Research On Dynamic Properties And Evolution Law Of Cumulative Damage Of Granite Under Cyclic Loading

Drilling and blasting technology has become an indispensable construction method for geotechnical engineering construction or mining production.The cumulative damage effect of surrounding rock induced by frequent blasting operations has an important impact on the stability of rock masses.Moreover,engineering rock masses often bear a certain static stress load or temperature load before being subjected to frequent disturbances.The underground rock mass in the complex engineering geological environment was took as the research object in this paper,and the dynamic properties and evolution law of cumulative damage of granite under cyclic impact and cyclic blasting loads was thoroughly studied with the help of Split Hopkison Pressure Bar(SHPB)system and outdoor blasting test.The main research content and research results of the paper are as follows:(1)The conic variable cross-sectional SHPB system was used to conduct cyclic impact tests on the heat-treated granite at different temperatures(T=20??600?)under five impact velocities.The effects of impact velocity and heat treatment temperature on the degradation characteristic of dynamic properties,evolution law of cumulative damage and energy dissipation characteristics under cyclic impact loading were studied.The experimental results indicated that the stress threshold of the heat-treated sample at600? to reach the damage was significantly lower than that of the heat-treated samples at 20??500? under cyclic impact loading.Under the same impact velocity,the heat-treated sample at 600? exhibited faster degradation characteristic of dynamic properties and temperature weakening phenomenon.Under the effect of fracture healing,the cumulative damage of the heat-treated samples at 200? and 300? increased slowly,showing obvious temperature hardening phenomenon.When the heat-treated sample at different temperatures under a single impact reach their respective energy thresholds,the total breakage energy dissipation density and fragmentation degree of the sample would increase with the increase in impact velocity.(2)The dynamic and static combined SHPB system was used to perform equal-amplitude cyclic impact on the granite sample with different axial pressures(?_A=0,30,60,90 and 120MPa)choosing the same peak voltage of the incident wave.The degradation characteristic of dynamic properties of the granite with different axial pressures under cyclic impact loading and the influence mechanism of axial pressure were analyzed.The experimental results indicated that the reflected wave in the impact waveform had no a clear“jump point”when the axial pressure was applied.The fundamental reason was the uneven instantaneous stress unloading of the elastic bar.For the sample subjected to axial pressure,the reflected wave when impacting without the sample was required for data processing.The degradation rate of dynamic properties of the sample with?_A=0 and 120MPa was faster,while the sample with?_A=60MPa was slower.The cumulative damage growth rate of the sample with?_A=60MPa was relatively small.When the axial pressure was less than the crack initiation stress,the dynamic properties of the sample under the cyclic impact loading was improved due to the compaction effect.However,when the axial pressure was greater than the crack initiation stress,the degradation rate of dynamic properties of the sample would accelerate due to the development of microcracks inside the rock.(3)The dynamic and static combined SHPB system was used to perform cyclic impact on the granite samples with different confining pressures(C_p=0,4,8,12 and16MPa)at the same impact velocity.The effect of confining pressure on the degradation characteristics of dynamic properties and evolution law of cumulative damage of the sample under the same cyclic impact loading was studied.The experimental results indicated that the total number of impacts of the sample was increased with the increase in confining pressure.Under the same cyclic impact loading,a higher confining pressure yielded a lower rate of increase of the average strain rate and maximum strain,as well as a lower rate of decrease of the peak stress and elastic modulus.The peak stress and maximum strain exhibited a good negative linear correlation and positive linear correlation with the average strain rate,respectively.The statistical damage constitutive model based on Weibull distribution and the acquisition method of related parameters have a good fit for the stress-strain relationship under impact load,and it has good applicability.Under the same cyclic impact load,the greater the confining pressure,the more slowly the cumulative damage of the sample increased.The sample with Cp=0MPa was pulverized,and the fragmentation was severe,while the sample with Cp=4,8,12,and 16MPa exhibited a pronounced mode of shear failure with less fragmentation.(4)An outdoor cyclic blasting test was carried out on the granite sample,and the change rate of sonic wave velocity was used as the damage variable to analyze the evolution law of cumulative damage of the rock mass under the cyclic blasting loading.The cyclic blasting was simulated numerically using the restart function in the ANSYS/LS-DYNA finite element program and RHT(Riedel-Hiermaier-Thoma)model.Finally,the simulation results were compared with the test results.The experimental results indicated that under the same number of blastings,the change rate of sonic wave velocity of the measuring point gradually decreased with the increase in the burst center distance.The change rate of sonic wave velocity of the rock mass including the blasting hole above explosive was greater than that of the complete rock mass below explosive.Using the restart function in the ANSYS/LS-DYNA finite element program and RHT model could well simulate the evolution process of cumulative damage and crack propagation,and the simulation results were in good agreement with the test results.