Quantitative Identification of Mesoscopic Failure Mechanism in Granite by Deep Learning Method Based on SEM Images

Since each rock type represents different deformation characteristics, prediction of the damage beforehand is one of the most fundamental problems of industrial activities and rock engineering studies. Previous studies have predicted the stress–strain behaviors preceding rock failure; however, quantitative analyses of the progressive damage in different rocks under stress have not been accurately presented. This study aims to quantify pre-failure rock damage by investigating the stress-induced microscale cracking process in three different rock types, including diabase, ignimbrite, and marble, representing strong, medium-hard, and weak rock types, respectively. We demonstrate crack intensity at critical stress levels where cracking initiates (σci), propagates (σcd), and where failure occurs (σpeak) based on scanning electron microscope (SEM) images. Furthermore, the progression of rock damage was quantified for each rock type through the fractal analyses of crack patterns on these images. Our results show that the patterns in diabase have the highest fractal dimensions (DB) for all three stress levels. While marble produces the lowest DB value up to σci stress level, it presents greater DB values than those of ignimbrite, starting from the σcd level. This is because rock damage in ignimbrite is controlled by the groundmass, proceeding from such stress level. Rock texture controls the rock stiffness and, hence, the DB values of cracking. The mineral composition is effective on the rock strength, but the textural pattern of the minerals has a first-order control on the rock deformation behavior. Overall, our results provide a better understanding of progressive damage in different rock types, which is crucial in the design of engineering structures.

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Quantifying the Rock Damage Intensity Controlled by Mineral Compositions: Insights from Fractal Analyses

Hydraulic fracturing is an effective means to increase natural gas production, and alternating fracturing is a special fracturing method. Compared with sequential fracturing, alternating fracturing is easier to form complex fracture networks. In order to investigate the generation mechanism of complex fracture networks during alternate fracturing, we conducted several multi-stage hydraulic fracturing experiments indoors with different fracturing sequences. Based on the cohesive element method, a numerical hydraulic fracturing model was established, from which the stress interference affected by the fracture spacing and horizontal in-situ stress were analyzed. The results show that: Alternate fracturing is more effective in controlling fracture geometry than sequential fracturing, which is also advantageous in shortening fracture spacing and avoiding the penetration of adjacent fractures. The critical factors affecting the branch fractures are the fracture spacing and the number of main fractures. Alternating fracturing is easier to produce branch fractures perpendicular to the main fracture. The stress interference among main fractures will change or reverse the stress field. The branch fractures closed due to the competition mechanism at the fracture tip are reactivated and extended perpendicular to the main fracture. Alternate fracturing is beneficial in activating microfractures near the main fractures, especially under a larger stress difference. The number of micro-fractures activated by alternating fracturing was more 14% and 21% than the sequential fracturing with the stress difference of 3 MPa and 11 MPa. The increase in stress difference and fracture spacing will decrease the fracture fractal dimension, and the fractal dimension of alternating fracturing is 0.054 more than that of sequential fracturing. 

Generation mechanism and influencing factors of fracture networks during alternate fracturing in horizontal wells

Cementitious composites reinforced with carbon nanotubes (CNTs) offer notable benefits compared to conventional cement-based materials in terms of durability and strength. Despite these advantages, the widespread implementation of CNTs in cement-based grouting engineering has not been carried out due to the limitations of dispersion issues. In order to facilitate the progression of carbon nanotubes (CNTs) from laboratory experiments to engineering applications, we proposed a method of using amino-functionalised functional groups to coat CNTs on the fly ash (FA) particles’ surfaces to assist the dispersion and prepare cost-effective, low-carbon emission, energy consumption-saving and high-performance cementitious slurry. The experimental results exhibit that compared with plain cement-based grouting materials, incorporating 0.023 wt% CNTs and 14.3 wt% FA can reinforce the compressive and tensile strength by 14.2–25.8% and improve the workability by about 21.0%. The application of coated CNTs induces nucleation and pore-infilling effects within the interfacial transition zone (ITZ) of the cementitious composites. This action leads to a reduction in ITZ width and a decrease in crack occurrence within the ITZ. Consequently, these effects contribute to an augmentation in the consolidation strength of the slurry. Acoustic emission and fracture surface fractal analysis further reveal that incorporating CNTs enhances the resist-loading ability of the cement matrix by increasing its energy absorption capability. The innovative coating method reported in this work is expected to facilitate the future application of CNTs in practical grouting engineering. 

Enhancing cementitious grouting performance through carbon nanotube-coated fly ash incorporation

Dynamic failure characteristics of high-strength concrete and high-strength rock based on fractal theory

Fractal characteristics of rocks and mesoscopic fractures at different loading rates

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Fractal characteristics of rocks and mesoscopic fractures at different loading rates