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At least 325 records · Page 18

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NASA NTRS1988

Construction of mass-age curves for the continental crust: An empirical model and an example from the western US

The methodology of determining crustal mass-age curves is discussed. Problems in doing this include determination of model ages and accounting for mixing of materials of different ages during crust forming processes. These difficulties can be overcome with some reasonable assumptions and estimates of rock volumes based on areal proportions. This technique was used to construct a reasonably well constrained mass-age curve for the southwestern U.S. based on isotopic measurements on over 100 samples. The results imply that the crust in this area grew episodically at 2.8, 1.8, and 0.1 Ga. It is estimated that it would take on the order of 10000 to 100000 individual Sm-Nd isotopic measurements to carry out a similar exercise for continental crust world wide.

Depaolo, D. J.↗

NASA NTRS1988

Lattice parameter variations during aging in nickel-base superalloys

The importance of the state of coherency on measurements of gamma/gamma-prime lattice mismatch has been experimentally demonstrated during aging at 1000 C of specimens of an alloy with composition Ni-(8.6)Cr-(5.3)Al-(10.1)Co-(11.7)W-(1.2)Ti-(0.7)Mo (wt pct). Lattice parameter measurements are given as a function of aging time, and the corresponding sample microstructures are presented. The results show that changes of the two phases during aging did not influence the lattice parameter measurements, indicating that aging specimens to produce a semicoherent gamma/gamma-prime structure provides a good approximation of the true, unconstrained lattice mismatch.

Nathal, M. V.↗

NASA NTRS1987

Effect of geometry on thermal aging behavior of Celion/LARC-160 composites

Laminates of Celion/LARC-160, fabricated in thicknesses from 4 to 16 ply and in unidirectional, x-ply and fabric ply configurations, were isothermally aged at temperatures of 204, 260 and 316 C for periods up to 15,000 hours. Weight-loss of the test panels was measured at selected intervals during aging. At the lower aging temperatures, it was observed that panel thickness and ply arrangement influenced the apparent stability: i.e., thicker panels degraded less than thin panels and unidirectional panels degraded less than x-ply or fabric reinforced panels. At higher aging temperatures, all panel configurations and thicknesses converged toward the same behavior.

Nelson, James B.↗

NASA NTRS1989

Age-related changes in human vestibulo-ocular and optokinetic reflexes: Pseudorandom rotation tests

The dynamic response properties of horizontal vestibulo-ocular reflex (VOR) and optokinetic reflex (OKR) were characterized in 216 human subjects ranging in age from 7 to 81 years. The object of this cross-sectional study was to determine the effects of aging on VOR and OKR reflex dynamics, and to identify the distributions of parameters which describe VOR and OKR responses to pseudorandom stimuli in a putatively normal population. In general, VOR and OKR response parameters changed in a manner consistent with declining function with increasing age. For the VOR this was reflected in declining response amplitudes, although the magnitude of the decline was small relative to the variability of the data. For the OKR the lag time of the response, probably associated with the time required for visual information processing, increased linearly with age at a rate of about 1 ms per year.

Peterka, R. J.↗

NASA NTRS1988

Association of sex and age with responses to lower-body negative pressure

The effects of age and sex on the human-body responses to -50 torr LBNP were investigated in subjects who have undergone LBNP tests at the Kennedy Space Center. The comparison of results obtained on women and age-matched men indicated that men had larger relative increases in calf circumference and greater increases in peripheral resistance during the exposure to LBNP than the women; on the other hand, women displayed greater increases in thoracic impedance and heart rate. The comparison of the results on men of different ages (between 29 and 56 y) indicated that older subjects had greater increases in peripheral resistance and less heart rate elevation in response to LBNP. It is suggested that the age-related circulatory differences were due to a reduction in vagal response and a switch to predominant sympathetic nervous system influence in older men.

Frey, Mary Anne Bassett↗

NASA NTRS1989

Update on terrestrial ages of Antarctic meteorites

Cosmic-ray produced Cl-36 (half-life = 3.01 x 10 to the 5th years) has been measured in 90 Antarctic meteorites by accelerator mass spectrometry. The terrestrial ages of the meteorites were calculated from the results. After excluding possible paired objects, 138 terrestrial ages from 18 different locations are available from C-14, Kr-81, and Cl-36 measurements for application to Antarctic meteorite and glaciological studies. The terrestrial ages of Allan Hills meteorites vary from 2000 years to 1 million years and are clearly longer than those of Yamato meteorites and other Antarctic meteorites. The oldest Allan Hills meteorites were found close to the eastern edge and in the southeast of the main icefield. Among all Antarctic meteorites measured to date, only L and LL chondrites have terrestrial ages older than 370,000 years.

Nishiizumi, K.↗

NASA NTRS1990

Age constraints for the present fault configuration in the Imperial Valley, California: Evidence for northwestward propagation of the Gulf of California rift system

Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial-Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the 4 to 5 million year age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculations of a maximum age of 80,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements, which also suggest that the current patterns of seismicity and faults in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest northwestward growth of the Imperial fault and migration of the Brawley Seismic Zone. It is suggested that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent.

Larsen, Shawn↗

NASA NTRS1989

Aging effects on the fracture toughness of SiC whisker reinforced 2XXX aluminum alloys

The effect of aging (at 150 C) time on the fracture toughness behavior of a 2XXX alloy (Al-3.55Cu-1.29Mg-0.01Fe-trace Mn) reinforced with 5 vol pct F-8 SiC whiskers was investigated by measuring hardness and electrical conductivity followed by fracture toughness tests on center-cracked specimens. The ageing time-hardening response plots showed that, independent of whisker orientation, the initial rapid increase in hardness was followed by a more gradual increase, with a broad hardness peak between 32 and 128 hrs of aging. Coincident with the hardness changes, the electrical conductivity initially decreased, reached a minimum, and then increased at aging times beyond 32 hrs. Examination by SEM indicated that the initial increase in hardness and decrease in conductivity was due to the GPB zone formation, while the subsequent increase in electrical conductivity and decrease in hardness (overaging) was due to S nucleation and growth.

Ratnaparkhi, P. L.↗

NASA NTRS1991

Natural aging and reversion behavior of Al-Cu-Li-Ag-Mg alloy Weldalite (tm) 049

This study was initiated to understand the natural aging and reversion behavior of Weldalite (trademark) 049 in tempers without cold work. Of particular interest are: (1) the microstructural basis for the high strength in the T4 condition; (2) an explanation of the reversion phenomenon; and (3) the effect of re-aging at room temperature after a reversion treatment. Mechanical properties were measured and transmission electron microscopy (TEM) analysis performed at various stages of microstructural development during aging, reversion, and subsequent re-aging.

Gayle, Frank W.↗

NASA NTRS1990

Distribution of terrestrial age and petrologic type of meteorites from western Libya

A group of 54 meteorites have been recovered from Daraj, Western Libya. After assessment of pairing of samples, using petrologic criteria, C-14 terrestrial ages were obtained on 13 samples selected from 9 different fall events. Eleven of the ages range from 3500 to 7600 years, with only two samples having ages in excess of 10,000 years. The cutoff in ages may be related to the timing of climatic changes in the Hammadah al Hamra.

Jull, A. J. T.↗

NASA NTRS1990

The effects of stellar age and metallicity on the infrared emission in the Magellanic Clouds

Infrared Astronomical Satellite (IRAS) maps of the Magellanic Clouds smoothed to a 5-min angular resolution and young star catalogs are used to study the effects of stellar age and metallicity on the infrared emission of these galaxies. The construction of the stellar catalog is described, along with the conversion from the observational to the theoretical temperature-luminosity diagram and the derivation of stellar ages. The infrared color-color diagram of the Clouds is investigated, and the effect of stellar age on their infrared emission is investigated. The effects of the UV radiation field and of the metallicity on the infrared emission are also considered with emphasis on the influence of such parameters as gas-to-dust ratio on the infrared colors. It is shown that the far-infrared fluxes of individual pixels in the Clouds' maps follow the same behavior as the integrated emission of normal galaxies in the IRAS color-color diagram and that the precise position of a particular pixel depends on the age of the youngest star in that pixel.

Sauvage, M.↗

NASA NTRS1991

A creep apparatus to explore the quenching and ageing phenomena of PVC films

A creep apparatus has been constructed for an in situ determination of length and length change. Using this apparatus, the creep behavior of PVC thin films associated with quenching and aging was studied. The more severe the quench through the glass transition temperature, the greater is the instantaneous elastic deformation and the subsequent creep behavior. As aging proceeds, the quenched films gradually lose the ductility incurred by quenching. These results agree well with the well-known phenomena of physical aging. Thus, the changes reflecting molecular mobilities due to quenching and aging can be properly monitored by such a creep apparatus.

Lee, H. H. D.↗

NASA NTRS1991

Ar-40/Ar-39 ages of six Apollo 15 impact melt rocks by laser step heating

Fifteen high resolution (21-51 step) Ar-40/Ar-39 age spectra are obtained for six Apollo 15 impact melt rocks of different compositions, using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48 percent or more of the Ar-39 released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21 and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3870 + or - 6 Ma. These three melt rocks differ in composition and likely formed in three separate impact events.

Dalrymple, G. B.↗

NASA NTRS1991

Age constraints for the present fault configuration in the Imperial Valley, California - Evidence for northwestward propagation of the Gulf of California rift system

Larsen, Shawn↗

NASA NTRS1992

The effect of helium diffusion on the ages of globular clusters

Evolutionary tracks for halo stars were calculated, and isochrones which include the effects of microscopic diffusion of helium were constructed. The isochrones were fitted to a metal poor (M92) and a moderately metal rich (NGC 288) globular cluster using an updated version of the Revised Yale Isochrone color calibration. Ages of the two clusters were also determined using the difference between the turnoff magnitude and horizontal branch magnitude, and the difference in color between the main-sequence turnoff and lower giant branch. Considering all methods and constraints, diffusion is argued to reduce the derived ages of M92 and NGC 288 by 0.5-1 Gyr. The maximum age reduction that diffusion could cause is 3 Gyr. Age estimates including diffusion indicate that M92 is 16 +/- 2 Gyr old, and that M92 is about 3 Gyr older than NGC 288, assuming that the clusters have the same O/Fe of +0.4.

Chaboyer, Brian↗

NASA NTRS1992

Apollo 15 impact melts, the age of Imbrium, and the Earth-Moon impact cataclysm

The early impact history of the lunar surface is of critical importance in understanding the evolution of both the primitive Moon and the Earth, as well as the corresponding populations of planetesimals in Earth-crossing orbits. Two endmember hypotheses call for greatly dissimilar impact dynamics. One is a heavy continuous (declining) bombardment from about 4.5 Ga to 3.85 Ga. The other is that an intense but brief bombardment at about 3.85 +/- Ga was responsible for producing the visible lunar landforms and for the common 3.8-3.9 Ga ages of highland rocks. The Apennine Front, the main topographic ring of the Imbrium Basin, was sampled on the Apollo 15 mission. The Apollo 15 impact melts show a diversity of chemical compositions, indicating their origin in at least several different impact events. The few attempts at dating them have generally not produced convincing ages, despite their importance. Thus, we chose to investigate the ages of melt rock samples from the Apennine Front, because of their stratigraphic importance yet lack of previous age definition.

Ryder, Graham↗

NASA NTRS1992

Physical aging and solvent effects on the fracture of LaRC-TPI adhesives

When amorphous materials are quenched below their glass transition temperature, excess enthalpy is trapped in the glassy material because the viscosity is too great to allow the material to remain in volumetric equilibrium. Over time, this excess free volume is reduced as the material slowly approaches its equilibrium configuration. This process, known as physical aging, leads to substantial changes in the constitutive behavior of polymers, as has been widely discussed in the literature. Less is known about the effects of this physical aging process on fracture and fatigue properties of aged materials. The original goal of the summer was to investigate the effects of physical aging on the fracture and fatigue behavior of LaRC-TPI, a thermoplastic polyimide developed at NASA-Langley. Preliminary results are reported, although a lack of equipment availability prevented completion of this task. In the process of making specimens, the current LaRC-TPI was observed to be extremely susceptible to environmental stress cracking. A study of the unique failure patterns resulting from this degradation process in bonded joints was conducted and is also reported herein.

Dillard, David A.↗

NASA NTRS1993

Ar-40/Ar-39 age spectra of Apollo 15 impact melt rocks by laser step-heating and their bearing on the history of lunar basin formation

Results are reported on 26 high-resolution (16-51 steps) Ar-40/Ar-39 age spectra obtained on 12 Apollo-15 melt rocks of different composition using a continuous laser system on submg fragments of recrystallized melt and single-crystal plagioclase clasts from impact melt rocks collected at the Apennine Front where the Imbrium and Serenitatis basins intersect. A table is presented with the summary of the Ar-40/Ar-39 spectrum data, which represent 891 individual temperature step analyses. Also presented are 20 of the 26 age spectra along with their respective K/Ca plots. Melt rock fragments and plagioclase clasts from seven of the 12 samples analyzed yielded reproducible, intermediate-T Ar-40/Ar-39 age spectrum plateaus, which were interpreted as crystallization ages that represent the times of impact of bolides onto the lunar surface.

Dalrymple, G. B.↗