Acoustic emission and fragment fractal characteristics of rock burst tendency coal samples under different strain rate loads

At present, the correlation analysis between the acoustic emission characteristics of the rock failure process and the fractal characteristics of sample fragments has been carried out. Some achievements have been obtained. But the quantitative description of the failure degree of coal samples with rock burst tendency under the condition of unidirectional loading with different strain rates and the quantitative relationship between the failure degree and the loading strain rate are few. In order to solve this problem, based on the MTS-C64. 106 electro-hydraulic servo system, the raw coal samples are subjected to uniaxial static load with different strain rates. In the test, the PCI-2 acoustic emission card is used to monitor the fracture process of the samples under load in real-time. The fractal theory is used to analyze the fracture fragments of the samples. The relationship between the fracture degree of the samples and the load strain rate is quantitatively evaluated. The results show the following points. ① On the basis of static load, the peak strength of sample failure increases with the increase of strain rate dynamic load. ② With the increase of the loading strain rate, the total number of AE decreases, and the number of high-energy AE events increases. The AE ringing count and energy amplitude undergo a consistent transition process of "slow increase-rapid increase-sudden increase." ③ The loading energy input rate of the sample is basically consistent with the increasing trend of acoustic emission ringing count and internal impact number. It will also experience the change of "slow increase-rapid increase-sudden increase." ④ The constant for the positive correlation between acoustic emission and vibration strength decreases with increasing loading strain rate. The constant which is negatively related to the ratio of the number of high and low energy vibrations, increases with increasing loading strain rate. The failure mode of the raw coal sample will experience a "shear failure - splitting failure - bursting failure" transformation. ⑤ When the loading strain rate is low, the upper part of the sample is destroyed. When the strain rate increases, the sample is gradually destroyed from the middle to the lower part. The failure process of the raw coal sample under the action of dynamic strain rate is mainly the brittle propagation behavior of cracks. ⑥ The fractal dimension of the impact fragment mass of the sample has a quadratic function relationship with the loading strain rate. There is an extreme value of the loading strain rate that maximizes the damage to the sample, with the test showing a strain rate extreme value of 2.8×10⁻³ s⁻¹.