Critical slowing down on acoustic emission characteristics of coal containing methane

The behavior of coal containing methane under triaxial compression with equivalent confining pressure was evaluated using YAW4306 microcomputer-controlled electrohydraulic servo stress testing machine and CTA-1 acoustic emission collecting system. In addition, stress strain and AE signals were monitored simultaneously during the loading process. The critical slowing down theory was introduced to analyze variances and autocorrelation coefficients of AE signal sequences. Experimental results indicate that because of the presence of methane, the compressive strength and elastic modulus of coal decrease with increases in pore pressure. During the failure of coal containing methane under equivalent confining pressure, acoustic emission response with time experiences compaction change, stable growth, accelerated growth, and remission, and a corresponding stress-strain curve is obtained. Adsorption swelling deformation and free gas expansion of the coal samples activate AE signals in the compaction change stage. Under triaxial compression with different confining pressure and pore pressure, the AE characteristics of coal samples exist phenomenon of critical slowing down. Variances and autocorrelation coefficients of AE counts to time sequences, increase before the failure of coal, and therefore, variances and autocorrelation coefficients can act as precursor signals of failure of coal. Compared with mutation points of AE accumulative signals as precursory signals, the time lag between the precursory signals and mutation point shorten, which accounts for 2-10% of the total loading time, in a reasonable range.