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Material Testing |
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Test facilities at T&M Engineering include facilities for ASTM E1559 testing for material outgassing and hypervelocity impact testing for material failure, calibration and impact studies, in general Outgassing Molecular contamination of materials due to outgassing pose a serious threat to sensitive devices, such as optics and thermal control surfaces. In the regime of outgas testing T & M Engineering can provide both ASTM E595 and ASTM E1559 testing. ASTM E595 ASTM E595 testing is the most common form of outgassing tests performed on materials used in the space program. There are two parameters that are determined; the total mass loss (TML) and the collected volatile condensable material (CVCM). In the ASTM E595 test, a sample is heated to 125C for 24 hours at a pressure of 7E-03 Pa (9E-05 torr). By comparing the initial and final masses, a differential mass is determined. The differential mass is the TML that is cited in E595 results. The second parameter, CVCM, is determined by using a collector plate that is held at 25C for the same 24-hour period. The CVCM is a measure of what fraction of the TML will condense on a 25C surface. The CVCM is expressed as a percentage of the pre-test mass of the sample and is determined by measuring the difference in mass of the collector plate before and after the test. For certain classes of materials, a large portion of the TML is water which will not condense on room temperature surfaces in vacuo. This is usually measured as a third parameter [water vapor regained (WVR)] by subjecting a sample, post-test, to a 50% humidity for 24 hours at 23C to determine the amount of water regained. The typical pass/fail criteria used for most space programs is to prescribe that materials have a TML < 1% and a CVCM < 0.1%. There are, however, limitations to E595 testing. First, there are materials that will pass E595 testing, but have been found to be very poor choices for use in spacecraft design. This occurs because a class of material exhibits slow outgassing initially. Accordingly, 24 hours is not a sufficient period of time to characterize the true outgassing properties. The other case exists for materials that have a low CVCM, but the condensed species have high absorption or turn opaque when exposed to UV radiation, and, hence are deleterious to critical surfaces. Alternatively, there are materials that fail E595 testing, but are commonly used on spacecraft. A material can have a high TML, but the material outgassed either doesn't condense readily on surfaces (a near zero CVCM) or the species is transparent (a low absorption coefficient). More importantly, E595 testing only tests materials at one specific temperature and this temperature regime is not consistent with the normal operating temperatures of spacecraft (i.e., spacecraft do not all have source temperatures of 125C and receptor temperatures of 25C). ASTM E1559 The ASTM E1559 method tests materials over a wider range of temperatures and over a longer period of time than the E595 tests. E1559 testing uses Quartz Crystal Microbalances (QCMs) to measure mass deposition as a function of temperature and time. The test method places four (4) QCMs in a vacuum chamber with a pressure similar to that experienced in the E595 testing. The test method always sets one QCM to 90 K (-183C) and one QCM to 298 K (25C) and allows the user to select the temperatures of the other QCMs. Each test (one temperature) lasts between 3-5 days. The output of an E1559 test is a condensable outgassing rate for each of the sample and collector temperatures. These rates can be entered into a contamination math model with modifications to take into account the view factors. E1559 testing is obviously more expensive and time consuming, but offers much greater insight into the outgassing characteristics of a material. The significant advantage of the E1559 test is that it provides information on the time evolution of the outgassing rate of materials. An example of how important ASTM E1559 testing is that as of 5 May 1999 all non-metallic materials to be flown on the International Space Station must undergo ASTM E1559 testing as detailed in NASA SSP 57003. T&M Engineering is unique in being able to provide ASTM E1559 testing to the space community. The only other locations offering such high precision, quality testing are government agency laboratories at NASA/JSC, NASA/GSFC and ESA/ESTEC.
Hypervelocity Impact (HVI) Hypervelocity impact damage has been well documented of the past years and has become a specialised field in it's own right. The damage to materials from particles can caused material failure through penetration, ionisation may cause electrical shorting, and small particles can scuff and damage optical surfaces and thermal control surfaces rendering them ineffective. T & M Engineering can supply impact testing for any material, for calibration purposes and impact studies, with particle velocity range of 1km/s up to 50km/s and particle masses from ~ E-13g to~ 2g utilising two stage light gas gun and a 2MV Van de Graaff accelerator.
Two Stage Light Gas Gun A light gas gun (LGG) is a common HVI simulator for "slow hypervelocity" (< ~10 km/s) and "large micro-projectiles" (> 10 microns). It is thus more suitable for space debris simulation than interplanetary dust particles (IDP's). Shield design for spacecraft and oblique impact studies are some of popular applications of the facility. The LGG available to T & M Engineering is a "two-stage" type as there is a theoretical limit to the velocity that can be achieved using rifle powder alone. Single shot or shotgun techniques are used for impact testing and a variety of diagnostics is available to measure the particle velocity during time of flight and also by incorporating a PZT in the stop plate to measure the impact momenta. The LGG routinely achieves the impact velocity ~7 km/s for 0.1 to E-6g mass ranges.
2 MV Van de Graaff Accelerator A 2 MV Van de Graaff electrostatic particle accelerator which routinely achieves a velocity of 1 - 25 km/s for projectiles of E-9 to E-13g, more toward the sporadic meteoroid momentum range than LEO debris. The most recent unpublished records of the velocities achieved are 215}10 km/s for a 1.8 E-17g particle and 106 +/-10 km/s for a 9.34 E-17g. The facility has been used for assessment of thin foil capture cells on LDEF and EuReCa as well as impact detectors on interplanetary probe like the PZT sensors on Giotto and a plasma sensor like the Cosmic Dust Analyzer (CDA) on Cassini. T & M Engineering can design fabricate and complete a full impact calibration test scenario model and recommend shielding configurations for payloads. |
