ASTM G 86 and ASTM G 74 (American Society for Testing and Materials standards G 86 and G 74) are commonly used to evaluate materials susceptibility to ignition in liquid and gaseous oxygen systems. The methods have been known for their lack of repeatability. The inherent problems identified with the test logic would either not allow precise identification or the magnitude of problems related to running the tests, such as lack of consistency of systems performance, lack of adherence to procedures, etc. Excessive variability leads to increasing instances of accepting the null hypothesis erroneously, and so to the false logical deduction that problems are nonexistent when they really do exist. This paper attempts to develop and recommend an approach that could lead to increased accuracy in problem diagnostics by using the 50 % reactivity point, which has been shown to be more repeatable. Although statistically for mechanical impact there were no differences at the 95 % significance level between the 50 % reactivity average values obtained with 20 samples and 30 samples, testing 30 samples reduced the coefficient of variation significantly in most instances. Consequently, determining the 50 % point with 30 samples will reduce the instances of accepting the null hypothesis erroneously. The initial tests conducted indicate that PTFE (polytetrafluoroethylene) and Viton A (copolymer hexafluoropropylene-co-vinylidenefluoride, for pneumatic impact) and Buna S (copolymer butadiene-co-styrene, for mechanical impact) would be good choices for additional testing and consideration in inter-laboratory evaluations. The approach could also be used to evaluate variable effects with increased confidence and tolerance optimisation.
Proficiency testing for evaluating aerospace materials test anomalies
2006
6 Seiten, 5 Tabellen, 18 Quellen
Conference paper
English
Raumfahrt , Raumfahrttechnik , Werkstoffeignung , Werkstoffauswahl , Materialprüfung , Standardprüfmethode , Stoß (Schlag) , Schlagfestigkeit , Entzündungsgefahr , Entzündbarkeit , Entflammbarkeit , Prüfverfahren , Genauigkeit , Sauerstoff , Druckgasbehälter , Polymereigenschaft , Polytetrafluorethylen , Hexafluoropropylen , Copolymer , Vinylidenfluorid , synthetischer Kautschuk , Copolymer-Butadien-Styrol , Unregelmäßigkeit
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