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Combination of soluble particles and absorbed solvents cause polymer cracking. New failure mechanism in glassy polymers identified. Cracking caused by presence of particles insoluble in polymer matrix but soluble in solvent. Experiments performed and equations describe cracking phenomenon set forth in concise report.
The effective fiber strength of alumina fibers in an aluminum composite was increased to 173,000 psi. A high temperature heat treatment, combined with a glassy carbon surface coating, was used to prevent degradation and improve fiber tensile strength. Attempts to achieve chemical strengthening of the alumina fiber by chromium oxide and boron oxide coatings proved unsuccessful. A major problem encountered on the program was the low and inconsistent strength of the Dupont Fiber FP used for the investigation.
Dynamic mechanical analysis instrumentation was used for the development of specific test methodology in the determination of engineering parameters of selected materials, esp. plastics and elastomers, over a broad range of temperature with selected environment. The methodology for routine procedures was established with specific attention given to sample geometry, sample size, and mounting techniques. The basic software of the duPont 1090 thermal analyzer was used for data reduction which simplify the theoretical interpretation. Clamps were developed which allowed 'relative' damping during the cure cycle to be measured for the fiber-glass supported resin. The correlation of fracture energy 'toughness' (or impact strength) with the low temperature (glassy) relaxation responses for a 'rubber-modified' epoxy system was negative in result because the low-temperature dispersion mode (-80 C) of the modifier coincided with that of the epoxy matrix, making quantitative comparison unrealistic.
Methods of improving the fire resistance of graphite epoxy composite laminates were investigated with the objective of reducing the volume of loose graphite fibers disseminated into the airstream as the result of a high intensity aircraft fuel fire. Improvements were sought by modifying the standard graphite epoxy systems without significantly negating their structural effectiveness. The modifications consisted primarily of an addition of a third constituent material such as glass fibers, glass flakes, carbon black in a glassy resin. These additions were designed to encourage coalescense of the graphite fibers and thereby reduce their aerodynamic float characteristics. A total of 38 fire tests were conducted on thin (1.0 mm) and thick (6.0 mm) hybrid panels.
The scientific capabilities of the Spacelab manned pallet are reviewed, together with the implications of an expansion of the research effectiveness with a free-flying platform. The premier Spacelab flight will carry out earth observations with a metric camera and SAR, atmospheric studies will be performed with imaging spectrometers, and space plasma physics will be examined by injecting particle beams or VLF waves into the near-Shuttle environment. Radiance and spectrum data will be gathered of the sun and UV and X ray information will be recorded from the stars. Experimentation will also be carried out for on-board crystal growth, metallurgy, and glassy material production, as well as the response of biological systems to zero-g conditions and hard space radiation. The telemetry, time, crewmember participation, and on-board controls required for Spacelab operations are outlined. Missions for a space platform for studying the atmosphere/space interface are described.
Annealing in nitrogen below the glass transition temperature T(g) is found to affect the matrix-dominated physical properties of postcured, (+ and -45)2s specimens of Thornel 300 carbon fiber/Fiberite 934 epoxy composite as well as neat Fiberite 934 epoxy resin. The damping behavior, stress-relaxation rate and enthalpic state of the matrix material are found to decrease as functions of annealing time. The time-dependent change in physical properties is explained on the basis of free-volume collapse related to the physical aging of the nonequilibrium glassy network of epoxy. Such physical aging in network epoxy has been demonstrated to be thermoreversible through the monitoring of its kinetics by differential scanning calorimetry, stress relaxation tests, and dynamic mechanical analysis.
The Apollo 16 site polymict breccia 66055 contains abundant brown KREEP glass, as well as related brown glassy breccias having fluid forms. Most of the rock is composed of clasts of poikiloblastic breccia and mesostasis-rich impact melt which predate 66055's assembly, while the KREEP glass was created in the rock-producing impact. KREEP compositions are not common at the Apollo 16 site, leading to the speculation that 66055 originated in terrains to the west of the landing site. The presence of a granitic glass fragment that is embedded in the KREEP glass suggests that it is igneous, with a local volcanic origin which is more consistent with spatial relationships deduced from orbital geochemical data than with a basin ejecta deposit origin.
Cooling rates required to form glassy or partly-crystalline bodies of 14 lunar compositions have been estimated using a previously introduced, simplified model. The calculated cooling rates are found to be in good agreement with cooling rates measured for the same compositions. Measurements are also reported of the liquidus temperature and glass transition temperature for each composition. Inferred cooling rates are combined with heat flow analyses to obtain insight into the thermal histories of samples 15422, 14162, 15025, 74220, 74241, 10084, 15425, and 15427. The critical cooling rates required to form glasses of 24 lunar compositions, including the 14 compositions of the present study, are suggested to increase systematically with increasing ratio of total network modifiers/total network formers in the compositions. This reflects the importance of melt viscosity in affecting glass formation.
Dynamic mechanical studies of thermosetting PMR 15 polyimide/Celion 6000 composites were used to characterize cure behavior. Variation in composite shear modulus with cure time for laminates partially cured under 3.45 MPa pressure was compared with that of laminates fully cured under pressure to select a partially cured system whose behavior corresponded with that of the pressure-cured composites. An empirical kinetic model was developed that relates relative composite shear moduli to cure time and temperature. This model, coupled with standard statistical techniques, was used in the determination of an overall activation energy, E = 145 kj/mole (35 kcal/mole) and order of reaction, n, for the cure reaction(s). The empirical relationship defining dynamic modulus as a function of time and temperature remained linear well into the glassy region. Changes in dynamic glass-transition temperature and in the breadth of the transition peaks with cure time are discussed. Shear modulus and damping were found to be more sensitive measures of relative extent of cure than Fourier transform infrared spectroscopy.
The Shuttle Infrared Telescope (SIRTF) is a 1-m class cryogenically cooled telescope to be operated from the Shuttle as a facility for infrared astronomy. By exploiting the very low infrared background of space, SIRTF will achieve 100 to 1000 times the sensitivity currently attainable at infrared wavelengths between 2 and 200 microns. The scientific requirements of SIRTF, the current design concept, and the scientific capabilities of the systems are reviewed. Recent experimental results are also reviewed showing that mirrors made of glassy materials may be suitable for use in large cryogenic telescopes such as SIRTF. Previously announced in STAR as N82-32207
A study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd(84.5)Si(15.5) eutectic composition is reported. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. It is found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with a previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.
An Rb-Sr isotopic analysis of deep-sea spherules (DSS) from the Pacific, vesicular iridium-rich glassy objects (VIRGO) from an Antarctic-Ocean diatomaceous-ooze core, and equigranular objects (EGO) associated with or included in VIRGO, is reported. The techniques of sample collection, preparation, chemical analysis, and isotopic analysis (by mass spectrometry) are described in detail, and the results are presented in tables. The Sr-87/Sr-86 determination has a precision of 1-2 percent in microsamples of 2-8 x 10 to the 11th Sr atoms. The DSS are found to have Sr concentrations and Sr-87/Sr-86 (0.730-0.757) typical of chondritic meteorites. Extreme Rb depletion is attributed to volatization during meteoroid atmospheric heating. The VIRGO, while having major-element compositions like that of meteorites and high Ir levels suggesting extraterrestrial origin, have Sr-87/Sr-86 = 0.703-0.705; and the associated EGO has Sr-87/Sr-86 = 0.701 + or - 0.001, typical of MORB or basaltic achondrites. Hence their extraterrestrial origin is not proved, and some lithic component other than the EGO must be the source of the high Ir concentration.
In response to a tensile stress glassy polymers exhibit crazing (surface cracks) which is a phenomenon of considerable practical and theoretical importance in the use of many plastics. An attempt is made to demonstrate how craze growth behavior is affected by the craze density (which is defined as the total number of crazes per unit surface area) surrounding the craze.
The technology development for phosphoric acid fuel cells is summarized. The preparation, heat treatment, and characterization of carbon composites used as bipolar separator plates are described. Characterization included resistivity, porosity, and electrochemical corrosion. High density glassy carbon/graphite composites performed well in long-term fuel cell endurance tests. Platinum alloy cathode catalysts and low-loaded platinum electrodes were evaluated in 25 sq cm cells. Although the alloys displayed an initial improvement, some of this improvement diminished after a few thousand hours of testing. Low platinum loading (0.12 mg/sq cm anodes and 0.3 mg/sq cm cathodes) performed nearly as well as twice this loading. A selectively wetproofed anode backing paper was tested in a 5 by 15 inch three-cell stack. This material may provide for acid volume expansion, acid storage, and acid lateral distribution.
The formation by melt quenching of such metastable structures as glassy or microcrystalline solids and highly supersaturated solutions is made possible by the extreme resistance of most melts to homophase crystal nucleation at deep undercooling. This nucleation resistance contrasts sharply with the very low kinetic resistance to the movement of crystal-melt interfaces, once formed, in metals and other fluid systems at even minute undercooling. The methods of nucleation study which have proven especially effective in bypassing nucleation by heterophase impurities thereby exposing the high resistance of melts to homophase nucleation may be summarized as follows: observation of the crystallization behavior of dispersed small droplets; drop tube experiments in which liquid drops solidify, under containerless conditions, during their fall in the tube; and observation of the crystallization of bulk specimens immersed in fluxes chosen to dissolve or otherwise deactivate (e.g., by wetting) heterophase nucleants. This method has proven to be remarkably effective in deactivating such nucleants in certain pure metals.
The laser microprobe was used to extend the Ar-39-Ar-40 method to small, low potassium glassy objects in order to determine the ages of Apollo 15 yellow volcanic, green volcanic group A, and green volcanic group D glasses. The apparent solidification ages for these glasses are 3.62 + or -0.07, 3.41 + or -0.12, and 3.35 + or -.18 aeons, respectively. The ages for group A and d green glasses agree well with previously reported ages for aliquots of 'bulk' green glass. No significant difference in the solidification age was found between the A and D groups. The average exposure ages for the yellow and green glasses from 15426 and 15427 were indistinguishable, with ages between 300 and 275 m.y.
The matrix-dominated properties of a network epoxy and its carbon fiber-reinforced composite are noted to be affected by sub-glass transition temperature T(g) annealing in a nitrogen atmosphere. The damping behavior, stress relaxation rate, and enthalpic state of the matrix material were found to decrease as functions of sub-T(g) annealing time. This time-dependent change in physical properties is explained on the basis of a free-volume collapse which is related to the recovery process of the nonequilibrium glassy network of the epoxy. The physical aging in network epoxies has been demonstrated to be thermoreversible. Aging kinetics are studied by means of differential scanning calorimetry, stress relaxation tests, dynamic mechanical analysis, and thermomechanical analysis.
Measurements taken by the International Ultraviolet Explorer spacecraft of the stars RY Sgr and R CrB have been analyzed using Mie theory. The extinction data, which show a 2400-2500 A peak, are consistent with a distribution of 5-60 nm glassy or amorphous carbon particles obscuring the stellar flux. The data are also fairly consistent with a cloud ejection model. Since the extinction data lack the commonly observed peak at 2170 A, it is proposed that this difference is due to the conditions present when the dust condenses. Interstellar carbon grains appear to originate in normal carbon stars which are carbon and hydrogen rich. In contrast, the grains around R CrB type stars seem to condense from a carbon-rich and hydrogen-poor vapor.