Investigation of the acoustic emission and fractal characteristics of coal with varying water contents during uniaxial compression failure

Abstract

To investigate the effect of water on the mechanical properties and acoustic emission (AE) characteristics of coal in the failure and deformation processes. Coal samples of different content were subjected to uniaxial compression tests and AE signals were monitored. The characteristics of the AE signals were further analyzed using fractal analysis. The results show that saturated coal samples have substantially reduced mechanical properties such as uniaxial compressive strength (UCS), dissipation energy, peak stress, and elastic modulus. Under loading, stress–strain curves are characterized by five distinct stages: (1) compaction; (2) linear elastic; (3) crack stable propagation; (4) crack accelerating propagation; and (5) post-peak and residual stages. Using phase-space theory, a novel Grassberger Procaccia (GP) algorithm was utilized to find the AE fractal characteristics of coal samples in different stages. It is significant to note that AE energy does not exhibit fractal characteristics in either the first or second stages. Contrary to the first two stages, the third stage showed obvious fractal characteristics. Fractal analysis of AE time sequences indicates that fractal dimension values change as stress increases, indicating the initiation of complex microcracks in coal. In the fourth stage, the fractal dimension rapidly declines as the strength reaches its limit, indicating the occurrence of macrocracks. However, fractal dimensions continued to decrease further or increased slightly in the fifth stage. Consequently, the coal begins to collapse, potentially resulting in a disaster and failure. It is, therefore, possible to accurately predict coal and rock dynamic failures and microcrack mechanisms by observing the subsequent sudden drop in the correlation dimension of the AE signals in response to different stages of loading.

Deformation-Failure Characteristics of Coal with Liquid CO2 Cryogenic-Freezing Process: An Experimental and Digital Study

“…(1) Mechanical characteristic parameter test: The synchronous experimental platform of uniaxial compression and acoustic emission are used, investigating the basic mechanical properties of original and treated coal samples under a normal temperature (30 • C). In the uniaxial compression load, the displacement rate is 0.002 mm/s, the acoustic emission system loads are at an interval of 1 µs, and the sampling frequency is set as 1 MHz [46]. The experimental system will automatically record the relative parameters, such as displacement, stress, and strain, as well as acoustic emission signals during the coal deformation-failure process.…”

Research on Damage Mechanism and Mechanical Characteristics of Coal Rock under Water Immersion

“…Under another external test condition (uniaxial compression strength, acoustic feature and failure form), the results of the uniaxial compression test showed that the attenuation of the uniaxial compressive strength and elastic modulus was very sensitive to increases in the water content [26,27]. In terms of acoustic characteristics, Wu et al [28], Chen et al [29], and Ali et al [30] investigated the stress-strain and acoustic characteristics of coal rocks with different water contents, and determined that the peak AE number and cumulative AE number of coal samples decreased with increasing water content, while the peak AE energy and cumulative AE energy of coal samples both tended to decrease with increasing water content. Gao et al [31] studied the effects of temperature and immersion on the AE and electromagnetic radiation signals of damage under coal and rock loading.…”