Brazilian Splitting Test Research Articles

An experimental study on mechanical properties and fracturing characteristics of granite under high-temperature heating and liquid nitrogen cooling cyclic treatment

Liquid nitrogen (LN2) fracturing is a promising technology for efficient development of hot dry rock reservoir geothermal resources. To investigate the cracking mechanism of LN2 on high-temperature granite, heating and LN2 cooling cyclic treatment were applied to granite samples. Based on Brazilian splitting test, and different methods were employed to describe the crack propagation and failure mechanism, including acoustic emission monitoring, laser profile scanning, and scanning electron microscopy. The results demonstrate that heating and LN2 cooling cyclic treatment deteriorates the mechanical properties of granite. Specifically, after 20 cycles, the average P-wave velocity of the samples decreases by 34%, with the amplitude of the waveform decreasing from 130 dB to 50 dB. Moreover, the uniformity and integrity of the waveform worsen after cyclic treatment. The tensile strength, peak strain, and brittleness index of granite samples decrease with cycle numbers. From 3 to 20 cycles, the absorbed energy of the samples reduces by 53%, with thermal damage reducing the energy storage limit. The cyclic cooling treatment of LN2 increases the surface roughness and width of crack of the samples, resulting in obvious plastic failure at both ends of the samples. Furthermore, the height parameter Sa and area ratio Sdr of the fracture surface increase by 53.8% and 69.5%, respectively, as the number of cycles increase from 3 to 20. Additionally, the fractal dimension of the granite samples increases from 2.155 to 2.181, indicating an increase in the complexity of the cross-section. Following heating and LN2 cooling cyclic treatment, the failure mode of granite changes from vertical splitting to shear failure, with decreased brittleness and increased plasticity and ductility.

A novel mesoscale modelling method for steel fibre-reinforced concrete with the combined finite-discrete element method

This paper proposes a novel mesoscale modelling method to study the effect of steel fibres on the fracture properties of steel fibre reinforced concrete (SFRC) based on an integrated finite-discrete element method (FDEM) framework. The present method models the steel fibre of SFRC as multiple bar elements, while simulating the cement matrix using triangle elements. A comprehensive bond-slip model is also introduced between the triangle element and bar element, enabling accurate reproduction of the interaction behaviour between the concrete matrix and steel fibre. Additionally, the progressive failure process of cohesive elements in FDEM is considered as crack propagation, confirming the superiority of FDEM from continuum to discontinuum. The results of the Brazilian split test and uniaxial compression test validate the capability of the FDEM framework for mode Ι and mode II fractures, while single fibre pull-out tests and 3-point-bending beam tests provide validation of the effectiveness of the integrated FDEM framework for the mesoscale modelling of SFRC. The discussion also covers employing the mesoscale modelling method to investigate the effect of steel fibre on the fracture properties of SFRC. Owing to its merits in quantitative analysis, the present method can serve as a feasible potential tool for mesoscale modelling analysis and SFRC design.

Cracking process and microstructural characteristics of granite under heating–cooling alternations

Studying the damage correlation mechanism of rocks subjected to heating–cooling alternations is of important significance for deeply understanding the permeability increase mechanism of reservoirs during heat extraction from hot dry rocks and assessing the reservoir stability. By carrying out the uniaxial compression test, Brazilian splitting test, nuclear magnetic resonance test, and scanning electron microscopy test on granite after heating–cooling alternations, the evolution laws of physical and mechanical properties as well as the microstructures of granite were studied. In addition, the multi-scale response characteristics and the damage correlation mechanism of granite after heating–cooling alternations were analyzed. Research results show that, when the heat treatment temperature rises to 600 °C, the physical and mechanical parameters, including the wave velocity, strength, and elastic modulus, reduce abruptly. The heat treatment temperature exerts more significant influences on the physical and mechanical properties of the granite than the heating–cooling alternation cycles. Both the temperature and alternation cycles are beneficial to the microcrack development and propagation, while the temperature more significantly affects the development degree of microcracks. Under the external load, the microscopic defects in the granite after heating–cooling alternations guide the propagation of macrocracks and cause more serious damage to the microstructures in the granite, exhibiting more complex failure modes. The results provide a theoretical basis for fracturing technology and stability evaluation of high-temperature reservoirs.

Effect of grouting on damage and fracture characteristics of fractured rocks under mode I loading

Grouting is widely used to improve the mechanical properties of fractured rocks, but its impact on the fracture behavior of fractured rocks remains incompletely understood. This research aims to investigate the effects of grouting on the damage and fracture characteristics of fractured rocks under mode I loading. We used epoxy resin and ultrafine cement to grout granite and sandstone centre-cracked Brazilian disc specimens. We conducted Brazilian splitting tests on intact specimens, as well as specimens with and without grouting under mode I loading. Acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to study the effect of grouting on rock fracture behavior. Furthermore, we employed an improved fracture toughness calculation method to evaluate the anti-fracture capacity of grouted specimens. The results show that grouting positively reinforces fractured rocks by increasing the shear crack ratio, mitigating the stress concentration effect at the crack tip, and fostering a more complete and stable development of the fracture process zone (FPZ). Materials with superior strength and bonding properties can effectively improve the mechanical performance of fractured rocks. Notably, epoxy resin significantly boosts the strength, deformation adaptability, and anti-fracture capacity of the specimens, outperforming ultrafine cement in terms of grouting effect. The entropy change of AE energy during the loading process indicates that the rock system undergoes three stages of order-to-disorder-to-order. The second stage and the abrupt changes in entropy value can serve as precursors to rock damage and fracture. This research enriches our understanding of how grouting influences the damage and fracture behavior of fractured rocks, providing valuable insights for risk assessment and disaster prediction in geotechnical grouting engineering.

Mechanical and fracture characteristics in center cracked circular discs of coal specimens with different sizes: An experimental investigation

The propagation of cracks on a small scale can result in dynamic disasters, such as rock bursts induced by large-scale fractures in coal seams. To gain insights into the mechanism of fracture-induced disasters in deep coal seams, this study investigates the micro-process and size effect of crack propagation in coal specimens featuring different sizes of center-crack circular discs (CCCD). We conducted Brazilian splitting tests on coal samples with varying dimensions and employed acoustic emission (AE) and digital image correlation (DIC) techniques to quantitatively describe the crack propagation process. Additionally, coal specimens of different diameters were compared for the size effect characteristics of different fracture parameters during the tensile failure process. The results show that as the specimen size increases, quasi-brittle failure characteristics become pronounced, tensile strength gradually decreases and stabilizes, while fracture toughness and fracture process zone length gradually increase and tend to stabilize. The macroscopic cracks in the coal specimens generally extend from the centre of the specimen to both ends along the precracks. Large energy events are mainly concentrated at the lower end of the precracks. As coal specimen size increases, the number of AE events decreases, while the proportion of large energy events gradually increases, and the critical crack opening quantity exhibits a linear increase with specimen size. By conducting the fracture size effect experiment on coal specimens, we extend the relevant mechanical parameters from laboratory-scale tests to field engineering scales. This provides fundamental parameters for the analysis of large-scale fracture process of in-situ coal based on fracture mechanics.

Acoustic emission characteristic of sandstone and sandstone like material under multi-path loading.

Using spline interpolation to select proportions of similar materials, a comparative analysis of the fracturing behavior of sandstone specimens and similar material specimens was conducted through Brazilian splitting tests under multi-path loading. The study revealed that during stepwise loading, both sandstone and similar materials exhibited memory effects and plastic deformation. However, under constant velocity loading, the relationship between force and displacement in sandstone showed linearity after compaction. Employing MATLAB optimization algorithms for the inversion of acoustic emission event information, the distribution of fracture points, and the evolution of cracks were analyzed. The findings indicated that under stepwise loading, both sandstone and similar materials exhibited banded distribution of peak frequencies, with sandstone concentrated in the mid-low-frequency range and similar materials leaning towards the low-frequency range. The amplitude-frequency characteristics of acoustic emission signals suggested that initially, sandstone produced low-frequency, low-amplitude signals. As cracks developed, these signals gradually transformed into high-frequency, high-amplitude signals, ultimately leading to macroscopic failure. The ringing counts and b-values of sandstone displayed an approximate "W" shape distribution, with a subsequent decrease in b-values during final failure. In contrast, the acoustic emission counts were inversely related to b-values. Similar materials exhibited slightly more acoustic emission counts than sandstone, with relatively lower b-values. The crack development process of both sandstone and similar materials was confirmed through these observations. From the perspective of section initiation and local damage, sandstone and similar materials exhibited similar failure characteristics. The proportions of quartz sand: cement: water = 9:1:0.9 in similar materials demonstrated the most similar characteristics to sandstone in terms of mechanical loading, acoustic emission features, and failure morphology. This suggests that these similar materials can be used as substitutes for sandstone in analogous simulation experiments. The study provides theoretical support for understanding rock fracture mechanisms, offers guidance for the selection and proportioning of similar materials, and holds significance for predicting and controlling rock fracture behavior in engineering applications.

Experimental study on the effects of chemical or microwave treatment on the tensile strength of hot dry rock

The Brazilian disc (BD) specimens were heated to high temperatures up to 100–500 °C followed by immersion in chemical solutions or irradiation in a microwave oven. The Brazilian splitting tests were then performed at high temperatures, and acoustic emission (AE) events were recorded. Meanwhile, the longitudinal wave velocity and open porosity of the rocks were measured, and the scanning electron microscope (SEM) was applied to identify the influence of chemical or microwave treatment on the microstructure in specimens. The experimental results show that: (1) the chemical or microwave treatment exacerbates the damage to the heat-treated rocks and that either treatment has significant effect on heat-treated rocks with diverse lithologies. (2) The specimens with a high proportion of soluble minerals are chemically dissolved and the specimens with a high percentage of powerful microwave absorption minerals were irradiated by the microwaves, resulting in lower longitudinal wave velocity, lower tensile strength, higher open porosity, and more severe microstructural damages. (3) Rock tensile strength is closely related to the breakdown pressure of hydraulic fracturing, implying that chemical or microwave-assisted fracturing techniques have significant impact on various hot dry rocks, and these auxiliary fracturing techniques considering rock lithology are novel and important for EGS projects.

Influence of microwave irradiation and water-based cooling on the fracturing behavior and failure mode transition of CSTBD granite

Utilizing microwave heating in conjunction with water-based cooling technology can effectively reduce the fracture toughness of hard rocks. However, the fracturing characteristics and failure modes of hard rocks following microwave heating and water-based cooling remain undisclosed. Granite, being one of the most common types of hard rocks, was selected for this study, employing Cracked Straight Through Brazilian Disc (CSTBD) specimens for laboratory testing. CSTBD specimens with various crack angles (0, 30, 60, and 90°) underwent treatment with varying microwave durations (30, 60, 180, 300, and 420 s). Subsequently, Brazilian splitting tests were conducted on the damaged specimens. To simulate the entire process of microwave heating and mechanical testing of CSTBD specimens, we developed a coupled Finite Element Method – Distinct Lattice Spring Model (FEM-DLSM) approach. The results indicate that microwave heating with water-based cooling significantly influences the fracture toughness of CSTBD granite. Mode-I fracture toughness decreases with microwave heating temperature at a low crack angle (pure tensile failure and tensile–shear mixed failure) and increases at a high crack angle (compressive–shear failure and compressive failure). Mode II fracture toughness decreases with microwave heating temperature at any crack angle. Microwave heating with water-based cooling causes a decrease in the transition angles from tensile–shear failure to compressive–shear failure and from compressive–shear failure to compressive failure. Thermally induced microcracks in the rock specimen are responsible for changes in fracture toughness and failure mode transitions. After microwave heating and water-based cooling, numerous microcracks are induced in the rock interior. While macroscopic cracks predominantly control failure, growing microcracks, by weakening the rock matrix, shift the initiation position of the failure crack from the tip of the macroscopic crack to other positions, following the principle of the weakest failure path.

Multiscale characterization of carbonate-rich shale: From micro structure to macro mechanical properties, a case study in Hongxing Block, China

As a new growth point of shale gas production, carbonate-rich shale has recently attracted great interest. However, gas production from this reservoir was poor since the properties of the carbonate-rich shale have not been systematically studied. This is an urgent need to be addressed. Downhole cores collected from an exploration well were analyzed by micro-structure experiments (X-ray diffraction, mineral quantitative evaluation, low-pressure nitrogen adsorption, and mercury intrusion piezometry test) and macro-mechanical tests (triaxial compression, Brazilian splitting, and fracture toughness test) to obtain multiscale characterization of carbonate-rich shale. The intrinsic relationship between the microstructural and mechanical characteristics of this reservoir has been revealed. The results showed that (1) The content of carbonate minerals exceeds 30%, just slightly lower than that of quartz. Calcite has large particle sizes and is closely cemented with quartz to form a dense rock skeleton. (2) The maximum adsorption volume, specific surface area, total pore volume and average pore diameter values are poor, which is unfavorable for shale gas accumulation. Limited porosity and permeability and great tortuosity can be obstacles to the release of accumulated gas. (3) High peak deviatoric stress, Young's modulus, and Poisson's ratio were contributed by dense rock skeleton. (4) Narrow maximum main fracture width is not favorable to place proppants and further to create effective gas transport channels. (5) High tensile strength and fracture toughness enhance high breakdown and extension pressure. These comprehensive insights are of great significance for both the exploration and development of carbonate-rich shale reservoirs.

Influence of granulated graphite on fracture behavior of MgO–C refractories based on Brazilian splitting test with digital image correlation method and acoustic emission technique

Flake graphite is prone to preferential orientation distribution owing to its flat geometric structure, resulting in significant anisotropy of the physical properties of MgO-C refractories, thus affecting their service performance. The preferential orientation of flake graphite within the material can be suppressed by granulation. In this study, the influence of granulated graphite on fracture behavior of MgO-C refractories was investigated based on the Brazilian splitting test combined with digital image correlation method and acoustic emission technique. The thermal shock resistance of the MgO-C refractories was compared using oil quenching method. The results showed that in the Brazilian splitting test, MgO-C refractories exhibited peak values for tensile strength, fracture energy, and maximum horizontal strain at fracture when θ=0º (θ: angle between loading and shaping direction), followed by a subsequent decrease with increasing θ. Moreover, the introduction of granulated graphite effectively mitigated the anisotropy of mechanical and deformation characteristics of MgO-C refractories. Additionally, the addition of granulated graphite reduced the maximum thermal expansion coefficient and its anisotropy in MgO-C refractories, and promoted the generation of a higher proportion of shear or mixed crack modes, thereby contributing to enhancing the thermal shock resistance of MgO-C refractories.

Numerical Modeling of Progressive Damage and Failure of Tunnels Deeply-Buried in Rock Considering the Strain-Energy-Density Theory

Exploring the rock failure mechanism from an energy perspective is crucial for ensuring the safe construction of tunnels under complex geological conditions. In this study, a progressive damage and failure model of rock elements is established using the strain-energy-density theory based on the thermodynamic theory. Specifically, the rock elements are considered to have failed when the strain energy density absorbed by the element is greater than the critical strain energy density. Besides, the damage evolution of rock elements is reflected through the reduction of elastic modulus, until the element only has a certain residual strength. Based on the above theory, the calculation program of rock damage and failure is developed in FLAC3D using the FISH language. The validity of the method for simulating the process of rock damage and failure is verified through the numerical simulation of Brazilian splitting tests. Finally, the model was applied to the overload test of the geo-mechanical model of the Liangshui Tunnel on Lanzhou-Chongqing Railway. The comparison between the numerical simulation and the test results has not only confirmed that the feasibility and accuracy of the model in simulating the progressive failure process of tunnel surrounding rock under high ground stress, but also its ability to visually display the damage degree, failure scope and evolution process of the surrounding rock. The research findings are of great significance in ensuring the safe construction of tunnel and will promote the efficient development of the underground engineering construction.

Influence of different cooling treatments on the mechanical properties of granite under Brazilian splitting test by grain‐based model 3D

AbstractDuring the operation of an enhanced geothermal system (EGS), the reservoir rock undergoes different cooling treatments based on the distance to the surface rock. The tensile capacity of reservoir rocks is a key parameter in estimating the parameters of the hydraulic fracturing process. Therefore, the gain‐based model 3D (GBM3D) was constructed to explore the micro behavior of granite under the heating/cooling process and the Brazilian splitting test. The numerical results demonstrate the ability of GBM3D to reproduce the grain interlocking and rotational resistance behavior, making it applicable to simulate the mechanical behavior of granite under uniaxial compression and the Brazilian splitting test. Temperature and cooling rates have significant effects on the stress‐strain curves. With the increase in temperature, the curves after peak strength translate from brittle to ductile, and the turning point of water‐cooled granite is lower than that of air‐cooled granite. When more flaws are present in the specimen, micro‐cracks tend to occur along the loading axial near the center of the specimen. This result can be used to explain the defect in numerical simulation, where cracks are easy to develop near the loading position under the Brazilian splitting test. This work can provide recommendations for hydraulic fracturing.

Mechanical properties and fracture surface roughness of thermally damaged granite under dynamic splitting

AbstractIn order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting, dynamic Brazilian splitting tests were conducted on granite samples after thermal treatment at 25, 200, 400, and 600°C. Results show that the dynamic peak splitting strength of thermally damaged granite samples increases with increasing strain rate, showing obvious strain‐rate sensitivity. With increasing temperature, thermally induced cracks in granite transform from intergranular cracks to intragranular cracks, and to a transgranular crack network. Thermally induced damages reduce the dynamic peak splitting strength and the maximum absorbed energy while increasing the peak radial strain. The fracture mode of the thermally damaged granite under dynamic loads is mode II splitting failure. By using the axial roughness index , the distribution ranges of the wedge‐shaped failure zones and the tensile failure zones in the fracture surfaces under dynamic Brazilian splitting can be effectively identified. The radial roughness index is sensitive to the strain rate and temperature. It shows a linear correlation with the peak splitting strength and the maximum absorbed energy and a linear negative correlation with the peak radial strain. can be used to quantitatively estimate the dynamic parameters based on the models proposed.

Acoustic emission and splitting surface roughness of sandstone in a Brazilian splitting test under the influence of water saturation

Water weakens rock masses, which induces a series of engineering geological hazards. In this study, Brazilian splitting and acoustic emission (AE) experiments were conducted on sandstone with different saturation levels (0%, 25%, 50%, 75%, and 100%), and the effect of saturation on the AE and fracture characteristics was investigated. Meanwhile, nuclear magnetic resonance (NMR) test was used to analyze the water absorption and distribution characteristics of the samples with different saturation. Three-dimensional (3D) morphological scanning and scanning electron microscopy (SEM) were performed to analyze the roughness of the splitting surface. The tensile strength of rocks decreased from 3.05 MPa to 0.98 MPa with a moisture content from dry to saturated conditions, which obeyed an exponential function. The hardness of the sandstone decreased from dry to saturated by 56.6%, whereas the plasticity of the high-saturation sandstone increased. The AE counts and cumulative energy distribution decreased from 3880 and 39,550 to 1248 and 476, respectively, from dry to saturated conditions. Based on the Ib value, the rock damage process can be divided into three stages: fluctuation, stabilization, and decline. As the saturation increased, the initial Ib value decreased from 0.1047 to 0.0755. The development of large fractures increased during the splitting of the highly saturated rocks. With increasing saturation, the fracture surface joint roughness coefficient (JRC) and fractal dimension increased. The mean JRC increased from 5.98 to 13.6025. The unsaturated JRC showed an obvious dispersion compared with the dry and saturated sandstones. Combined with micro-macro characterization, it was found that an increase in saturation caused the rock to generate a weak structure. The damage gradually evolved from transgranular to intergranular fractures, leading to the macroscopic performance of rougher splitting surfaces and lower AE.

Comparative analysis of three different types of self-healing concrete via permeability testing and a quasi-steady-state chloride migration test

Concrete is the most used construction material in the world and with the growing world population, the demand for housing and infrastructure keeps increasing. Due to its low tensile strength, concrete cracks easily and these cracks have a negative effect on the durability. Different studies have already been conducted on the development of self-healing concrete, which has the ability to heal its own cracks. Unfortunately, due to a lack of standardized test methods, the boundary conditions in these studies are often different, making it difficult to do an accurate comparison. In this research, three different healing agents are tested using the same test methods under the same boundary conditions: a powder of organic/inorganic composite material (WM), solidified calcium sulfoaluminate capsules (SMU) and bacterial pellets (KU). The healing efficiency of the agents is tested with (1) a low-head water flow test on prismatic specimens cracked in a three-point bending test with subsequent active crack width control, (2) a Korean water permeability test on cylindrical specimens cracked by a Brazilian splitting test, (3) a chloride diffusion test and (4) a quasi-steady-state chloride migration test. The results showed that WM performed superior with respect to the reference mix and the other healing agents in the two water permeability tests and the quasi-steady-state chloride migration. The performance of SMU and KU was depending on the healing condition. When fully saturated during healing, the KU specimens had a good behaviour and SMU performed poorly. For the cylindrical specimens, with the specimen partly immersed in water, SMU and KU had a similar performance which was better in comparison to the reference.

Macro/Microfracture evolution and instability behaviors of high-temperature granite under water-cooling subjected to Brazilian splitting test using the DIC technique.

To investigate the evolution and stability characteristics of granite thermal damage, a series of Brazilian splitting tests is conducted on high-temperature granite samples using digital image correlation (DIC) technology. The results show that the Brazilian tensile strength and P-wave velocity exhibit a clear decline beyond a temperature threshold of 450~600°C, with a linear relationship between them. The presence of micro-cracks alters the stress transfer path, disrupting the stress balance on the Brazilian disc and leading to complex fracture patterns. At temperatures below 450°C, high strain areas and the development of micro-cracks occur at both the upper and lower loading ends of the granite Brazilian disc. However, these phenomena are only observed at the upper loading end when the temperature exceeds 450°C. Thermal cracks also cause changes in the internal structure of rock samples, and temperature variations can affect both the P-wave velocity and tensile strength. In terms of the relationship between P-wave velocity and Brazilian tensile strength (BTS) of high-temperature granite under water cooling, the negative exponential function model proposed in this study fits the experimental data very well.

Effects of heat-treatment on physical and mechanical properties of limestone

The susceptibility of buildings to fire makes it crucial to examine the physical and mechanical (P&M) properties of building stones under high temperature, which is necessary to predict and evaluate the safety and stability of stone structures. In this study, uniaxial compression (UC) and Brazilian splitting (BS) tests were performed on heat-treated limestone specimens and digital image correction (DIC) techniques were utilized to measure specimen deformation. Correlation analysis (CA) and principal component analysis (PCA) were conducted on the mechanical parameters obtained from the UC and BS tests, resulting in the establishment of three principal components (y1c, y2c and y1t) that comprehensively reflect the compressive and tensile properties; these were then utilized to analyze the sensitivity of mechanical properties to temperature changes. Furthermore, based on prior experimental data regarding the fundamental physical parameters of granite, one principal component (y1w) that comprehensively reflects fundamental physical characteristics was established and utilized to predict the uniaxial compression strength (σc) and tensile strength (σt). The results indicate that the compressive properties (σc, Poisson's ratio, elastic modulus and principal compressive strain) and tensile properties (σt and tensile strain) are all affected by temperature changes; as tensile strength is sensitive to temperature changes, special attention should be paid to fire and high temperature of stone buildings with the components subjected to the tensile stress; basic mechanical properties (σc and σt) can be well predicted by a principal component (y1w) which provides a new idea for calculating indirectly the residual mechanical properties of building stones after high temperature or fire.

A Voronoi tessellated model considering damage evolution for modeling meso-mechanical mechanism of the sandstone

The strength of sandstone samples considering different particle sizes is required in many engineering applications. A rock model based on the Voronoi tessellated model is established for studying the failure mechanism of sandstone samples. Uniaxial compression, Brazilian splitting and triaxial compression tests are carried out to evaluate the influence of different crystal sizes on macroscopic parameters of sandstone. The uniaxial compressive strength, elastic modulus and tensile strength of the specimens show an increasing trend with the decrease in the crystal size. Failure with redistributed stress has been observed, and strain energy density (SED) can help explain the failure mechanisms. Additionally, a constitutive model is compiled to reproduce the damage evolution taking into account the heterogeneity in elastic modulus and rock strength, which is in good agreement with the experimental results. In the engineering application, it is found that the damage on the surface of the slope is triggered by the action of vibration excitation. The dynamic response of the slope is more intense if the vibration frequency reaches its optimal frequency. The proposed constitutive model is used to quantitatively evaluate the accumulation of damage of the Mining-by tunnel. This work is anticipated to be extensively utilized in other engineering applications.

Experimental and numerical study of disk specimen with rough structural plane under splitting test

To study the effects of the anisotropic matrix and structural planes on the splitting strength and failure mode of rocks, Brazilian splitting tests were carried out with seven different loading angles on specimens of rock-like materials with rough structural planes. The surface strains of the samples during the failure process were monitored and analysed with the help of a high-speed camera and digital image correlation (DIC) technology. The test results showed that the Brazilian splitting strength (BSS) decreased gradually with an increased loading angle. According to the crack morphology, the samples showed three failure modes, and the structural plane and the loading angle (θ) had an important effect on the failure mode. When θ < 75°, the sample failure was mainly affected by the matrix, and when θ > 75°, the sample failure was mainly controlled by the structural plane. The numerical simulation of the sample with a structural plane was carried out by the PFC2D particle flow program, the micro parameters were calibrated using a back propagation (BP) neural network model. The internal cracks of the sample under a splitting load were mainly matrix tensile microcracks and structural plane shear microcracks, and the tensile microcracks in the side with the weak matrix appeared significantly earlier than those in the side with the strong matrix. With increasing loading angle, the proportion of tensile microcracks in the matrix increased, while the proportion of shear microcracks in the matrix decreased, especially in the weak matrix. The microcracks at the structural plane mainly changed from tensile microcracks to shear microcracks, and the development degree of microcracks along the structural plane was more significant than that on the weak matrix with increasing loading angle. The results of the study can provide a reference for rock stability evaluation and utilization.

Investigation of Fracture Evolution and Failure Characteristics of Rocks under High-Temperature Liquid Nitrogen Interaction

The utilization of liquid nitrogen as a sustainable and water-free fracturing medium exhibits immense promise in engineering applications. In this investigation, Brazilian split tests and acoustic emission tests were conducted to explore the impact of liquid nitrogen cooling on the internal structure and mechanical properties of rock specimens. To examine the influence of liquid nitrogen cooling on the tensile strength of rocks, displacement-load curves were obtained from samples subjected to varying cycles of high-temperature liquid nitrogen cooling using Brazilian split tests. Acoustic emission experiments were conducted to investigate the characteristics of granite samples exposed to various cycles of high-temperature liquid nitrogen cooling. Based on these findings, the impact of liquid nitrogen cooling on the internal structure of rock masses was analyzed. The findings of this study demonstrate that high-temperature liquid nitrogen thermal treatment significantly modifies the microscopic structure and mechanical properties of rocks, with potential implications for overall stability and reliability. Notably, an observable decline in tensile strength was observed as the number of cycles of high-temperature liquid nitrogen treatment increased. These findings underscore the substantial impact of liquid nitrogen cooling on the behavior of rocks. High-temperature liquid nitrogen treatment effectively promotes the generation of microcracks within rocks, thereby increasing their permeability. During the experiment, granite specimens primarily exhibited shear-type fractures when subjected to high-temperature freeze-thaw cycles induced by liquid nitrogen.