Study On The Evolution Law Of Double Fractures Under Two-Dimensional Stress Based On Acoustic Emission Technique

Rock is a complex and discontinuous geological medium.It is the most basic natural material involved in rock engineering.After long-term and complex geological environment evolution,the mechanical and structural properties of rock have changed,and some initial structural defects(such as fissures,faults,joints,etc.)are generated inside the rock mass.When subjected to external forces,these defects are amplified and have a large and inconsequential effect on the rock.Therefore,through the theoretical analysis,laboratory experiments,numerical simulation and other methods,the paper studies the crack propagation evolution and acoustic emission characteristics of double-fracture rock materials under two-dimensional stress.The research results can provide theoretical and practical guidance for the support and design of rock mass engineering.(1)Through the biaxial loading test of double fractured rock specimens,the crack propagation characteristics,penetration mode and specimen failure type during loading were analyzed.Five rock bridge crack penetration modes(? type,?-1 type,?-2 type,?-1 type,?-2 type)and three types of damage(T type,S type,TS type)were classified.The test results show that when the length of the rock bridge is b,the rock bridge penetration mode of the 90°and 120°rock bridge dip angle specimens is dominated by oblique shear crack propagation(?-1 type),and the 150°rock bridge dip angle specimen rock bridge The through mode is dominated by the wing crack propagation(I type);when the length of the rock bridge is 2b,the rock joints of the inclined cracks(30°,45°,60°)are mostly wing crack propagation overlapping shear crack propagation and penetration(?-1 type,?-2 type).(2)Combining the stress-strain curve and the amplitude-stress-time and AE spatial distribution maps of each test piece,the two-fracture evolution process and acoustic emission characteristics(acoustic emission signal gathering position,peak variation,etc.)under two-dimensional stress are analyzed.It can be seen that the stress-strain curve of the double prefabricated fracture specimen under the action of two-dimensional stress exhibits an "S" shape and undergoes four stages:compaction stage,elasticity stage,yield stage,strain softening stage,corresponding to the characteristics of the four-stage acoustic emission signal during the crack propagation process:intermittent acoustic signal phase,stable acoustic signal phase,unsteady acoustic signal phase,and abrupt acoustic signal phase.(3)Combined with PFC numerical analysis software,the whole process of crack initiation,expansion,penetration and specimen failure of double prefabricated fractured rock specimens was reproduced.The crack morphology,expansion mode and specimen failure type were analyzed.The number of samples is small,but through numerical simulation and similar material simulation test analysis,a basically consistent evolution law of crack propagation is obtained.