Inversion Of Micro Fracture Mechanism Of Rock Under Different Loading Conditions Based On Acoustic Emission

Acoustic emission(AE)refers to the phenomenon that strain energy is released in the form of stress wave during the formation and propagation of cracks in materials.The appearance of macro cracks in rock materials is the result of the initiation,propagation and interpenetration of a large number of micro cracks,while the evolution and development of micro cracks in rock are often accompanied by acoustic emission phenomenon.Through the analysis of acoustic emission phenomenon,not only the understanding of rock failure process can be deepened,but also the fracture mechanism of rock can be explored from the meso scale.Therefore,in this paper,the mechanism of micro fracture of rock is taken as the theme,the acoustic emission tests of sandstone and granite with different mineral particle sizes under different loading conditions are carried out,the temporal and spatial evolution rules of acoustic emission events are analyzed,the types and scales of micro fracture of rock are determined by combining with the inversion theory of moment tensor,and then the relationship between micro fracture scale and mineral particle size is analyzed.The main work can be summarized as follows:1.The particle size of sandstone and granite is measured by optical microscope.According to the results,the two types of rocks are divided into three types:fine,medium and coarse.The mineral composition of rocks was analyzed by XRD,and the content of mineral composition was determined.2.The acoustic emission tests of granite and sandstone with different particle sizes were carried out under three-point bending and direct shear conditions.It is found that under the two loading conditions,the influence of different particle size and different diagenesis mechanism on the micro fracture mechanism is not obviously different.The shear type cracks account for more than 50%of the micro fracture cracks.3.By analyzing the temporal and spatial evolution of AE events,the spatial distribution of low-level events,medium-level events and high-energy level events in rock loading process is summarized.It is found that a large number of low-level events and individual high-energy level events occur in the early stage of loading,and a large number of medium-level events appear in the middle stage,which are similar to through shape,no matter in three-point bending or direct shear conditions Therefore,the risk assessment of rock failure is carried out.4.Based on the law of conservation of energy,a quantitative model of new-born shear microcracks is established,and the micro fracture scale of rocks with different particle sizes under direct shear is calculated.It is found that the micro fracture scale is smaller than the mineral particle size observed under the microscope.