Study On Mechanical And Damage Properties Of Coal-rock Combined Body Subjected To Cyclic Loading

The coal production often involves a number of mine working faces,roadways and working conditions.In actual coal mining process,coal-rock combined body will be inevitably affected by frequent dynamic loadings of periodic fracture of roof,mining disturbances,blasting vibration and mechanical disturbances.Under repeated or continuous dynamic loadings,the internal crack in coal-rock combined body will extend,the length and number of crack will increase,resulting in the increase of the degree and extent of damage.When the ultimate deformation or the accumulated damage reaches the threshold value,coal-rock combined body will failure,seriously,resulting in the occurrence of dynamic disasters,such as the large-area roof weighting,coal bumps.However,there are few studies on mechanical and damage properties of coal-rock combined body subjected to cyclic loading.To this end,in this paper,the coal bumps on coal face 1323 of the Gencun Coal Mine in Henan Province are taken as engineering background,and the theory analysis and experimental tests in the lab are used to investigate mechanical properties of coal-rock combined body subjected to cyclic loading.Firstly,the tests on the mechanical experiments of the rock-coal-rock samples under high stress and cyclic dynamic disturbance of small amplitude,and the axial strain,the irreversible deformation and failure modes were analyzed.The damage evolution law of coal-rock combined body under fatigue loading was studied.Then,the study on mechanical properties of coal-rock combined body subjected to the three-dimensional coupled static-dynamic loads,the coupled static and cyclic dynamic loads will be investigated.In the end,based on the test results,the damage constitutive model considered the strain rate,the mechanical properties of rock and coal,the confining pressure was established.The damage properties of coal-rock combined body subjected to the coupled static and cyclic dynamic loads were revealed.The main achievements are as follows:(1)The ultrasonic detector was used to study the P-wave velocity of rock,coal,coal-rock combined body.The scanning electron microscope was used to investigate the microscopic structure of rock and coal.Based on the nuclear paramagnetic resonance technique,the porosity and the amplitude of T2 spectrum were obtained.The uniaxial compression tests were conducted by using the rock,coal,coal-rock combined body.The strength,elastic moduli and failure modes of rock,coal,coal-rock combined body were analysed.The laws of energy evolution of samples under compression tests were obtained.(2)The servo-controlled RMT150-C system was used to conduct the cyclic loading experiments.The effect of stress amplitude on the fatigue mechanical behaviors of coal-rock combined body was studied.The threshold value of the stress ratio of coal-rock combined body under fatigue tests was obtained.The relations between the axial strain,the irreversible deformation,failure modes and thecyclic numbers under different stress amplitudes were obtained.The damage evolution law of coal-rock combined body under fatigue loading was studied.The influence mechanism of the stress amplitude on the fatigue mechanical behaviors of coal-rock combined body were revealed.the effect ofcyclic loads on the softening and reinforcement mechanisms of coal-rock combined body was discussed.(3)Based on the modified SHPB testing system,the tests on mechanical properties of coal-rock combined body subjected to dynamic loads,one-dimensional coupled static-dynamic loads,three-dimensional coupled static-dynamic loads were conducted.The strength,the deformation,failure modes and the energy evolution of coal-rock combined body were investigated.The effects of strain rate on peak strength and deformation modulus were obtained.The relations between dissipated energy density and the incident energy were determined.The high speed camera was used to record the failure behavior,and the failure properties of coal-rock combined body subjected to dynamic loads,one-dimensional coupled static-dynamic loads were investigated.(4)Based on the modified SHPB testing system,the tests on mechanical properties of coal-rock combined body subjected to three-dimensional static loads andcyclic dynamic loads were conducted.The effects of the incident energy and confining pressure on the strength,the deformation,failure modes and the energy evolution of coal-rock combined body were investigated.The relations betweencyclic numbers and the incident energy,confining pressure were determined.The characteristics of stress waves and stress-strain curves were found.The relations between the peak stress,average strain rate,deformation modulus,peak strain,accumulated dissipated energy density and thecyclic numbers,the incident energy,confining pressure were obtained.The broken characteristics of coal-rock combined body were investigated.The change mechanisms of mechanical properties of coal-rock combined body subjected to three-dimensional static loads andcyclic dynamic loads were discussed.(5)Based on the statistical damage constitutive model,a new damage constitutive model considered the strain rate,the mechanical properties of rock and coal,the confining pressure was established.It is indicated that the results of this damage constitutive model and test results are consistent.The variation characteristics of coal-rock combined body between the damage and the strain were obtained.The relations between the accumulated damage and the incident energy,confining pressure were determined.The model can well describe the relation between the accumulated damage and relative cyclic numbers.