Fractal characteristics of acoustic emission in different damage degrees of impact coal

In order to study the fractal characteristics of acoustic emission with different damage degrees in the uniaxial compression process of coal samples with impact tendency. the uniaxial compression test is carried out on the impact-prone coal. the damage model of acoustic emission ringing count and the fractal dimension calculation model of the time series of acoustic emission parameters are used to study the damage evolution in the process of coal sample failure and the fractal dimension of different damage degrees. The research shows that whenever there is a large rupture of coal samples,the acoustic emission will increase gradually. The generation of large ringing count is the result of energy release during crack propagation and aggregation. Most of the frequent acoustic emission phenomena of impact coal occur in the later stage of the whole process of stress-strain; the damage strain process of coal samples can be divided into three stages: initial damage stage,damage evolution stage,and continuous damage growth stage. The acoustic emission ringing counts of each stage are1.61%,14.83%,and 83.56% respectively. The duration of the sustained growth stage of the coal impact damage is less,but the acoustic emission ringing is more; as the degree of coal sample damage increases,the fractal dimension of acoustic emission increases first. When the damage of coal sample reaches 30%,it suddenly drops,and then keeps fluctuating. When it reaches 80%,after reaching the local peak,a sudden drop occurred and the overall trend showed a tilted M shape. The phenomenon that the fractal dimension reaches a peak and then a sudden drop can be used as a precursor to the instability of the coal sample.