# Research on Rock Strength Test Based on Electro-Hydraulic Servo Point Load Instrument
## 4 Results and Discussion
### Effect of Loading Rate and Rock Sample Size on \(I_{\text{s(50)}}\)
#### 4.3.2 Effect of Rock Sample Size on \(\mathrm{I_{s(50)}}\)  
Referring to Figure 7, and using the size \(50^{3}\) mm\({}^{3}\) as the reference point of each rock sample, the top-down blue line (\(\mathrm{I_{s(50)3}}\)), green line (\(\mathrm{I_{s(50)4}}\)), red line (\(\mathrm{I_{s(50)2}}\)), and black line (\(\mathrm{I_{s(50)1}}\)) in the image are noted as \(\mathrm{I_{3}}\), \(\mathrm{I_{4}}\), \(\mathrm{I_{2}}\), and \(\mathrm{I_{1}}\), respectively. At the same size, with the increase in loading rate, the ranking order of \(\mathrm{I_{s(50)}}\) solved by different methods for each rock sample remained the same; while with the decrease in size, the top-down ranking order changed and basically followed the pattern: \(\mathrm{I_{3}}\), \(\mathrm{I_{4}}\), \(\mathrm{I_{2}}\), \(\mathrm{I_{1}}\)-\(\mathrm{I_{4}}\), \(\mathrm{I_{3}}\), \(\mathrm{I_{2}}\), \(\mathrm{I_{1}}\)-\(\mathrm{I_{4}}\), \(\mathrm{I_{3}}\), \(\mathrm{I_{2}}\), \(\mathrm{I_{1}}\)-\(\mathrm{I_{4}}\), \(\mathrm{I_{2}}\), \(\mathrm{I_{3}}\), \(\mathrm{I_{1}}\). It can be found that the ranking order of \(\mathrm{I_{2}}\) showed an increasing trend with the decrease in size, \(\mathrm{I_{3}}\) showed a decreasing trend with the decrease in size, and the ranking order of \(\mathrm{I_{1}}\) and \(\mathrm{I_{4}}\) remained basically the same.  
This is due to the fact that \(\mathrm{I_{s(50)}}\) solved by different methods contains two components, the uncorrected point load strength index \(\mathrm{I_{s}}\) and the correction factor. \(\mathrm{I_{s(50)1}}\) and \(\mathrm{I_{s(50)2}}\) are corrected based on the point load strength index \(\mathrm{I_{s1}}\) (\(\mathrm{I_{s1}}\) is calculated using the equivalent diameter method), while \(\mathrm{I_{s(50)3}}\) and \(\mathrm{I_{s(50)4}}\) are corrected based on the point load strength index \(\mathrm{I_{s2}}\) (\(\mathrm{I_{s2}}\) is calculated using the loading-spacing method). Under the same size, with the increase in loading rate, the damage load increases, the change in loading rate only affects \(\mathrm{I_{s}}\), and the correction coefficients of different methods only contain the size parameters, so the arrangement order remains the same. Under the same loading rate, the relationship between the magnitude of \(\mathrm{I_{s1}}\) and \(\mathrm{I_{s2}}\) always remained \(\mathrm{I_{s1}}\) \(<\)\(\mathrm{I_{s2}}\) as the size decreased, and the relationship curves between the load strength index and size of the three rock sample points, as shown in Figure 8. The order of the correction coefficients of different methods changed; the relationship curves between the correction coefficients and the size for the three rock sample points are shown in Figure 9.  
Figure 7: Relationship between \(\mathrm{I_{s(50)}}\) and loading rate for three rock samples with different sizes.  
Figure 8: Relationship between two point load strength indexes \(\rm I_{8}\) and size for the three rock samples under different loading rates.  
Figure 9: Three correction factors versus size for the three rock samples at different loading rates.