## 4 Results and Discussions
### Discussion-Effect of Plastic Deformation on the Normal Penetration  
In the case of the normal impact, the projectile with an ogive nose showed the deepest penetration. The plastic deformation of the projectile has a significant effect on the penetration depth. The penetration depth of the projectile had a close relationship with its kinetic energy. Before the collision, the projectile had kinetic energy related to the velocity. If there was no energy loss, the kinetic energy changed to penetration energy and the deformation energy of the projectile. As the plastic deformation of the projectile increased, the deformation energy of the projectile increased. The remaining energy was converted into penetration energy. Figure 11 shows the deformed shapes of the cylinder shape, blunt shape, truncated shape, and ogive shape with the CRH value of 3 (CRH = 3) in normal impact. The transmission depth is the highest in CRH = 3, followed by Truncated, Blunt, and Cylinder, as shown in Figure 10. In the case of the cylinder geometry, large plastic deformation occurs after impact. As a result, the shape of the projectile is significantly altered. The initial shape cannot be maintained. In the case of blunt geometry, the diameter of the projectile body was increased after impact. Plastic deformation at the tip of the projectile occurs in the truncated geometry, thereby increasing the lower diameter. However, in the case of CRH = 3, the plastic deformation at the center was also very small. The projectile retained its original shape. Figure 11: Deformation of the projectile after the normal penetration: (**a**) Cylinder, (**b**) Blunt, (**c**) Truncated, and (**d**) CRH = 3. Figure 10: Penetration depth/mass distribution with respect to the projectile geometries. Figure 12 presents the velocity, and the displacement of the projectile tip for the cylinder geometry, blunt geometry, truncated geometry, and ogive geometry (CRH = 3). The velocity of the cylinder geometry reduced very quickly. Finally, the velocity of the projectile was converged to zero. For the projectile having an ogive nose (CRH = 3), showed a moderate reduction of the velocity. Consequently, the penetration depth was the largest. The reaction force of the projectile is proportional to the acceleration of the projectile. The cylinder geometry showed the largest acceleration, which is the first derivative of the velocity. It means that a large force occurred at the cylinder geometry. The graph in Figure 13 shows the relationship between the plastic deformation energy per mass and the depth of the penetration depth per mass. In Figure 11d, the maximum equivalent plastic strain is 0.027. The deformation energy of the projectile is very small. Therefore, the remained energy is converted to penetration energy and has good penetration performance. In the case of cylinder shape, plastic deformation is very large, indicating a large amount of plastic deformation energy. Before impact, all have kinetic energy relative to the same mass, where the larger the dispersion to plastic modified energy, the smaller the transmission depth can be found. If there were no nose in the projectile like the cylinder geometry, a strong reaction force due to the impact occurred at the earlier stage of the collision. This energy is converted into the plastic strain deformation of the material. In the case of the ogive shape, the angle of the nose between the surface of the nose and the center axis of the projectile is small. The cavity pressure occurred due to the impact is low compared  
Figure 12: Velocity and penetration depth with respect to time: (**a**) Velocity distribution and (**b**) Penetration depth. Figure 13: Relationship between penetration depth/mass and plastic deformation energy/mass: (**a**) Cylinder, (**b**) Blunt, (**c**) Truncated, and (**d**) CRH = 3.  
to other cases. This greatly increases the depth of penetration.