Investigation of tensile and fracture mechanical properties of bituminous coal at different strain rates

Coal is one of the most important energy sources in China, and bituminous coal accounts for 80% of China's total coal. However, there are still some deficiencies in the previous understanding of the failure and mechanical characteristics of bituminous materials under external loads of different strain rates (especially high strain rates). To investigate the tensile and fracture mechanical properties of bituminous coal at different strain rates, the GCTS multifunctional rock mechanical loading device and Hopkinson bar dynamic loading system were employed. The crack extension process and crack distribution after damage of bituminous coal specimens were also analyzed by high-speed camera and CT scanning instrument. The results show that the peak stress distribution of the Brazilian disc (BD) bituminous coal samples ranged from 3.5 MPa to 5 MPa. The main crack expansion path of the internal splitting is not straight due to the non-homogeneity of the coal sample itself. And the secondary cracks of the specimen are more developed and mostly located in the middle region with transverse spreading. The average peak load of the notched semicircular bending (NSCB) coal samples tends to decrease with increasing depth of the notch. In addition, dynamic crack initiation toughness of NSCB bituminous coal specimen decreases continuously as the initiation time increases. The relationship between fracture toughness and crack initiation time is an approximate exponential function. The normalized dynamic fracture toughness test value of bituminous coal is higher than that of steel material, but lower than that of granite