial and circumferential coefficients of thermal expansion when the fibers are oriented circumferentially. In multilayer tubes, stresses are quite high and just two constants characterize the overall bending and axial deformations of the tubes.

Hyer, M. W.↗
NASA NTRS1987

Thermally induced stresses and deformations in angle-ply composite tubes

Cure-induced uniform temperature change effects on the stresses, axial expansion, and thermally-induced twist of four specific angle-ply tube designs are discussed with a view to the tubes' use as major space structure components. The stresses and deformations in the tubes are studied as a function of the four designs, the off-axis angle, and the single-material and hybrid reinforcing-material construction used. It is found that tube design has a minor influence on the stresses, axial stiffness, and axial thermal expansion characteristics, which are more directly a function of off-axis angle and material selection; tube design is, however, the primary influence in the definition of thermally-induced twist and torsional stiffness characteristics. None of the designs is free of thermally induced twist.

Hyer, M. W.↗
NASA NTRS1987

Remote detection of air pollution stress to vegetation - Laboratory-level studies

An experimental investigation of the role of leaf chemistry, anatomy, moisture content, and canopy density on spectral reflectance in healthy and pollution stressed western conifer needles and broad-leafed species of California coastal sage scrub is presented. Acid mist at a level of pH 2.0 is found to more severely effect chlorophyll loss and leaf death than ozone at a level of 0.2 ppm for a four-week period. Both pollutants cause water loss, affecting Bands 4 and 5 in nonlinear ways. The infrared bands initially rise as free water is lost, and subsequently, scattering and reflectance decline. The net effect is shown to be a reduction in TM 4/3 and a rise in TM 5/4 with pollution stress. Under more severe pollution stresses, the decline of leaf area indices due to accelerated leaf drop accentuates the expected TM 4/3 and TM 5/4 changes.

Westman, Walter E.↗
NASA NTRS1988

Delamination stresses in semicircular laminated composite bars

Using anisotropic elasticity theory, delamination stresses in a semicircular laminated composite curved bar subjected to end forces and end moments were calculated, and their radial locations determined. A family of design curves was presented, showing variation of the intensity of delamination stresses and their radial locations with different geometry and different degrees of anisotropy of the curved bar. The effect of anisotropy on the location of peak delamination stress was found to be small.

Ko, William L.↗
NASA NTRS1988

A test for interfacial effects and stress transfer in ceramic matrix composites

A test specimen was devised for measuring stress transfer between a high modulus fiber and a ceramic matrix. Single filaments of SiC were embedded in chemically vapor deposited SiC on a thin plate of molybdenum. The CVD overcoating which encapsulated the fiber was continuous with a coating of SiC on the molybdenum. When placed in a microtensile test device and loaded in the fiber direction, the fiber fracture characteristics provide information on the fiber/matrix adhesion and stress transfer. Problems were encountered due to the formation of a weak boundary between the SiC and the molybdenum which obviated any meaningful tensile tests. Also, the high CVD temperature used in fabricating these specimens restrict the fiber, matrix (and substrate) to materials having similar thermal coefficients of expansion in order to minimize thermal stresses.

Source record↗
NASA NTRS1988

Chronic stress as a factor in aircraft mishaps

Naval aviation is an unusually stressful career because of the inherent demands of the work. Stress is recognized as a cause of mishaps which involve pilot error. A questionnaire was adapted from Rahe and Homes' list of stressful life events in order to determine the relationship between pilot behavioral, personality, and life change factors on the one hand and responsibility for accidents on the other. A number of factors regarding interpersonal relationships, changes in personal behavior, personality factors, and life changes were found to discriminate between pilots who were and were not at fault in accidents.

Alkov, Robert A.↗
NASA NTRS1988

A test for interfacial effects and stress transfer in ceramic matrix composites

The efforts to fabricate single embedded filament specimens of carbon and SiC fibers were unsuccessful largely due to the thermal stresses resulting from differences in thermal coefficient of expansion. Other factors appear to have been involved including embrittlement of the metal substrate by the H2 gas in the chemical vapor deposition flow stream and reaction layers formed between the silicon carbide and the metal substrate. The carbon fiber may have been attacked by the CVD reactant. It is concluded that these differential stresses are so large as to make the embedded fiber test impractical for the study of interphase effects and stress transfer in fiber ceramic matrix systems.

Bascom, Willard D.↗
NASA NTRS1987

The effects of crack surface friction and roughness on crack tip stress fields

A model is presented which can be used to incorporate the effects of friction and tortuosity along crack surfaces through a constitutive law applied to the interface between opposing crack surfaces. The problem of a crack with a saw-tooth surface in an infinite medium subjected to a far-field shear stress is solved and the ratios of mode I stress intensity to mode II stress intensity are calculated for various coefficients of friction and material properties. The results show that tortuosity and friction lead to an increase in fracture loads and alter the direction of crack propagation.

Ballarini, Roberto↗
NASA NTRS1987

Low-field magnetoacoustic residual stress measurement in steel

Much of the recent development of the magnetoacoustic technique has been devoted to refine this technique as a reliable and practical tool for measuring bulk residual stress in steel components. For this, the effects of structural and metallurgical properties on the magnetoacoustic interaction have been studied for various types of steel samples. Also, progress is being made to obtain quantitative residual stress measurements in railroad wheels. This paper reviews the physical basis and the experimental results of the magnetoacoustic stress measurements in steels.

Namkung, M.↗
NASA NTRS1987

Secular cooling of the earth as a source of intraplate stress

As a result of secular cooling and contraction of the earth, an increasingly extensional horizontal stress is imposed on the central portions of the plates. While the rate of increase of this stress is small in most situations, it is enhanced for large plates and during any episodes of accelerated global cooling. This source of stress may have contributed to the rifting and breakup of long-lived supercontinental plates.

Solomon, Sean C.↗
NASA NTRS1988

On the cyclic stress-strain behaviour of a Ni-base superalloy at room temperature

The cyclic stress-strain behavior of Nimonic alloy PE16 was studied at room temperature and at different aging conditions to determine whether the plateau in the cyclic stress-strain curve (CSSC) reported by Arbuthnot (1982) is typical of the room temperature behavior and/or some specific initial microstructural states. Specimen blanks were heat-treated in batches in Ar/H2 (98/2) atmosphere to produce gamma-prime precipitates of different average sizes, but with the volume fraction of gamma-prime precipitates kept constant at about 7 percent at all the heat-treatment conditions. Total axial strain controlled LCF tests were conducted under fully reversed loading (R = -1) at a constant strain rate of 0.004/s, using a servohydraulic machine. The load response in tension and compression was recorded continually, and stress-strain hysteresis loops were recorded at frequent intervals. In the present investigation, the CSSCs of the P16 alloy at room temperature did not display the plateaus reported by Arbuthnot.

Singh, Vakil↗
NASA NTRS1988

Spectral changes in conifers subjected to air pollution and water stress: Experimental studies

The roles of leaf anatomy, moisture and pigment content, and number of leaf layers on spectral reflectance in healthy, pollution-stressed, and water-stressed conifer needles were examined experimentally. Jeffrey pine (Pinus jeffreyi) and giant sequoia (Sequoiadendron gigantea) were exposed to ozone and acid mist treatments in fumigation chambers; red pine (Pinus resinosa) needles were artificially dried. Infrared reflectance from stacked needles rose with free water loss. In an air-drying experiment, cell volume reductions induced by loss of turgor caused near-infrared reflectance (TM band 4) to drop after most free water was lost. Under acid mist fumigation, stunting of tissue development similarly reduced band 4 reflectance. Both artificial drying and pollutant fumigation caused a blue shift of the red edge of spectral reflectance curves in conifers, attributable to chlorophyll denaturation. Thematic mapper band ratio 4/3 fell and 5/4 rose with increasing pollution stress on artificial drying. Loss of water by air-drying, freeze-drying, or oven-drying enhanced spectral features, due in part to greater scattering and reduced water absorption. Grinding of the leaf tissue further enhanced the spectral features by increasing reflecting surfaces and path length. In a leaf-stacking experiment, an asymptote in visible and infrared reflectance was reached at 7-8 needle layers of red pine.

Westman, Walter E.↗
NASA NTRS1987

Accelerated crack growth, residual stress, and a cracked zinc coated pressure shell

During a partial inspection of a 42 year old, operating, pressurized wind tunnel at NASA-Ames Research Center, a surface connected defect 114 in. long having an indicated depth of a 0.7 in. was detected. The pressure shell, constructed of a medium carbon steel, contains approximately 10 miles of welds and is cooled by flowing water over its zinc coated external surface. Metallurgical and fractographic analysis showed that the actual detect was 1.7 in. deep, and originated from an area of lack of weld penetration. Crack growth studies were performed on the shell material in the laboratory under various loading rates, hold times, and R-ratios with a simulated shell environment. The combination of zinc, water with electrolyte, and steel formed an electrolytic cell which resulted in an increase in cyclic crack growth rate by as much as 500 times over that observed in air. It was concluded that slow crack growth occurred in the pressure shell by a combination of stress corrosion cracking due to the welding residual stress and corrosion fatigue due to the cyclic operating stress.

Dittman, Daniel L.↗
NASA NTRS1988

Influence of geometric nonlinearities on skin-stiffener interface stresses

A method for computing skin-stiffener interface stresses in stiffened composite panels is developed. Both geometrically linear and nonlinear analyses are considered. Particular attention is given to the flange termination region where stresses are expected to exhibit unbounded characteristics. The method is based on a finite-element analysis and an elasticity solution. The results indicate that the inclusion of geometric nonlinearities is very important for an accurate determination of the interface stresses. Membrane flattening of the panel tends to reduce the tendency of the stiffener to separate.

Cohen, D.↗