## 3 Results and Discussion
### Impact Characteristics under Cryogenic and High Temperatures
#### 3.2.2 Damage Morphology  
With the target plate at cryogenic temperature, three impact tests were undertaken using the composite projectile. The projectile perforated the target plate in tests C2 and C3 (see Table 2), and the projectile rebounded in test C1. Figure 16 shows the damage morphology, where the left-hand images show the damage at the impact surface of the target, and the right-hand images show the damage at the exit surface. The picture at the bottom shows a cross-sectional view of the damage. Figure 16: Damage to 2D triaxial braided composites at cryogenic temperature. Figure 15: Energy absorbed by 2D triaxial braided composite target at various temperatures. Figure 16a shows damage caused by test C1, in which the projectile failed to perforate the target. In this case, the projectile caused relatively little damage to the target. Only a slight crater appears due to the shearing action of the projectile at the impact surface, whereas no visible damage appears at the back surface of the target. We deduce from these results that the impact velocity is far below the ballistic limit. When the composite target plate is perforated, a strip opening appears on the incident surface that matches the size of the rectangular projectile. At the exit surface of the target, a wide-ranging, elliptically shaped, broom-like damage area appears, and yarn fracture and delamination are evident. In the C3 test (with a larger incident velocity), more severe crack growth appears at the two ends of the perforation strip, and the damage crack is also significantly larger than that of the C2 test. A total of four ballistic-impact tests were performed at high temperature. In tests H3 and H4 (see Table 2), the projectiles perforated the target. Figure 17 shows the damage morphology for tests H2 and H4. In tests H2 (impact velocity of 146 m/s) and H1 (impact velocity of 130 m/s) tests, the projectile failed to perforate the target but rebounded. As for the tests under cryogenic conditions, rectangular craters of various depths appear on the impact surface of the target. The crater depth of the H2 target (for a larger impact velocity) significantly exceeds that of the H1 test. Therefore, we consider that, under conditions of rebound, the depth of the impact crater increases with increasing impact velocity. When the target was perforated in tests H3 (impact velocity of 170 m/s) and H4 (impact velocity of 185 m/s), a clear, neat strip perforation was produced on the front surface of the target, corresponding to the shape and size of the projectile, and clear cracking and delamination appeared on the exit surface. In addition, when the projectile impacts the target plate, the projectile itself also suffers fiber breakage and delamination failure. Figure 17: Damage to 2D triaxial braided composite target plate at high temperature due to impact of rectangular projectile. Figure 18 shows an enlarged view of the local damage to the composite target at cryogenic and high temperatures. The images on the left (right) show the damage to the front (back) surface of the target. For tests at cryogenic conditions, damage at the impact surface consists of a tear opening with longitudinal and transverse cracks. The main failure modes of the target at the front surface are fiber fracture and matrix cracking. For tests at high temperature, the damage at the impact surface clearly differs from that at cryogenic conditions. A neat and complete rectangular perforation is caused by shearing. The main failure modes in this case are fiber shearing failure and matrix crushing failure. Extensive yarn fracture and pull-out is evident at the exit surface. The tough fracture surface indicates that the yarn failure is the result of tensile forces. Matrix cracking and delamination failure are also significant failure modes at the target exit surface. Moreover, the extent of damage and the damage area of the target in test C3 are greater than in test C2.