Predicting ballistic impact failure of aluminium 6061-T6 with the rate-independent Bao-Wierzbicki fracture model

The main objective of the paper is to exploit a dedicated material calibration to predict an extreme loading condition: the damage of a helicopter tail rotor transmission shaft due to a small arm ballistic impact. A typical material point in the damage area undergoes to complex stress and strain history, starting from a large compressive stresses through shear deformation during crack extension all the way to the tensile fracture of the remaining ligament. The plasticity and the fracture characterization require to perform material tests covering the wide range of stress state. Therefore, a part of the paper is devoted to the description and analysis of plasticity and fracture test of different type of specimens. This includes test on round smooth and notched specimens under combined torsion/tension-compression, flat dogbone and a curved tensile specimen cut from the wall of the shaft. The test data have been used to calibrate the phenomenological Modified Mohr-Coulomb model and the empirical Bao-Wierzbicki fracture models. The result of the numerical simulation of the projectile impact was then compared with the experiments on the ballistic impact of 7.62 ball projectile against the helicopter shaft. A very good agreement was found for both damage shape and residual velocity.