12.1.5.3.1. General Requirements
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12.1.5.3.1.1. A detailed and comprehensive stress analysis of each pressure vessel and pressurized structure shall be conducted under the assumption of no crack-like flaws in the structure.
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12.1.5.3.1.2. The analysis shall determine stresses resulting from the combined effects of internal pressure, ground or flight loads, and thermal gradients.
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12.1.5.3.1.3. Both membrane stresses and bending stresses resulting from internal pressure and external loads shall be calculated to account for the effects of geometrical discontinuities, design configuration, and structural support attachments.
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12.1.5.3.1.4. Loads shall be combined by using the appropriate design limit or ultimate safety factors on the individual loads and comparing the results to allowable loads.
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12.1.5.3.1.5. Safety factors shall be as determined in 12.2.
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12.1.5.3.1.6. Safety factors on external (support) loads shall be as assigned to the primary structure supporting the pressurized system.
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12.1.5.3.2. Metallic Pressure Vessels and Pressurized Structures Stress Analysis
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12.1.5.3.2.1. For metallic pressure vessels and pressurized structures, classical solutions are acceptable if the design geometries and loading conditions are simple and the results are sufficiently accurate (as determined by PSWG and Range Safety) to warrant their application.
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12.1.5.3.2.2. Finite element or other equivalent structural analysis techniques shall be used to calculate the stresses, strains, and displacements for complex geometries and loading conditions.
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12.1.5.3.2.3. As necessary, local structural models shall be constructed to augment the overall structural model in areas of rapidly varying stresses.
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12.1.5.3.2.4. Minimum material gauge as specified in the design drawings shall be used in calculating stresses.
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12.1.5.3.2.5. The allowable material strengths shall reflect the effects of temperature, thermal cycling and gradients, processing variables, and time associated with the design environments.
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12.1.5.3.2.6. Minimum margins of safety associated with the parent materials, weldments, and heat-affected zones shall be calculated and tabulated for all pressure vessels and pressurized structures along with their locations and stress levels.
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12.1.5.3.2.7. The margins of safety shall be positive against the strength and stiffness requirements of 12.1.7 and 12.1.8.
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12.1.5.3.3. Composite Hardware Stress Analysis
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12.1.5.3.3.1. For composite overwrapped pressure vessels (COPVs) and pressurized structures made of composite materials, the state-of-the-art methodology using composite laminate theory shall be used.
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12.1.5.3.3.2. Interlamination normal and shear stresses as well as in-plane stress components shall be calculated.
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12.1.5.3.3.3. Effects of ply orientation, stacking sequence, and geometrical discontinuities shall be accounted for.
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12.1.5.4. Flight Hardware Pressure System and Pressurized Structure Fatigue Analysis. When conventional fatigue analysis is used to demonstrate the fatigue-life of an unflawed pressure vessel or pressurized structure, nominal values of fatigue-life characteristics including stress-life (S-N) and strain-life (Se - N) data of the structural materials shall be used.
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12.1.5.4.1. These data shall be taken from reliable sources or other sources approved by the payload project and the PSWG.
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Fatigue-life characteristics data are available from reliable sources such as MIL-HDBK-5.
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12.1.5.4.2. The analysis shall account for the spectra of expected operating loads, pressure, and environments.
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12.1.5.4.3. Fatigue damage cumulative technique (such as Miner's rule) is an acceptable method for handling variable amplitude fatigue cyclic loadings.
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12.1.5.5. Flight Hardware Pressure System and Pressurized Structure Safe-Life Analysis
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12.1.5.5.1. When crack growth safe-life analysis is used to demonstrate the safe-life of a pressure vessel or a pressurized structure, undetected flaws shall be assumed to be in the critical locations and in the most unfavorable orientation with respect to the applied stress and material properties.
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12.1.5.5.2. The size of the flaws (cracks) shall be based on the appropriate NDE techniques and flaw detection capabilities.
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12.1.5.5.3. The crack growth safe-life analysis shall be based on fracture mechanics methodology that has been submitted to the PSWG for PSWG and Range Safety review and approval.
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12.1.5.5.4. Nominal values of fracture toughness and fatigue crack growth rate data associated with each alloy, temper, product form, and thermal and chemical environments shall be used in the safe-life analysis.
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12.1.5.5.5. Pressure vessels or pressurized structures that experience sustained stresses shall also show that the corresponding maximum stress intensity factor (Kmax) during sustained load in operation is less than the stress-corrosion cracking threshold (KISCC) data in the appropriate environment, Kmax < KISCC.
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12.1.5.5.6. A crack growth software package accepted by the PSWG and Range Safety shall be used to conduct the safe-life analysis.
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12.1.5.5.7. Aspect ratio (a/2c) changes shall be accounted for in the analysis.
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12.1.5.5.8. Retardation effects on crack growth rates from variable amplitude loading shall not be considered without approval by the payload project.
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12.1.5.5.9. Tensile residual stresses shall be included in the analysis.
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12.1.5.5.10. The safe-life analysis shall be included in the stress analysis report. In particular, loading spectra, environments, assumed initial flaw sizes, crack-growth models, fatigue crack growth rate, and fracture data shall be delineated. A summary of significant results shall be clearly presented.
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12.1.6. Flight Hardware Pressure Vessel and Pressurized Structure Loads, Pressures, and Environments
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12.1.6.1. The entire anticipated load-pressure-temperature history and associated environments throughout the service life shall be determined in accordance with specified mission requirements.
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12.1.6.2. At a minimum, the following factors and their statistical variations shall be considered:
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12.1.6.2.1. The environmentally induced loads and pressures.
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12.1.6.2.2. The environments acting simultaneously with these loads and pressures with their proper relationships.
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12.1.6.2.3. The frequency of application of these loads, pressures, environments, and their levels and duration.
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12.1.7. Flight Hardware Pressure Vessel and Pressurized Structure Strength Requirements
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12.1.7.1. All pressure vessels and pressurized structures shall possess sufficient strength to withstand limit loads and maximum expected operating pressure (MEOP) in the expected operating environments throughout their respective service lives without experiencing detrimental deformation.
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12.1.7.2. All pressure vessels and pressurized structures shall also withstand ultimate loads and design burst pressure in the expected operating environments without experiencing rupture or collapse.
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12.1.7.3. Pressure vessels and pressurized structures shall be capable of withstanding ultimate external loads and ultimate external pressure (destabilizing) without collapse or rupture when internally pressurized to the minimum anticipated operating pressure.
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12.1.7.4. All pressure vessels and pressurized structures shall sustain proof pressure without incurring gross yielding or detrimental deformation and shall sustain design burst pressure without rupture.
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12.1.7.5. When proof tests are conducted at temperatures other than design temperatures, the change in material properties at the proof temperature shall be accounted for in determining proof pressure.
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12.1.7.6. Pressurized structures subject to instability modes of failure shall not collapse under ultimate loads nor degrade the functioning of any system due to elastic buckling deformation under limit loads.
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12.1.7.7. Evaluation of buckling strength shall consider the combined action of primary and secondary stresses and their effects on general instability, local or panel instability, and crippling.
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12.1.7.8. Design loads for buckling shall be ultimate loads, except that any load component that tends to alleviate buckling shall not be increased by the ultimate design safety factor.
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12.1.7.9. Destabilizing pressures shall be increased by the ultimate design factor, but internal stabilizing pressures shall not be increased unless they reduce structural capability.
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12.1.7.10. The margin of safety shall be positive and shall be determined by analysis or test at design ultimate and design limit levels, when appropriate, at the temperatures expected for all critical conditions.
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12.1.8. Flight Hardware Pressure Vessel and Pressurized Structure Stiffness Requirements
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12.1.8.1. Pressure vessels and pressurized structures shall possess adequate stiffness to preclude detrimental deformation at limit loads and pressures in the expected operating environments throughout their respective service lives.
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12.1.8.2. The stiffness properties of pressure vessels and pressurized structures shall be such as to prevent all detrimental instabilities of coupled vibration modes, minimize detrimental effects of the loads and dynamics response that are associated with structural flexibility, and avoid adverse contact with other vehicle systems.
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12.1.9. Flight Hardware Pressure Vessel and Pressurized Structure Thermal Requirements
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12.1.9.1. Thermal effects, including heating rates, temperatures, thermal gradient, thermal stresses and deformations, and changes in the physical and mechanical properties of the material of construction shall be considered in the design of all pressure vessels and pressurized structures.
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12.1.9.2. These effects shall be based on temperature extremes that simulate those predicted for the operating environment plus a design margin as specified in MIL-STD-1540, Test Requirements for Space Vehicles, or equivalent.
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12.1.10. Physical Arrangement of Flight Hardware Pressure Systems and System Components
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12.1.10.1. Flight Hardware Pressure System and System Component General Requirements
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