Study On Size Effect Of Mode Ⅰ Fracture And Acoustic Emission Characteristics Of Granite

In the Brazilian granite splitting test of the current study,the sample size selected in the laboratory test is much smaller than the actual engineering rock mass,and the study considering size effect of fracture and acoustic emission characteristics of rock is relatively insufficient.Therefore,it is of great significance to carry out theoretical and experimental research for size effect of fracture and acoustic emission characteristics of granite.In this paper,the size effect of mode I fracture and acoustic emission characteristics of rock have been systematically studied.INSTRON 1346 type electro-hydraulic servo machine has been used to investigate the CSTBD（Cracked Straight Through Brazilian Disc）granite samples.Brazilian splitting tests were carried out with granite samples under five loading rates(30,165,300,3000,30000 N·s^-1).To carry out the test for size effect of fracture of granite by the selection of an appropriate loading rate according to the test results.On this basis,four CSTBD granite samples with diameter sizes ranging from 64 to 132mm（64,94,119,132 mm）were used for the experimental and theoretical study on size effect.The main contents and concluding results are as follows:（1）Based on the Brazilian splitting test,CSTBD granite samples under different loading rates was designed,and the failure characteristics of the whole process in the granite samples under different loading rates were studied.Study on mechanical properties,fracture toughness and acoustic emission characteristics of granite under different loading rates,and the fracture toughness of samples under different loading rates was obtained.A logarithmic function relationship was established to describe the change of fracture toughness with the loading rate.Combined with acoustic emission characteristic parameters,the fracture mechanical behavior of granite is analyzed.Finally,the maximum loading rate is obtained to study the size effect of fracture and acoustic emission characteristics of rock.（2）Granite samples with the same thickness-to-diameter ratio and different diameters were selected to design Brazilian splitting tests with different sizes.On the utilization of MTS（Maximum Tangential Stress）criterion,MMTS（Modified Maximum Tangential Stress）criterion,and FIMTS（Further improved Maximum Tangential Stress）criterion determine the fracture toughness near the leading edge of the crack tip of granite of different sizes.The results show that the fracture toughness of granite samples present size effect.On this basis,it is found that FIMTS criterion can better predict the fracture toughness of small size granite samples.（3）Brazilian splitting tests were carried out on granite samples of different sizes to analyze the variation rules of acoustic emission characteristic parameters at each stage of the whole failure process.The results show that the splitting damage mechanism of granite is affected by size,and increases in the cumulative acoustic emission ringing count with increase in sample size under splitting load.According to the characteristic parameters RA and AF values,the high RA and low AF values of ae signals were identified for small-size samples at the initial loading stage,while the low RA and high AF values of ae signals were identified for large-size samples.In initial compaction and elastic deformation stages,the peak frequency is mainly 250-350 k Hz;The band of ratio of 250-350 k Hz decreases,while the band of 0-50 k Hz increases with the closure of micro-cracks forming micro-cracks.When the micro-cracks are fused together to form macro-cracks,a large number of acoustic signals in the band of 0-50 k Hz are generated.（4）Size effect in the rock fracture of microscopic mechanism was explained based on fracture morphology by scanning electron microscopy.The specular area of the small size granite samples are widely distributed,and the serrated area is relatively small.In the large granite samples,the number of steps in the serrated area increase,and the smooth area on the steps is relatively small.In the peak frequency of acoustic emission signal,most of the acoustic emission signal sources in the band of 250-350 k Hz are tiny damages caused by the splitting of different mineral particles along the grain boundary of the crystal.The acoustic emission signal sources in the band of 0-50 k Hz in the peak frequency are mostly the crystal grain boundary splitting friction of different mineral particles.