TWO principal methods for providing safety against catastrophic aircraft structural fatigue — safe-life and fail-safe — are treated. The author concludes that the safe-life method is generally inadequate, while the fail-safe method is practical and sound. Some test results of the fail-safe attributes of certain materials, stress levels, and design details are summarized, and some observations on fail-safe testing techniques are made.
From a fail-safe standpoint, it is concluded that 24ST aluminum alloy is superior to 75ST, that certain stress levels and design features will fail safe, and that underwater pressure testing is probably unnecessary because dynamic over-stresses in fail-safe tests are probably small and fatigue cracks can be quickly simulated in the laboratory.