Experimental Study On Fracture Evolution Of Coal-rock With Prefabricated Crack Under Uniaxial Loading

As the most common engineering medium in mining,coal-rock is a kind of original damaged body with a lot of holes and cracks.The expansion of fractured coal-rock cracks is the main cause of coal pillar instability,coal and gas outburst,rock burst and other mine disasters,exploring and mastering the law of fracture evolution of fractured coal-rock is the premise of preventing and predicting coal-rock disasters,which has very important engineering significance.Therefore,in this paper,the preparation of the prefabricated crack coal-rock specimens with different dip angle,using the combining method of theoretical analysis,laboratory test and numerical simulation,systematic study on the fracture evolution of uniaxially loaded pre-crack coal-rock from the aspects of coal-rock crack propagation characteristics,acoustic emission characteristics and numerical simulation comparison analysis,and the main results are as follows:(1)According to the stress-strain curves of prefabricated crack coal-rock with different dip angles,the deformation process of coal-rock can be divided into four stages,the generation and propagation of cracks are mainly concentrated in the stage of elastic unstable failure.When the prefabricated crack dip angle increases,the average strength and strain of the specimen decrease at first,then increase,then decrease and then increase,show a "w" wave pattern.Prefabricated crack have a certain impact on the stress and strain of coal-rock specimens,and the influence degree is different when the dip angle of prefabricated crack is different.(2)Study on the crack propagation characteristics of coal-rock specimens with different dip angle prefabricated crack under uniaxial loading,the main cracks of coal-rock specimens are anti-wing cracks and wing cracks,which distribute in the form of anti-symmetry along prefabricated crack.When the prefabricated crack inclination angle is low,the cracking initiation stress of the specimen is low,anti-wing crack preferential cracking,more cracks occur,cracks propagation is sufficient and the specimen is axial tensile failure.When the prefabricated crack inclination angle is high,the cracking initiation stress of the specimen is high,the wing crack initiation is preferential,the cracks are few,the cracks propagation is insufficient,and the specimen is slip shear failure.(3)Study on the acoustic emission characteristics of coal-rock specimens with different dip angle prefabricated crack under uniaxial loading.When the prefabricated crack inclination angle is low,the non-uniform deformation of the specimen is increased because of prefabricated crack,the AE amplitude is more active and concentrated in the compaction period of micro-defects,when the prefabricated crack inclination angle is high,become more sparse.The crack initiation AE energy of anti-wing crack is much higher than that of wing crack,and the former is about 2-3 times of that of the latter,shear cracks and secondary cracks occur before and after failure of specimens,the characteristics of AE show sudden change and peak value.The cumulative AE count increases "step by step" with the increase of the damage degree of the specimen,which is divided into two stages:stable phase and unstable phase.(4)The results of numerical simulation,such as crack propagation and crack initiation angle,are basically consistent with those of laboratory tests.On the whole,the crack opening of the anti-wing cracks is generally larger than that of the wing crack.In the early stage of loading,a low stress concentration zone will be formed and its edge will be subjected to tensile stress.When the prefabricated crack inclination angle is low,it is parallelogram,the anti-wing crack is produced when specimens crack initiation.When the prefabricated crack inclination angle is high,it is fan-shaped,the wing crack is produced when specimens crack initiation.At the later stage of loading,the specimens with different dips angle prefabricated crack will form different concentration zones of horizontal displacement.When the prefabricated crack inclination angle is low,the maximum value of the horizontal displacement is located near the central axis of the specimen,the specimen is easy to form waist drum and form tensile failure.When the prefabricated crack inclination angle is high,the maximum value of the horizontal displacement gradually shifts to both ends,the specimen is prone to slip and form shear failure.