# Study on Bulge State of PCnNi3 MoV Target Plates Penetrated by Square Fragments
## 2 Fragment Penetration Test
All characterized planes were conducted on the longitudinal sections of the gun steel target plate, i.e., observing from the top view as shown in Figure 3. Before SEM observations, the samples were carefully ground and polished first mechanically to a mirror-like finish, and finally etched by 4% HNO\({}_{3}\) + ethanol solution. Figure 4 shows the macro-morphology and microstructure of the gun steel target plate after impact by the tungsten fragment at different striking velocity. The fragment with a velocity of 277 m/s resulted in a bulge height of only 0.26 mm, thus the microstructure deformation was almost negligible. It can be identified by the identical microstructure to the matrix, consisted of randomly orientated lamellar pearlites and equiaxed ferrites (Figure 4(a1)). It is necessary to note that the initial steel was fabricated by rolling and subsequent thermal-treatment, leading to the initial fibers were parallel to the rolling plane and perpendicular to the penetration direction. With the increasing of fragment velocity up to >406 m/s, obvious penetration hole can be observed in their contact regions as shown in Figure 4b,c. The initial horizontal fibers were seriously distorted and compressed to align to the contact surface, as sketched by Figure 4d. Besides, the pile-up formation around the penetration hole is pronounced, as the plastic zone is confined and the material deforms to the free surface [16]. The target steel impacted by a fragment with a velocity of 597 m/s, as shown in Figure 4c, showed almost identical microstructure characters to Figure 4b, except some micro-cracks were identified around the contact region and marked by red arrows. It can be reasonably understood that these cracks resulted from the points of the high local plastic strain and high local stress concentration due to the high penetration speed, since  
Figure 2: Schematic diagram of the fragment penetrating the target plate. the material flow to the free surface was suppressed and the materials were compressed to overcome the constraint effect from the plate thickness. The local microstructures as marked by red squares were investigated in detail at high magnifications and given in Figure 4(a1,b1,b11,b2,b21). Figure 4(b1,b2) showed that the initial random orientated pearlite were compressed to align to the contact surface, consistent with that the horizontal macro-fibers were deformed and adjusted to the same direction as shown Figure 4b-d. Higher magnification for the contact surface region in Figure 4(b11,b21) demonstrated that the deformation introduced by penetration was heterogeneous and gradient. The deformation at the surface region was the most severe and the initial lamellar pearlites were even fragmented to equiax  
Figure 3: Failure modes of the target plate. 400 nm. The thickness of the nano-grains layer was measured to be about 10-20 \(\upmu\)m, while the thickness of compression affected layer was about 500 \(\upmu\)m.  
A high-precision altimeter was used to measure the bulge height of the target plate. The resolution of the altimeter was 0.01 mm, which was suitable for this study. The results are shown in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline
**No.** & \begin{tabular}{c} **Fragment Length** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Target Plate** \\ **Thickness (mm)** \\ \end{tabular} & \begin{tabular}{c} **Striking Velocity** \\ **(m/s)** \\ \end{tabular} & \
\begin{tabular}{c} **Striking Kinetic** \\ **Energy (J)** \\ \end{tabular} &
\begin{tabular}{c} **Bulge Height** \\ **(mm)** \\ \end{tabular} \\ \hline
1-1 & 6 & 6 & 277 & 153 & 0.26 \\
1-2 & 6 & 6 & 370 & 274 & 0.56 \\
1-3 & 6 & 6 & 406 & 330 & 0.8 \\
1-4 & 6 & 6 & 452 & 410 & 1.23 \\
1-5 & 6 & 6 & 455 & 414 & 1.3 \\
1-6 & 6 & 6 & 455 & 414 & 1.52 \\
1-7 & 6 & 6 & 468 & 438 & 1.3 \\
1-8 & 6 & 6 & 503 & 506 & 1.47 \\
1-9 & 6 & 6 & 527 & 556 & 1.62 \\ \hline \end{tabular}
\end{table} 
Table 3: Bulge height of the gun steel target plate.