Characteristics and mechanism of coal sample impact failure induced by low stiffness loading
In order to study the mechanical properties and failure characteristics of the coal-rock combination with different stiffness ratios,coal samples and rock samples with different elastic modulus were combined to construct composite coal-rock samples for tests.Uniaxial compressive tests were carried out to evaluate the response of the combined samples including the macro and micro failure characteristics.The debris of coal samples was collected after test to quantify the spacial and scale distribution characteristics.The ejection process of the debris during failure was recorded using a high-speed camera and then used to analyze the ejection parameters such as the volume,velocity,momentum and kinetic energy of the debris which were then used to quantitatively evaluate the damage intensity of coal samples with different stiffness ratio combinations.The deformation characteristics of the loading process are obtained by using the strain gauge pasted on the surface of the sample,and the energy evolution is calculated by combining the load data recorded by the testing machine.The impact failure mechanism of the middling coal sample of the combined sample is analyzed from the energy perspective.The research findings are as follows:(1)Impact failure occurs to middling coal samples of combined samples,and the rock is compressed during the loading stage to accumulate elastic strain energy.When the coal sample is damaged,more than 90％of the accumulated energy releases to work on the coal sample with the rebound deformation of the rock,leading to the impact failure of the coal sample.(2)The axial deformation of composite specimens is a comprehensive deformation of rock and coal samples.The axial deformation of coal samples is relatively constant at peak strength,and the difference in axial deformation of composite specimens under different combination modes mainly depends on the rock.The axial strain of rock at peak strength decreases with the increase of elastic modulus.(3)As the elastic modulus of the rock decreases,the pre peak deformation increases in an inverse proportional function,the accumulation of elastic strain energy increases,and the post peak energy release rapidly increases,leading to a nonlinear intensification of the coal sample failure intensity as the elastic modulus of the rock decreases.(4)The nonlinear characteristics of parameters such as axial strain of rock at peak strength and failure intensity of coal samples with respect to the evolution of rock elastic modulus in composite samples are prominent:when the elastic modulus of rock and coal samples is at the same level,the response of each parameter to the change of rock elastic modulus is significant;When the elastic modulus of rocks is significantly higher than that of coal samples,the response of various parameters to the elastic modulus of rocks weakens.
rock mechanicscomposite coal-rock sampleimpact failuremechanical propertydeformation characteristicsenergy evolutiondestructive intensity
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