Separation and velocity dependence of the drag force applied to a rigid ovoid of Rankine nosed projectile penetrating an elastic–perfectly-plastic target

Yarin et al. have developed an analytical solution for normal penetration of a rigid projectile of the shape of an ovoid of Rankine into an incompressible elastic–perfectly-plastic target. Here, the closed form expressions from this solution are used to analyze separation and velocity dependence of the drag force applied to the projectile. It is shown that for penetration velocities V below a critical value V_s, the target material maintains full contact with the projectile's surface, the drag force is due solely to the resistance of plastic flow in the target and it is independent of the velocity V. For V equal to V_s the separation point jumps from the projectile's tail to a point closer to its tip. For increasing values of V the separation point moves even further towards the projectile's tip causing a strong dependence of the drag force on velocity. In contrast, results of the cavity expansion model applied to this problem indicate that separation cannot occur and that the drag force depends on velocity for all values of V. These results emphasize the importance of accurately modeling the flow field around the projectile.