Experimental Study On Dynamic Compression Characteristics And Damage Rupture Of Granite With Different Weathering Degrees

In order to investigate the influence law of weathering degree on the dynamic damage characteristics of rock mass,this thesis based on the National Natural Science Foundation of China project(52064015)and takes granite with different weathering degrees as the research object.The dynamic characteristics and damage rupture characteristics of granite specimens with different degrees of weathering were investigated by single impact and cyclic impact tests with the Hopkinson bar(SHPB)experimental system.And the changes of microstructure such as porosity and pore size of weathered granite specimens before and after impact were investigated with the help of Nuclear Magnetic Resonance Instrument(NMR),Scanning Electron Microscopy(SEM)and other instruments.This study provides some scientific and theoretical support for the geotechnical engineering of ionized rare earth mine "blasting and seepage enhancement",uneven weathering surrounding rock excavation and so on.The following conclusions were mainly drawn.(1)Under static compression,as the degree of weathering increases,the stress required by the rock body gradually decreases and the strain gradually increases when it enters the elastic stage.The total energy,elastic strain energy and dissipation energy absorbed by the granite specimens before the peak point show a non-linear decreasing trend with the increase of weathering degree;The peak stress and elastic modulus of granite specimens gradually decrease with the increase of weathering degree,and the damage mode gradually changes from longitudinal tensile damage to tensile-shear composite damage.The shear action,the number of shear cracks and the degree of fragmentation of the granite specimens are proportional to their degree of weathering.(2)Under the same impact,the crack extension phase of the stress-strain curve of the granite specimen changes from approximately linear to downward concave with increasing weathering,and the rebound of the unloading phase is inversely proportional to the degree of weathering of the specimen,gradually transforming from type II stress-strain curve to type I.The higher the degree of weathering of granite specimens,the smaller the dynamic peak stress and modulus of elasticity and the larger the strain;with the increase of impact speed,the dynamic peak stress of granite specimens increases exponentially,the modulus of elasticity increases and then decreases with the increase of impact speed,and the maximum strain increases linearly.The reflected energy,absorbed energy and absorbed energy per unit volume gradually increased with the increase of impact velocity,and the slope of tangent line gradually increased with the increase of weathering degree,while the transmitted energy increased exponentially with the increase of impact velocity;The average particle size of granite specimens decreased with the increase of impact velocity and strain rate,while the fractal dimension increased with the increase of impact velocity and strain rate and then tended to be constant;(3)The crack extension stage of weathered granite specimens under cyclic impact tends to be concave with the increase of impact times,and the rebound phenomenon in the unloading stage gradually decreases and gradually changes from type II stress-strain curve to type I.The change trend of absorbed energy per unit volume of slightly and weakly weathered granite specimens with the increase of impact times can be divided into two stages of slow growth and rapid growth,and the absorbed energy per unit volume of strongly weathered granite specimens increases rapidly with the increase of impact times.The energy dissipation rate of weathered granite increases exponentially with the number of impacts,and the higher the degree of weathering the greater the rate of energy dissipation;The development law of cracking under cyclic impacts of micro-weathered,weakly weathered and strongly weathered granite specimens can be summarized as follows: longitudinal tensile cracking → longitudinal tensile crack development → shear cracking → crack penetration specimen damage.(4)From a macroscopic point of view,using the elastic modulus to define the rock damage,it was found that the damage of the slightly and weakly weathered granite specimens increased with the number of impacts in an inverted S-shaped growth,while the strongly weathered granite specimens showed an approximate linear growth,lacking a slow growth phase of damage.The pore distribution curve gradually shifts to the right with the increase of impact number,the pore categories of granite specimens with different weathering degrees are similar,mainly small,medium and large pores,and the pore area is directly proportional to the weathering degree and impact number.The greater the degree of weathering of the granite specimen,the rougher the fracture surface,the greater the number of pores and the increase in pore size,the disappearance of the step-like pattern used to impede the rate of crack expansion of the specimen,the cracks that are oblique to the fracture surface begin to sprout,the number of cracks and the degree of development is proportional to the degree of weathering of the specimen.Combined with the macroscopic crack expansion pattern of the specimen,it can be seen that with the increase of the weathering degree of the specimen,the damage form of the granite specimen changes from tensile damage to tensile-shear composite damage form.