Empirical Damage Tolerance

A purely empirical method is developed for the crack growth damage tolerance substantiation of metal flightcritical fatigue-loaded helicopter dynamic components. The method captures the key substantiation elements that are essential parts of successful safe-life fatigue substantiations. Namely: independent of analytical design methods; full-scale tests of multiple specimens; strain survey and substantiating parameter; simple constant-amplitude tests; use of measured flight loads; well-defined safety margins; and ability to analyze for loads and usage differences without new testing. Such a method is needed because the purely analytical methods used to date may not provide consistent and repeatable results which can provide high-confidence, highly reliable, inspection intervals for routine general use in helicopter applications. The empirical method proposed relies on curve fitting to the full-scale crack propagation test data, and results in a da/dN equation or equations for each component.