Junjun Liu

Laser-induced rock breaking is governed by transient thermal gradients and the resulting crack evolution, but the link between surface temperature fields and three-dimensional fracture remains unclear. Cylindrical sandstone specimens were irradiated at 250–1000 W for 10–30 s using a 4 mm fixed laser spot. Time-resolved imaging, infrared thermography, end-surface/sidewall observations, and polarized-light microscopy were combined to track fracture evolution. Increasing laser power advanced the first visible macroscopic crack from approximately 20 s at 250 W to 6, 5, and 4 s at 500, 750, and 1000 W, respectively. Cracks initiated near the laser-affected zone, propagated radially across the end surface, and selected dominant cracks continued along the sidewall into the specimen depth, forming a radial–depthwise fracture pattern. At 500 W, the central surface temperature approached 920 °C after 20–30 s, while the 200–700 °C zone continued to expand outward. Petrographic observations indicate that crack paths are strongly controlled by grain boundaries, cemented interfaces, and mineral-scale heterogeneity. The results show that laser-induced sandstone fracture is a progressive multiscale process driven by nonuniform thermal stresses and modulated by the heterogeneous grain–cement framework.

Intact deep sandstone cores are scarce, heterogeneous with poorly repeatable, limiting systematic laboratory studies of deep-rock mechanical behavior. This study selected dense sandstone recovered from 1050 m in the Pingdingshan mining area as the prototype system for the development of a targeted-sandstone-constrained screening strategy for simulated deep sandstone. The strategy integrates mineral-composition matching, orthogonal mixture design, mechanical testing, PCA-based comprehensive similarity evaluation, GMM classification, stress–strain curve comparison and fracture-morphology verification. Candidate materials were prepared using a cement–silica-fume matrix with quartz sand, K-feldspar, Na-feldspar, nanoclay and superplasticizer. Results show that the water–binder ratio dominated uniaxial compressive strength, tensile strength and elastic modulus, whereas superplasticizer and nanoclay had secondary effects. The PCA-based index assigned weights of 52.9%, 29.2% and 17.9% to uniaxial compressive strength, elastic modulus and brittleness index, respectively. Among the 25 mixtures sampled, S5 showed the highest mechanical similarity, with a simulation index of 71.65% and a stress–strain curve similarity of 0.958. GMM clustering identified S5 and S10 as the closest high-strength, high-stiffness and high-brittleness group, while S10 better reproduced natural crack geometry. These results indicate that the optimal simulated sandstone depends on the target response and provide a task-oriented route for reproducible simulated deep sandstone.

Studying the rough structure characteristics of rock fracture surfaces under microwave irradiation is of a great significance for understanding the rock-breaking mechanism. Therefore, this work takes fracture surface as the research object under three failure modes: microwave irradiation, uniaxial loading and microwave-uniaxial loading. The undulation and roughness are used to describe the morphological characteristics of the fractured surface. Research shows that the fracture surface under microwave irradiation is the smoothest, which exhibits undulation and rough fractal dimension in the range of 1–7[ ]mm and 2.0005–2.0657, respectively. The fractal anisotropy of the fracture surface is the most obvious under microwave irradiation. The roughness along the fracture surface is about 1.02 times that is perpendicular to the fracture direction. In the perspective of size effect characteristics, the roughness of the fracture surface under microwave irradiation shows a positive correlation with the selected size. The results provide a certain basis for interpreting the mechanism of microwave rock breaking.

Rock breaking is one of the most basic issues in deep underground engineering. Water plays an important role in the rock response under microwave radiation. Consequently, microwave radiation experiments using red sandstone with different water contents were conducted. The damage characteristics and ultrasonic properties of red sandstone after microwave radiation were primarily investigated, and the representative conclusions were drawn as follows: With the increase in water content, the time of complete formation of the rupture surface of the rock sample gradually decreased, and the decreasing range gradually increased. When the fracture surface is completely formed, the samples with a higher water content have more powdery rock cuttings and less surface roughness. The damage degree of the samples does not increase significantly with the increase in the water content when the sample is radiated at the same time. As the microwave radiation time is increased, the damage degree of the sample will increase significantly. Through the ultrasonic velocity test, it can be suggested that the sample exhibits obvious zonal damage characteristics under the action of a microwave. Generally speaking, it is a very effective means of improving the degree of microwave attenuation of the rock by increasing the water content of the rock mass.

Fractures are widely distributed in coal, and studying the seepage characteristics of fluids in fractures is of great significance for unconventional natural gas extraction and prevention of gas disasters. In this work, based on the W-M fractal function, a Y-shaped fracture model with different roughness is established. The seepage characteristics of Y-shaped fracture with different roughness are interpreted from two perspectives. Firstly, the seepage law of the Y-shaped fracture with the same roughness is studied. Secondly, the fluid competitive diversion capacity caused by the difference of branch fracture roughness is discussed. The results show that the fracture roughness is an important factor affecting the seepage characteristics of the fracture. When the roughness of the fracture is identical, the outlet flow rate, velocity, and Re are all positively correlated with pressure, and the flow regime is unchanged. The increase in the fracture roughness will lead to a significant flow rate and momentum loss, resulting in maximum loss up to 45.44, besides, enhance the flow resistance of the fracture, Re increasing by 771, 713, 489, and 355, respectively, at four patterns. And there is a threshold D_M between 1.1 and 1.2 so that the major influencing factor on hydraulic conductivity changes. In addition, the roughness difference between the branch fractures of the Y-shaped fracture is the key factor to control the fluid competitive diversion capacity. The larger the roughness difference, the greater difference in flow velocity, and the more significant change in flow rate proportion, which proportion differs by 4, 44, and 54, respectively, and the stronger smooth branch fracture competitive diversion capacity, and the lower the rough branch fracture hydraulic conductivity. As the inlet pressure increases, the two branch fractures Re gradually increases but Eu decreases, which Eu in smooth branch fracture is smaller is 15, 19, 20, and 21 smaller than that of rough branch, respectively, and the rough branch fracture competitive diversion capacity is weakened. Both roughness difference and inlet pressure will affect the competitive diversion capacity of the Y-shaped fracture. These results are expected to provide new insights for the exploitation of underground fluid resources.