FATIGUE CRACK PROPAGATION UNDER TORSIONAL SPECTRUM LOAD OF A HELICOPTER TAIL ROTOR SHAFT AFTER BALLISTIC DAMAGE

Experimental tests of impact of a 7.62 NATO projectile perforating a helicopter tail rotor shaft are briefly presented. Subsequently the damaged specimen has been tested with a multiaxial testing machine, in order to evaluate its capability to sustain a recovery mission, simulated by applying a torsional fatigue spectrum. During the test the nucleation and propagation of fatigue cracks have been monitored. Finite Element analyses have been carried out to simulate both impact and crack propagation for a further analytical automated simulation of these conditions. According to the multiaxial state of stress induced by torque, residual stress field and shape of the damage, mixed mode propagation has been considered, by means of an equivalent SIF. Numerical results are in good agreement with experimental data.