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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The role of fiber and matrix in crash energy absorption of composite materials

Static crushing tests were conducted on tube specimens fabricated from graphite/epoxy, Kevlar/epoxy and hybrid combinations of graphite-Kevlar/epoxy to examine the influence the fiber and matrix constitutive properties and laminate architecture have on energy absorption. Fiber and matrix ultimate failure strain were determined to significantly effect energy absorption. The energy absorption capability of high ultimate failure strain materials (AS-6/F185 and AS-6/HST-7) was less than materials having lower ultimate failure strain. Lamina stacking sequence had up to a 300 percent change in energy absorption for the materials tested. Hybridizing with graphite and Kevlar reinforcements resulted in materials with high energy absorption capabilities that have postcrushing integrity.

Farley, G. L.↗
NASA NTRS1986

Effect of specimen geometry on the energy absorption capability of composite materials

Static crushing tests were conducted on graphite and Kevlar reinforced epoxy tubes to examine the influence of specimen geometry on the energy absorption capability of composite materials. Tube inside diameter to wall thickness (D/t) ratio was determined to significantly affect the energy absorption capability of composite materials. As D/t ratio decreases, the energy absorption capability increases nonlinearly. The energy absorption capability of K/E tubes was found to be geometrically scalable but energy absorption of Gr/E tubes was not geometrically scalable.

Farley, G. L.↗
NASA NTRS1986

Microwave absorption measurements of melting spherical and nonspherical hydrometeors

Measurements were made of the absorption behavior of melting and freezing hydrometeors using resonant cavity perturbation techniques at a wavelength of 2.82 cm. Melting ice spheres with equivalent melted diameters between 1.15 and 2.00 mm exhibit a period of strong absorption during melting as predicted by prior theoretical calculations. However, the measured magnitude of the absorption peak exceeds the predicted value. Absorption measuremets of melting oblate and prolate ice ellipsoids also exhibit enhanced absorption during melting.

Hansman, R. J., Jr.↗
NASA NTRS1987

Measurement of the absorption coefficient of acoustical materials using the sound intensity method

In this study the possibility of using the two-microphone sound intensity technique to measure the normal incidence and the random incidence sound absorption coefficient was investigated. The normal incidence absorption coefficient was determined by measuring the intensity incidence on the sample and the intensity reflected by the sample placed in an anechoic chamber. The random incidence absorption coefficient was determined by measuring the intensity incident on the sample and the intensity reflected by the sample placed in a reverberation chamber. Absorption coefficient results obtained by the sound intensity technique were compared with standard techniques, namely the reverberation chamber and the standing wave tube. The major advantages of using the sound intensity technique are that it permits 'in situ' measurements and the absorption coefficient for a large range of frequencies can be obtained from a single measurement.

Atwal, Mahabir S.↗
NASA NTRS1988

Interstellar absorption lines in high-resolution IUE spectra of cataclysmic variables

High-resolution ultraviolet spectra of five cataclysmic variables obtained with the IUE are used to investigate the character of the interstellar medium in the vicinity of the sun. These spectra reveal narrow absorption features of neutral and singly ionized interstellar species, and, in SS Cyg, narrow velocity-shifted absorption features of C IV, Si IV, and Si III. Using the column densities implied by the absorption features of the neutral and singly ionized species, values for the depletion of Si, Mg, Mn, and Fe from the gas phase of the interstellar medium in the vicinity of the sun are obtained. These hydrogen column densities are of particular importance in constraining the soft X-ray luminosity of cataclysmic variables because of the severe attenuation of the soft X-ray flux of cataclysmic variables by photoelectric absorption in the interstellar medium. In addition, using the column densities implied by the absorption features of C IV, Si IV, and Si III in the spectrum of SS Cyg, the existence of an expanding H II region of interstellar gas photoionized by he EUV and soft X-ray flux of this cataclysmic variable are inferred.

Mauche, Christopher W.↗
NASA NTRS1989

Energy-absorption capability and scalability of square cross section composite tube specimens

Static crushing tests were conducted on graphite/epoxy and Kevlar/epoxy square cross section tubes to study the influence of specimen geometry on the energy-absorption capability and scalability of composite materials. The tube inside width-to-wall thickness (W/t) ratio was determined to significantly affect the energy-absorption capability of composite materials. As W/t ratio decreases, the energy-absorption capability increases nonlinearly. The energy-absorption capability of Kevlar epoxy tubes was found to be geometrically scalable, but the energy-absorption capability of graphite/epoxy tubes was not geometrically scalable.

Farley, Gary L.↗
NASA NTRS1990

Methane overtone absorption by intracavity laser spectroscopy

Interpretation of planetary methane (CH4) visible-near IR spectra, used to develop models of planetary atmospheres, has been hampered by a lack of suitable laboratory spectroscopic data. The particular CH4 spectral bands are due to intrinsically weak, high overtone-combination transitions too complex for classical spectroscopic analysis. The traditional multipass cell approach to measuring spectra of weakly absorbing species is insufficiently sensitive to yield reliable results for some of the weakest CH4 absorption features and is difficult to apply at the temperatures of the planetary environments. A time modulated form of intracavity laser spectroscopy (ILS), has been shown to provide effective absorption pathlengths of 100 to 200 km with sample cells less than 1 m long. The optical physics governing this technique and the experimental parameters important for obtaining reliable, quantitative results are now well understood. Quantitative data for CH4 absorption obtained by ILS have been reported recently. Illustrative ILS data for CH4 absorption in the 619.7 nm and 681.9 nm bands are presented. New ILS facilities at UM-St. Louis will be used to measure CH4 absorption in the 700 to 1000 nm region under conditions appropriate to the planetary atmospheres.

Obrien, James J.↗
NASA NTRS1990

The hot DOA1 degenerate HZ 21 - A search for circumstellar/photospheric metals and peculiar absorption at He II

A high-resolution IUE spectrum of the hot DO1 degenerate HZ 21 was obtained by combining US1 + European 2 low-background observing shifts. The SWP image reveals a rich spectrum of interstellar absorption lines with an average velocity in the line of sight to HZ 21 of -30 km/s. However, there is no clear evidence of any highly or lowly ionized metal features which could be attributed to circumstellar, wind, or photospheric absorption. There is, however, a broad absorption trough at He II (1640) which was not unexpected, given the clear presence of He II (4686) absorption in this star's optical spectrum. The velocity width of He II (1640) appears consistent with photospheric absorption wings which appear to flank the geocoronal Ly-alpha emission feature. The He II (1640) feature reveals what appears to be a broad (310 km/s) emission reversal. Evidence is provided that the emission reversal is probably real.

Fritz, M. L.↗
NASA NTRS1990

Ultraviolet interstellar absorption toward HD 163522, a halo star at 9 kiloparsecs in the direction l = 350 deg and b = -9 deg

UV interstellar absorption line measurements obtained by the IUE satellite toward HD 163522 in the direction l = 349.6 deg and b = -9 deg are analyzed. The UV stellar spectrum of HD 163522 places the star beyond the Galactic center at a distance away from the Galactic plane of z = -1.5 kpc. An absorption-line spectrum with S/N approximately equal to 15-25 is produced. The UV interstellar absorption lines for neutral, and weakly and highly ionized gas are examined, as well as the correlation between profiles and the Galactic rotation. Simple model calculations are performed to quantitatively determine the characteristics of the profiles. The observed absorption-line profiles for Al III, Si IV, C IV, and N V are converted into plots of optical depths vs velocity. A comparison of these plots for the weak and strong member of each doublet reveals that the profiles are not strongly influenced by the presence of unresolved saturated absorption, and the optical depth profiles can therefore be directly converted into measures of N(v), the column density per km/s. The converted column densities are examined.

Savage, Blair D.↗
NASA NTRS1989

Propagation and absorption of electron-cyclotron maser radiation during solar flares

The propagation and absorption of the maser radiation during solar flares are examined through linear theory and electromagnetic particle simulations. It is shown using linear theory that strong absorption of the radiation should occur as it propagates toward the second harmonic layer, where the magnetic field is half as strong as in the emission region. Only radiation propagating nearly parallel to the magnetic field in a low-temperature plasma may be able to escape under certain limited conditions. Finite temperature effects can cause radiation propagating nearly perpendicular to the magnetic field to refract, causing enhanced absorption. Particle simulations are then used to evaluate the nonlinear response of the plasma as the maser radiation propagates through the absorption layer. It is shown that some of the maser radiation is able to escape through a process of absorption below the second harmonic of the local gyrofrequency and re-emission above it. The fraction able to escape is much higher than that predicted by linear theory, although the amount of escaping energy is only a small fraction of the incident energy.

Mckean, M. E.↗
NASA NTRS1991

Search for correlated UV and x ray absorption of NGC 3516

NGC 3516, a low-luminosity Seyfert galaxy, is one of a small fraction of Seyfert galaxies that exhibit broad absorption in a resonance line. In order to determine whether the UV and x ray absorption in NGC 3516 are related, 5 IUE observations were obtained, quasi-simultaneously with 4 Ginga observations. The results are presented and discussed. The following subject areas are covered: short-term UV variability; emission lines; galactic absorption lines; the C IV, N V, and Si IV absorption features; lower limit on the carbon column density; estimate of the distance from the absorber to the continuum source; variability in the continuum and absorption; a comparison with BAL QSO's; and the x ray-UV connection.

Martin, Christopher↗
NASA NTRS1991

The resonant absorption of p-modes by sunspots with twisted magnetic fields

A simplified inhomogeneous sunspot model with an axial current (twisted magnetic field) is considered. The absorption of incoming acoustic modes in a narrow resonance layer inside the sunspot flux tube is investigated, and the energy loss is estimated. For nonaxisymmetric modes the results are consistent with previous calculations. However, contrary to previous work, it is demonstrated that the existence of an azimuthal component of the magnetic field can lead to significant absorption of even the axisymmetric modes. If the absorption rate calculated in this paper is used in conjunction with the observed wavelength dependence of the absorption coefficient, it is found that the sunspot flux tube must have significant twist in the subsurface layers. Furthermore, the presence of twist in the magnetic field leads to a natural explanation for the observed dependence on m, the azimuthal wave mode number, and the magnitude of the absorption coefficient can be accounted for in a self-consistent way.

Chitre, S. M.↗
NASA NTRS1991

A description of the correlated k distributed method for modeling nongray gaseous absorption, thermal emission, and multiple scattering in vertically inhomogeneous atmospheres

A radiative transfer method for treating nongray gaseous absorption and thermal emission in vertically inhomogeneous multiple scattering atmospheres is described. Probability density distributions of absorption coefficient strength are derived from line-by-line calculations to construct line-by-line and band model based k distributions. The monotonic ordering of absorption coefficient strengths in these k distributions implicitly preserves the monochromatic structure of the atmosphere at different pressure levels, thus simulating monochromatic spectral integration at a fraction of the line-by-line computing cost. The k distribution approach also permits accurate modeling of overlapping absorption by different atmospheric gases and accurate treatment of nongray absorption in multiple scattering media. It is shown that the correlated k distribution method is capable of achieving numerical accuracy to within 1 percent of cooling rates obtained with line-by-line calculations throughout the troposphere and most of the stratosphere.

Lacis, Andrew A.↗
NASA NTRS1991

Resonant behaviour of MHD waves on magnetic flux tubes. I - Connection formulae at the resonant surfaces. II - Absorption of sound waves by sunspots

The present method of addressing the resonance problems that emerge in such MHD phenomena as the resonant absorption of waves at the Alfven resonance point avoids solving the fourth-order differential equation of dissipative MHD by recourse to connection formulae across the dissipation layer. In the second part of this investigation, the absorption of solar 5-min oscillations by sunspots is interpreted as the resonant absorption of sounds by a magnetic cylinder. The absorption coefficient is interpreted (1) analytically, under certain simplifying assumptions, and numerically, under more general conditions. The observed absorption coefficient magnitude is explained over suitable parameter ranges.

Sakurai, Takashi↗
NASA NTRS1991

Relative contributions of discrete and continuum absorption to the photodissociation of vibrationally excited O2 (v = 12-20)

In order to test the proposal that photodissociation of highly vibrationally excited O2 in the Schumann-Runge (SR) band is a source of upper stratospheric ozone, cross sections for the band and continuum absorption are calculated. Calculated maximum SR cross sections are in the 1-2 x 10 to the -18th sq cm range, with the O2(B) state contributing 2-3 orders of magnitude greater absorption than the 1 3Pi(u) state. For v-double prime = 12, it is shown that the SR band and continuum cross sections merge smoothly at 250 nm. It is shown that numerous processes, the most important of which is nonlinear photoabsorption, ultimately lead to the prediction that narrow laser line absorption in the 248-nm region should generate comparable O atom yields whether absorption is into the bands or the continuum. In the atmosphere, SR band absorption is the dominant process.

Saxon, R. P.↗
NASA NTRS1991

The effects of crushing speed on the energy-absorption capability of composite tubes

The energy-absorption capability as a function of crushing speed was determined for Thornel 300-Fiberite 934 (Gr-E) and Kevlar-49-Fiberite 934 (K-E) composite material. Circular cross section tube specimens were crushed at speeds ranging from 0.01 to 12 m/sec. Ply orientations of the tube specimens were (0/ +/- theta)2 and (+/- theta)2 where theta = 15, 45, and 75 deg. Based upon the results of these tests, the energy-absorption capability of Gr-E and K-E was determined to be a function of crushing speed. The magnitude of the effects of crushing speed on energy-absorption capability was determined to be a function of the mechanisms that control the crushing process. The effects of crushing speed on the energy-absorption capability is related to whether the mechanical response of the crushing mechanism that controls the crushing process is a function of strain rate. Energy-absorption capability of Gr-E and K-E tubes ranged between 0 and 35 percent and 20 and 45 percent, respectively, depending upon ply orientation.

Farley, Gary L.↗
NASA NTRS1991

Observation of the visible absorption spectrum of H2O(+)

The water cation, H2O(+), has been studied, using laser absorption spectroscopy in a velocity-modulated discharge. It is shown that it is possible to observe the absorption spectrum of an ion that is not a terminal ion, despite the weak absorption oscillator strength, and despite the use of a relatively noisy dye laser. The relative intensities of the absorption lines have been measured to an accuracy of 13 percent. It is concluded that if the absorption cross section of a single transition can be measured absolutely, then the entire manifold will be known absolutely.

Das, Biman↗
NASA NTRS1992

PKS 0483-436 - A high-redshift quasar with strong X-ray absorption

The first X-ray spectrum of a high-redshift (z = 2.85) quasar is reported. The Rosat PSPC spectrum of PKS 0438-436, covering 0.3-9 keV in the quasar's rest frame, reveals unexpected absorption of about 1 x 10 exp 22/sq cm, assuming it occurs at the source. Only one other high-luminosity quasar (of greater than about 50 observed by Einstein) shows significant absorption in its X-ray spectrum. Of the common line-of-sight absorbers, only highly ionized Ly-alpha forest clouds may be able to explain this amount of absorption. Candidates for an intrinsic absorber are discussed. Absorption at about 1 keV (rest frame) is due primarily to heavy elements. (O, Ne, Mg, Si, S) raising the possibility of measuring early universe abundances via X-ray absorption in this and like quasars. PKS 0438-436 may be a high-redshift member of a population of quasars which can contribute to the X-ray background above 2 keV, without being detectable by previous imaging missions.

Wilkes, Belinda J.↗