Abstract
Acoustic emission (AE) can reflect the dynamic changes in a material’s structure, and it has been widely used in studies regarding coal mechanics, such as those focusing on the influence of loading rate or water content change on the mechanical properties of coal. However, the deformational behavior of coals with various strengths differs due to the variation in microstructure. Hard coal presents brittleness, which is closely related to certain kinds of geological disasters such as coal bursts; soft coal exhibits soft rock properties and large deformation mechanical characteristics. Therefore, conclusions drawn from AE characteristics of a single coal sample have application limitations. This paper studies the deformation patterns and AE characteristics of coals with different strengths. A uniaxial compression experiment was carried out using coal samples with average uniaxial compressive strengths of 30 MPa and 10 MPa; the SAEU2S digital AE system was used to measure the AE counts, dissipation energy, and fracturing point distributions at each deformation stage of the different coals. The results show that the bearing capacity of hard coal is similar to that of the elastic stage and plastic deformation stage, but it may lose its bearing capacity immediately after failure. Soft coal has a relatively distinct stress-softening deformation stage and retains a certain bearing capacity after the peak. The AE counts and dissipation energy of hard coal are significantly higher than those of soft media, with average increases of 49% and 26%, respectively. Via comparative analysis of the distribution and development of internal rupture points within soft coal and hard coal at 15%, 70%, and 80% peak loads, it was observed that hard coal has fewer rupture points in the elastic deformation stage, allowing it to maintain good integrity; however, its rupture points increase rapidly under high stress. Soft coal produces more plastic deformation under low loading conditions, but the development of the fracture is relatively slow in the stress-softening stage. We extracted and summarized the AE characteristics discussed in the literature using one single coal sample, and the results support the conclusions presented in this paper. This study subdivided the deformation process and AE characteristics of soft and hard coals, providing a theoretical guidance and technical support for the application of AE technology in coal with different strengths.
Highlights
When a rock mass is subjected to external load, the internal stress of the material is redistributed and its structure is altered, causing stress concentration in the rock mass and sudden energy release accompanied by elastic waves
More than 80% of the roadway is excavated in the coal seam, and, as such, the stability and failure mechanism of the coal mass are critical to ensuring roadway stability
The average compressive strength of the coal samples from the former is over 30 MPa, while that of the coal samples from the latter is around 10 MPa, and, as such, they represent hard and soft coals in China, respectively, presenting typical acoustic emission (AE) reflections in the process of deformation
Summary
When a rock mass is subjected to external load, the internal stress of the material is redistributed and its structure is altered, causing stress concentration in the rock mass and sudden energy release accompanied by elastic waves. This phenomenon is called acoustic emission (AE), which has been implemented as an effective technique to study the deformation and failure mechanism of rock, coal, and other geo-materials [1,2]. As the stress path induced by mining activities is a key factor affecting coal mass stability [15,16,17], AE technology has been used to detect the stress state of coal mass [18,19]
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