# Research on Rock Strength Test Based on Electro-Hydraulic Servo Point Load Instrument
## 1 Introduction  
A point load test is a method to obtain the compressive strength of rocks indirectly, which is widely used in engineering construction because of its simple sample preparation and compact and portable test instrument for field testing [1]. In recent years, scholars at home and abroad have conducted much research on the point load test of rocks. [PERSON] and [PERSON] [2] proposed that the point load strength index I\({}_{8}\) is equal to the damage load P divided by specimen-loading spacing squared, \(D^{2}\). The International Society of Rock Mechanics (ISRM) [3] and American Society for Testing and Materials (ASTM) [4] stated that the point load strength index I\({}_{8}\) is equal to the damage load P divided by the square of the equivalent core diameter of the specimen \(D_{e}^{2}\), and then the correction index m is proposed based on different sizes, and finally the standard point load strength I\({}_{8(50)}\) is equal to the point load strength index I\({}_{8}\) multiplied by the correction factor F. [PERSON] and [PERSON] [5] conducted point load tests and uniaxial compressive strength tests onigneous, metamorphic, and sedimentary rocks, and used regression analysis to investigate the effect of different rock types on their correlations. [PERSON] et al. [6] conducted point load tests and uniaxial compressive strength tests on sandstones of different sizes, invoking the unified size effect law. They investigated the effect of contact area on the size-effect behavior of point load results by using a new method to determine the point load strength index, which led to opposite size-effect trends. [PERSON] et al. [7] conducted point load tests and numerical studies on different shaped specimens, and the results of the study showed that the point load strength index increased with increasing test model size, and that the shape of the particles did not have a significant effect on their tensile strength. [PERSON] et al. [8] investigated the effect of model size and particle size on the point load index through numerical simulation tests, and the results showed that the standard point load strength \(\mathrm{I_{s(50)}}\) increased with both model size and particle size. [PERSON] et al. [9] showed by the effects of rock sample shape factor \(\beta\) and loading point spacing \(\mathrm{D}\) on rock sample point load strength index \(\mathrm{I_{s}}\) that specimen \(\mathrm{I_{s}}\) gradually decayed with the increase in both \(\beta\) and \(\mathrm{D}\) in a power function law, and finally stabilized. [PERSON] et al. [10] conducted a large number of point load tests on rock samples with three different geometric shapes. Theoretically and experimentally, the results showed that the point load strength index was insensitive to the shape of the rock samples, and the research results verify the effectiveness of point load test on irregular specimens. [PERSON] and [PERSON] [11] performed point load tests on rock samples of different shapes to analyze the stress state inside the samples and demonstrated that the samples were mainly subjected to compressive stresses near the loading point and mostly uniform tensile stresses in the region beyond the loading axis. [PERSON] [12] revealed the process of point load damage under the action of point load and tensile and compressive stress zones, which are formed inside the rock sample. As the load continues to increase, compressive damage cracks are formed near the loading cone, and the cracks form a sliding line as the load increases. The cracks acting with the tensile stress in the center of the specimen gradually penetrate, and the specimen is damaged. [PERSON] [13] used the axisymmetric finite element method to determine the stress distribution in cylindrical rock discs with different diameter-thickness ratios under double-point loading, and discussed the characteristics of the tensile radial and tangential stress distribution. The stress field tends to stabilize when the ratio of diameter to thickness is greater than 1.