The effect of cleat structure on mechanical properties of coal based on three-dimensional printing technology

The mechanical properties and failure characteristics of coal have a significant impact on the efficiency of coalbed methane extraction. Cleat structures, which are widely distributed in coal reservoirs, exert a notable influence on the mechanical behavior of coal. In this study, coal-like specimens with various cleat structures were fabricated using three-dimensional printing technology, and uniaxial compression tests were conducted to investigate the effects of cleat quantity, cleat angle, and cleat combination on the mechanical behavior of coal. The experimental results reveal that as the cleat quantity increases, both the uniaxial compressive strength (UCS) and elastic modulus (E) of the specimens exhibit a decreasing trend. With increasing cleat angle, the UCS and E follow a V-shaped trend, initially decreasing and then increasing. Specimens with cleat angles of 0° and 30° primarily undergo compressive failure, while those with cleat angles of 45°, 60°, and 75° predominantly experience shear failure along the cleats. At a cleat angle of 90°, tensile splitting failure occurs. Combination cleats alter the stress distribution and load transfer paths, leading to enhanced crack propagation and coalescence during loading, which significantly reduces the load-bearing capacity of the specimens. The maximum acoustic emission (AE) energy shows a negative correlation with increasing cleat quantity, while the frequency and complexity of AE events also increase. The cleat angle has a significant nonlinear effect on the maximum AE energy, which fluctuates with increasing angles. The presence of a combination increases the compaction space required during loading, resulting in enhanced AE activity during the initial loading phase. In the early to mid-loading stages, the speckle deformation field exhibits indistinct zonal characteristics, with microcracks initiating on both sides of the preexisting cleats. As the peak stress is approached, the speckle deformation field displays clear zonal characteristics, with an increase in the number of microcracks and their coalescence into macroscopic fractures. In the post-peak stage, the displacement on both sides of the cleats further increases, leading to the complete formation and propagation of macroscopic fractures.