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Evolution of USAF Damage Tolerance design philosophy

In the early days of aviation, the concept of fatigue life was not a prime consideration in design. Aircraft did not typically have a long service life and were designed based on ultimate strength. Early aircraft failures occurred due to component/system failures, combat damage, and pilot error. Aircraft did not typically last long enough for fatigue to become an issue. As the design of aircraft became more sophisticated and pilot skill increased, aircraft began to last longer and fatigue became a critical concern.

After several accidents in the 1950s, particularly the Comet and B-47 aircraft, the USAF changed to a fail-safe and safe-life design philosophy where fatigue performance was carefully managed. The fail-safe design philosophy incorporated the idea of redundant structure to reduce the risk of a catastrophic failure if a primary structural component failed. The safe-life design philosophy was based on Miner’s Rule. Fatigue testing of materials, components, and assemblies were used to develop a finite service life.

In the 1960s, it became obvious after several failures, most notably the F-111 crash in 1969, that the safe-life design philosophy was not adequate to deal with the fatigue issues occurring in USAF aircraft structures. These events resulted in the USAF adopting the Damage Tolerance design philosophy for aircraft structures in 1975. As a result, the bulk of USAF fatigue critical structural components have migrated to the damage tolerance management philosophy.

Damage tolerance is defined as the ability of a structure containing a crack or an anomaly in the material to resist fracture. This is a salient difference from the safe-life design philosophy, which assumes the structure is free of defects. For damage tolerance, cracks are assumed to be present in the material and fracture mechanics analysis and testing are utilized to determine whether these cracks will grow to critical size resulting in fracture of the part. The objective of the damage tolerance approach is to detect cracks in critical parts before they propagate to failure. The three key elements of a damage tolerance design framework are fatigue crack growth behavior (typically utilizing linear elastic fracture mechanics), residual strength, and non-destructive inspections.

Hill Engineering has experience in all aspects of damage tolerance analysis and other related topics. Please look through our website for more specific information and examples including: additional information on fatigue analysis, the use of compressive residual stress to enhance performance, and details about fatigue testing. Please contact us for additional information.