A top-level safety requirement for aircraft is that the probability of structural failure, resulting in the loss of aircraft, shall be less than 1 × 10−7 per flight hour. A reasonable approach to justify fatigue scatter factors is to derive them from this top-level safety requirement. The minimum scatter factor that can be justified using Weibull analysis is a function of the material data and stress level. Typical results at realistic stress levels range from 8 to 16 lifetime analytical margin to the characteristic life of the S-N data set. Weibull analysis results demonstrate that a 4-life time test provides high probability of detecting a design margin shortfall. Scatter Factors are also derived for design loads and life tracking loads. The analysis discussion addresses single-point-failure safety critical structure; however, a Weibull based probability analysis for multiple-load-path structure with dependent failure mechanisms is also included. Graphical and statistical proofs justify the material scatter factors. The F-22 program uses Weibull based probability analysis for design margins, testing, certification and life management. We call the risk based scatter factor approach High Durability Margin Analysis (HDMA).

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