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An experimental work is presented that aims to improve our understanding of water–rock reaction, under both neutral and acidic conditions, and its effects on the mechanical deterioration of sandstone in Chongqing, southwest China, where a lot of heritage rock statues are carved in the same strata. SCB tests were conducted to evaluate the weakening of fracture toughness of Chongqing sandstone after water and acid erosions, and a high-speed camera was employed to capture its cracking behaviors under different hydrochemical conditions. Microstructural observation was also carried out to investigate the cracking path at a micro-scale, and the chemical analyses of sandstone soaking water in a duration of up to 120 days allows to reveal the evolving process of water–rock reaction. The results show that compared to acid solutions of pH = 3 and 5, the strength weakening effect of distilled water on Chongqing sandstone is more severe. Crack propagation rates captured by the high-speed camera vary between 20 and 60 mm/s in the SCB tests. There is no significant difference in the data for dry and water-saturated samples, and the same for the samples in the two acid fluids. Yet the crack growth rates of the latter two groups are apparently greater than those of the former two. The chemical reaction between water / acid solutions and Chongqing sandstone is mainly pertinent to the cementation and grain boundaries, which weakens grain contacts and promote crack propagation. This mechanism results in a low KIC, long crack length, i.e., more tortuosity along grain boundaries, and high crack propagation rate. [ABSTRACT FROM AUTHOR]

A particle-based discrete element model combined with digital image processing to take into account the grain geometry and size distribution is used to investigate the mechanical and microcracking behavior of Beishan granite (BG), a typical crystalline rock potential for nuclear waste disposal in China. First, the procedure to construct a digital image-based grain-based model is introduced. The unsupervised K-means clustering algorithm is used to determine mineral boundaries, and four micro-parameters are assigned according to the Weibull distribution to reflect micro-heterogeneity of rock. Then, the model is calibrated to the macro-properties of BG obtained from laboratory tests, and consistency between model simulation and laboratory test results of both microcracking processes and failure modes is found. It is shown that intragrain cracks are initiated successively in quartz, feldspar, and mica; and their numbers in quartz and feldspar are much more than that in mica. Crack cluster, forking cracks and cataclastic microcracking are typical in quartz, while the "voids" is found in mica. Grain property heterogeneity has a significant impact on micro-cracking behavior and macro-properties of the model, with greater heterogeneity resulting in a lower uniaxial compression strength and elastic modulus, and a more scattered distribution of microcrack inclination. The modeling procedures presented here provide an effective method for investigating mechanical and damage behavior of granites, with specific consideration of heterogeneity involving the distribution of mineral grains and their mechanical properties. [ABSTRACT FROM AUTHOR]

The effect of grain size heterogeneity on the mechanical and microcracking behavior of Lac du Bonnet (LdB) granite during heating, cooling, and uniaxial compression loading is investigated using a grain-based model. By defining a grain size heterogeneity index H, the heterogeneity induced by variation of grain size distribution can be explicitly incorporated into the grain-based model quantitatively. Five modelled LdB granite samples with different grain size heterogeneity indices ranging 0.19–1.22 are established. The results suggest that the grain size heterogeneity and treatment temperature have critical impacts on the mechanical properties of modelled LdB granite samples. With the increase of temperature and grain size heterogeneity index, the mechanical properties show a decreasing trend. Several empirical formulations are developed to predict the temperature-dependent mechanical properties of modelled LdB granite samples with different grain size heterogeneity indices. Generally, modelled LdB granite samples with greater grain size heterogeneity index has a lower temperature of brittle-ductile transition. In terms of microcracking behavior, both temperature and grain size heterogeneity affect it, including number, density, orientation, and distribution. As the temperature increases, the number and related density of thermal stress-induced cracks increase. Generally, the amount of thermal stress in modelled LdB granite samples during heating increases with the increases of grain size heterogeneity index. The controlling factor of the failure pattern of samples gradually changes from the presence of thermal stress-induced cracks to grain size heterogeneity as the model progresses from relatively homogeneous (H ranging 0.19–0.76) to heterogeneous (H ranging 0.93–1.22). Highlights: Explicitly incorporate grain size heterogeneity induced by grain size distribution into a grain-based model. Reveal the effect of grain size heterogeneity on mechanical properties and microcracking behavior of Lac du Bonnet granite at different temperatures. Propose empirical formulations to quantify the temperature-dependent mechanical properties of Lac du Bonnet granite with grain size heterogeneity index. [ABSTRACT FROM AUTHOR]

Understanding the microcracking mechanisms and associated acoustic characteristics involved in the creep of heterogeneous granite at the grain scale is of high importance in practical engineering. To perform this task, a series of creep numerical simulations were conducted on granite with a single pre-existing open flaw using a grain-based parallel-bonded stress corrosion model. We investigated the effect of flaw inclination angle on the creep behavior and discussed the significance of microstructure. The results suggest that the axial strain, secondary creep strain rate and time-to-failure all depend strongly and nonlinearly on the flaw inclination angle. Creep failure is mainly induced by the initiation, propagation and coalescence of microcracks, and the dominant types are inter-grain tension cracks and intra-grain tension cracks. The b value, source types and distribution of acoustic emission events are associated with the flaw inclination angle and driving-stress ratio. Tensile sources account for the majority of the total number of acoustic emission sources, while shear and implosive acoustic emission sources are larger in magnitude. The model with higher heterogeneity creeps at a faster rate under the same applied constant axial stress. The higher number of acoustic emission events at each creep stage of such model indicates that the relatively heterogeneous microstructure provides more paths for the growth of microcracks during creep loading. As the heterogeneity increases, the secondary creep strain rate rises and the b value decreases. The position of the final macroscale shear band of pre-cracked granite is more affected by pre-existing flaw compared to its microstructure. Highlights: An extended grain-based parallel-bonded stress corrosion (GB-PSC) model was proposed to simulate the creep and associated acoustic behavior of granite. Voronoi tessellations to provide a more realistic representation of the microstructure of granite. Effects of open flaw inclination angle on creep behavior of pre-cracked granite were systematically investigated and the significance of microstructure were discussed. The acoustic emission characteristics of mineral boundaries and interiors during creep loading were obtained using the moment tensor analysis. [ABSTRACT FROM AUTHOR]

Granite is a heterogeneous material characterized by a significant population of mechanically distinct grain boundaries, which exert a substantial influence on crack propagation. An extended grain-based model was developed by integrating the geometric heterogeneity, represented by Voronoi tessellations, and the mechanical heterogeneity of grain boundary, as described by Weibull distribution. This study numerically investigates the influence of mechanical heterogeneity in grain boundary on the mechanical properties and microcracking behavior of semi-circular bend granite samples under mode I loading. The mechanical heterogeneity of grain boundary was assessed by independently varying the heterogeneity index, defined by the shape parameter m in the Weibull distribution. The findings indicate that the force–displacement curve and fracture toughness are primarily influenced by the parallel bond tensile strength and cohesion of grain boundary contacts. When the value of m in Weibull distribution is between 1.5 and 2, a compromise between failure patterns and diverse grain boundaries characterization can be reached. Considering the heterogeneous mechanical properties of grain boundary numerically is able to simulate the local stress concentration of the real rocks, thus accelerating the generation of microcracks and reflecting the brittleness of rock failure. Otherwise, the fracture toughness of the rock will may be overestimated. Generally speaking, models that consider grain boundary heterogeneity will produce larger fracture process zone and microscopically complex crack propagation patterns during loading, and these phenomena are more consistent with experimental observations than those models in which grain boundaries are homogeneous. Highlights: The mechanical heterogeneity of grain boundary in granite is quantitatively characterized by the Weibull distribution; The heterogeneous grain boundary could favor the local stress concentration, thus accelerating the generation of microcracks; Considering both geometric heterogeneity of granite and mechanical heterogeneity of grain boundary result in the generation of larger fracture process zone and microscopically complex crack propagation patterns under mode I loading. [ABSTRACT FROM AUTHOR]