## 3 Dynamic Tests with the Use of Hopkinson Split Pressure Bar (SHPB)  
Dynamic tests were performed using the split Hopkinson bar (SHPB) method [38]. The applied classic system consisted of a bullet and two bars between which a samples was placed (Figure 10) [38; 39]. The bullet was powered by a gas gun. The bars and the bullet made of V720 maraging steel had a diameter of 25 mm and a length of 2000, 2000, and 350 mm, respectively. Copper pulse sections with a diameter of 5 mm and a thickness of 3 mm were used to dampen high-frequency Pochhammer-Chree oscillations and to increase the incident pulse rise time [39; 40]. The waves propagating in the bars were recorded with the use of a full Wheatstone bridge (EA-06-060 LZ-120, Vishay, Malvern, PA, USA) as well as a data acquisition system that included the LTT500 high-speed amplifier from LTT Labortechnik Tasler GmbH (Wurzburg, Germany) and the NI USB-6366 measurement card produced by National Instruments (Austin, TX, USA). During the tests, the signals were sampled with a time interval of 0.5 \(\upmu\)s. The cylindrical samples used in the tests had a diameter of 23 mm and a height (L)-to-diameter (D) ratio of 1, 3/4, and 1/2 (Figure 10). The front surfaces of the samples were ground before testing in order to ensure their proper plane parallelism. Twelve tests were carried out for each material, sample geometry, and load values. Figure 9: The chart depicting the relationship between the quantity of pores and their sizes for (**a**) Apricot Pink, (**b**) Barwald, and (**c**) Kandla Grey. During the experiment, a bullet propelled to speed V hits the input bar, generating an incident pulse \(\varepsilon_{l}\). This impulse propagates into the incident bar until it reaches the bar-sample boundary. Due to differences in impedance, the pulse partially reflects from the bar-sample boundary, and partially penetrates into the sample and passes to the other bar as a transmitted pulse \(\varepsilon_{T}\). The impulse reflected from the incident bar-sample boundary moves as a stretching wave. \(\varepsilon_{R}\) moves towards the striker. The recorded signals allow the stress \(\sigma_{S}\), strain \(\varepsilon_{S}\) and strain rate \(\dot{\varepsilon}_{S}\) of the sample to be determined on the basis of the relationships commonly applied in the SHRB method [38,40,41]:  
\[\sigma_{S}(t)=\frac{A_{B}E_{B}}{2A_{S}}[\varepsilon_{I}(t)+\varepsilon_{R}(t)+ \varepsilon_{T}(t)] \tag{2}\]  
\[\dot{\varepsilon}_{S}(t)=\frac{C_{B}}{L_{S}}[\varepsilon_{I}(t)-\varepsilon_{ R}(t)-\varepsilon_{T}(t)] \tag{3}\]  
\[\varepsilon_{S}(t)=\frac{C_{B}}{L_{S}}\int\limits_{0}^{t}{[\varepsilon_{I}( \tau)-\varepsilon_{R}(\tau)-\varepsilon_{T}(\tau)]d\tau} \tag{4}\]  
where \(A_{B}\) is the cross-sectional area of the bar, \(E_{B}\) is the Young's modulus of the material from which the bars are made, \(C_{B}\) is the wave velocity of the bars, \(A_{S}\) is the cross-sectional area of the sample, and \(L_{S}\) is the sample length. The energy of the incident \(W_{I}\), reflected \(W_{R}\), and transmitted \(W_{T}\) wave can be expressed as:  
\[W_{I}=A_{B}C_{B}E_{B}\int\varepsilon_{I}^{2}dt \tag{5}\]  
\[W_{R}=A_{B}C_{B}E_{B}\int\varepsilon_{R}^{2}dt \tag{6}\]  
\[W_{T}=A_{B}C_{B}E_{B}\int\varepsilon_{T}^{2}dt \tag{7}\]  
where \(W_{I}\) is the energy associated with the incident pulse, \(W_{R}\) is the energy of the reflected pulse, and \(W_{T}\) is the energy of the transmitted pulse. Assuming that energy losses are small, the energy transferred to the sample is \(W_{I}-W_{R}\), and the energy absorbed by the sample is:  
\[W_{d}=W_{I}-W_{R}-W_{T}\]  
Figure 10: The  
According to [42], the energy absorbed by the sample can be expressed as:  
\[W_{d}=2A_{B}C_{B}E_{B}\int\varepsilon_{T}\varepsilon_{R}dt \tag{8}\]  
[PERSON] [43] demonstrated that more than 95% of the energy transferred to the sample is associated with fracture and damage energy. Therefore, \(W_{d}\) can be considered to correspond to dissipated energy for rock fragmentation. Tables 3-5 summarize data obtained in dynamic tests.