Cross-interface propagation behavior of tension cracks during Brazilian experiments of coal–concrete bi-materials with different cementation strengths

Understanding the cross-interface propagation behavior of tension cracks during hydraulic fracturing in coalbed methane exploitation involving multiple coal seams is crucial for enhancing fracturing efficiency. This study investigated the propagation behavior of cracks at the coal–concrete interface under varying interface conditions through numerical simulations and laboratory experiments. The crack propagation patterns were analyzed, and mechanical parameters were determined from laboratory experiments conducted at seven different angles. Based on the experimental results, the microparameters for the Particle Flow Code 2D were calibrated, and a numerical model for the Brazilian test of coal–concrete bi-materials using a flat-joint model was developed. Numerical simulations were conducted to examine the crack propagation patterns and processes at seven interface angles and fourteen interface cementation strengths. The results indicated that the cross-interface propagation behavior of tension cracks during the Brazilian test of coal–concrete bi-materials is significantly influenced by the interface angle and cementation strength. Crack initiation points tend to occur at the interface, within the coal, or simultaneously, depending on the interface cementation strength. The propagation behavior of cracks at the interface can be classified into three main types: pass-through, propagation along the interface, and deflection at the interface, depending on the interface angle and cementation strength. Furthermore, this study provides valuable insights for guiding hydraulic fracturing practices.