Quantitative Evaluation Method Of Hard Rock Brittleness Based On Stress-strain Curve Study

Brittleness is one of the important mechanical properties of rock.It has been proved that brittleness can significantly affect rock drillability and borehole stability in hydraulic fracturing,rock burst and other engineering.Therefore,it is a current important research topic to deeply analyze and evaluate rock brittleness.The evaluation of hard rock brittleness is mainly based on the theory of stress-strain curve,hardness,energy change,mineral composition,and so on.Among them,based on the stress-strain curve has been widely used due to its characteristics of easy access and strong practicability.However,most of the brittleness indicators based on stress-strain curves need post-peak data,while the post-peak curve cannot be obtained after the failure of the tock due to the stiffness limitation of the testing machine,and the reliability of the evaluation results is low.Besides,,there are many methods to determine the threshold of axial strain and stress,and the results obtained by different methods are different,which needs further verification and analysis.Based on this,this paper carries out uniaxial and triaxial compression tests on basalt,granite,sandstone,syenite porphyry,marble and shale using MTS testing machine,and determines the axial strain of rock samples by comparing LVDT,plate displacement and extensometer.Then,the acoustic emission signals of rock samples in uniaxial compression tests are monitored synchronously,and the stress threshold value of rock is determined by the variation law of acoustic emission parameters.Final,considering the ratio between the growth rate of damage stress to the peak stress and the elastic modulus,a new evaluation method of brittleness index based on the pre-peak characteristics of stress-strain curve is proposed.The main conclusions are as follows:(1)Determine the selection of axial strain(deformation)of rock in the compression test of MTS testing machine.The axial strain of the rock sample before the peak strength should adopt the extensometer to obtain the deformation data.The axial strain after the peak strength should adopt the LVDT to measure axial deformation data.Meanwhile,the deformation of rock samples after peak strength can also be represented by the measured data of platen displacement.The axial strain measured by an extensometer is not accurate after the peak strength is reached.(2)Determine the stress threshold value of rock in compression test.The 1?3 stress threshold value of rock can be determined by acoustic emission energy,absolute acoustic emission energy,count,amplitude,AF value and RA value,while the evolution law of acoustic emission b value is difficult to determine the stress threshold value of rock.(3)A brittleness index B_pbased on the pre-peak characteristics of rock stress-strain curve is proposed,which ranges from 0 to 1.In the triaxial compression tests of different rocks,the effectiveness of B_pis verified by comparing the brittleness index B_pwith the index B_dbased on the post-peak stress-strain curve.The uniaxial compression test shows that B_pis more discriminative than B_din evaluating the brittleness of extremely brittle rocks.Besides,the rock brittleness decreases with the increase in confining pressure.(4)Brittleness index B_pis adopted to evaluate the brittle anisotropy of shale.At the same dip Angle,the confining pressure has a positive linear relationship with shale,while has no effect on the closure stress.Under the same confining pressure,the initiation stress,damage stress and peak stress of shale show obvious fluctuation characteristics with the increase in dip Angle,,while the dip Angle has no impact on the closing stress.Moreover,brittle anisotropy of shale can be well evaluated by brittleness evaluation index B_p.