Effect of stress triaxiality cut-off value in the fracture criterion on predicting the ballistic behavior of Al2024-T351 plate impacted by blunt- hemisphere- and ogival-nosed projectiles

Finite element analysis has been carried out to study the effect of stress triaxiality cut-off value in the fracture criterion on the ballistic behavior of 9.94 mm thick Al2024-T351 plates impacted by rigid blunt-, hemisphere- and ogival-nosed projectiles. The plastic and fracture behavior of plates are described by a modified Johnson-Cook (MJC) model and our recently proposed DF2021 criterion, which are implemented into ABAQUS/Explicit finite element code with VUMAT subroutine. The stress triaxiality cut-off parameter in the DF2021 criterion is varied as 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 with a larger stress triaxiality cut-off parameter corresponding to a lower stress triaxiality cut-off value. The effect of stress triaxiality cut-off values on the ballistic behavior of the targets against three different nose-shaped projectiles is evaluated and compared in terms of ballistic limit velocities, residual velocities, and failure patterns in the impact process. It is observed that the ballistic limit velocity increases with the stress triaxiality cut-off value under the impact of blunt-nosed projectiles while it first decreases and then increases with the stress triaxiality cut-off value under the impact of the hemisphere- and ogival-nosed projectiles. The formed shear plug or fragments exhibit different profiles with variations of the stress triaxiality cut-off value under the impact of hemisphere- and ogival-nosed projectiles, but uniformly exhibit a cylinder shape under the impact of blunt-nosed projectiles. The numerical results are compared with available experiments. It is found that the predictions from the DF2021 criterion with a cut-off parameter of 0.1 have the best agreement with the experiments. The reason causing the difference in the ballistic behavior due to the change of stress triaxiality cut-off value is discussed by analyzing the stress states of failed elements and the energy dissipation. The objective of the study is to evaluate the necessity of considering the stress triaxiality cut-off value in a fracture criterion in predicting ballistic behavior through finite element simulations.