Experimental Study on Damage Evolution Characteristics of Coal Samples under Impact Load under Different Surrounding Pressures

To study the response characteristics of coal samples to impact load under different surrounding pressures, the self-developed magnetic drop hammer impact device was used to measure the change of ultrasonic wave velocity before and after drop hammer impact, the internal damage of coal samples was quantitatively characterized, and then, the damage factor, damage difference, and crack development direction of briquette samples were analyzed. Based on the Griffith strength theory, the relationship between stress intensity factor, crack development angle, and surrounding pressure ratio of coal samples with composite cracks was deduced. It was found that a certain surrounding pressure would slow crack expansion and reduce the amount of damage increase. The damage degree of briquette samples caused by impact load under no surrounding pressure is the largest, followed by that at the surrounding pressure ratio of 0 and 2 and that at the surrounding pressure ratio of 1 is the smallest. When the amount of damage difference of the sample is positively correlated with the number of impacts, the crack development is directional; when the amount of damage difference of the sample is nonpositively correlated with the number of impacts, the direction of crack development is random. It is therefore reasonable to predict the direction of crack expansion by the trend in the amount of damage difference. Besides, the difference in impact load between briquette and raw coal samples under different surrounding pressure ratios was discussed, and the fractal dimension of the sample surface under different surrounding pressure ratios was compared and analyzed. It is found that the fractal dimension of raw coal is larger than that of briquette, indicating that the crack development of raw coal is higher after impact; the damage amount of raw coal is larger than that of briquette, indicating that raw coal is easier to be damaged after impact.