[TFSR 15] All threaded fastening systems shall be designed to withstand the entire service life, including the life scatter factor specified by the program or project, and service environment without fatigue failure.
[Rationale: Static rupture is not the only structural failure mechanism for threaded fastening systems. In some instances, due to design or service loads, failure due to fatigue has been observed.]
Fatigue life requirements for threaded fastening systems are the same as those in the general structural requirements levied by NASA on the hardware developer (e.g., NASA-STD-5001). Typically, verification of service life may be by either of the following methods:
- Fatigue analysis using the fatigue analysis factor and service life factor specified in the applicable structural requirements (e.g., NASA-STD-5001), or
- Fatigue testing using the service life factor specified in the applicable structural requirements (e.g., NASA-STD-5001).
Alternatively, with the prior approval of the delegated NASA Technical Authority, similarity to the hypothetical fastened joint described below and in Appendix C may be used to verify a fatigue failure is unlikely to occur in lieu of fatigue analysis or testing. This approach does not relieve any fracture control requirements.
Fasteners used in hardware that is under fracture control are verified for fatigue life if they are designated as fracture critical and the required crack growth (damage tolerance) analysis is performed (e.g., NASA-STD-5019, Fracture Control Requirements for Spaceflight Hardware). Note that fracture control requirements may not be levied by NASA for some projects, particularly projects for uncrewed spaceflight hardware. Fasteners under fracture control that are designated as non-fracture critical (e.g., fail-safe) typically can satisfy fracture control without a crack growth analysis. However, the fatigue life requirement of this NASA Technical Standard must still be verified to substantiate adequate design of the fastening parts for the expected service life and environments.