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The selection of cylinder design was driven by the nature of the proposed testing and the means of accomplishing it. As previously stated, the intent was to evaluate various materials in actual pressure vessel performance scenarios under cryogenic conditions.
It was first determined to test the candidate pressure vessels under typical operational loads/strains. In order to quantify this, the logic shown in the diagram below was developed.
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In normal aerospace design a maximum expected operational pressure is first established (MEOP, Pt C on the diagram). This is typically arrived at by system requirements and analytical definition of the expected variations. A designated factor of safety is next identified (typically 1.2 to 1.5 for aerospace applications depending on many different factors), and this in turn establishes the minimum acceptable pressure vessel performance level (Pt. D, -3 sigma). Established material and manufacturing capabilities are next factored into the equation to establish the overall pressure vessel statistical capabilities, and the pressure vessel is designed accordingly. It is exactly these "established material capabilities" that this effort is in fact trying to establish.
For this exercise it was determined to "work this equation backwards". In other words, the baseline pressure vessel capability at ambient temperatures would first be established, thereby defining points D, E, and F for each fiber/resin material system to be explored. Next, a factor of safety of 1.5 was chosen. A 1.5 factor was chosen in lieu of lower factors because the potential for degradation posed by the cryogenic environment was not know ahead of time, and it was important to have as much of a safety factor on burst as was justifiable. This established point C, the maximum test pressure (corresponding to the maximum operating pressure under real world conditions). Since there was no way to know the operational spread that might be required of a "typical" cryogenic vessel, it was assumed that the coefficient of variation would be in the 3% range; this established the NOP or point B in the diagram.
In summary, for the efforts documented herein, the statistical spread of each material system through actual testing at ambient conditions was first designed. A 1.5 factor of safety was then applied to establish the "maximum operation pressure" corresponding to the maximum test pressure. By applying an assumed 3% coefficient of variation, the "nominal operational pressure" was established (corresponding to the nominal target test pressure).
It was decided very early to use compressed air to test the pressure vessels under evaluation. Since the source of compressed air was limited to 4500 psig or less, this established the pressure range in which to test.
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HyPerComp Engineering, Inc.; SBIR NAS8-03027 Final Report
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