Study On Monitoring Method Of Three-Dimensional Stress In Surrounding Rock Based On Fiber Bragg Grating Sensing Technology

During the process of coal mining,the study of the stress state of rock mass is of very significance to reveal the distribution characteristics and the evolution law of the surrounding rock stress field as well as roadway support design.The existing monitoring methods mainly monitor the vertical stress and cannot realize the threedimensional stress monitoring,and these instruments adopt electrical sensing with the disadvantages of poor ability to resist electromagnetic interference,low precision and poor long-term stability,and cannot meet the requirements of the monitoring of threedimensional stress state of mine.Based on rheological stress recovery theory and Characteristics of axial stress sensitivity of fiber Bragg gratings,this thesis proposed FBG-based stress monitoring method for surrounding rock,and by employing this monitoring theory,FBG Three-dimensional stress sensor of cube was developed and verified its accuracy and reliability with numerical simulation and physical experiment.The structure of FBG three-dimensional stress sensor introduced in this paper is 1 fiber Bragg gratings located on 6 sides of the cube base as a sensing element in a specific direction.By building the mechanical model of three-dimensional stress sensor and introducing the strain formula of any straight line in 3D space,the numerical relationship between strain monitoring values and three dimensional stress state parameters of three dimensional stress sensors is deduced.Also,the coefficient matrix between the wavelength drift of each fiber grating and the three-dimensional stress state parameters was deduced with the combination of basic theory of strain measurement of fiber bragg grating.Through the ANSYS Workbench simulation software,the numerical analysis model of the sensor is set up to analyze the sensing characteristics of the threedimensional fiber bragg grating stress sensor under the uniaxial load and the hydrostatic pressure,as well as the strain distribution of the fiber bragg grating and the sensor matrix.According to the fiber Bragg grating strain obtained by numerical simulation,the calculated three-dimensional stress value is basically the same as the theoretical value with the error of less than 1%,which proves the rationality of its structure.A numerical analysis model of measuring stress of sensor embedded surrounding rock is established.By analyzing the interference range of surrounding rock exerted by sensor,the optimum matrix material is selected,and the ratio of the stress to the actual stress measured by the sensor is given under certain conditions.There is a linear relationship between the deformation of the fiber Bragg grating and the uniaxial compression load.According to this characteristic of the sensor,the sensing performance of the sensor was tested by the uniaxial compression experiment instead of the three axis compression test.The sensor was subjected to uniaxial cyclic loading along X,Y and Z axes respectively,and the sensor performance was tested.The test results show that the sensor is of good repeatability and linearity under the external load,and the sensitivity of the sensor to X,Y and Z axis is 25.51,25.97 and 24.86 pm/MPa respectively.The relative error of the measured stress is less than 4%,which can meet the requirement of measuring the stress of surrounding rock.