Dynamic Progressive Failure and Energy-Driven Damage Evolution of Coal–Sandstone Composite Specimens Under Impact Loading: Coupling Effects of Component Ratio

Fractal Characterization of Stress–Energy Response and Progressive Damage in Coal–Rock Mass Before and After Pre-Splitting Blasting of Hard Roof: From Laboratory Fragmentation to Field Fractures

Hard roof strata in deep coal mines commonly cause rib failure and roof collapse, restricting extraction efficiency. This study employs theoretical analysis, numerical simulation, and field experiments to investigate the fractal evolution of damage in coal–rock mass under loading and blasting disturbances, with the aim of quantifying progressive failure and optimizing roof control. Key findings include: (1) The fractal dimension D of fragment size distribution increases monotonically with loading rate, with fine-particle proportion rising from 48.5% to 52.3%, indicating more thorough fragmentation at higher rates. (2) Load intensity, elastic modulus, and seam thickness govern coal bearing capacity and energy accumulation, with D serving as a quantitative damage indicator. (3) Pre-splitting blasting shifts the stress peak away from the working face, with shear fractures dominating the fracture network and tensile fractures playing a secondary role. (4) Field application at Zhangji Coal Mine (9 coal seam, 7–23.5 m sandstone roof) confirms the effectiveness of segmented fan-shaped borehole pre-splitting blasting in controlling roof behavior; fractal dimension derived from borehole images quantifies fracture propagation. Dynamic adjustment of blasting parameters based on geological core samples is recommended to enhance fracture network complexity and improve roof control efficiency under varying hard rock conditions.

Failure analysis of coal–rock composites with different coal thicknesses under uniaxial loading based on resistivity response and damage evolution

Variations in coal-seam thickness affect the load-bearing structure and electrical response of coal–rock composites, contributing to dynamic disasters in coal-thickness variation zones. To investigate failure mechanisms and resistivity precursors, conductive coal–rock analogue materials with similar mechanical properties were prepared. Composite specimens with rock-to-coal thickness ratios of 1:1, 1:2, and 1:3 were tested under uniaxial compression with synchronous resistivity monitoring, and PFC3D simulations were used to analyze crack evolution and damage accumulation. As the coal proportion increased, peak strength decreased from 18.73 to 15.62 MPa, and failure strain decreased from 2.45 to 2.20%. The total input energy and elastic energy storage capacity also declined, indicating weakened load-bearing and energy storage capacities and earlier damage localization. During loading, resistivity evolves through compaction‑induced reduction, elastic stabilization, pre‑peak rise and failure fluctuation. The failure-stage resistivity increased by 1.27–1.33 times, and the pre-peak resistivity rise preceded the macroscopic stress drop, providing a precursor window for instability identification. PFC3D results reproduced the experimental responses and failure modes, showing that increased coal thickness promoted earlier crack clustering and coalescence, with shear cracks dominating near peak stress. The normalized damage variable based on broken particle bonds corresponded well with resistivity evolution. A resistivity increment index was developed to quantify the transition from stable fluctuation to sustained increase, enabling identification of the precursor stage from stable crack growth to accelerated unstable coalescence. These findings provide a basis for failure-mechanism analysis and resistivity-based warning in coal-thickness variation zones.

Mechanical response and damage evolution mechanism of multi-layer combined coal mass under static and dynamic loading

During underground excavation, structural instability and failure of coal-rock masses are major causes of engineering accidents. This study focuses on the coal mass ahead of the excavation face and divides into a multi-layer composite structure to systematically analyze the mechanical response characteristics of the combined coal mass under static and dynamic loading. Drop hammer impact tests were conducted to investigate the crack propagation and failure morphology of the combined coal mass. The results show that, under static loading, single-layer coal mainly exhibited X-shaped shear failure, whereas the combined coal mass showed V-shaped layered spalling failure. Under dynamic loading, compressive failure dominated in the central region, while shear failure mainly occurred at the four corners, and the damage was more pronounced along the minor axis direction. Two crack patterns were observed on the coal surface, simple radial cracks and complex intersecting radial and circumferential cracks. Under the combined effects of rectangular geometry and impact loading, the radial cracks of the specimen show a structural tendency to preferentially propagate along the minor axis direction. Under impact loading, the combined coal mass exhibited an overall spindle-shaped failure morphology in the axial direction. In the damage zone, the superposition of tensile stress and energy accumulation made the coal mass more susceptible to failure, resulting in a greater number of radial and circumferential cracks. These findings provide a scientific theoretical basis for revealing the incubation mechanisms of coal-rock dynamic disasters and optimizing disaster prevention and control measures under mining-induced disturbance conditions.

Mechanical response and failure mechanisms of sandstone under water–rock interaction: a multi-scale study and constitutive modeling

Study on mechanical response and damage models of sandstone-shale interbedding under different cyclic loading modes

Loading-angle-dependent rate effects on dynamic mixed-mode I/II fracture behavior of coal-measure sandstone

Sandstone roof strata in deep coal mining are frequently subjected to impact disturbances, and their dynamic mixed-mode fracture response is closely related to crack initiation, propagation, and instability of surrounding rock. Previous studies have addressed rate-dependent dynamic fracture and angle-dependent mixed-mode fracture largely in isolation, but how loading rate and loading angle interact to govern the transition from opening-dominated to shear-dominated cracking, and how this coupled behavior manifests across different length scales has not been fully investigated. Therefore, this study conducted dynamic mixed-mode fracture tests on cracked straight-through Brazilian disc sandstone specimens with loading angles of 0°–90° and loading rates of 20–100 GPa/s using a split Hopkinson pressure bar system. High-speed photography, scanning electron microscopy, and three-dimensional fracture-surface fractal analysis were combined to characterize crack propagation, Mode I/II stress intensity factors, effective fracture toughness, and fracture morphology. The results indicate that the dynamic Mode I stress intensity factor decreases monotonically with increasing loading angle and changes sign near 45°, indicating a transition from opening-dominated to shear-dominated fracture. The dynamic Mode II stress intensity factor first increases and then decreases with loading angle, reaching its peak within approximately 45°. The dynamic effective fracture toughness follows a logarithmic relationship with loading rate within the tested range (R 2  = 0.94–0.99), with the strongest rate sensitivity coefficient a ranges from 0.59 to 3.08 across all loading angles and peaks at 3.08 near 45°, indicating the strongest rate sensitivity at the mixed-mode transition. The fracture-surface fractal dimension increases with loading rate and reaches its maximum within the 45–60° interval, where crack branching, grain pull-out, and local fragmentation are most pronounced. These findings demonstrate how loading rate and loading angle jointly govern dynamic mixed-mode I/II fracture in sandstone and provide laboratory-scale evidence for understanding rate-dependent fracture of sandstone roof strata under impact loading.