## 2 Experimentation
### SHPB Tests  
The SHPB apparatus [36] has been widely used to investigate the dynamic behaviors of materials at high strain rate (\(10^{2}\)-\(10^{4}\) s\({}^{-1}\)). The SHPB apparatus consists of a strike bar, an incident bar, a transmission bar, an absorbing bar and a shock absorber. Figure 5 shows a schematic of SHPB apparatus for dynamic compressive tests. In this study, all bars have diameters of 14.5 mm, and the incident bar and transmission bar both have a length of 1.5 m. The striker bar is fired from the gas gun system, and then the striker bar impacts the free end of the incident bar. Upon impact, a longitudinal elastic compressive stress wave is created, which propagates along the incident bar toward the incident bar/specimen interface; once the compressive stress wave has reached the incident bar/specimen interface, and the compressive has been divided into two parts: one part is reflected back into the incident bar as a tensile stress wave, and the other part of the stress wave which is not reflected is transmitted into the specimen, and the wave that is transmitted into specimen propagates reaches the specimen/transmission bar interface along the loading direction of the specimen. When the wave reaches the interface between the transmission bar and the specimen, one part of the stress wave is reflected into the specimen, and the other part of the stress wave is transmitted into the transmission bar. Finally, the transmission bar along its axis displaces until the other end of it reaches the absorbing bar, and the stress wave is absorbed by the shock absorber. The stress wave within the specimen undergoes a complex reverberation, until the dynamic compressive strength of the specimen is reached. It should be noted that the dynamic deformation of 3D braided composites is in a longitudinal direction; more specific details are shown in Figure 13.  
During dynamic compressive tests, two strain gauges, which were mounted diametrically opposite to each other on the incident bar and transmission bar to cancel any bending strains, were employed to measure the incident, reflect and transmit signals, respectively. The data were captured by using a high-speed data acquisition card, employing a sampling rate of 1 MHz. A high-speed camera was employed to capture the deformation and damage evolution process during impact. In this paper, we chose a resolution of \(1792\times 448\), and the sampling frequency was 20000 fps. Besides, two LED lights were placed on both sides of the high-speed camera to enhance the light on the specimen, in order to obtain clearer photos.  
In this study, the strain signals from strain gauges were analyzed based on one-dimensional wave propagation assumption. The stress (\(\sigma_{\mathrm{s}}\)), strain (\(\varepsilon_{\mathrm{s}}\)) and strain rate (\(\dot{\varepsilon}_{\mathrm{s}}\)) could be obtained through the following equations, respectively [37]:  
Figure 5: Schematic of SHPB(split Hopkinson pressure bar)apparatus.  
\[\sigma_{\rm s}(t)=\frac{A_{0}}{A_{\rm s}}E_{0}\varepsilon_{\rm T}(t), \tag{1}\]  
\[\varepsilon_{\rm s}(t)=-2\frac{C_{0}}{I_{\rm s}}\int_{0}^{t}(\varepsilon_{\rm I }(t)-\varepsilon_{\rm T}(t))dt, \tag{2}\]  
\[\dot{\varepsilon}_{\rm s}(t)=-2\frac{C_{0}}{I_{\rm s}}(\varepsilon_{\rm I}(t)- \varepsilon_{\rm T}(t)), \tag{3}\]  
where \(C_{0}=\sqrt{E_{0}/\rho_{0}}\) is the longitudinal stress wave velocity in the bar, \(E_{0}\) is [PERSON]'s modulus of the bar, \(E_{0}=206\) GPa, \(\rho_{0}\) is density of the bar, \(\rho_{0}=7850\) kg/m\({}^{3}\), \(I_{\rm s}\) is the length of the specimen, \(A_{0}\) and \(A_{\rm s}\) are the cross-sectional area of the bar and the specimen, respectively. Moreover, \(\varepsilon_{\rm I}(t)\) and \(\varepsilon_{\rm T}(t)\) are strain gauge signals of the incident and transmitted pules, respectively.