Fatigue crack propagation in a helicopter component subjected to impact damage

Damage tolerant methodology is increasingly used in aeronautical components, especially due the fact that the Aviation Regulation requires such an assessment in case an accidental damage occurs. At present, there is a strong and actual interest in applying such procedures to helicopter components that are subjected to high frequency cyclic loads. In this paper, an investigation on a damaged transmission shaft for a tail rotor transmission of an actual helicopter has been carried out focusing on the fatigue crack propagation. A complete sequence of experimental tests was performed in order to create an actual ballistic damage and to subsequently check the damage tolerant behaviour. The shaft was damaged by oblique ballistic impact and was subsequently subjected to torsional fatigue loading. During the fatigue cycles several cracks propagated from the ballistic damages. Both of these steps (impact and fatigue loading) were also simulated by a complex modelling approach based on Finite Element Models and fracture mechanics theory. The comparison between the experimental and numerical results shows a good agreement but it underlines the need for a very refined modelling technique capable to replicate all the features associated with the damage in order to reliably simulate the subsequent propagation phase.