| Rock is a complex material composed of various mineral grains,pores and cementitious materials,which has its particularity and complexity.Geometry on the surface of the rock fracture fractal characteristics,the fractal dimension can describe quantitatively the complexity of the rock fracture surface,the prefabricated crack sandstone mechanical characteristics,damage rule and fractal geometry,the fractal dimension of fracture surfaces and containing crack rock mechanical properties,the relationship between has strong theoretical significance and practical significance.This project takes sandstone as the object,and sandstone is made into standard cylindrical specimens.9 specimens with complete shape and similar weight are selected and numbered from 0 to 8.The electroplated diamond wire saw was used to preform the crack of No.0 to No.8 specimen,and the damage of No.0 to No.8 specimens was 0,0.060%,0.099%,0.196%,0.332%,0.513%,0.743%,1.045% and 1.356%.The sandstone test piece was subjected to uniaxial compression test using SAW-2000 rock servo test system,and the damage process of the test piece was recorded by high-definition camera.The fracture surface of prefabricated fractured sandstone with different damage level was obtained under uniaxial compression.The fracture surface was observed by XPV-900 E computerized transflective polarizing microscope,and the microscopic image of rock fracture surface under different magnification conditions was collected.The ImageJ software binarizes the obtained microscopic image and calculates its fractal dimension.Based on the study of mechanical properties,failure laws and fractal characteristics of prefabricated fractured sandstones,the following conclusions can be drawn:(1)The complete test piece No.0 has the highest peak intensity,and its value is 167.21 MPa.The peak intensities of the pre-formed crack specimens 1 to 8 were successively reduced,which were 164.53 MPa,157.17 MPa,139.51 MPa,124.64 MPa,107.71 MPa,77.51 MPa,74.69 MPa and 61.03 MPa,respectively.Compared with no.0 specimen,no.1 ~ 8 specimen reduced the peak strength by 1.6%,6.0%,16.6%,25.5%,35.6%,53.6%,55.3% and 63.5%.As the level of damage increases,the peak strength of sandstone specimens gradually decreases,but does not decrease uniformly.(2)The peak strength and peak strain of the fractured sandstone specimens are lower than the intact rock specimens,and the peak strength and peak strain of the specimens with different damage level are also different.With the increase of damage level,the peak strength and peak strain of sandstone specimens gradually decrease.With the increase of damage level,the effects on peak strength and peak strain are gradually reduced.However,the intensity and peak strain are not exactly the same as the damage.(3)The development trend of the stress-strain relationship curve of the pre-formed fracture test piece is similar to that of the complete test piece,but the crack closure stage and the elastic continuous stage of the pre-formed fracture test piece are shorter than the complete test piece duration,and the pre-fabricated sandstone test is carried out with the increase of the damage level.The duration of the elastic phase is gradually reduced.(4)The main crack of the intact sandstone specimen appeared parallel to the loading direction.As the test progressed to the two ends of the specimen,the overall failure occurred and the failure mode was splitting failure.The main crack containing the pre-formed crack test piece through the pre-formed crack is generated by the end of the pre-formed crack and extends to the upper and lower ends,and the failure form is also the splitting damage.As the peak intensity decreases,the degree of fracture of the sandstone is reduced and the degree of bending of the crack is gradually reduced.(5)The fractal dimension of the fracture surface decreases with the decrease of the peak strength of the specimen.The linear regression equation fitting the fractal dimension of the fracture surface with the peak strength of the specimen at 50,100,400 and 600 times magnification are y=0.00093x+1.62553,y=0.000513x+1.62049,y=0.00037x+1.62553,y=0.00066x+1.54281,and the correlation coefficients are as high as 0.9604,0.92789,0.9628,0.9628.In other words,there is a strong correlation between the fractal dimension of the fracture surface and the fitting linear regression equation of the peak strength of the specimen at each magnification.(6)When the microscope magnification is increased,the scanning range becomes smaller,and the pore structure of the fracture surface can be observed in more detail.The complexity of the fracture surface structure is reduced,and the pore structure filling ability of the skeleton structure is also reduced.Therefore,the fractal dimension calculation result increases with the magnification.And gradually decrease.(7)When the damage level is 0,the fractal dimension is the largest,and as the damage degree increases,the fractal dimension gradually decreases.As the damage degree increases,its influence on the fractal dimension of the fracture surface gradually decreases. |
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