Materials incompatibility with oxygen has been attributed to many high profile NASA failures including Apollo 1 and Apollo 13. Most metals and all polymeric materials are flammable in 100% oxygen with sufficient pressure and temperature. Oxygen is necessary for the combustion of fuels in most conventional rocket engines and for the sustainment of humans with life support systems. Therefore, NASA requires that usage of materials in oxygen system applications shall be based on flammability and combustion test data in a relevant oxygen environment to avoid materials incompatibility with oxygen leading to a catastrophic system failure.
Understanding of metals produced by additive manufacturing (AM) methods, such as Powder Bed Fusion Technologies, are now mature enough to be considered for qualification in manned spaceflight oxygen systems. NASA Space Launch System, Commercial Resupply, and Commercial Crew programs are using AM components in propulsion systems, which are likely to include various life support systems in the future. Without systematic flammability and ignition testing in oxygen there is no credible method for NASA to accurately evaluate the risk of using AM metals in oxygen systems.
NASA White Sands Test Facility (WSTF) develops and maintains the expertise and facilities to evaluate material flammability and ignitability in pressurized oxygen environments for the AM materials in question. Through experimentation WSTF is working to identify and investigate specific oxygen compatibility issues unique to AM materials.
Historically flammability statistics have relied on binomial pass/fail responses and resulted in low statistical power and confidence without extensive testing. Using a sequential design of experiments approach, systematic flammability testing in oxygen has been performed at WSTF for selective laser melted Inconel 718. Identification of a meaningful continuous response, sequential orthogonal experimentation, and test matrix randomization allowed for the rigorous evaluation of many factors and their effect, on flammability. Factors studied include base chemistry, post processing methods, heat treatments, and surface treatments. Several additional statistical methods were employed during experimentation, including parametric life analysis and regression model selection. By leveraging modern experimental design philosophy a useful flammability model was developed for Inconel 718. This new methodology for evaluating factors influencing flammability was efficient (required fewer tests) and yielded greater statistical confidence, to make actionable engineering decisions, than previous test methodology. The methodology and results will be discussed.
- : Jonathan Tylka, Kenneth Johnson
- : National Aeronautics and Space Administration
- : Jonathan Tylka
- : experimental_design
- : introductory/practitioner
- : firstname.lastname@example.org
- : 575-524-5762
Sequential Approach to Identify Factors Influencing Flammability of Selective Laser Melted Inconel 718