# Calibration Method of PFC3D Micro-Parameters under Impact Load
## 1 Introduction  
Particle discrete element PFC3D software uses many contacting particles to describe the mechanical properties of real material media. This description differs from the existing testing and specification methods. It is difficult to directly obtain the particles' micro-parameters. Therefore, it is necessary to calibrate the micro-mechanical parameters, such as particle and contact properties, through three typical numerical rock mechanics tests: triaxial compression, Brazilian splitting, and tensile testing. [PERSON] et al. evaluated the influence of the micro-mechanical parameters of the PFC model on its macro-mechanical parameters through sensitivity analysis [1]. [PERSON] et al. proposed a new linear parallel bond model (CM-LPBM) calibration method for the PFC micro-mechanical parameters [2]. [PERSON] et al. established the calculation formulas for the microscopic and macroscopic parameters of rock materials through regression analysis and quantitatively analyzed the micro-parameters of PFC3D [3]. [PERSON] et al. studied the influence of PFC micro-parameters on UCS and Brazilian splitting test results [4]. [PERSON] et al. proposed a set of standardized micro-parameter calibration procedures for LPBM and the smooth joint model (SJM) [5]. [PERSON] et al. used MATLAB and a genetic algorithm to write the micro-parameter calibration program of the concrete model and the inversion of its macro-parameters [6]. [PERSON] et al. used the cellular automata method to establish a discrete elementmodel of clustered particles and proposed a fast and effective calibration method for rock micro-parameters [7]. [PERSON] estimated the nonlinear relationship between the microscopic and macroscopic parameters of the PFC model using the statistical center composite design (CCD) method [8]. [PERSON] et al. proposed an optimization method combining Box-Behnken experimental design and expectation function to quickly calibrate micro-parameters [9]. [PERSON] et al. proposed a combinatorial optimization method (GT-RSM-MD) to effectively calibrate micro-parameters [10]. In addition to the PFC software, other discrete element methods also need to calibrate micro-parameters. [PERSON] et al. developed the y-Geo FDEM code to obtain the model parameters for the bonded-particle DEM and FDEM method [11]. [PERSON] et al. proposed a calibration method to calibrate DEM parameters, considering the coupling effect of micro-parameters [12]. However, the microscopic parameters calibrated using static mechanical properties of rocks are often unsuitable for high strain-rate numerical tests. Therefore, this problem must be solved according to the dynamic mechanical properties of rock. The primary test method used to obtain the dynamic mechanical properties of rock is the split Hopkinson pressure bar test (SHPB). The SHPB test has been used to study the dynamic mechanical properties of rock. [PERSON] et al. used the SHPB test to conduct the dynamic indirect tensile test on coal samples. They studied the influence of the bedding structure in coal seams on their dynamic indirect tensile strength [13]. [PERSON] et al. used the SHPB system to study the dynamic mechanical properties of horizontal and vertical bedding coal samples [14]. [PERSON] et al. used SHPB to conduct dynamic impact tests on gas-bearing coal [15]. In addition, many scholars have used the SHPB test to study materials such as coal, granite, limestone, marble, and concrete [16; 17; 18; 19; 20; 21]. [PERSON] et al. used SHPB to carry out impact compression tests on laminated sandstone with different bedding inclination angles and used a high-speed camera to capture the fracture process of samples [22]. [PERSON] et al. used the SHPB device to conduct cyclic impact tests on soil with different axial compression and impact velocities [23]. [PERSON] et al. used the SHPB test to study the strength and microstructure of coal samples under an impact compression load [24].