Probabilistic damage tolerance analysis of aircraft structures

Abstract

It is generally agreed that the probability of complete failure of a main structural part of an aircraft must be kept extremely low. Figures from 10⁻³ to 10⁻⁵ during the whole service life have been mentioned, depending on the type of mission considered. Long ago it became obvious that aircraft structures will inevitably suffer from fatigue damage much earlier than their economical life has been exhausted from other aspects. When it was discovered, originally for commercial transport planes, that the Safe Life concept is feasible only in those cases where replacement of the member is easy and cheap, the Fail-Safe design was introduced. Originally this design concept implied structural redundancy. When one structural element failed there would be one or several other elements to carry the load. With the advent of computerized structural optimization, redundant elements will, however, become an anomaly. The only way to fully achieve a fail-safe airframe, seems to be by ensuring a slow and inspection controlled fatigue crack propagation in the structure. The concept Damage Tolerance will be used below mainly to represent such a structure.