Experimental Study On Dynamic Fracture And Damage Mechanism Of Granite Under Different Loading Rates

It is of great significance to further systematically study the dynamic fracture and damage evolutions of rock masses with respect to loading strain rates,and reveal the loading effect on various dynamic fracture parameters.Thus,granite materials from Gansu Beishan and Beijing Fangshan were mainly employed as specimens in this paper,and loading systems with different loading strain rate(drop-weight system,split Hopkinson pressure bar system,cutting seam cartridge loading)were used in sequence to investigate the fracture and damage mechanism of granite specimens under different loading conditions experimentally.The main conclusions are as follows:(1)Dynamic I-mode fracture experiments were conducted on three-point-bend rock-like specimens(PMMA)under drop-weight hammer loading using strain gage method,dynamic caustics and numerical methods.By comparisons between these three methods on calculations of dynamic fracture parameters(strain responses around crack tips,crack propagation velocities,dynamic stress intensity factors),the reliability and rationality of strain gage method for measuring dynamic I-mode fracture parameters of rock-like brittle materials were verified.This research conclusion provides a new method on testing the dynamic fracture parameters of rock masses under drop-weight hammer loading condition.(2)Strain gage method was introduced into the testing procedure of dynamic I-mode experiments of Beishan three-point-bend specimens under drop-weight hammer loading conditions.With combination of dynamic digital image correlation(DIC)method,the evolutions of I-mode dynamic fracture parameters were studied.The results show that,horizontal displacement gradient becomes severe during the cracking process,and a strain belt forms along the cracking path.The typical "fire-flame" shaped patterns are found around the crack tips in strain fields.The strain concentration degree decreases obviously due to the quick release of elastic energies accumulated around the crack tip when the pre-existing crack initiates.With the development of the running crack,the strain concentration degree increases and remains stable during the late cracking process.Meanwhile,the dynamic stress intensity factor(DSIF)evolution with respect to time goes up and decreases in a sudden,after then,a gradual increase and a final stable period can be found in the DSIF evolution curve.The dynamic fracture initiation toughness of Beishan three-point-bend specimen is 7.18 GPa·m1/2 under the loading rate of 119 GPa·m1/2/s in drop-weight hammer device.The related fracture energy release rate is 0.67 kJ/m2.(3)The dynamic fracture toughness evolutions of Beishan semi-circular specimens(NSCB,CCNSCB)with respect to loading rates were studied using SHPB device with combination of dynamic DIC method.The loading rate ranged from 162.82 GPa·m1/2/sto 341.68 GPa·m1/2/s.The results show that,the dynamic fracture initiation toughness increases with the rise of loading rates.With the comparisons between NSCB and CCNSCB specimens in dynamic fracture toughness,it can be known that the differences between the experimental results are relatively margin,which demonstrates that the size effect can be minimized by carefully unify the typical size of NSCB and CCNSCB when calculating the DSIF.In addition,the turning point of DSIF with respect to the loading rate is 200 GPa·m1/2·s-1 for Beishan semi-circular specimens.Besides,the evolutions of DSIFs around the crack tips show obvious periodic change:the evolution of DSIFs with respect to time witnesses a gradual increase and a sudden drop,and finally increases rapidly until the specimens are totally fractured into halves.The dynamic fracture toughness obtained by both SHPB and DIC is in good agreement with each other.(4)To simulate the dynamic mechanical responses in rock masses subjected to pre-load conditions,the I-mode dynamic fracture SHPB experiments were conducted on Beishan CSTBD specimens.The influences of uniaxial pre-load on I-mode fracture parameters of Beishan CSTBD specimens were analyzed and studied.The results show that,under the constant loading rate in SHPB device,the dynamic fracture initiation toughness own a good linear relation with uniaxial pre-load,and the fitting expression is given as follow:KICd=2.46+0.117?p,R2=1(5)To simulate the dynamic responses of rock masses with different in-situ stress conditions under explosion loading rate,the influence of biaxial pre-load on dynamic fracture and damage mechanisms of granite specimens containing two boreholes were studied using cutting seam cartridge and rock acoustic detection technique.The results show that,the acoustic reductions reach the maximum values at areas near blasting sources in granite specimens,and the degree of acoustic reduction decreases when the biaxial loads are raised.Meanwhile,the widths and the curving degree of blast-induced cracks are also decreases when biaxial loads rise,which illustrates that there is a strong inhibiting effect from biaxial loads on fracture and damage evolution in granite specimens containing two boreholes under directional fracture blasting load.(6)To further reveal the internal damage evolution mechanism of rock masses under blasting loading rate,both granite cuboid and cylindrical specimens with single borehole were subjected to the directional blasting load.Using the computerized tomography(CT)technique,the meso damage mechanism was revealed.The results show that,the crack distribution density and damage degree are weakened along the borehole,and the entire crack distribution in granite specimens show a sand glass pattern with the utilization of three-dimensional reconstruction based on CT images.The main crack and damage regions are concentrated along the cutting seam cartridge direction.Besides,the damage degree in cylindrical granite specimen is lower than that in cuboid specimen.