Experimental Study On The Relationship Between Mode Ⅰ Fracture Toughness Of Coal-rock And Newly Added Surface Area After Crushing

In the process of coal and gas outburst,the energy dissipation is mainly manifested in the crushing and throwing of coal,especially the crushing.At present,the way to obtain the crushing energy based on the drop weight method shows a large error,so the more accurate method to obtain the crushing energy of coal should be based on the fracture toughness test,but because coal is a complex geological material,the fracture toughness is not easy to measure.However,the surface area of particles is easy to calculate in the process of coal crushing.Therefore,taking marble,sandstone,non outburst-prone coal and outburst-prone coal as research objects in this paper.Three-point bending test and crushing test were carried out with notched semi-circular bend（NSCB）specimens and 1-2 mm/0.425-1 mm particle size specimens respectively by using AG-250k N IS testing machine and ball mill.Based on fracture mechanics and rock fragmentation,the relationship between mode I fracture toughness of coal-rock and newly added surface area after crushing were studied through theoretical analysis and laboratory test.The main research results are as follows:（1）Based on the method recommended by ISRM in 2014 to measure the fracture toughness of rock and other geotechnical materials with NSCB,three-point bending fracture tests were carried out on marble,sandstone,non outburst-prone coal and outburst-prone coal with AG-250k N IS testing machine.And the fracture toughness of four specimens were calculated,which were 1.26 MPa·m^1/2,0.82 MPa·m^1/2,0.31MPa·m^1/2 and 0.12 MPa·m^1/2 respectively.（2）Through the comprehensive analysis of load displacement curve and NSCB specimens after test,it can be found that marble,sandstone,non outburst-prone coal and outburst-prone coal all experienced three stages of pore compaction,linear elasticity and post peak drop.And the crack initiation of the four specimens started from one side of the crack tip and gradually expanded to the loading end.In addition,the crack propagation path coincided with the straight line of the prefabricated crack.（3）Four kinds of samples with two particle sizes of 1-2 mm/0.425-1 mm were ground and broken by ball mill.Through screening treatment,the change curves of mass percentage and surface area percentage of particles with different particle sizes were calculated and drawn.The results show that the variation curves of different particle sizes are similar,and the percentage of surface area of particles with particle size less than 0.075 mm is the largest.（4）Based on fracture mechanics and rock fragmentation,it is theoretically concluded that the second power of fracture toughness is directly proportional to the reciprocal of new surface area after fragmentation.And then the fitting was carried out based on the test data.The results show that there is a good proportion relationship between the second power of fracture toughness and the reciprocal of the new surface area of the 1-2 mm/0.425-1 mm particle size samples after grinding,and the correlation coefficient R² of the fitting linear curve are 0.974 and 0.878,which is consistent with the theoretical analysis.（5）As coal is a kind of complex geological material,fracture toughness is not easy to measure.Based on the above research results,the fracture toughness of coal can be estimated by the newly increased surface area after crushing obtained by grinding test,so as to obtain the energy required for coal crushing,and provide experimental basis for further understanding the energy dissipation in outburst process.