## 4 Discussion
### The Variation of Voltage and Current Generated by the Specim under the Action of Load  
During the deformation and damage process of the specimen under load, in addition to direct-current pulsed electromagnetic radiation, there will also be low-frequency non-uniform attenuation oscillation. Below is an analysis of the variation patterns between open-circuit voltage (\(U_{\sigma}(t)\)), short circuit current (\(I_{\sigma}(t)\)), and load (\(P\)). Figure 10 illustrates the curve of the open-circuit voltage (\(U_{\sigma}(t)\)) with respect to load (\(P\)), where this voltage represents the electromotive force generated by the specimen under loading conditions. It should be noted that the prerequisite for measuring the open-circuit voltage is to isolate the sample's two ends and the loading pad of the pressure machine through insulating boards. From this figure, it can be seen that electromotive force generally increases with higher loads but exhibits significant fluctuations. The curve of the short-circuit current (\(I_{\sigma}(t)\)) versus the load (\(P\)) is shown in Figure 11. Here, the short-circuit current refers to the current that forms a natural loop circuit between the test piece and the pressure machine when the upper and lower plates of the pressure machine are not separated from the test piece by an insulating plate under the load. The overall trend of the current increases as the load increases, but it fluctuates greatly. Based on the experimental results, it is clear that both cement mortar specimens and other rock specimens exhibit a reset in particle structure when subjected to small loads, leading to the generation of piezoelectric effects. This means that electromagnetic radiation is consistently produced as the specimen undergoes loading. Moreover, the current intensity generated by specimens of different rock types under various load excitations varies, resulting in differences in electromagnetic radiation intensity. The phenomenon of the current intensity not always increasing for the same specimen under different load excitations is associated with the reset state of the specimen's internal microstructure configuration. Furthermore, the voltage amplitude and current intensity are not always directly correlated. For instance, the 10# cement mortar specimen, when \(P=4\) kN, has a voltage amplitude of up to 400.8 mV, but only a current of 78 \(\mu\)A; when \(P=4\) kN, the voltage  
Figure 11: The relationship between current (\(I_{\sigma}\)) and load (\(P\)): (**a**) 8# granite specimen, (**b**) 9# granite specimen, (**c**) 11# red sandstone sample, (**d**) 10# cement mortar sample. Figure 10: Relationship between voltage (\(I_{\sigma}\)) and load (\(P\)): (**a**) 8# granite specimen, (**b**) 9# granite specimen, (**c**) 11# red sandstone sample, (**d**) 10# cement mortar sample. amplitude of the 8# granite specimen is only 9 mV, but it has a higher current of 88 uA. This discrepancy is associated with the rock type and internal microstructure configuration of the sample. It can be seen that it is feasible to analyze and continuously track the compressive deformation and damage of different rock samples by using the parallel circuit model consisting of dynamic physical quantities \(U_{\sigma}\), \(I_{\sigma}\), \(C_{\sigma}\), \(L_{\sigma}\), and \(R_{\sigma}\). Additionally, this study has selected common granite and sandstone as the subjects of research. However, the complexity of structural characteristics in the study subjects, such as samples with heterogeneous rock structures, and the discrepancies between the sample sizes in experimental conditions and actual engineering practices, have limited the applicability of the experimental results in assessing the electromagnetic radiation patterns of larger-sized samples under compression. In actual engineering applications, the methods of applying compressive loads are diverse and do not completely align with experimental conditions.