Along with the advance of the working face, coal experiences different loading stages. Laboratory tests and numerical simulations of fracture and damage evolution aim to better understand the structural stability of coal layers. Three-dimensional lab tests are performed and coal samples are reconstructed using X-ray computer tomography (CT) technique to get detailed information about damage and deformation state. Three-dimensional discrete element method (DEM)-based numerical models are generated. All models are calibrated against the results obtained from uniaxial compressive strength (UCS) tests and triaxial compression (TRX) tests performed in the laboratory. A new approach to simulate triaxial compression tests is established in this work with significant improved handling of the confinement to get realistic simulation results. Triaxial tests are simulated in 3D with the particle-based code PFC^3D using a newly developed flexible wall (FW) approach. This new numerical simulation approach is validated by comparison with laboratory tests on coal samples. This approach involves an updating of the applied force on each wall element based on the flexible nature of a rubber sleeve. With the new FW approach, the influence of the composition (matrix and inclusions) of the samples on the peak strength is verified. Force chain development and crack distributions are also affected by the spatial distribution of inclusions inside the sample. Fractures propagate through the samples easily at low confining pressures. On the contrary, at high confining pressure, only a few main fractures are generated with orientation towards the side surfaces. The evolution of the internal fracture network is investigated. The development of microcracks is quantified by considering loading, confinement, and structural character of the rock samples. The majority of fractures are initiated at the boundary between matrix and inclusions, and propagate along their boundaries. The internal structure, especially the distribution of inclusions has significant influence on strength, deformation, and damage pattern.

The infrared radiation response characteristics and mechanisms of sandstone samples under uniaxial compression were investigated using infrared thermal imaging and strain measurement techniques. The stress–strain curves were divided into different damage stages (i.e., crack closure, elastic deformation, stable crack growth, unstable crack growth, and post-failure) by the damage thresholds determined using the transverse and vertical strains. Experimental results show that the infrared radiation response characteristics of sandstone include the superposition of different mechanisms, which lead to either temperature increase or decrease. The response mechanisms of infrared radiation vary depending on the damage stage. During crack closure and elastic deformation stage, temperature variations are mainly governed by thermoelastic effects and the nearly adiabatic compression of air in the pore space. In the stable and unstable crack growth phase, the temperature variations are dominated by two reverse acting processes: heat production by frictional sliding and cooling by the expansion of gases in pore spaces. This leads to complex inhomogeneous radiation and consequently temperature distribution at the sample surface with local hotspots.

This paper presents Cerchar scratch tests performed on sandstone samples with a constant thrust force of 70 N. The scratch test is reproduced using discrete element method (DEM) models. Simulated scratching force–(depth-) displacement curves evolve in the same way as those of experimental results. Numerical scratching force is also comparable with laboratory results. All these accordance indicate that the presented model can represent the experiments. Microscopic observation of the scratch indicates pullout of grains and the associated tensile failure dominates the scratching process. Particle detachments and microcrack development in numerical simulations also verified this tensile-dominated failure mode. Compared with reported rock cutting studies with fixed cutting depth mode, the cutting force shows a much smaller variation in this fixed thrust force mode. This finding indicates that the applied thrust force plays a significant role in the cutting process, rock failure, and the tool–rock interaction. Factors that may result in a much larger scratch depth in simulations are carefully discussed. A parametric study was conducted to investigate the effect of scratching velocity, amplitudes of indentation velocity, particle sizes, and applied thrust load on the scratch behavior.