Experimental Study On Mechanical Properties And Crack Damage Evolution Mechanism Of Fractured Red Sandstone Under Different Uniaxial Loading Rate

Red sandstone is a common rock material used as a natural bearing material in civil engineering foundation.In engineering practice,red sandstone is affected by excavation disturbances（blasting vibration and excavation speed）,thus causing differences in the micro-macro damage evolution within the rock system.At the same time,the complex defects of different states endowed inside the red sandstone are important factors affecting the safety of rock engineering.Therefore,in this paper,we consider the influence of load rate changes caused by excavation disturbance and internal defects of red sandstone,prefabricate eight kinds of inclined fractured red sandstone specimens,conduct uniaxial compression and acoustic emission tests on fractured red sandstone under the influence of five loading rates,and analyze its mechanical characteristics,crack expansion and energy evolution characteristics.The results can provide reference for the evaluation of the safety and stability of the actual rock mass of the project,and provide a basis for the selection of loading rate for rock mechanics test.The main contents and conclusions of the experimental study are as follows.（1）Uniaxial compression tests of fractured red sandstone with strain rates of(10^-6/s,10^-5/s,and 10^-4/s)were conducted,and the mechanical properties of the fractured red sandstone all had a loading rate strengthening effect.The mechanical strengthening effect of red sandstone is not obvious in the strain rate class of 10-5/s,and the loading rate of indoor test is recommended to control the strain rate in the class of 10-5/s.（2）The acoustic emission ringing counts of the fractured red sandstone were statistically analyzed,and the evolution of acoustic emission counts of the damage process has critical characteristics.The time from the beginning of loading to the first critical point（the point where the cumulative ringing counts first increase）becomes longer with the increase of inclination angle,indicating that the time lag of the fractured rock samples into the critical state increases;the time of sub-critical state from the first critical point to the second critical point（the point where the cumulative counts increase significantly）shortens with the increase of inclination angle,which is more favorable for damage prediction than the fractured rock samples with small inclination angle.（3）According to the classification method of crack types and damage modes in fractured rocks under unidirectional stress state and the analysis of crack initiation angle.In this test,a total of six types of cracks were found in the cleaved red sandstone:wing,resisting tensile,coplanar secondary,non-coplanar secondary,transverse and far-field.The number of surface cracks decreases with increasing strain rate steps.The ultimate damage mode of the fractured red sandstone shows three 9 types of damage,and the increase of the dip angle drives the fracture body to change from S-type to T-and M-type damage modes,and the damage mode of the rock sample gradually transitions from T-type tensile damage to S-type shear damage with the increase of the loading rate.The fracture initiation angle is 85°when the fracture dip angle is 0°,and 1°～2°when the dip angle increases to 90°.The calculated fracture initiation angle of red sandstone fracture based on the maximum distortion energy quasi-measurement is in good agreement with the experimental value.（4）The damage process of fractured red sandstone is consistent with the critical characteristics.The time from the beginning of loading to the first critical point（cumulative ring count first apprentice point）becomes longer with the increase of the inclination angle,that is,with the increase of the inclination angle,the time lag of the fracture body into the critical state is enhanced;from the first critical point to the second critical point（cumulative count significant apprentice point）of the sub-critical state time with the increase of the inclination angle and shorten,the smaller inclination angle fracture body is more conducive to damage prediction.（5）uniaxial stress state,fractured red sandstone acoustic emission cumulative energy in the compressive stage grows faster,but into the linear elastic stage almost no acoustic emission events,in a long period of calm,near the peak stress,the cumulative energy is stepped surge,and the number of steps with the increase in fracture dip and less,the cumulative energy versus time in line with the Boltzmann function distribution characteristics.With the increase of fissure dip angle,Boltzmann function distribution near the peak stress is steeper,the higher energy acoustic emission event occurs in a shorter time span,the tendency of rock explosion is enhanced.（6）Based on the principle of continuous damage mechanics,a damage model characterized by acoustic emission cumulative energy of fractured red sandstone is established,and the validation of the model is in good agreement with the experimental results.The physical significance of the damage parameters was analyzed,and the bt value represents the time position of the large-scale energy release of fractured red sandstone,and the bt gradually increases from 0.88 to 0.97 as the inclination angle increases from 0°to 90°;the c value characterizes the fast and slow energy release of fractured red sandstone and decreases from 36.25 to 5.35 as the inclination angle increases,indicating that the energy storage time of fractured red sandstone grows and the energy release from small-scale multi-stage release to large-scale transient release.