Research on Damage Tolerance (or Flaw Tolerance) applied to Rotorcraft
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. AgustaWestland is committed to work on this issue aiming to enhance the safety of aviation.
The fatigue design methodology most commonly applied to the helicopter community was based on 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. Compliance with this requirement can be demonstrated by means of tests and analysis, using the structural concepts already experienced for a long time in the fixed wing aircraft. Typical helicopter and airplane structures are quite similar, but considerable differences exist in the usage, and consequently also in the fatigue spectra. 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 component geometry, material data and complex loading spectrum were distributed to the helicopter structure with the objective of assessing the capability of current structural analytical tools to predict fatigue crack growth and thus incorporate damage tolerance into helicopter design. A research program has been carried out in collaboration between AgustaWestland and universities for the study of fatigue crack growth under typical helicopter spectra on the critical components of the principal structures such as the main and tail rotors, the MGB mounting rods and the rear fuselage.
A full fatigue and flaw tolerance assessment has been successfully achieved for the certification of the AW139, based on a combination of conventional safe-life and flaw tolerance approaches. A reliable and cost effective approach has been implemented for the assessment of rotor components and all structural parts subjected to high vibratory loading, providing an improvement of fatigue reliability and allowing defining a practical inspection plan for the operative rotorcraft.
More information about AgustaWestland research on Damage Tolerance can be found on several technical papers:
(2008) Application of Damage Tolerance principles to the design of helicopters (International Journal of Fatigue, Volume 31)