Experimental Study On Compressive Properties And Damage Cracking Of Weakly Weathered Granite Under High Strain Rate

Damage and fracture mechanics of weathered rock under high strain rate are involved in all the engineering blasting activities in the weathering crust of granite.Based on the National Natural Science Foundation of China?No.51404111?and taking weakly-weathered granite as the research object,studies the compression performance and damage fracture characteristics under high strain rate by using SHPB.The dynamic and static compressive properties of weathered granite and dense granite,the dynamic characteristics of weakly-weathered granite with different porosity,the damage evolution law of weakly-weathered granite under cyclic impact and its dynamic damage evolution constitutive model and strength criterion were compared and analyzed,the following conclusions are obtained.?1?SHPB test technique was used to conduct uniaxial compression tests on weakly-weathered and dense granite with similar porosity at strain rates of 5.0×10^-5s^-1 and26.07 s^-1?75.16 s^-1,and the strength,deformation characteristics and energy dissipation law of the two were compared and analyzed.The results show that the reflection amplitude value of weakly weathered rock is higher than that of dense rock,and the transmission amplitude value is lower.Under different strain rates,the dynamic peak stress and elastic modulus of the two kinds of rocks increase first and then decrease,while the strain rate,peak strain and maximum strain increase exponentially,the dynamic modulus of elasticity is weakly related to the strain rate and tends to increase.The energy dissipation of the two kinds of rocks has significant strain rate effect,and the degree of fragmentation is positively related to strain rate.?2?In this paper,the distribution of pore group hierarchical microstructure of 2%to 8%weakly-weathered granite was considered by using NMR technology.Two groups of rock samples with different pores were screened out to conduct damage and failure shock respectively.The propagation characteristics of stress wave and the evolution law of the stress-strain curve with the increase of porosity under different shock were obtained.The results show that the strength,deformation and strain rate of weakly-weathered granite decrease with the increase of porosity;the absorption energy of damage impact is positively correlated with porosity,while the impact energy of failure is opposite;the elastic modulus is correlated with the initial damage degree,but not with strain rate.?3?The effective damage threshold of weakly weathered granite is 0.041 J/cm³determined by energy method,the mechanical properties of rocks under equal/unequal velocity cyclic impact were studied.Under constant velocity cyclic impact,the elastic modulus and secant modulus decrease with the increase of impact times,the strain rate increases first and then decreases,and the absorption energy increases continuously;under unequal velocity cyclic impact,the elastic modulus and peak stress increase first and then decrease,and the strain and strain rate increase continuously;Under cyclic impact The damage evolution has three typical stages,and a two parameter damage evolution model was established.The damage variable defined by the maximum strain method can be used to uniformly describe the damage evolution rule of equal/unequal velocity cyclic impact,when damage degree equal to 0.45 is the threshold of accelerated failure.?4?Based on the continuous damage mechanics,the damage was defined by the reduction of elastic modulus,and the damage evolution equations related to strain and strain rate were established.Based on the assumption of equivalent strain,a segmented dynamic damage evolution constitutive model of weakly weathered granite was constructed,and a variety of strength criteria were discussed,as well as the calculation of charge amount,crushing circle radius and charge parameters in surrounding controlled blasting holes in the design of geotechnical blasting.