Accelerated Creep Test Methodologies for Space Habitat Softgood Structural Materials

Space habitatinflatable structures require complex material configurations and manufacturing processes. There is a need to develop a standardized accelerated creep test methodology with analysis capability to compute the master creep curves for high strength Vectran webbing to ensure long-term habitat structural stability. Texas Research Institute Austin, Inc. (TRI Austin) proposes to develop a modified Step-Isothermal Method (SIM) and alternative Accelerated Life Testing methods that can be used for Vectran webbing and yarn based on our Phase I efforts. In Phase II of the program TRI Austin is teaming with Dr. Brown of Clemsons Center for Advanced Engineering Fibers and Films to characterize the tensile properties of Vectran fibers, as well as using techniques to identify the effect of fiber microstructure, molecular architecture, and intermolecular interactions on Vectran long-term creep behavior. Bally Ribbon Mills will provide expertise in webbing design and manufacturing processes used in its construction and currently manufactures the 24K 2-inch webbing for ILC Dovers prototype space habitat. OTEX, who manufactured the 12.5K 1-inch Vectranused in Phase I, will assist by supplying new webbing and yarn with QA/QC lot testing data. The Phase II program has 14 technical objectives based on our discoveries and theories from the Phase I evaluations that will be addressed in Phase II. In Phase IIwe can test and address the impact of manufacturing defects, long-term storage, packaging (folding), transportation to space, and final deployment in space with accelerated aging protocols. These test articles will then be assessed with the developed testing protocols to determine mission profile-based reliability performance calculated based on probability and confidence level to estimate field use life. There is a need to develop and validate a standardized accelerated creep test methodology with analysis capability to compute the master creep curves for high strength Vectran webbing. Classic accelerated creep testing protocol results do not concur with room temperature fix load long-term creep test failure data. An enhanced accelerated test method will be required that can determine accurate long-term creep performance data without overaccelerating the Vectran. The Phase II program has 10 technical objectives listed below based on our discoveries and theories from the Phase I program, we will first focus on the Vectran yarn accelerated creep then move to webbing. The Technical objectives for the Phase II: Perform SIM testing 1500 Denier Vectran yarn. Perform modified SIM testing on Vectran yarn/webbing at fix elevated temperature and at moderate and high UTS loads Temperature shifting modified SIM tests yarn and webbing Perform series of SIM tests using inert gas in environmental chamber Evaluate webbing architecture, construction, and manufacturing process Determine if thermal effects induced during SIM testing process have an impact on predicted creep behavior of the webbing. Evaluate the sonic modulus to ascertain the level of molecular orientation in the Vectran fibers. Hardware development to include grips, load frames, advanced strain measurement, and thermography monitoring techniques Validate modified SIM test method utilizing NASA’s long-term fix load data and TRI Austin in-house fix load creep testing Document protocol for modified SIM testing of Vectran webbing Proposed Deliverables Report detailing methods and hardware used to accelerate creep testing of Vectran webbing. Analysis of data generated towards validating an accelerated creep test for Vectran webbing.
Benefits: The primary NASA application is inflatable Softgoods for Next Generation Habitation Systems. This research development effort will develop and document creep test methodology and analysis capability to compute master creep curves for Vectran webbing generating relevant lifetime material performance predictions. The developed testing methodologies should be included in Vectran webbing qualification testing and certification plan for human-rated inflatable space structures. It is anticipated that with a variety of corporations including Bigelow Aerospace LLC, ILC Dover, Maxar Technologies, Inc., The Boeing Co., Sierra Nevada Corporation, Northrop Grumman Corp., Lockheed Martin, Blue Origin, and Virgin Galactic, among others, entering the area of space travel, that demand for evaluating the creep behavior of inflatable structures will increase in the coming years.