Study On Mechanical Characteristics Of Dynamic Tensile And Mode ? Fracture Of Coal Samples Under Impact Loading

The dynamic tensile and fracture characteristics of coal include the tensile properties and mode I fracture toughness of coal,as well as the deformation,strength and energy dissipation of coal during the process of deformation and failure.These mechanical properties,as one of the hot topics in mining engineering and rock mechanics research,are of great significance for the selection of blasting engineering parameters in coal mine,the stability control of coal mass,the roadway support and the prevention of rockburst.Dynamic tensile and mode I fracture characteristics,the energy dissipation law and the various influencing factors of coal containing bedding under impact loading are systematically studied.The conclusions are as follows:?1?According to the theory of thermodynamics and statistical physics,the evolution and nucleation process of micro cracks in rock materials are discussed.The growth rate of the defect is obtained as v=v₀exp[-?U-???/?kT?],and the fractal characteristics of rock fracture nucleation are analyzed.According to the theory of fracture mechanics,the stress and displacement field distribution of I type crack in an infinite plate,and the analytical solution of stress and displacement at the end of a crack are calculated.The dynamic fracture criteria of rock materials is summarized from microcosmic and macro scale,the rock rate response constitutive model based on micromechanics and the macroscopic fracture criterion of rock like materials are obtained.Three criteria for crack initiation in rock under impact load are concluded:maximum stress intensity factor criterion,shortest time criterion and minimum force criterion.A fracture dynamic model of coal mass presplitting blasting is established,and the critical quasi-static pressure of the explosion in the hole is obtained as K_{I d}/??af?a/r??^1/2by using the maximum stress intensity factor criterion.?2?Aiming at investigating the characteristics of tensile strength and fracture properties of coal under quasi-static loading,the Brazilian disc?BD?testing and the semi-circular bending?SCB?test were carried out.Moreover,the SCB specimens with various depths of notch were tested to analyze the fracture properties of coal.The fracture toughness K_ICC and J-intergral fracture toughness of coal were calculated.The effects of notch depth on the fracture toughness results were also discussed.The strain gauges were utilized to monitor the deformation features of specimens.The computed tomography scanning was also carried out on the failure specimens to figure out the cracks distribution.Results show that SCB method is more suitable to obtain the tensile strength of coals compared with BD test.In the current testing situation,the coefficient of variation of fracture toughness K_IC is the smallest as the dimensionless notch depth?equal to 0.28.Nevertheless,the K_IC fracture toughness can be affected by the notch depth and the dimensionless stress intensity factor?SIF?.The results also indicate that the J-intergral fracture toughness is more suitable to evaluate the fracture properties of coals.?3?In order to study the tensile failure characterization of coal under impact loading,the split Hopkinson pressure bar was applied in the test of dynamic Brazilian disc split tension test of coals.The influence of three factors such as impact velocity,bedding and saturated water on dynamic tensile mechanical properties of coal is discussed.The energy dissipation law of coal samples in the process of impact deformation with different influence factors is analyzed.The results show that the dynamic tensile strength of natural and saturated water coal samples both increase with the increase of impact velocity,strain rate and loading rate.Compared with the natural dry coal sample,the saturated water coal sample has a larger post peak deformation,and its dynamic tensile strength is greater than that of the natural dry coal sample,presumably due to the Stephen Effect.When the bedding inclination of coal is in a certain angle with the loading direction,the final failure mode is coupled with the tensile failure and the shear failure phase.Compared with the impact velocity,the bedding angle has a weak influence on the dynamic tensile strength of coal.The damage variable of coal sample increases with the increase of impact velocity,and the overall damage variable of saturated water coal samples increases exponentially with the increase of impact velocity.?4?The dynamic initiation fracture toughness?IFT?of 135 coal samples was tested by using the notched semi-circular bending?NSCB?method and a split Hopkinson pressure bar?SHPB?system,so as to explore the influence of loading rate and bedding orientation on the fracture toughness of coal samples under dynamic loading.The dynamic IFT of NSCB coal specimens with various bedding angles and crack lengths under different loading rates were analyzed and discussed.A high-speed high-resolution digital camera was used to record the processes of cracking initiation and propagation in coal specimens.The experimental results indicate that the effects of bedding plane on the cracking pattern decreases with the increase of both loading rate and crack length in the specimens.The impact velocity dominates the evolution of the dynamic IFT of coal,and the anisotropy caused by bedding planes in the specimens plays a more critical role in the evolution of dynamic IFT than the crack length.The effects of bedding angles on the dynamic IFT decrease with the increase of impact velocity.The growth behavior of dynamic IFT of coal distinctly changes after the loading rate exceeds a critical value.In addition,a model of rate dependency was established to describe the dynamic IFT of coal.?5?In order to study the fractal propagation characteristics of crack in coal samples under impact loading,and the change of velocity,acceleration and fractal dimension of crack propagation.Split Hopkinson pressure bar?SHPB?system was applied to determine the prepared notched semi-circular bending specimens.The high-speed camera was used to record the propagation characteristics of cracks.The image processing method and fractal dimension calculation software are combined to further analyze the effects of bedding and loading rate on the fractal characteristics of crack propagation in coal.The experimental results presented that the presence of bedding has a remarkable impact on the crack propagation.The crack velocity of coal samples with45°bedding angle is of the maximum,however,the crack velocity of samples with 0°bedding angle is of the minimum.Bedding angles also have obvious influence on the fractal dimension of cracking path,the bedding angle of 45°being the largest,and those of 22.5°and 67.5°having the intermediate values,while those of 0°and 90°being the smallest.Several points of instantaneous fractal crack velocity are close to the Rayleigh wave velocity?C_r?.The crack velocities of coal specimens with bedding angles of 22.5°,45°and 67.5°are prone to the high value.?6?Based on the CDEM numerical simulation,the evolution process of the dynamic stress and strain field of the Brazil disk coal sample under impact load and the propagation,reflection and superposition characteristics of stress wave in coal are studied.It is found that the results are in good agreement with the experimental results.When the bedding inclination of coal is in a certain angle with the loading direction,the final failure mode is coupled with the tensile failure and the shear failure phase.In addition,some secondary cracks along the weak plane can be observed by the numerical simulation,which is due to the additional shear failure caused by the bedding.With the increase of the impact velocity,the cracking time of the crack in coal is decreasing,but the test value of tensile strength increases continuously.Based on the CDEM method,the evolution of stress and strain fields in the dynamic fracture process of NSCB coal samples and the propagation process of stress waves in coal are simulated.It is found that when the bedding plane is parallel to or perpendicular to the loading direction,the crack propagation path is more straight,that is,the fractal dimension is smaller.However,the crack propagation path will be inclined along the weak surface,the ultimate path is more bending and the fractal dimension is larger when the bedding plane is in a certain angle with the loading direction.With the increase of the impact velocity,the test value of the dynamic fracture toughness of coal samples also increases.In addition,the different confinement methods have significant influence on the simulation results of mode I fracture toughness.