PSI - Issue 71

Sarath Chandran Nair S. et al. / Procedia Structural Integrity 71 (2025) 340–347

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1.5 x stress due to thermo-mechanical loads (ground condition) or 1.5 x stress due to thermo-mechanical loads ± stress due to FAT sine vibration level 1/2.2 (Used to obtain equivalent test pressure at room temperature for lines that require testing at LN2 temperatures)

Environmental Correction Factor (ECF)

Flight Acceptance stress at RT Higher of

a.1.5 x stress due to thermo-mechanical loads applying ECF for Cryo temperature lines. b.1.05x stress due to thermo-mechanical loads for cryo temperature lines c.1.5 x stress due to thermo-mechanical loads for RT lines.

Factors for flight acceptance and design qualification as per NASA standard document - NASA STD 5012B (2016) for composites/ bonded structures are used to finalise the compliance criteria. Factors used for polyimide pipes are 2.0 and 1.5 for design qualification and flight acceptance, respectively. These factors for human rating are higher than NASA standards. As per NASA standard NASA-STD-5012B dtd. June 2016, if testing in the operational environment is not feasible, tests can be performed in a non-operational environment if an ECF is applied. An ECF is a factor to be multiplied by the test load to compensate for the environmental effect on the strength capability at test conditions versus the operating condition. Design qualification of these polyimide lines was done through deflection tests to demonstrate the design for the human space programme. 5. Flight acceptance test plan for polyimide pipes for human space mission Finite element analysis reveals the stress/strain on each polyimide pipe. For acceptance of each pipe, the required stress is simulated on the pipe with a safety factor by either the pressure test method or the deflection test method. Pressure test method is used for acceptance test of polyimide pipes for generic cryogenic stages. Test pressure is finalized on the acceptance stress to be simulated for each pipe. But considering the safety actor for human space missions, the acceptance test pressure will be higher, and many of the polyimide pipes will not withstand this high pressure. This is due to the failure at polyimide – metallic adaptor interface. On the other hand, these polyimide lines will not experience such huge pressure during the operating condition. Hence, the polyimide pipes are selected for acceptance test by deflection test method to demonstrate the acceptance criteria for human space mission. Flight acceptance of polyimide pipes is done through deflection test. Test criteria are to simulate required deflections, which are predicted through finite element analysis. Loading up to proof load (1.5 times operating stress) followed by NDT and leak check to ensure the hardware integrity and to demonstrate the margin. The test procedure is given in table 3. Table 3 : Flight acceptance test plan Test no. Test description 1 Pre NDT checks (Ultrasonic testing & Radiography inspection) 2 Leak check with GN2 by pressure hold method 3 Finite element analysis to predict load and deflection to achieve through deflection test 4 Preparation of test and instrumentation plan 5 Proof load deflection test (either at room temperature or LN2 temperature) based on test conditions 6 Test data processing and comparing with predictions 7 Ultrasonic inspection& Radiography test 8 Leak check with GHe by sniffing method Deflection test procedure Finite element analysis was carried out for polyimide line, considering the realised hardware thickness as measured through UT inspection. Based on the FE analysis, the load required to obtain the required acceptance stress is finalised, and deflection corresponds to the load in the loading direction and perpendicular to the loading direction are predicted. 6.