Experimental And Numerical Study On Dynamic Splitting Behavior Of Heat Treatment Granite

Rock is a heterogeneous brittle material containing crystals as well as defects such as pores,cracks and grain boundaries at the microscopic scale.Tensile fracturing often occurs due to its tensile strength is far less than the compressive strength.In addition,modern engineerings such as development of geothermal resources,underground storage of high radioactive nuclear waste and reconstruction after tunnel fire,are all affected by temperature and load.In order to investigate the medium-fine granite dynamic splitting tensile properties and failure modes after heat treatment at different temperatures,as well as to study the characteristics of energy dissipation and the influence of temperature and loading rate on energy dissipation of granite after heat treatment,the experimental and numerical study of dynamic splitting are carried on heat treatment granite.The research work carried out in this paper and the conclusions are as follows:Firstly,the impact splitting tests were carried out on granite specimens subjected to six different heat treatments(25,100,300,500,700 and 900?)at 3 different loading rates(5.4m/s,7.7m/s and 13.7m/s)using ?50mm variable cross-section split Hopkinson pressure bar(SHPB).The results of mechanical properties analysis showed that: When the temperature is less than 500?,the failure mode of the granite specimen in the dynamic splitting test is symmetrical crack along the radial direction at lower loading rate.As the increase of loading rate,the splitting failure modes at the ends of the specimen are nearly triangular crushed zone.After 500? of heat treatment,specimens show a broken pieces form at high loading rate.With the increase of heat treatment temperature,the longitudinal wave velocity of rock samples decreased,damage value increases.Thermal damage evolution law is accorded with logistic curve model.In the studied temperature range,the splitting strength decreased with the increase of temperature under different loading rates.The relationship between the tensile strength and damage and the allometry function curve fit well after the temperature of 100?.Meanwhile,the dynamic tensile strength is also sensitive to strain rate.Secondly,the energy properties of heat-treated granite in dynamic splitting test is analyzed based on the structural failure criterion for rock.The results of study on the characteristics of energy dissipation showed that the dissipation energy of granite samples decreases with the increase of heat treatment temperature,but increases with the increase of projectile velocity.An increase in change rates of incident energy and temperature results in a smaller rock fragmentation,and the number of broken pieces showed an increasing trend.With the increase of change rates of incident energy,the effect of temperature is more obvious.Under the impact load,the energy utilization rate is less than 30%,and it shows the opposite effect to the loading rate law.In addition,the splitting strength and the transmission energy of the granite are approximately linearly increasing.It is pointed out that the rock tensile damage capacity can be more clearly and accurately reflected from the perspective of energy dissipation.Finally,the dynamic splitting failure process of granite and the effect of end effect are numerically simulated based on the finite element analysis software ANSYS/LS-DYNA and ABAQUS.The results of numerical simulation showed that the stress concentration during the contact of the SHPB bars and the specimen can be avoided by using non-equal mesh.In addition,the dynamic behaviors of granite at room temperature can be well described using the Cowper-Symonds viscoplastic constitutive model which considering strain rate effect.Moreover,the existence of the end effect of the sample makes the tensile strength larger than the theoretical value in dynamic splitting test.