Crack Growth / Damage Tolerance Analysis Methods as Applied to V-22 Fuselage and Empennage

During the design phase of the V-22 tiltrotor aircraft both fatigue and fracture mechanics analyses were conducted on the metallic structure. As crack initiation, not crack growth, was the primary criterion and since the fuselage/empennage is designed with fail-safe principals, a less rigorous approach to fracture mechanics was required than would be expected for single load path parts (i.e. dynamic components). An analysis which provides an assessment of remaining life and residual strength after initiation and propagation of a crack was required for all flight critical components. In addition to the typical discrete maneuver type spectrum, the V-22 airframe experiences continuous high frequency oscillatory loads due to 3/rev loads and tail buffeting. In order to realistically evaluate crack growth under this type of loading it was necessary to define a buffet/vibratory load spectrum. This paper details analysis methods (spectrum development, stress intensity factor solutions, residual strength calculations) used on the V-22 fuselage and empennage and provides results of analysis by way of crack growth curves, critical crack sizes, and residual strength margins.