Influence Of Loading Rate And Size Effect On Mechanical Properties Of Coal And Rock

Rock is a complex geological body with random distribution of pores,joints and other defects.These special structures make the rock have obvious size effect.The results show that the deformation process and failure mode of rock samples with different sizes are obviously different,and their sizes have significant influence on the mechanical properties of rock such as strength and strain.At the same time,the mechanical properties of rock,such as strength,are affected by the loading rate,showing great differences.Therefore,it is of great theoretical and practical significance to study the size effect of rock mechanical properties under different loading rates.(1)Uniaxial compression test,Brazilian splitting test and acoustic emission test were carried out under different loading rates.Firstly,the influence of size effect on the mechanical properties of coal rock is studied.The results show that the uniaxial compressive strength,peak strain and tensile strength of coal rock decrease with the increase of size,while the elastic modulus increases at first and then decreases with the increase of height diameter ratio of coal rock.On this basis,a theoretical model of the size effect of mechanical parameters in uniaxial compression is proposed.The theoretical curves are compared with the experimental results,and it is found that the two agree well.At the same time,in the uniaxial compression test,the ratio of height to diameter is 2,when the ratio of height to diameter is less than 2,the change of mechanical parameters is obvious,and when the ratio is greater than 2,the change of mechanical parameters slows down.(2)The effects of loading rate on the mechanical properties of coal and rock and the acoustic emission characteristics under uniaxial compression are studied,and the effects of loading rate and size on the mechanical parameters of coal and rock are analyzed.The results show that the higher the loading rate is,the more obvious the size effect is.Secondly,the compressive strength and tensile strength increase with the increase of loading rate.The compressive strength and tensile strength show a logarithmic function relationship with the change of loading rate,and the tensile strength show a quadratic polynomial function relationship with the change of loading rate.The smaller the sample size,the more obvious the trend of compressive strength and tensile strength with the increase of loading rate.At the same time,in the uniaxial compression test,the peak strain decreases with the increase of loading rate,showing a negative correlation.The maximum number of acoustic emission ringing and the cumulative number of acoustic emission ringing increase with the increase of loading rate.Before and after the peak strength,the number of acoustic emission ringing changes abruptly.(1)RFPA2D numerical simulation software was used to simulate the uniaxial compression and Brazilian split test process,and the effects of different loading rates and size effects on the mechanical properties such as peak strength and acoustic emission characteristics were studied.The numerical simulation results are compared with the experimental results,and the two results are basically consistent.At the same time,the analysis shows that before and after the peak strength,there is a large area of crack in the specimen,and the number of acoustic emission ringing changes abruptly,which can be regarded as the precursor of rock instability.When the ratio of height to diameter is less than or equal to 2,the number of acoustic emission gradually increases and then decreases until it decreases to 0 in the whole process;when L/D is more than 2,there is a very obvious acoustic emission event after the peak strength,which belongs to the sudden change law,which has certain reference significance for the prediction of rock fracture instability by using acoustic emission.