Research On A Method Of Rock Brittleness Evaluation Based On The Stress-strain Curve

Brittleness plays an important role in the brittle failure process of rocks,and is also one of the important mechanical properties of rocks and a key indicator in rock engineering such as hydraulic fracturing,tunneling machine borehole drilling and rockburst prediction.Therefore,aiming at the applicability of the brittleness index,this paper summarizes and analyzes the existing brittleness indices based on different experimental methods,such as strength,stress-strain,hardness,mineral composition,inner friction angle,elastic modulus and Poisson’s ratio.The evaluation method based on the stress-strain,whose key parameters can be easily obtained and quantified,is widely used for brittle characteristic.It proposes a brittleness index based on the post-peak stress drop rate of the rock stress-strain curve and the difficulty of brittle failure.In order to verify the accountable and application of the brittleness index,we change the mesoscopic parameters to stand for different types of rock specimens in numerical simulation with PFC2D.The results show that the brittleness index B_L can reflect the difference of numerical specimens.On the other hand,changing the confining pressure with mesoscopic parameters unchanged,the brittle degree differences between numerical specimens is simulated.The result shows that the brittleness index is increased with the confining pressure increased.In order to verified the reliable of brittleness index(B_L),the value of brittleness index(B_L)and the brittle failure character of rock specimens are compared and analysis,which is obtained from three sets of experiments.These three sets of experiments are the same type rock specimens under different confining pressure,different type rock specimens under uniaxial compression and confining pressure.So that the application and reliable of brittleness index are further verified.The major researches are listed as follow:(1)In this paper,a rock brittleness evaluation method which is based on the ratio of stress drop rate and strain drop rate and peak strain which represents the difficult degree of brittle failure is proposed.(2)Through the numerical simulation research of PFC2D,macroscopic parameters are calibrated by orthogonal experiment method.Under confining pressure of 5MPa,the brittle degree and macro-mechanical properties are simulated through changing the mesoscopic parameters.By comparison with the value of rock brittleness index and the difference of numerical specimen’s brittle,the application and reliable of brittleness index B_L are verified.The influence of different meso-parameters on brittleness characteristics of rock specimens is studied.The results show that the parallel bond stiffness,parallel bond strength and friction coefficient have a great influence on the brittleness of the rock.As the parallel bond stiffness and the parallel bond strength increase,the degree of brittleness decreases;as the coefficient of friction increases,the degree of brittleness increases.On the other hand,changing the confining pressure with mesoscopic parameters unchanged,rock failure character are simulated under confining pressure of 5MPa、10MPa、15MPa and 20MPa.The results show that the brittleness of rock decreases with the increase of confining pressure,and the brittleness index B_L reflect this tend well.(3)This paper used the brittleness index B_L to evaluate the brittle characteristic of the same specimens under different confining pressure and different specimens under uniaxial compression and confining pressure,and study the brittle failure character.The reliable and application of the brittleness index B_L are further verified.