Mechanical Behavior Of Rock Research Articles

Fracturing Processes in Specimens with Internal vs. Throughgoing Flaws: An Experimental Study Using 3D Printed Materials

AbstractThe fracturing behavior and associated mechanical characterization of rocks are important for many applications in the fields of civil, mining, geothermal, and petroleum engineering. Laboratory testing of rocks plays a major role in understanding the underlying processes that occur on the larger scale and for predicting rock behavior. Fracturing research requires well-defined and consistent boundary conditions. Consequently, the testing design and setup can greatly influence the results. In this study, a comprehensive experimental program using an artificial material was carried out to systematically evaluate the effects of different parameters in rock testing under uniaxial compression. The parameters include compression platen type, specimen centering, loading control method, boundary constraints, and flaw parameters. The results show that these testing conditions have a significant effect on the mechanical behavior of rocks. Using a fixed compression platen helped reduce bulging of the material. Centering of the specimen played a critical role to avoid buckling and unequal distribution of stress. Slower displacement rates can control the energy being released once failure occurs to prevent the specimen from exploding. Also, the frictional end effects were investigated by comparing friction-reduced and non-friction-reduced end conditions. Very importantly, the study also identified variations in crack initiation and propagation between specimens with internal flaws and specimens with throughgoing flaws. This investigation showed that wing cracks appeared in specimens with throughgoing flaws, while wing cracks with petal cracks were associated with the internal flaws. It also showed that the mechanical properties are influenced by the inclination of the flaws and established that specimens with internal flaws generally exhibit higher strength compared to specimens with throughgoing flaws. The systematic analysis presented in this work sheds light on important considerations that need to be taken into account when conducting fracture research and adds knowledge to the fundamental understanding of how fractures occur in nature.

Investigating Mechanical Characteristics of Rocks Under Freeze–Thaw Cycles Using Grain-Based Model

AbstractBased on the discrete element method (DEM), a water-contained grain-based model (GBM) is developed in this study to evaluate the effects of freeze–thaw cycles (FTCs) on the mechanical characteristics of the rock. A set of freeze–thaw and uniaxial compression tests is carried out to explore the impact of micro damage caused by FTCs on the mechanical prosperities of rock samples. By monitoring the development and distribution of micro-cracks during freeze–thaw test and uniaxial compression test, the damage mechanism of FTCs is revealed from a microscopic perspective, which shows that FTCs deteriorate the strength and brittleness parameters as exponential functions. The parametric analysis is carried out to explore the influence of porosity and mineral components on the mechanical behaviors of rock against freeze–thaw and uniaxial loadings. Based on the parametric analysis results, it is found that UCS, Young’s modulus, and total strain energy at peak stress decrease with the increase of porosity and clay content, which emphasizes the contributions of porosity and mineral components on the mechanical properties of rock samples. It is proved that the water-contained grain-based model developed in this study can capture the damage caused by FTCs on the mechanical performance of rock from a microscopic perspective, which provides novel perspectives on the phenomenon of rock degradation in response to fluctuations in temperature.

Experimental study of the end effect on the mechanical behaviors of rocks under true 3D compressions

AbstractIn true triaxial compression tests, all three principal stresses are imposed independently. This allows for a more comprehensive analysis of the material's mechanical properties. The end effect in true triaxial compression tests is a crucial phenomenon that impacts the accuracy and reliability of the test results. In this study, a series of true triaxial compression tests is conducted to examine the influence of the end friction on the mechanical properties. The laboratory results show that the presence of the end friction could bring about an apparent increase in rock strength and also restrict the deformation in each direction showing that the stiffness (the slope of the curves) increased slightly. The rock strength could be enhanced from 24.7 to 90.7 () when the end friction is increased, which is mainly caused by the lateral interface friction. The failure mode and fracture angle of the specimen are also influenced by the end effect, showing that under high friction conditions, the failure is more ductile, and a larger fracture angle is observed. At last, in comparison with the published experimental data, the actual specific friction angle (corresponding friction coefficient is about 0.19) for the direct specimen–metal contacts in a true 3D test is numerically identified, which is empirically reasonable and higher than the tested range 0.146–0.157 obtained from double‐shear test system.

Quantification of artificially induced microcracks and their effects on hard rock fragmentation performance and mechanical response characteristics

Microcracks have a significant effect on the physical and mechanical properties of rocks. Inducing artificial microcracks in rocks can reduce rock strength and decrease the energy required for rock fragmentation. Nevertheless, few attempts have been made to investigate the effect of microcracks on rock fragmentation performance and mechanical behavior, especially from the perspective of microcrack quantification. For this end, we heated granite samples to induce different degrees of microcracks. However, the parameters of microcracks are difficult to control. For the purpose of accurately characterizing the distribution of microcracks, we measured the microcrack parameters in detail. We visualized microcracks using the fluorescent epoxy method and quantified the microcrack parameters by image processing. Such a method suffered at the expense of losing some of the extremely small microcracks. But, on the other hand, a much larger field of view area is assured, allowing the data to be more representative. After the quantification of microcracks, we performed indentation tests on samples containing microcracks to analyze the mechanical response of tool-rock interaction and crater parameters. The results show that the lengths of individual thermally induced microcracks are distributed between 0.2 and 1.1 mm. At higher temperatures, thermal microcracks are dominated by intragranular microcracks leading to a more uniform distribution. The presence of microcracks can affect the fracture characteristics of rocks and a threshold exists. Compared with the unheated sample, a fivefold increase in the density and total length of microcracks reduces the peak force by 88% and 54%, and the absorbed energy by about 90% and 68%, respectively. Additionally, the comprehensive parameter (i.e., fractal dimension) is not an effective parameter in analyzing microcracks, and a single absolute metric (e.g., crack density, length, number, etc.) is recommended. This study is expected to provide a new perspective to understanding the relationship between microcrack parameters and rock fragmentation, and to provide a reference for applied research on new drilling techniques.Our study enhances the efficiency of drilling in hard rocks for geothermal resources by elucidating the role of microcracks in rock fragmentation. This research aligns with the goals of sustainable and cost-effective subsurface engineering, contributing to a net-zero energy future.

Effect of pore-bearing fluids on the mechanical behavior of siliceous cemented sandstones containing different clay contents and types

The clay minerals contained in the reservoir rock of CO2 geological storage formations can adsorb fluids and swell, which weakens the rock and promotes deformation. This phenomenon has not been widely investigated due to the complexity of the mineral components of the rock. To study the relationship between fluids in rock pores and the mechanical behavior of clay-bearing reservoir rocks, three types of siliceous cemented sandstones mainly composed of clay minerals and quartz, both dry and saturated with various fluids (i.e. N2, scCO2, H2O), were subjected to conventional triaxial compression tests. The results show that the fluids weakened the mechanical strength of the sandstones, and the degree of weakening depended on the clay composition and the type of fluid, with no significant relationship to the clay content. For the sandstone containing montmorillonite, compared to the dry condition, scCO2 and H2O reduced the peak strength by up to 7.16% and 15.33%, respectively, while for the sandstone containing only kaolinite as clay component, scCO2 and H2O reduced the peak strength by 3.34% and 3.18%, respectively. As a control, N2 weakened the strength of the three types of sandstones between 1.46% and 4.04%. By considering the failure characteristics of sandstone at the macro- and microscales, the failure mechanism of the sandstones was explained from two perspectives: the intercrack surface energy reduction and the swelling of clay. This study offers new perspectives for assessing the mechanical stability of fluid-bearing reservoirs and clarifies the role of clay in the connection between fluids in rock pores and the mechanical behavior of rocks.

True triaxial test and DEM simulation of rock mechanical behaviors, meso-cracking mechanism and precursor subject to underground excavation disturbance

Mechanical excavation or blasting generates stress waves that rapidly dissipate in the surrounding high-geostress rock, causing microdynamic disturbances. These disturbances trigger microcracks within the damaged rock, even inducing engineering disasters. However, the fracture behaviors and mechanisms of rockmass subjected to excavation disturbances under three-dimensional geostress remains unclear. Therefore, this study proposed a true triaxial static–dynamic combined loading method to capture the entire process of tunnel excavation damage and the continuous fracturing induced by microdynamic disturbances. True triaxial static–dynamic combined tests and numerical simulations using PFC3D-GBM were employed systematically to analyze the influence of principal stresses σ1, σ2, and σ3 on the disturbances mechanical behaviors of the gabbro. The disturbance failure under true triaxial stress indicated a three-stage pattern of deformation: deceleration, constant velocity, and acceleration. With increased σ1 or decreasing σ2 and σ3, the disturbance bearing capacity of the gabbro significantly decreased. Moreover, an increase in σ1 and σ3 corresponded to an increased proportion of intergranular shear cracks, whereas an increase in σ2 resulted in a notable increase in intragranular tensile cracks. Small-magnitude events tended to disperse during the deceleration and constant-velocity stages, whereas larger-magnitude events were concentrated during the acceleration stage. The AE parameter b-value initially increased and then decreased during disturbance fracture process. The excavation disturbance of the tunnel intensified the depth of the damage zone and the energy released, thereby increasing the risk of a catastrophic deep fracture.

Characterizing thermo-hydro-mechanical behavior of rock using a grain interface-based discrete element model (GIB-DEM)

Understanding the meso-structure evolution and macro-mechanical property changes in rocks under thermo-hydro-mechanical (THM) treatment is crucial for advancing and managing deep geological projects. However, due to complex discontinuity networks in rocks, achieving an accurate characterization that quantitatively correlates the micro-macro mechanical behaviors under THM conditions remains challenging. This study introduces a groundbreaking multi-scale analysis method called the grain interface-based discrete element model (GIB-DEM) to tackle this issue. By utilizing digital imaging processing (DIP) and meso-mechanical measurements, the GIB-DEM directly determines the meso-mechanical parameters of bonds, eliminating the need for complex calibration found in traditional methods. After validating the GIB-DEM through experimental and numerical Brazilian Splitting tests, we investigate the macro- mechanical behaviors of rock under different THM conditions. Our results indicate that the increase in the tensile strength at 75 °C is attributed to the tensile strength increase of quartz grain interfaces. The development of micro-cracks on the interfaces of feldspar and feldspar-quartz accounts for the decrease in tensile strength of rock under thermal and hydraulic treatment. Overall, this pioneering research provides valuable insights for optimizing Enhanced Geothermal System (EGS) projects and effectively managing Hot Dry Rock (HDR) resources.

Mechanical behavior evolution and failure characteristics of saturated and dry rocks under different water pressure environments

A complex and changeable water environment is a key factor for safe rock engineering. Variations in water distribution and pressure pose serious challenges in evaluating rock properties and failure modes. To investigate the effect of real water‒rock interactions on rock mechanical behavior, the self-developed hydraulic loading test equipment was designed. A series of mechanical experiments (unjacketed) were carried out on three representative types of rocks under different water pressure environments, and the jacketed test was also performed for the saturated rocks. The experimental results showed that with increasing water confining pressure, the stress‒strain curve of the saturated rocks increased, while that of the dry rocks first decreased and then increased, and that of the sandstone became more obvious. Furthermore, when the water confining pressure increased, the peak stress and elastic modulus of the saturated rocks increased linearly, and those of the dry rocks first decreased and then increased more sharply. Interestingly, the water confining pressure weakened the local tensile failure and aggravated the shear failure. The relationship between the water confining pressure and strength also indicated that the change of saturated rocks more slowly when in contact with the water confining pressure. From the above results, the pore water pressure correction coefficients (μ) of sandstone, marble and granite were 0.98, 0.96 and 0.93, respectively, in the water pressure environment and rock connection. The failure mechanism of saturated rocks is similar to that of rocks without confining pressure due to the balance of the water confining pressure and pore water pressure, and pore water pressure mainly acts as piestic water. For dry rocks, the penetration of pore water firstly weakens the mechanical properties of rocks when pore water pressure is smaller. A greater confining pressure enhances the role of traditional confining pressure for granite and dry rocks.

Pore structure expansion and evolution in sandstone with prefabricated crack under freeze-thaw cycles based on CT scanning

Pore structure expansion and evolution in sandstone with prefabricated crack under freeze-thaw cycles based on CT scanning

Introducing the area under stress–velocity curve: Theory, measurement and association with rock properties

AbstractSince many years ago, ultrasonic velocity has been used to investigate the physical and mechanical behaviour of rocks, thereby playing an important role in reservoir characterization and seismic interpretation. In order to develop the knowledge of ultrasonic tools, we performed a noble analysis on the ultrasonic behaviour of rocks under confining stress and evaluated a distinctive property of porous media that is measured as the area under the stress–velocity curve (here defined as S*). We further investigated its relationship with elastic and mechanical behaviours of rock. To validate the theoretical framework developed in this work, 20 core plugs from various rock units with complex microstructures were subjected to triaxial compressional tests to calculate their area under the curve. Calculations were made for crack‐closing, elastic and post‐elastic stages (e.g. pore collapse) along the ultrasonic velocity–stress curve. Moreover, the selected samples had their microstructure investigated by thin‐section studies to quantify their porosity and pore type. The results were analysed to check for the effect of pore type on S* in different stages of the stress–velocity curve. Based on the outputs of the analysis of variance and Pearson's correlation coefficient analysis, the curve had its shape and underlying area closely related to the porosity and pore geometry. Indeed, the results showed that the shale and sandstone with micro cracks and carbonate with stiff pores correspond to smaller and larger areas under the curve in crack‐closing and inelastic stages, respectively. Cross‐correlating the results to compressibility (inverse of bulk modulus), it was figured out that the calculated area under curve was well consistent with the compressibility. In addition, S* represents both static and dynamic behaviours of the rock, and the results revealed that the shape and curvature of the stress–velocity curve give valuable information about the rock microstructure. Another finding was the fact that the type of fluid and wave velocity seemingly affect the S*. Our findings can help interpret wave velocity behaviour in reservoir rocks and other stressful porous media.

Mode-I failure and mechanical behavior of sandstone under cyclic loading: Laboratory testing and DEM simulation

To study the mode-I failure and mechanical behavior of rock under cyclic loading, the cracked chevron notched Brazilian disc (CCNBD) of sandstone was employed in this paper, and thus three different upper-limit loads cyclic tests were carried out, including 95 %SUL(static ultimate load) ∼ 35 %SUL, 85 %SUL ∼ 25 %SUL, 75 %SUL ∼ 15 %SUL. The 3D discrete element model of CCNBD specimen was built to study mechanical properties and cracking behavior at a meso-scale. Meanwhile, the effect of cyclic loading on microstructure characteristics and fracture process zone (FPZ) was discussed. The results show that cyclic loading has a weakening effect on fracture toughness, and a modified fracture toughness calculation method was proposed considering the effect of irreversible plastic deformation under cyclic loading. There are obvious stages of microcracking behavior under cyclic loading, specific for the condition of 85 %SUL ∼ 25 %SUL, and the microcrack density is positively correlated with the number of cycles. Meanwhile, the effect of cyclic load on sandstone microstructure contains weak-structure fracture effect and compaction effect, and they exist simultaneously and work together. In addition, cyclic loading has a great influence on the length of FPZ, and the length of FPZ under cyclic loading is greater than that under monotonic loading.

Three-dimensional numerical simulation on failure mechanical characteristics of fissured sandstone specimens under true triaxial conditions

Understanding intermediate principal stress influences on deformation characteristics and failure mechanism of fractured rock from deep engineering often necessitates long term on-site detection and complex experimental research, although very limited efficacy can be obtained. With presently available experimental data on conventional triaxial cuboid noncoplanar fissured sandstones, this study further investigated the effects of differential stress (σ2 − σ3) and rock bridge inclination angle (β30°, β60°, β90°, β120°, and β150°) on their damage evolution processes and failure modes based on 3D discrete element methods (PFC3D). Numerical simulation results indicated that the peak strength of the fissured sandstone under the same state of horizontal principal stress shows an overall increasing pattern with the increase of the inclination angle of the rock bridge, while its peak strain displays a decreasing and then increasing trend. Besides, in this work, five failure modes, nine crack types and two kinds of stress–strain curves are summarized which are significantly affected by the horizontal principal stress, rock bridge inclinations or cooperative interaction. Differential stress value of 15 MPa is an important threshold for determining the residual strength of the stress–strain curve from present to absent. The cracking and force chain evolution processes of five failure modes were analyzed in order to discover the corresponding failure mechanisms. Finally, differential stress and flaw inclination influences on rock mechanical parameters and failure behavior have significant implications for strategies of the design, operation, and maintenance of rock engineering under severe conditions in the underground.

Mechanical behavior of rock under uniaxial tension: Insights from energy storage and dissipation

Many rock engineering projects show that the growth of tensile cracks is often an important cause of engineering disasters, and the mechanical behavior of rocks is essentially the transmission, storage, dissipation and release of energy. To investigate the tensile behavior of rock from the perspective of energy, uniaxial tension tests (UTTs) and uniaxial compression tests (UCTs) were carried out on three typical rocks (granite, sandstone and marble). Different unloading points were set before the peak stress to separate elastic energy and dissipated energy. The input energy density ut, elastic energy density ue, and dissipated energy density ud at each unloading point were calculated by integrating stress-strain curves. The results show that there is a strong linear relationship between the three energy parameters and the square of the unloading stress in UCT, but this linear relationship is weaker in UTT. The ue and ud increase linearly with the increase in ut in UCT and UTT. Based on the phenomenon that ue and ud increase linearly with ut, the applicability of Wetp index in UTT was proved and the relative energy storage capacity and absolute energy distribution characteristics of three rocks in UCT and UTT were evaluated. The tensile behavior of marble and sandstone in UTT can be divided into two stages vaguely according to the energy distribution, but granite is not the case. In addition, based on dissipated energy, the damage evolution of three types of rocks in UCT and UTT was discussed. This study provides some new insights for understanding the tensile behavior of rock.

Hydromechanical simulation of a carbonate petroleum reservoir using pseudo-coupling

In the domain of petroleum reservoir management, numerical flow simulation stands as the primary tool for prediction and control. Traditionally, rock compressibility has been the key geomechanical parameter, often applied as a single, unchanging value across the entire reservoir model throughout the simulation. This study introduces a novel methodology that enhances reservoir simulation accuracy by incorporating rock mechanical behavior through a pseudo-coupling approach, without increasing computational costs. This approach empowers reservoir engineers to consider rock hydromechanical behavior in daily reservoir studies.The pseudo-coupling updates both porosity and permeability based on tables that correlate pore pressure to porosity and permeability multipliers, thereby replacing the conventional reliance on compressibility. It begins with reservoir-rock samples selection guided by lithological composition, followed by mechanical and hydromechanical laboratory tests, and numerical adjustments with ABAQUS stress analysis software. The resulting mechanical parameters serve for generating pseudo-coupling tables tailored to each facies.Validation experiments comparing pseudo-coupling simulations with conventional and fully coupled models reveal convergence in porosity and permeability variations with pore pressure between both coupled approaches. Extensive assessments further compare pseudo-coupling simulations against conventional reservoir simulations using a synthetic model based on a carbonate post-salt reservoir, denoted as Field B. The pseudo-coupling approach emerges as a more robust model capable of discerning preferential flow paths, all while maintaining computational efficiency. Moreover, these preferential flow paths were also predicted by the reservoir team responsible for the field development, but through a trial-and-error procedure to match production history, whilst the pseudo-coupling predicted it based on rock mechanical behavior.In this way, the proposed methodology enables an enhancement in the accuracy and efficiency of petroleum reservoir management by considering geomechanical effects, without incurring any additional computational costs when compared to conventional reservoir simulation.

A human-machine interaction method for rock discontinuities mapping by three-dimensional point clouds with noises

Rock discontinuities control rock mechanical behaviors and significantly influence the stability of rock masses. However, existing discontinuity mapping algorithms are susceptible to noise, and the calculation results cannot be fed back to users timely. To address this issue, we proposed a human-machine interaction (HMI) method for discontinuity mapping. Users can help the algorithm identify the noise and make real-time result judgments and parameter adjustments. For this, a regular cube was selected to illustrate the workflows: (1) point cloud was acquired using remote sensing; (2) the HMI method was employed to select reference points and angle thresholds to detect group discontinuity; (3) individual discontinuities were extracted from the group discontinuity using a density-based cluster algorithm; and (4) the orientation of each discontinuity was measured based on a plane fitting algorithm. The method was applied to a well-studied highway road cut and a complex natural slope. The consistency of the computational results with field measurements demonstrates its good accuracy, and the average error in the dip direction and dip angle for both cases was less than 3°. Finally, the computational time of the proposed method was compared with two other popular algorithms, and the reduction in computational time by tens of times proves its high computational efficiency. This method provides geologists and geological engineers with a new idea to map rapidly and accurately rock structures under large amounts of noises or unclear features.

Prediction of the Unconfined Compressive Strength of Salinized Frozen Soil Based on Machine Learning

Unconfined compressive strength (UCS) is an important parameter of rock and soil mechanical behavior in foundation engineering design and construction. In this study, salinized frozen soil is selected as the research object, and soil GDS tests, ultrasonic tests, and scanning electron microscopy (SEM) tests are conducted. Based on the classification method of the model parameters, 2 macroscopic parameters, 38 mesoscopic parameters, and 19 microscopic parameters are selected. A machine learning model is used to predict the strength of soil considering the three-level characteristic parameters. Four accuracy evaluation indicators are used to evaluate six machine learning models. The results show that the radial basis function (RBF) has the best UCS predictive performance for both the training and testing stages. In terms of acceptable accuracy and stability loss, through the analysis of the gray correlation and rough set of the three-level parameters, the total amount and proportion of parameters are optimized so that there are 2, 16, and 16 macro, meso, and micro parameters in a sequence, respectively. In the simulation of the aforementioned six machine learning models with the optimized parameters, the RBF still performs optimally. In addition, after parameter optimization, the sensitivity proportion of the third-level parameters is more reasonable. The RBF model with optimized parameters proved to be a more effective method for predicting soil UCS. This study improves the prediction ability of the UCS by classifying and optimizing the model parameters and provides a useful reference for future research on salty soil strength parameters in seasonally frozen regions.

Numerical Analysis of the Shape Effect on the Mechanical Behaviors of Rocks in True 3D Compression Test

Rock strength parameters are usually indispensable for rock engineering design. Under the experimental testing conditions, the shape effect could significantly affect the measured results. Previous findings from uniaxial tests reveal that the measured rock strength gradually decreases with the increase in the slenderness ratio, which is mainly ascribed to the end effect. However, it is still unclear how strong the influence of the end effect on the shape effect is in true 3D tests. In the present study, rock mechanical behaviors in response to the variation in slenderness ratio are detailly examined in true 3D tests. The calculated results show that rock strength progressively decreases as the slenderness ratio increases. But the rock strength in true 3D tests still deviates far from the actual even though the slenderness ratio goes beyond 2, which is mainly caused by the interface friction along the two extra σ2-normal specimen faces. It is interesting that the slenderness ratio increases lead to an increase in the measured stiffness as well. The calculated results suggest that symmetry in the experimentally defined typical arc-shaped curves σ1(σ2) at constant σ3 are neglected most likely due to the stronger end effects in true 3D tests, and the accurate rock strength parameters are not obtained only through using slender specimens.

Mechanism of increasing or inhibiting laser-weakened rocks by saturated fluids and mechanical behavior of rocks after laser damage

Laser technology has a wide range of application prospects as a novel means of assisting rock breaking. The realization of efficient laser-assisted breaking of deep hard rocks is based on revealing the mechanism of the saturated fluid increase or inhibition of laser-weakened rock. The laser irradiation of basalt with different water contents was used to analyze the mechanical response characteristics and investigate the destruction law of basalt under different laser power irradiations to explore the mechanism of the influence of water on the effect of laser rock breaking. The results show that the presence of water can significantly increase the drilling depth, and a laser irradiation of 1250 W can penetrate 56.5 mm deep into the rock in 30 s. The uniaxial compressive strength of basalt was reduced by a maximum of 38.23 % and the modulus of elasticity by a maximum of 74.99 % after the laser irradiation. Under low- power conditions, water has a positive effect on the laser rock-breaking efficiency. After 30 s of a laser irradiation in the range of 250–750 W, the uniaxial compressive strength of the water-saturated rock (WSR) samples was 12.64–13.43 % less than that of the original rock (OR) samples, and that of the natural state rock (NSR) samples was 1.09–3.93 % less than that of the OR samples. Both acoustic emission events and basalt energy significantly increased after a laser irradiation, and water promoted the expansion of rock microcracks. The degree of rock fragmentation increased with an increasing laser power. Laser irradiation can significantly reduce the strength of the rock, and water can effectively promote a low-power laser to break the rock, which is effective in reducing the difficulty and improving the efficiency of rock breaking. The obtained results provide theoretical and technical support for the application of the laser technology in the effective crushing of deep hard rock, and efficient and safe exploitation of deep resources.

Correlational fractal characteristics and damage progression of granite with different grain sizes based on acoustic emission monitoring

Mineral grain size caused by the geological diagenetic process is one of the fundamental reasons leading to the differences in rock mechanical behaviors, which brings challenges to the stability of underground engineering, especially in rock masses with continuous grain size changes. This study conducted uniaxial compression tests on granite samples obtained from the same invaded anticline with different grain sizes. During the tests, the acoustic emission (AE) signals were acquired synchronously. The correlation between damage evolution and AE data was then studied by frequency-spectrum analysis, fractal theory methods, and G-P algorithms. The mechanism of grain size effect on granite failure evolution was discussed. The results reveal that as the grain size increases, the strength and the elastic modulus decrease while the plastic deformation characteristic is enhanced. Grain size also affects the frequency distribution of AE signals and its time-varying characteristics. With the increase in grain size, the proportion of high-frequency signals declines during loading. The fractal dimension value of dominant frequency during loading fluctuates within the range from 0.5 to 2.25 and generally shows an overall ‘rise-fall-rise-fall’ mode. In addition, as the grain size increases from fine-grained to medium-grained, the fractal dimension changes from an increasing to a decreasing trend earlier in the compaction and elastic stages. Especially in coarse-grained samples, after a brief increase, the fractal dimension decreases continuously until approaching the overall failure. The frequency domain characteristics and correlation fractal variation of granites reflect the damage and crack evolution inside the rock sample. More scattered and unordered micro-cracks generated in samples with finer grains facilitate the dispersion of applied stress and the reduction of local stress concentration, resulting in higher strength and stiffness of the sample.

Study on macro–micro mechanical behavior of rock like samples with hole and cracks

Study on macro–micro mechanical behavior of rock like samples with hole and cracks