Revisiting the L/D effect at high L/D

Abstract
Previous computer simulations of the L/D effect (L/D = projectile aspect ratio) demonstrated that reduction in penetration efficiency at large L/D should be explainable within the framework of plasto-dynamic theory. But those simulations did not show what could be the mechanism responsible for penetration velocity deceleration and the reduction in penetration efficiency. Based on computational work that we did some years ago, we assume that the deceleration mechanism has to do with accumulation of projectile material at the bottom of the crater, and that this mechanism is controlled by the flow stress of the projectile (Y_p). We conducted four simulations in which we changed Y_p from 1.0 to 2.5 GPa. From the results we draw several conclusions. 1. The penetration velocity deceleration rate is proportional to Y_p. 2. The deceleration mechanism seems to be caused by projectile material accumulation at the bottom of the crater. 3. We've not shown this, but it seems that projectile material accumulation and penetration velocity deceleration would decrease when D_o/D increases (where D_c = crater diameter). 4. Looking at material location plots, we see that after a long time, material flow near the bottom of the crater becomes somewhat unstable. This would indicate that rods with L/D>40 may become inefficient, unless they are shot at velocities of 2 km/s or higher.