Effect of Lode angle in predicting the behaviour of stiffened 921A steel target plates in ballistic impact by truncated ogive projectiles

Abstract

Two sets of penetration experiments with truncated ogive projectiles were carried out on large unstiffened and stiffened target plates made of 921A steel; the residual velocity of the projectile was recorded, and the failure mode of the target was analysed. The experimental results show that the unstiffened target plate exhibited obvious petaling failure, whereas the stiffened plate exhibited a coupled complex failure mode because of its spatial structure. Many recent studies have shown that the failure strain of metals can be affected by the Lode angle in addition to the stress triaxiality, so this study introduced the Lode angle into the fracture criterion to investigate whether it affects the ballistic behaviour of the target plate. A numerical model was established in the Ansys LS-DYNA finite-element software, with the mechanical properties of 921A steel characterized by the Johnson–Cook (JC) constitutive equation and the failure strain controlled by either the JC fracture criterion independent of the Lode angle or the modified Mohr–Coulomb (MMC) fracture criterion dependent on the Lode angle. The numerical and experimental results shows that both fracture criteria predict well the residual velocity after penetration. However, the failure mode of the target plate obtained with the JC fracture criterion exhibits a reaming process, as reflected in the local upward dish deformation, and although the target plate also forms multiple petals, they are all connected and gathered inward, acting as a whole. The results calculated with the MMC fracture criterion predict the experimental ones well, with the unstiffened plate exhibiting obvious petaling failure. Finally, the MMC fracture criterion is added to the numerical model of the stiffened plate, and the calculation results show that the ballistic impact behaviour of the target plate can still be predicted effectively.