Ground-based durability projection of ePTFE on ISS based on Hubble Space Telescope degradation data

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 approximately 300 mum after 10 years.