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Solar Flare X-Ray Titles |
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Titles:
![[horizontal rule]](file:///W:/epbranch/images/rule.gif){kind=small} K. K. de Groh, K., Banks, B., Dever, J. and Hodermarsky, J., "Use of Hubble Space Telescope Degradation Data for Ground-based Durability Projection of EPTFE on ISS," Paper presented at the 9th International Symposium on Materials in a Space Environment, Noordwijk, The Netherlands, June 16-20, 2003. Use of Hubble Space Telescope Degradation Data for Ground-based Durability Projection of EPTFE on ISS Ground-based environmental durability tests have indicated that exposing materials in accelerated tests to environmental model predicted spacecraft mission exposures of known degradation sources does not simulate the extent of damage that occurs in the space environment. One approach to overcoming the difficulties in simulating the space environment using ground-based testing is to calibrate the facility using data from actual space exposed materials to determine exposure levels required to replicate degraded properties observed in space. This paper describes a ground-to-space correlation method that uses a multiple step process to determine the durability of expanded-polytetrafluoroethylene (ePTFE) for International Space Station (ISS) applications based on ground-based x-ray irradiation and heating exposure that simulates bulk embrittlement as occurs in fluorinated ethylene propylene (FEP) thermal insulation covering the Hubble Space Telescope (HST). This method was designed to damage the back surface of equivalent thickness ePTFE to the same amount of scission damage as occurred in HST FEP (based on elongation data) and then correct for differences in ground test ionizing radiation versus space radiation effects, temperature variations, space ionizing radiation environment variations (spacecraft altitude, inclination and duration), and thickness variations. The analysis indicates that after a 10 year mission, the ISS ePTFE will have an extremely embrittled front surface, with surface cracks induced under any given strain, and a very ductile back surface. This study also found that a thermal induced strain of 0.1 will develop in the ePTFE, and under this strain condition, microscopic cracks will start developing very early in the mission at the exposed surface and develop to a depth of ≈ 300 μm after 10 years. K. K. de Groh, M. Martin, “The Effect of Heating on the Degradation of Ground Laboratory and Space Irradiated Teflon FEP,” Presented at the 6th ICPMSE Conference, Toronto, Canada, May 1-3, 2002; NASA TM-2002-211704, July 2002. The Effect of Heating on the Degradation of Ground Laboratory and Space Irradiated Teflon FEP The outer most layer of the multilayer insulation (MLI) blankets on the Hubble Space Telescope (HST) is back surface aluminized Teflon¨ FEP (fluorinated ethylene propylene). As seen by data collected after each of the three servicing missions and as observed during the second servicing mission (SM2), the FEP has become embrittled in the space environment, leading to degradation of the mechanical properties and severe on-orbit cracking of the FEP. During SM2, a sample of aluminized-FEP was retrieved from HST that had cracked and curled, exposing its aluminum backside to space. Because of the difference in optical properties between FEP and aluminum, this insulation piece reached 200 ˇC on-orbit, which is significantly higher than the nominal MLI temperature extreme of 50 ˇC. This piece was more brittle than other retrieved material from the first and third servicing missions (SM1 and SM3A, respectively). Due to this observation and the fact that Teflon thermal shields on the solar array bi-stems were heated on-orbit to 130 ˇC, experiments have been conducted to determine the effect of heating on the degradation of FEP that has been irradiated in a ground laboratory facility or in space on HST. Teflon FEP samples were x-ray irradiated in a high vacuum facility in order to simulate the damage caused by radiation in the space environment. Samples of pristine FEP, x-ray irradiated FEP and FEP retrieved from the HST during SM3A were heat treated from 50 to 200 ˇC at 25ˇ intervals in a high vacuum facility and then tensile tested. In addition, samples were tested in a density gradient column to determine the effect of the radiation and heating on the density of FEP. Results indicate that although heating does not degrade the tensile properties of non-irradiated Teflon, there is a significant dependence of the percent elongation at failure of irradiated Teflon as a function of heating temperature. Irradiated Teflon was found to undergo increasing degradation in the elongation at failure as temperature was increased from room temperature to 200 ˇC. Rate of degradation changes, which were consistent with the glass I transition temperatures for FEP, appeared to be present in both tensile and density data. The results indicate the significance of the on-orbit temperature of Teflon FEP with respect to its degradation in the low Earth orbital space environment. K. K. de Groh and J. D. Gummow, “Effect of Air and Vacuum Storage on the Tensile Properties of X-Ray Exposed Aluminized-FEP,” Special Issue: High Performance Polymers 13 (2001) S421-S431. Effect of Air and Vacuum Storage on the Tensile Properties of X-ray Exposed Aluminized-FEP Metallized Teflon® FEP (fluorinated ethylene propylene), a common spacecraft thermal control material, from the exterior layer of the Hubble Space Telescope (HST) has become embrittled and suffers from extensive cracking. Teflon samples retrieved during Hubble servicing missions and from the Long Duration Exposure Facility (LDEF) indicate that there may be continued degradation in tensile properties over time. An investigation has been conducted to evaluate the effect of air and vacuum storage on the mechanical properties of x-ray exposed FEP. Aluminized-FEP (Al-FEP) tensile samples were irradiated with 15.3 kV Cu x-rays and stored in air or under vacuum for various time periods. Tensile data indicate that samples stored in air display larger decreases in tensile properties than for samples stored under vacuum. Air-stored samples developed a hazy appearance, which corresponded to a roughening of the aluminized surface. Optical property changes were also characterized. These findings indicate that air exposure plays a role in the degradation of irradiated FEP, therefore proper sample handling and storage is necessary with materials retrieved from space.
Pepper, S. V., Wheeler, D. R., and de Groh, K. K., “Quantitative Study of Degradation of PTFE and FEP Films by Soft X-Rays,” Proceedings of the 8th ISMSE & 5th ICPMSE, Arcachon, France, June 5-9, 2000. The deterioration of mechanical properties experienced by the thermal control blankets on the Hubble Space Telescope may be due, at least in part, to the soft x-ray component of the space environment. We have performed a quantitative study of the degradation of the mechanical properties of both Teflon PTFE and FEP films exposed to soft x-rays in high vacuum. A quantitative x-ray intensity spectrum of the radiation at the specimen surface (in photons/cm2-sec-eV) was calculated with the NIST Desk Top Spectrum Analyzer and experimentally verified in-situ with a silicon photodiode radiation detector. Most of the exposures were performed with a tube potential of 15.3 kV and with the radiation filtered through a Be plate to eliminate the softer x-rays that would be absorbed by the front surface of the specimen. Knowledge of the intensity spectrum allowed a calculation of the distribution of energy deposition within the specimen film. This distribution was reasonably uniform throughout the film for these conditions. The quantitative measure of radiation dose most closely correlated with the degradation of mechanical properties was found to be the energy absorbed per unit area – in kJ/m2 – throughout the film’s thickness (areal dose). The strength of the films was assessed by a hydrostatic burst test devised in our laboratory. It was found that the strength of the 75 mm thick PTFE films fell by 25% at an areal dose of 10 kJ/m2. This corresponded to an interior dose 58.3 kGy. The strength continued to decrease with exposure until it reached about 10% of its initial value at areal doses of 350 kJ/m2. In contrast, the 50 µm FEP films were much more robust than the PTFE films, retaining much of their original strength up to areal doses of 50 kJ/m2.
Dever, J. A., de Groh, K. K., Banks, B. A., Townsends, J. A., Barth, J. L., Thomson, S., Gregory, and T., Savage, W., "Environmental Exposure Conditions for Teflon FEP on the Hubble Space Telescope", prepared for the 44th International SAMPE Symposium, May 23-27, 1999. The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in FEP materials retrieved from solar-facing surfaces on HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a Failure Review Board was convened to address the problem of degradation of MLI on HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, environmental exposure conditions for FEP surfaces on HST were estimated including; number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons, and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the Hubble Space Telescope, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on HST.
Pieces of the multilayer insulation (MLI) that is integral to
the thermal control of the Hubble Space Telescope (HST) have been
returned
by two servicing missions after 3.6 and 6.8 years in orbit. They reveal
that the outer layer, which is made from 5 mil (0.13 mm) thick
aluminized
fluorinated ethylenepropylene (FEP) Teflon®, has become
severely
embrittled. Although possible agents of embrittlement include
electromagnetic
radiation across the entire solar spectrum, trapped particle radiation,
atomic oxygen, and thermal cycling, intensive investigations have not
yielded
unambiguous causes. Previous studies utilizing monoenergenic photons in
the 69-1900 eV range did not cause significant embrittlement, even at
much
higher doses than were experienced by the HST MLI. Neither did x-rays
in
the 3 to 10 keV range generated in a modified electron beam evaporator.
An
antidotal aluminized FEP sample that was exposed to an intensive dose
from
unfiltered Mo x-ray radiation from a rotating anode generator, however,
did show the requisite brittlement. Thus, a study was undertaken to
determine
the effects of x-ray exposure on the embrittlement of aluminized FEP in
hopes that it might elucidate the HST MLI degradation mechanism.
Tensile
specimens of aluminized 5 mil thick FEP were exposed to a constant
fluence
of unfiltered x-ray radiation from a Mo target whose maximum energy
ranged
from 20-60 kV. Other samples were annealed, thermally cycled (100x)
between
77-333 K, or cycled and irradiated. Tensile tests and density
measurements
were then performed on the samples which had been irradiated had the
drastically
reduced elongation-to-break, characteristic of the HST samples. Thermal
cycling may accelerate the embrittlement, but the effect was near the
scatter
in the measurements. Annealing and thermal cycling had no apparent
effect.
Only the samples which had been irradiated and annealed showed
significant
density increases, likely implicating polymer chain scission and
annealing.
Metallized Teflon® fluorinated ethylene propylene
(FEP) thermal control insulation is mechanically degraded if exposed to
a sufficient fluence of soft x-ray radiation. Soft x-ray photons (4 to
8 Ĺ in wavelength or 1.55 to 3.2 keV) emitted during solar flares have
been proposed as a cause of mechanical properties degradation of
aluminized Teflon® FEP thermal control insulation on the
Hubble Space Telescope (HST). Such degradation can be characterized by
a reduction in elongation-to-failure of
the Teflon® FEP. Ground laboratory soft x-ray exposure tests
of
aluminized Teflon® FEP were conducted to assess the degree
of
elongation degradation, which would occur as a result of exposure to
soft x-rays in the range of 3 to 10 keV. Test results indicate that
soft x-ray exposure in the 3 to 10 keV range, at mission fluence
levels, does not alone cause the observed reduction in elongation of
flight retrieved samples. The soft x-ray exposure facility design,
mechanical properties degradation results, and implications will be
presented. ![[horizontal rule]](../images/rule.gif){kind=small}
de Groh, K. K., Banks, B. A., Sechkar, E. A., and Scheiman, D. A., "Simulated Solar Flare X-Ray and Thermal Cycling Durability Evaluation of Hubble Space Telescope Thermal Control Candidate Replacement Materials", prepared for the 4th International Space Conference on Protection of Materials and Structures from the LEO Space Environment sponored by the University of Toronto and the Institute for Aerospace Studies, Toronto, Canada, May 23-24, 1998, NASA/TM-1998-207426. During the Hubble Space Telescope (HST) second servicing
mission (SM2), astronauts noticed that the multi-layer insulation (MLI)
covering the telescope was damaged. Large pieces of the outer layer of
MLI (aluminized Teflon® fluorinated ethylene propylene
(Al-FEP)) were cracked in several locations around the telescope. A
piece of curled up Al-FEP
was retrieved by the astronauts and was found to be severely
embrittled,
as witnessed by ground testing. The national Aeronautics and Space
Administration
(NASA) Goddard Space Flight Center (GSFC) organized a HST MLI Failure
Review
Board (FRB) to determine the damage mechanism of the Al-FEP in the HST
environment, and to recommend a replacement thermal control outer layer
to be installed on HST during the subsequent servicing missions.
Candidate thermal control replacement materials were chosen by the FRB
and tested for environmental durability under various exposures and
durations by GSFC and NASA Glenn Research Center (GRC). This paper
describes durability testing at GRC of
candidate materials which were exposed to charged particle radiation,
simulated
solar flare x-ray radiation, and thermal cycling under load. Samples
were
evaluated for changes in solar absorptance and tear resistance.
Descriptions
of environmental exposures and durability evaluations of these
materials
are presented. |
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