Characteristics and evolution mechanism of acoustic emission time-frequency signal during coal failure process
(2.School of Civil and Resource Engineering, University of Science & Technology Beijing, Beijing, China 100083)
(3.School of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong Province, China 266590)
(4.School of Civil Engineering, North China University of Technology, Beijing, China 100144)
(5.School of Emergency Management and Safety Engineering, China University of Mining & Technology (Beijing), Beijing, China 100083)
(6.School of environment, Liaoning University, Shenyang, Liaoning Province, China 110136)
【Abstract】The time-frequency domain characteristics of an acoustic emission signal and their essential relationship with mechanical properties of coal are the basis for predicting and warning the instability of coal. On the basis of acoustic emission monitoring tests for the compressive failure of coal with different partings and original cracks, the time-frequency domain evolution of an acoustic emission signal is discussed in detail combined with the digital signal theory, rock mechanics and other related approaches, and the wavelet transform method is introduced. The mechanical expressions of amplitude and frequency of a stress wave are established, which is induced by the elastic energy released from crack growth. The results showed that with the increase in weak partings or cracks in coal, its strength and elastic modulus decrease. The softening characteristic is obvious in the post-peak phase, and there is a signal surge point at which a signal transforms from a low-amplitude oscillation to a high-amplitude pulse of acoustic emission. The higher strength of coal can lead to the higher amplitude of an energy signal, the more cumulative total energy, the larger amplitude of waveform, the longer interval between the two adjacent peaks of signal waveform, and the fewer small oscillations mixed in the signal. The wavelet basis functions of db5 and sym2 have the highest similarity with the time-domain waveforms at the surge point and peak point respectively, which is more suitable for the study of acoustic emission signals of coal. The main frequency band of signal is 0–70 kHz. The lower loading stress level indicates the wider signal frequency distribution, and the signal band distribution gradually moves to the main frequency with the increase in stress. The variation range of stress-wave amplitude is determined by the elastic modulus and the crack propagation rate. The crack size determines the variation trend of amplitude and frequency, and the crack propagation rate is a key parameter to determining the stress wave frequency. Then three parameters affect the time-frequency characteristics of acoustic emission signals. Based on the experimental results, the qualitative description of the frequency and amplitude of AE signals with crack characterization parameters is established, which provides a theoretical basis for improving the accuracy of the AE monitoring method. Therefore, the quantitative application of the theory is the focus in the future research.
【Keywords】 acoustic emission; coal failure; wavelet time-frequency transform; crack propagation; stress wave;
(Translated by HAN R)
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