Creep acoustic emission and fractal characteristics of coal rock under high temperature pre-damage

The thermal damage and triaxial creep acoustic emission experiments of coal samples at different temperatures were carried out to investigate and analyze the mechanical characteristics of coal and rock after high temperature pre-damage. The results show that the thermal damage of coal and rock increases exponentially with the increase of temperature; the brittle-ductile failure characteristics of coal and rock appear at lower temperature(≤ 200℃) and the accelerated creep characteristics are not obvious; the brittle-brittle failure characteristics of coal and rock appear at higher temperature(>200℃) and the accelerated creep characteristics are more obvious; the value of steady creep rate is between pre-damage D and pre-damage D. The higher the temperature is, the smaller the acoustic emission ringing counting rate of coal and rock is, the lower the low frequency and low amplitude changes, and the three creep stages tend to develop in the same frequency. The cumulative ringing counts Nm and pre-damage D decrease in a negative exponential function. The fractal dimension values change in three stages with creep test, namely, decrease-dynamic stability-re-decrease with the three creep stages. Correspondingly, the turning point of fractal dimension from steady state to decrease can be used to judge the failure of coal and rock. The higher the temperature(the greater the pre-damage value), the larger the fractal dimension d_f, the more disorderly the failure of rock sample. The research results are of great significance to reveal the long-term mechanical behavior of coal after pyrolysis and gasification at different temperatures.