Application of damage tolerance principles to the design of helicopters

The design of helicopters against fatigue phenomena is a particularly important and complex problem, due to the peculiar load spectra, composed by a high number of low-amplitude cycles. The fatigue design methodology most commonly applied by the helicopter community was based on the Safe-Life philosophy, applied anyhow with a particular approach. Since 1989, the Airworthiness Regulations evolved towards the application of Damage Tolerance principles also to rotorcraft. This change has forced the helicopter industries to review their design methodologies, and to face new problems, linked with fracture mechanics applications to their typical structures. Flaws, accidental damages and manufacturing discrepancies must be accounted for, in addition to the retirement life based on Safe-Life. The paper reviews the current requirements and presents and discusses the methodologies that the helicopter industry adopts for demonstrating compliance with such regulatory requirements. In addition, recommendations are given on research and development activities required for refining the defined methodologies and for their better implementation