## 2 Experimental Procedure
### Test Apparatus
#### 2.2.2 Dynamic Test  
The Split Hopkinson pressure bar (SHPB) apparatus is widely applied to study the dynamic behavior in strain rates ranging from 100-10,000 s\({}^{-1}\)[39]. Due to the different hardness in the out-of-plane and in-plane directions, steel bars and aluminum bars were applied, respectively. The values of density and Young modulus for the steel and aluminum bars were \(7.85\times 10^{3}\) kg/m\({}^{3}\), 206 GPa and \(2.8\times 10^{3}\) kg/m\({}^{3}\), 72 GPa, respectively. The striker bar had a length of 0.4 m for aluminum and 0.5 m for steel. The input and output bars had lengths of 1.5 m and 1.5 m, respectively. Their diameters were all 14.5 mm. In addition, a \(\Phi 10\times 1\) mm copper thin circular slice and a \(\Phi 10\times 1\) mm thin circular hard paper slice were used as the pulse shapers for the out-of-plane and in-plane compression, respectively. For each experiment, a fresh pulse shaper was used [22; 40]. The recording of stress waves was performed using a pair of two half bridges that compensated for the bending effect of the bars [41], as shown in Figure 2. To reduce the non-uniform stress distribution due to friction [42; 43; 44], Vaseline was applied to the two contact faces of each specimen. The SHPB apparatus (Luoyang Liwei Technology Co., Ltd., Luoyang city, Henan province of China) is shown in Figure 3; a high-speed camera (Phantom V2511, Vision Research, Inc., Wayne, New Jersey, USA) was applied to record the morphology under high-strain-rate loading [18]. Upon impact of a projectile at the free end of the input bar, a compressive longitudinal incident wave \(\varepsilon_{i}\) in the input bar is developed. This incident wave induces a reflected wave \(\varepsilon_{r}\) in the input  
Figure 3: SHPB experimental apparatus. Figure 2: Schematic diagram of Split Hopkinson pressure bar (SHPB) experimental apparatus. bar and a transmitted wave \(\varepsilon_{t}\) in the output bar [17]. The stress-strain curve of the specimen can be calculated from the forces and the velocities at both specimen faces. The classical calculation is based on the hypothesis of homogeneity of stress and strain within the specimen [17]. The calculation equations are in the form of Equations (1)-(3) [17; 22; 28], where \(l_{s}\) and \(A_{0}\) are the thickness and area of the specimen, while \(c\), \(A\), and \(E\) are the sound speed, area, and Young modulus of bar. The energy absorption can be calculated using Equation (4) [21]. \[\dot{\varepsilon}(t)=\frac{c}{l_{s}}\Big{(}\dot{\varepsilon}_{t}-\dot{ \varepsilon}_{r}-\dot{\varepsilon}_{t}\Big{)}. \tag{1}\]  
\[\varepsilon(t)=\frac{c}{l_{s}}\int_{0}^{t}(\varepsilon_{i}-\varepsilon_{r}- \varepsilon_{t})dt. \tag{2}\]  
\[\sigma(t)=\frac{A}{2A_{0}}E(\varepsilon_{i}+\varepsilon_{r}+\varepsilon_{t}). \tag{3}\]  
\[W=AE\varepsilon\int_{0}^{t}\Big{[}\varepsilon_{i}(t)^{2}-\varepsilon_{r}(t)^{ 2}-\varepsilon_{t}(t)^{2}\Big{]}dt. \tag{4}\]  
Furthermore, when it comes to aligning the wave head, there are many methods. [PERSON] [45] considered that, when changing the position of the three wave heads, if the stress-strain curves generated by the two-wave method and three-wave method could be aligned at the maximum strain point, the head of the three waves could be considered as aligned. Some researchers calculated the wave head differences by measuring the distances between the strain gages and the ends of the bars in contact with the specimen, as well as by measuring the wave speed of the bars and evaluating the wave speed of the specimen [46]. Under ideal conditions, this method could get more accurate results. However, if the wave speed of the specimen is not clear enough [47] or the measurements of the lengths are not accurate enough, applying the same value for each specimen could lead to the result of each specimen being inaccurate.