This Section is intended to give the designer a broad understanding of Fatigue and Fracture Control analysis of fasteners, presenting in addition some hand calculation methods suitable for preliminary design purposes only.
The areas of fatigue and fracture mechanics are very much inter-related, in many cases, the relevant analysis tasks are complementary but usually a potential fracture critical item (PFCI) is designed to only one of these methods.
The differences between fatigue and fracture mechanics analyses are in their basic philosophies; fatigue estimates the life to failure of an initially uncracked item while fracture mechanics calculates whether or not an initial crack of a given size can propagate in a catastrophic manner under service loading.
In general, it is important to verify threaded fastener joints for adequate fatigue. Fasteners that are of aerospace quality and design that are used on a single mission (i.e. non-reusable items) and verified to remain elastic and without gapping during their life are generally not fatigue (initiation) critical since they are not subject to significant cyclic loading other than launch and acceptance testing, unless there are significant other cyclic loads e.g. on orbit.
It is important that potentially fracture critical (PFCI) joints are designed in accordance with ECSS-E- ST-32-01 “Fracture Control” (Reference 11.1).
Where possible, it is recommended that the latest version of the approved ESA/NASA computer program be used. The current version is ESACRACK, which incorporates ESALOAD, NASGRO1 and ESAFATIG (see References 11.2 and 11.9).