Monitoring Mechanism And Experimental Analysis Of Rock Crack Propagation With Fiber Bragg Grating

In China,geological catastrophes and coal mine mishaps are common,with the majority of them being caused by unknown crack propagation mechanisms and rock mass internal stress states.The use of modern monitoring technology to better understand the mechanism and principle of rock crack propagation and to take certain preventative actions will greatly limit the incidence of disasters.Fiber Bragg grating sensing technique offers the distinct benefits of high accuracy,resistance to external interference,and flexibility,making it ideal for monitoring the deformation and collapse of rock masses.When employing fiber grating to monitor a fracture,however,the crack propagation and damage evolution of the rock mass are nonlinear deformations,and the mechanical interaction mechanism and spectrum evolution characteristics of the crack and optical fiber must be addressed.This work investigates the grating monitoring mechanism of crack propagation in rock mass and conducts important experimental investigation based on this.The inhomogeneous strain was obtained under the action of the nonlinear deformation characteristics of the spectrum,and the inhomogeneous strain reflectivity spectrum and reflection coefficient formula were inferred using the Runge-Kutta method and transfer matrix method;The linear law of bandwidth broadening with increasing local tensile and shear strain was found by linear fitting after a calibration experiment of crack propagation grating monitoring.The mechanical models of type I type Ⅱ,and I-Ⅱcomposite cracks and FBG were established based on fracture mechanics and shear hysteresis theory,and the stress states of FBG at the crack tip were obtained when different types of cracks propagated,and the strain truth relations of FBG out of the crack tip were obtained using the strain transfer model.It is carried out a rock mechanics experiment of rock crack propagation grating monitoring.The loading process was tracked while the fiber Bragg grating is pasted over several types of fracture samples.The crack propagation process’ s spectrum nonlinear deformation and evolution features were derived.Changes in band width and peak reflectivity were used to study the fracture propagation process.Through the monitoring of the sticking angle of the fiber Bragg grating,it was concluded that when the fiber Bragg grating was pasted vertically with the crack,the band width of the grating was the largest,but it is easy to fracture.The monitoring effect is the best when 60 °,indicating that the sticking angle had a certain influence on the monitoring process.The damage variable D was defined to statistically analyze the damage degree of different types of crack propagation.The results showed that the accumulation process of damage variables during mode I crack propagation was concentrated in the pre peak stage,without typical initial compaction stage and elastic stage;Based on the damage variables characterized by the broadening of grating bandwidth,the whole process of rock compression failure was divided into three stages:damage stability evolution,damage aggravation evolution and post peak residual strength.This study is of great significance for the theoretical system of sensing monitoring of rock crack damage relationship and the formation of intrinsically safe mine disaster monitoring method.