Study On The Evolution Law Of The Fracture Process Zone Developing Into Macro-Fractures

Coal and rock mass hydraulic fracturing technology has significant effects,simple construction technology,and high safety factor.It has been widely used in underground mine prevention and control of rock bursts and improvement of top coal caving.In practical engineering applications,the expansion results of hydraulic fractures affect the final improvement effect of the entire hydraulic fracturing project on the coal and rock mass structure.Therefore,studying the fracture forms of hydraulic cracks in the framework of fracture mechanics is the basis for describing the mechanism of hydraulic crack propagation.Before the crack propagation of rock-like quasi-brittle materials,there is a discontinuous plastic deformation zone(fracture process zone)near the crack tip.The existence of the plastic deformation zone makes linear elastic fracture mechanics unable to accurately describe the fracture behavior of rock-like materials.Based on damage mechanics and fracture mechanics,this paper focuses on the influence of the fracture process zone on the formation of the real fracture surface,including the design of the fracture process zone monitoring experimental system,the analysis of the spatiotemporal evolution characteristics of micro-fracture,the analysis of the deformation evolution characteristics of the fracture process zone and the prefabrication Four parts are analyzed for the deformation evolution characteristics of the crack tip.Aiming at the fracture behavior of rock materials,the experimental methods and monitoring methods for the study of rock fracture characteristics are determined.The three-point bending loading method is used to characterize the crack propagation process,and the combination of acoustic and optical(two angles)methods are used to monitor the deformation and evolution characteristics of the rock fracture process zone during the crack propagation process,and the design of the fracture process zone monitoring experimental system is completed.Based on the monitoring experimental system of the fracture process area,the characteristic law of type I fracture strength of the sandstone specimen is proposed,combined with the characteristic parameters of acoustic emission,the response law of the micro-fracture event of the sandstone specimen is determined,and combined with the characteristic law of the acoustic emission energy,the micro-fracture event is obtained.The law of temporal and spatial evolution under different load levels;the distribution characteristics of the micro-fracture(seismic source)fracture mechanism are determined,combined with the energy distribution histogram,the size and effective width of the micro-fracture area are determined,and the influence of different focal mechanisms on the fracture behavior is analyzed.Based on the fracture process area monitoring experimental system and the microfracture spatiotemporal evolution characteristics,using digital image correlation methods,the distribution characteristics of the horizontal strain field and displacement field on the surface of the specimen are determined,and then the fracture process areas of the sandstone specimens at different loading stages are developed.Evolution morphology characteristics and crack opening displacement characteristics;based on the horizontal displacement field of the specimen,the development and expansion images of the discontinuous area of the sandstone specimen under various load levels are analyzed,and the fracture process area is determined based on the cohesive crack model identification method The development form determines the size of the fully developed fracture process zone,and obtains the law of the length of the fracture process zone with load conditions.Based on the deformation evolution characteristics of the fracture process zone and the cohesive fracture model theory,the digital image correlation method was used to determine the deformation evolution characteristics of the prefabricated crack tip of the sandstone specimen,and the trend of the horizontal deformation field in the prefabricated crack tip area was described;the analysis obtained the prefabricated crack tip level Displacement trend line change characteristics,the cracking deformation law of the prefabricated crack tip is obtained;based on the cohesive crack model,the nonlinear fracture characteristics such as the critical opening displacement of the crack tip and the characteristics of the load-CTOD softening curve are studied,and the cohesive crack is given The relationship between the three uncertain factors of the model.This paper has 53 figures,5 tables,and 85 references.