## 3 Results and Analysis
### Analysis of Failure Modes of Liquid Cabin
In Tests 1 and 3, the kinetic energy ratio of the projectiles is 0.82, and the maximum deflection ratio of the rear plates is 0.60, which indicates that when the rear plate is approaching the ballistic limit, the impact force on the rear plate is increased by the projectile's pushing of the water; thus, the rear plate is subjected to the high pressure of the water and the impact of the projectile, resulting in two failures; the coupling effect of the two failures aggravates the damage to the rear plate. Figure 17: Peak value and specific impulse of pressure wave on each wall of the tank. Figure 19 shows the failure morphology of the liquid cabin after the penetration of the projectile with a L/D ratio of 4.8 and a velocity of 1600 m/s and at different attack angles (No. 19-23). As the attack angle increases, the breach in the front plate enlarges. During normal penetration, the front plate undergoes shear plugging-thin film bulging deformation, and the breach is circular. When an attack angle exists, the load applied by the projectile changes from a point load to a line load; thus, the area in contact with the target increases, and the shear plugging breach is rectangular. After the water entry of projectile, which is subjected to the shock wave pressure and cavitation load, the front plate undergoes bulging deformation; at the same time, the plate is subjected to the surface load of the water. Based on the classical yield line theory, the plastic hinge line in the rectangular plate is as shown in Figure 20, where the plate tears along the plastic hinge line. Since more water is displaced by the water entry projectile with an attack angle, the initial shock wave pressure and cavitation load on the front plate increase with the increasing attack angle: in the range of 15-30\({}^{\circ}\), the front plate shows \"1\"-shaped tear along the plastic hinge line; in the range of 45-60\({}^{\circ}\), the front plate is subjected to a greater load, and thus shows the \"X\"-shaped tear along the plastic hinge line. Figure 18: Deformation/failure diagram and deflection contour of the front and back plates. (the deflection unit: mm). (**a**) Deformation and failure diagram of the front plate in Test 1; (**b**) contour of deformation and deflection of the front plate in Test 1; (**c**) deformation and failure diagram of the front plate in Test 3; (**d**) contour of deformation and deflection of the front plate in Test 3; (**e**) deformation and failure diagram of the rear plate in Test 1; (**f**) contour of deformation and deflection of the rear plate in Test 1; (**g**) deformation and failure diagram of the rear plate in Test 3; (**h**) contour of deformation and deflection of the rear plate in No. 3. Figure 19: Failure morphology of the front and rear plates at different attack angles (deflection unit: cm). As the attack angle increases, the breach in the rear plate gradually decreases for the following reason: the projectile with an attack angle is subjected to greater drag in water; therefore, the projectile velocity decays faster; moreover, the front plate undergoes overall failure in advance under high attack angles; the liquid pressure is unloaded from the front plate, which reduces the damage to the rear plate; in the case of normal penetration, the rear plate undergoes dishing-petaling deformation; with the increase in the attack angle, the breach in the rear plate changes from the petal shape to the strip shape; in the range of 30-45\({}^{\circ}\), the rear plate undergoes dishing- \"1\" tearing; at 60\({}^{\circ}\), the projectile cannot perforate the rear plate; therefore, the rear plate exhibits dishing-bulging deformation.