DEPARTMENT OF THE AIR FORCE SMC REGULATION 800-29
Headquarters Space and Missile Systems Center (AFMC)
Los Angeles Air Force Base, California 90009-2960
STRESS AND FRACTURE MECHANICS ANALYSES OF
PRESSURE VESSELS AND PRESSURIZED STRUCTURES
This regulation establishes policies, procedures, and assigns responsibilities for SMC program office acquisitions that contain pressure vessels or pressurized structures in launch or space vehicle applications.
1. Policy: All SMC space and launch vehicle programs shall require stress analyses, fracture mechanics analyses and life analyses to be conducted on pressure vessels and pressurized structures.
2. Background: Weight limitations generally prevent current pressure vessels and pressurized structures in missile and space systems from meeting DOL/OSHA requirements. SMC has made commitments which require design and operational planning for conditions as safe as those specified by OSHA without meeting the requirement for large design safety factors. Determination of equivalent safety requires detailed stress analysis and comprehensive fracture mechanics analysis. MIL-STD-1522A, "Standard General Requirements for Safe Design and Operation of Pressurized Missile and Space Systems," specifies the detailed requirements for pressure vessels. However, requirements for pressurized structures are considered to be inadequate.
Conventional Fatigue-life Analysis - An analysis to determine life of unflawed structures using nominal values of fatigue-life characteristics of the structural materials. Parent materials, weldments and heat-affected zones shall be analyzed as appropriate.
Failure Mode Analysis - An analysis based on fracture mechanics to determine potential failure modes in terms of leakage (leak-before-burst (LBB) failure mode) or complete fracture (brittle fracture mode). Results of stress analysis shall be used to perform the failure mode analysis.
Pressurized Structure - A structure designed to sustain both internal pressure and vehicle structural loads. The main propellant tank of a launch vehicle is a typical example.
Pressure Vessel - A structural component whose primary function is to store pressurized fluids and:
(a) Contains stored energy of 19,130 joules (14,240 foot-pounds) or greater based on adiabatic expansion of a perfect gas; or
(b) Contains a gas or liquid which will create a mishap(accident) if released; or
(c) Will experience a design limit pressure greater than 100 psi.
Safe-life Analysis (Fracture Mechanics Safe-life Analysis) - An analysis to determine the life of a component under functional and operational constraints. The safe-life analysis shall be performed under the assumption of pre-existing initial flaws or cracks in the pressure vessel or the pressurized structure. In particular, the analysis shall show that the pressure vessel or the pressurized structure with flaws placed in the most unfavorable orientation with respect to the applied stress and material properties, of sizes defined by the acceptance proof test or non-destructive inspection and acted upon by the spectra of expected operating loads and environments, will meet the safe-life requirement. The loading spectra, material strengths, fracture toughness and crack growth rates of the parent material and weldments, stress levels, and the compatibility of the structural material with the thermal and chemical environments expected in service shall be taken into consideration. Nominal values of fracture toughness and crack-growth rate data corresponding to each alloy system, temper and product form shall be used along with a life factor of four (4) on specified service life.
Safety Factors - Safety factors for pressure vessels are specified in MIL-STD-1522A. (Note: since safety factors for pressurized structures are not documented elsewhere, they are listed below.) For pressurized structures, safety factors will be as follows:
SAFETY FACTORS FOR PRESSURIZED STRUCTURES
Application Yield Ultimate(1)(2)
Manned 1.25 1.40
Unmanned 1.10 1.25
[Notes: (1) When safety of launch site personnel is a design consideration, the safety factor shall conform to the requirements of ERR 127-1 or WRR 127-1 (as appropriate).
(2) If internal pressure is a stabilizing or relieving influence on stress, then no ultimate safety factor is applied to the internal pressure portion of the load.]
Stress Analysis - An analysis to determine stresses resulting from the combined effects of ground loads or vehicle flight loads and internal pressure, and the associated thermal gradients. Stress analysis will assume no crack-like flaws in the structure. Loads will be combined by using the appropriate yield or ultimate safety factors on the individual loads.
4. Responsibilities and Procedures:
a. Each SMC program office using pressure vessels in space applications shall comply with the requirements of MIL-STD-1522A.
b. Each SMC program office using pressurized structures in space applications shall:
(1) obtain a stress analyses for the pressurized structure.
(2) obtain a failure mode analysis that shall be based on the results of the stress analyses obtained in 4.b.(1).
(3) obtain either, conventional fatigue-life analysis, or safe life analysis as outlined below:
(A) Conventional fatigue-life analysis: if the failure mode analysis indicates a leak-before-burst (LBB) failure mode, and if the pressurized fluid is non-toxic or non-hazardous, further fracture mechanics safe-life analysis is not required. Conventional fatigue-life analysis shall be performed on the unflawed structures instead.
(B) Safe-life analysis: if the failure mode analysis indicates brittle fracture mode or hazardous LBB failure mode, a fracture mechanics safe-life analysis shall be conducted.
c. Each SMC program office using pressure vessels or pressurized structures in space applications shall obtain an analysis report. The input parameter data, assumptions, rationale, methods, references, summary of significant analysis results, etc. used during the analyses of 4.a, 4.b.(1), 4.b.(2) and 4.b.(3), shall be delineated in the analysis report. Additionally, if a fracture mechanics safe-life analysis [4.b.(3)(B)] is required, the report shall delineate the following:
(1) fracture toughness and crack-growth rate data;
(2) loading spectrum and environments;
(3) initial flaw sizes;
(4) analysis assumptions and rationale;
(5) calculation methodology;
(6) references; and,
(7) summary of significant results.
The analysis report shall be provided at the PDR and periodically revised and updated during the life of the program at subsequent major design reviews.
d. Instructions for applying and executing this regulation
are provided by SMC Regulation 540-15 (Systems Engineering Policy).
Any program director who does not apply and execute this regulation,
as required by SMCR 540-15, shall immediately request a waiver
from the Commander.