Study On Mechanical Properties And Crack Propagation Characteristics Of Fractured Sandstone Under Uniaxial Compression

Rock mass is a geological body with heterogeneity,discontinuity and anisotropy.After complex geological tectonics,there are many different defects in its interior,and the most common form of defects is cracks.It is very important to study the mechanical properties and crack propagation of fractured rock mass.In this regard,many experts and scholars have carried out research on fractured rock mass,but most of the research is on straight cracks and rock-like materials,while the research on S-shaped cracks and rock materials with fluctuations is relatively lacking.Research on S-shaped fractured rock mass has an important impact on revealing the development process of rock mass failure.It is very necessary to study the mechanical properties and crack propagation characteristics of S-shaped fractured rock mass,which has important theoretical and practical significance.In this paper,prefabricated flat-fractured red sandstone and prefabricated S-shaped fractured red sandstone are taken as the research objects,and the whole process of uniaxial compression test of prefabricated fractured red sandstone samples is monitored based on 3D-DIC(3D Digital Image Correlation)technology and acoustic emission technology.At the same time,the laboratory test was combined with the numerical simulation,the Particle Flow Code3D(PFC3D)was used to conduct numerical simulation,and the mechanical properties and crack propagation characteristics of prefabricated flat fracture and prefabricated S-shaped fracture sandstone under uniaxial compression were deeply studied from macroscopic and mesoscopic levels.The results are as follows:(1)The peak strength,peak strain and elastic modulus of sandstone samples with single straight fracture,double straight fracture and single S-shaped fracture basically increase with the increase of fracture Angle.The peak strength,peak strain and elastic modulus of the double-S-shaped fractured sandstone samples are also positively correlated with the fracture Angle on the whole,but when the fracture Angle is 45°,the elastic modulus shows a sign of decreasing obviously.(2)The stress-strain curves of both intact sandstone samples and prefabricated fractured sandstone samples are highly consistent with the corresponding ringing count and cumulative ringing count.When there is an obvious new macroscopic crack on the surface of the sample,the stress curve will drop sharply,and the corresponding ringing count and cumulative ringing count will increase sharply.The sudden change in the ringing count occurred at exactly the same time as the sudden change in stress.(3)For intact sandstone samples,the failure mode belongs to tension-shear mixed failure.For a single straight fractured sandstone sample,the failure mode presents a changing trend of tensile failure and tension-shear mixed failure as the fracture Angle increases.For sandstone samples with double straight fractures,the failure mode presents a changing trend of tensile failure,shear failure and tension-shear mixed failure with the increase of fracture Angle.For the single S-shaped fractured sandstone sample,the failure mode is tensile failure when the fracture Angle is 0° and 45°.When the fracture Angle is 30°,60° and 90°,the failure mode is tensile shear mixed failure.For the double S-shaped fractured sandstone samples,the failure modes are tensile failure at 0°,30° and 45°.When the fracture Angle is 60° and90°,the failure mode belongs to tensile shear mixed failure.(4)When the fracture Angle is 0° and 60°～90°,the failure mode,crack type and germination location of single straight fractured red sandstone samples and double straight fractured red sandstone samples are basically the same.When the crack Angle is from 30°to 45 °,the failure mode,crack type and germination location are different.When the fracture Angle is 0°～45°,the failure mode,crack type and germination location of single S-shaped fractured red sandstone samples and double S-shaped fractured red sandstone samples are basically the same.When the crack Angle is from 60° to 90°,the failure mode,crack type and germination location are different.When the fracture Angle is 0°～30°,the failure mode,crack type and germination location of single straight fractured red sandstone samples and single S-shaped fractured red sandstone samples are basically the same.When the crack Angle is 45 ° ～90 °,there is a big difference in failure mode,crack type and germination location.When the fracture Angle is 0°～30° and 60°～90°,the failure mode,crack type and germination location of single straight fractured red sandstone samples and single S-shaped fractured red sandstone samples are basically the same.When the crack Angle is 45 °,there are great differences in failure mode,crack type and germination location.(5)The numerical simulation results show that the microcracks in the sample model initially start from the prefabricated crack tip and its surroundings,and then gradually appear in other parts of the model with the uniaxial compression process.For the prefabricated fractured red sandstone samples,the number of microcracks after the peak strength is much more than that before the peak strength in the uniaxial compression simulation process,and the number of microcracks will increase sharply whenever the stress-strain curve shows the phenomenon of stress drop.