PSI - Issue 28

N.A. Sazhenkov et al. / Procedia Structural Integrity 28 (2020) 1572–1578 N.A. Sazhenkov et al./ Structural Integrity Procedia 00 (2019) 000–000

1575

4

a different coating arrangement. All shots were fired for a range of projectile speeds of 97... 150 m/s. During ballistic loading of the samples, the video of the impact process was carried out in order to determine the kinematic characteristics of the projectile and sample fragments during the impact. The assessment of the fracture of the samples was carried out based on the analysis of the geometric characteristics of the cracks on the specimen’s surface and delamination zones. 3. Experimental results and discussion The parameters of the tests performed are shown in Table 1. Negative values of the residual velocity refer to the projectile, which rebounded off the target specimen. The main interest was in the estimation of the ballistic limit and the amount of energy that the sample is able to absorb. The results obtained for the uncoated target were used as a benchmark for evaluating the effectiveness of the protective coating. The ballistic limit for this case was 120.6... 128.1 m/s.

Table 1. Experimental results summary

Test №

Coating location

Initial velocity, m/s

Incident energy, J

Residual velocity, m/s

Residual energy, J

Absorbed energy, J

Penetration

1 2 3 4 5 6 7 8 9

97,6

261,20 499,73 398,81 449,95 447,85 498,25 559,14 616,95 501,21 396,82 449,95

-14,9 55,9 -15,1 28,6 -14,8 -16,7 -22,1 48,7

6,09

255,11 414,05 392,55 427,52 441,84 490,60 545,75 551,92 408,97 386,93 442,93

No

135,2 120,6 128,1 127,8 134,8 142,8 135,2 120,3 128,1 150

85,68

Yes

No coating

6,25

No

22,43

Yes

6,01 7,65

No No No

Face

13,39 65,03 92,24

Yes Yes

58

10 11

Back

-19

9,90 7,02

No

16

Yes

As it can be seen, the most effective location for the protective coating is when it applied on the face surface of the target. The ballistic limit between 142.8…150 m/s with the amount of absorbed energy of 545.75 J was obtained for this case. Thus polyurea coating with a thickness of 1.2 mm allowed increasing the ballistic limit of the target by 19% with an increase in the plate weight and areal density only by 6.7%. When the coating is on the back surface of the specimen the ballistic limit is quite the same as if there is no coating at all. The amount of absorbed energy was increased just by 15.5 J for 11-th test in comparison with the 4-th one when the penetration occurs. Most likely this was due to deformation and delamination of the coating from the back surface which looks like the minor mechanism of the absorbing the ballistic energy. Selected frames of a high-speed collision process of a steel ball with a target plate are shown in Figure 4. Two cases are compared: an impact with an uncoated plate at a speed of 128.1 m/s (Fig. 4, a) and with a polyurea coated plate on the front surface at a speed of 127.8 m/s (Fig. 4, b) (tests 4 and 5, table 1). It can be seen that in both cases the collision initiates oscillations of the sample. They represent the superposition of vibrations with the first and second bending eigenmodes. In both cases, the free edge of the target reaches its maximum deflection in the direction of impact, first along the 2nd bending mode 2.12 ... 2.48 ms after the collision, and then along the 1st mode by 4.42 ... 4.44 ms after collision. The deflection of the free edge of the uncoated target specimen was 22.0 mm and 26.9 mm at times of 2.12 and 4.42 ms, respectively. Even in the third frame (Fig. 4 a. 0.92 ms), the sample is pierced. The deflection of the