Shear Microcracks Research Articles

Damage index correlation in massive granite-porous backfills under hydromechanical triaxial cyclic loading using acoustic emissions and X-ray computed tomography

The characterization accuracy of damage indices in massive and porous composite biomaterials under complex osmotic conditions has long been a challenge in engineering materials science. Establishing correlations between damage indices calculated using different methodologies is crucial to validate and improve defects characterizations. In this study, we investigated the damage index correlation of cylindrical massive granite-porous backfill structural composites under both dry and seepage conditions. A series of triaxial cyclic loading and unloading tests were conducted on specimens under hydromechanical conditions with varying osmotic pressures (0, 1, 3, 5, 7, and 9 MPa) and a confining pressure of 9 MPa involving acoustic emission monitoring and phase-based 3D nondestructive CT scanning. The results revealed the AE energy was lower under osmotic conditions compared to dry conditions. The AE Damage Index (DAE), defined by separating tensile and shear microcracking signals, significantly increased from 0.32 to 0.55 with rising osmotic pressure. 3D mesoscopic fracture visualization provided detailed insights into the fractures in granite and backfill interfaces. The number of small-angle cracks increased with osmotic pressure, and the crack inclination distribution range expanded from 63°- 85° to 49°–80°. The average crack angle showed a significant monotonic decrease with increasing osmotic pressure. An Energy Dissipation Damage Index (DED) was defined, and its correlation with the DAE under dry conditions was modeled using a reciprocal function DED=a/(DAE+bσ3)+c. This novel approach offers insights into the transition points between tensile and shear-dominated failure stages. Furthermore, an exponential function DV=aDAEb(b<0) was used to model the correlation between AE Damage Index and Visual Damage Index (DV) under osmotic conditions. The damage indices derived from different methods can be mathematically correlated, providing a comprehensive evaluation of material damage states. The study found the flow transition between different natural-human medias results in different damage mechanisms, which are all obey the correlation law. These damage mechanisms offered constructive recommendations for engineering applications based on varying ratios of osmotic pressure to confining pressure.

Microcracking evolution and clustering fractal characteristics in coal failure under multi step and cyclic loading

During the deep mining process, coal mass encounter intricate geo-environmental stress, such as periodic weighting loading and repeatedly excavation unloading–reloading cycles, which weakens coal’s mechanical integrity and predisposing it to severe coalburst accidents. To investigate the microcracking damage mechanisms and predictive indicators in coal failure under in-situ stress analogs, the multistage step and cyclic loading experiments are conducted on cubic coal specimens. Acoustic emission (AE) technology is employed to track the spatiotemporal-energy evolution of stress-induced damages and discern the microcracking nature through AF/RA assessments, and the power-law scaling relation of AE activity near the catastrophic failure of coal is investigated. Then the clustering fractal structures of microcracking events in the stressed coal are quantified across temporal, spatial and energetic domains, utilizing correlation integral methodologies and b-value derivations from magnitude-frequency relation. Findings indicate that irrespective of the loading mode (step or cyclic), escalating stress triggers an intensification of irreversible fatigue deformations. AE characteristic parameters manifest a gradual rise, culminating in a precipitous peak coinciding with the critical failure point. This escalation adheres to a power-law correlation between AE occurrence frequency and time to failure, observable in the immediate pre-failure seconds, reflecting a universal attribute of coal fracture. Prior to ultimate failure, a marked increase in shear microcracks is discernible, despite tensile-dominated cracks (constituting about 80 % of total microcracks) prevailing as inferred from the variation of AF/RA values, aligning with an inferred “X” conjugate wedge splitting pattern from AE event density and energy mapping. The microcracking events in the loaded coal exhibit a clustering fractal structure that spans across temporal, spatial, and energetic (or magnitude) domains. Notably, the temporal fractal dimension, spatial fractal dimension, and b-value (i.e., a parameter characterized the energetic fractal dimension) all follow a parallel decrease pattern as the loading stress escalates, with a pronounced diminution becoming especially evident as the specimen approaches its catastrophic failure threshold. This insight offers fresh perspectives for predicting rock/coal dynamic disasters, emphasizing the necessity of concurrently monitoring the shift from diffuse microcracking to localized failure across time, space and energy domains. These research findings contribute to a deeper understanding of microcracking damage evolution and failure mechanism of loaded coal, and provide a foundational basis for early warning of rock failure such as the coalburst disasters.

Thermal-damage effects on fracturing evolution of granite under compression-shear loading

Shear-dominant instability disasters in high-temperature conditions pose a significant challenge for deep underground rock projects, necessitating a thorough understanding of rock fracture evolution and the impacts of thermal damage. To investigate how thermal damage influences shear fracturing behavior and geophysical responses, experiments were conducted on thermally-treated granite under compression-shear loading conditions. The detailed failure process was meticulously monitored using integrated acoustic emission (AE), electric potential (EP), and digital image correlation (DIC) techniques. The effects of thermal treatment on the patterns of AEs, EPs, and DIC strain fields during the loading process were analyzed. Micromechanics related to the influence of thermal treatment on mechanical properties and geophysical responses were explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). By defining damage variables through multiparameter analysis using AE, EP, and DIC, the study delved into predictive insights on the failure process of thermal-damage granite. The results indicate that the physical properties of granite undergo significant changes as the treatment temperature increases, with the peak stress gradually decreasing and the plastic characteristics increasing. These changes can be attributed to thermal damage, which causes internal water escape, expansion of mineral particles, and crack formation. Additionally, increasing the thermal treatment temperature leads to a decrease in both AE and EP intensity, as well as a reduction in the occurrence of high-magnitude AE events, while the cumulative AE count rapidly increases during the plastic stage. Notably, thermal treatment induces significant alterations in mineral composition, intergranular fractures, and the formation of a gradual zigzag crack path, resulting in the intensified development of shear microcracks. This observation is supported by the varying of AE characteristic parameter AF/RA and the analysis of DIC displacement decomposition. Moreover, the three defined damage variables exhibit accelerated growth prior to catastrophic failure, with the order of precursor occurrence observed as AE, DIC, and EP. However, the occurrence of the precursor time for these three geophysical signals becomes earlier with increasing treatment temperature. These research findings shed light on the mechanism of compression-shear failure of thermal-damage rock and provide valuable references for predicting precursors of the shear-dominant instability disasters such as induced earthquakes.

Failure Mechanism and Control Mechanism of Intermittent Jointed Rock Bridge Based on Acoustic Emission (AE) and Digital Image Correlation (DIC).

Deep foundation pit excavation is an important way to develop underground space in congested urban areas. Rock bridges prevent the interconnection of joints and control the deformation and failure of the rock mass caused by excavation for foundation pits. However, few studies have considered the acoustic properties and strain field evolution of rock bridges. To investigate the control mechanisms of rock bridges in intermittent joints, jointed specimens with varying rock bridge length and angle were prepared and subjected to laboratory uniaxial compression tests, employing acoustic emission (AE) and digital image correlation (DIC) techniques. The results indicated a linear and positive correlation between uniaxial compressive strength and length, and a non-linear and negative correlation with angle. Moreover, AE counts and cumulative AE counts increased with loading, suggesting surges due to the propagation and coalescence of wing and macroscopic cracks. Analysis of RA-AF values revealed that shear microcracks dominated the failure, with the ratio of shear microcracks increasing as length decreased and angle increased. Notably, angle exerted a more significant impact on the damage form. As length diminished, the failure plane's transition across the rock bridge shifted from a complex coalescence of shear cracks to a direct merger of only coplanar shear cracks, reducing the number of tensile cracks required for failure initiation. The larger the angle, the higher the degree of coalescence of the rock bridge and, consequently, the fewer tensile cracks required for failure. The decrease of length and the increase of angle make rock mass more fragile. The more inclined the failure mode is to shear failure, the smaller the damage required for failure, and the more prone the areas is to rock mass disaster. These findings can provide theoretical guidance for the deformation and control of deep foundation pits.

Simulation of Shear and Tensile Fractures Using Ductile Phase Field Modelling with the Calibration of P Wave Velocity Measurement and Moment Tensor Inversion

The appropriate understanding and formulation of rock discontinuities via FEM is still challenging for rock engineering, as continuous algorithms cannot handle the discontinuities in rock mass. Also, different failure modes of rock samples, containing tensile and shear failure, need to be computed separately. In this study, a novel double-phase field damage model was introduced with two independent phase field damage variables. The construction of the proposed model follows the thermodynamics framework from the overall Helmholtz free energy, with elastic, plastic and surface damage components. The proposed model is calibrated via traditional damage variables, based on ultrasonic wave velocity measurement and acoustic emission monitoring, and both show great consistency between simulation results and laboratory observations. Then the double-phase field damage model is applied to COMSOL software to simulate microcrack propagation in a pre-fractured rock sample. Both lateral and wing cracks are observed in this study, manifested as shear- and tensile-dominated cracks. We also observed different microcracking mechanisms in the proposed numerical models, such as tensile and shear cracking, the influence of plastic strain and the percolation between tensile and shear microcracks. Overall, this study provides valuable insights into the mechanics of microcracking in rocks, and the proposed model shows promising results in simulating crack propagation.

Revealing the failure mechanism of high strength seawater coral aggregate reinforced concrete slabs based on acoustic emission technology: Parameter analysis and DBN-BPNN classification

In this study, high strength seawater coral aggregate concrete (SCAC) with a compressive strength of 80 MPa was prepared, and seawater coral aggregate reinforced concrete slab (SCARCS) was designed with epoxy-coated steel bars. Combining with the AE technology, monotonic flexural static tests were conducted on SCARCS specimens with three varying reinforcement ratios (i.e., 0.85%, 1.13%, and 1.41%). Results showed that the increasing the reinforcement ratio led to an increase in peak bearing capacity and a decrease in center deflections at the peak. The loading process can be divided into four stages based on the varying magnitudes of the b-value and Ib-value. The primary mode of failure was tensile microcracks at various stages, and the ratio of shear microcracks reached its maximum value in the final stage of loading. Besides, based on the signal intensity analysis, it can be observed that the scale and complexity of cracks propagation inside the SCARCS specimen increased as the reinforcement ratio rose. In addition, back propagation neural network (BPNN) classification model optimized by deep belief network (DBN) algorithm was proposed to predict the damage degree of SCARCS during loading, and the average prediction accuracy of the proposed model was shown to be over 89%. The classification outcomes demonstrated that with an increase in the reinforcement ratio of SCARCS, the proportion of AE signals in Cluster 3 increased, while the proportion of AE signals in Cluster 2 dropped. Notably, both the average frequency and energy presented an ascending trend. The findings of this study provided valuable insights for designing and assessing the safety of SCAC structures for essential island and reef engineering infrastructure.

Damage evolution of rock-encased-backfill structure under stepwise cyclic triaxial loading

Rock-encased-backfill (RB) structures are common in underground mining, for example in the cut-and-fill and stoping methods. To understand the effects of cyclic excavation and blasting activities on the damage of these RB structures, a series of triaxial stepwise-increasing-amplitude cyclic loading experiments was conducted with cylindrical RB specimens (rock on outside, backfill on inside) with different volume fractions of rock (VF = 0.48, 0.61, 0.73, and 0.84), confining pressures (0, 6, 9, and 12 MPa), and cyclic loading rates (200, 300, 400, and 500 N/s). The damage evolution and meso-crack formation during the cyclic tests were analyzed with results from stress-strain hysteresis loops, acoustic emission events, and post-failure X-ray 3D fracture morphology. The results showed significant differences between cyclic and monotonic loadings of RB specimens, particularly with regard to the generation of shear microcracks, the development of stress memory and strain hardening, and the contact forces and associated friction that develops along the rock-backfill interface. One important finding is that as a function of the number of cycles, the elastic strain increases linearly and the dissipated energy increases exponentially. Also, compared with monotonic loading, the cyclic strain hardening characteristics are more sensitive to rising confining pressures during the initial compaction stage. Another finding is that compared with monotonic loading, more shear microcracks are generated during every reloading stage, but these microcracks tend to be dispersed and lessen the likelihood of large shear fracture formation. The transition from elastic to plastic behavior varies depending on the parameters of each test (confinement, volume fraction, and cyclic rate), and an interesting finding was that the transformation to plastic behavior is significantly lower under the conditions of 0.73 rock volume fraction, 400 N/s cyclic loading rate, and 9 MPa confinement. All the findings have important practical implications on the ability of backfill to support underground excavations.

Characteristic stress variation and microcrack evolution of granite subjected to uniaxial compression using acoustic emission methods

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of microcrack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment.

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 &lt; 75°, the sample failure was mainly affected by the matrix, and when θ &gt; 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.

Experimental study on the effect of anchors on the creep mechanical properties and acoustic emission characteristics of fractured sandstone under uniaxial compression

This study conducts laboratory tests to investigate the creep and acoustic emission (AE) of unanchored and anchored fractured sandstone samples under uniaxial compression by taking the Triassic fine sandstone collected from the Xiaolangdi Reservoir area as the research object and ensuring the same testing conditions using an RLJW-2000 rock rheometer and a PCI-Ⅱ AE instrument. Moreover, this study identified the effects of anchors on the creep mechanics and AE characteristics of fractured sandstone samples by analyzing the strain, long-term strength, AE event count, AE rate, AE energy rate, and rising angle – average frequency (RA-AF) characteristics of each sample before and after anchoring. According to the study results: (1) Under the same stress level, the anchored sample's instantaneous strain, creep strain, and total strain are smaller than those of the unanchored sample. Anchors exerted the largest inhibitory effect on creep strain, followed by the total strain, while anchors inhibited instantaneous strain to the smallest extent. (2) Compared to the unanchored sample, the anchored sample experienced a significant increase (as high as 50%) in long-term strength. Anchor reinforcement significantly improves the long-term strength of samples. (3) Under the same stress level, the AE event count and cumulative AE event count of the anchored sample are smaller than those of the unanchored sample. Anchors significantly inhibited microcracks initiation, propagation, overlapping, and penetration in samples. (4) The maximum AE rate of the anchored sample is only 69.30% of that of the unanchored sample, whereas the peak AE energy rate of the anchored sample is only 23.21% of that of the unanchored sample. Anchors reduce the AE event count generated and the AE energy released by samples per unit of time, exerting a significant crack-arrest effect on them. (5) The presence of anchors changes the type and number of microcracks in the samples' creep process. Anchors greatly inhibit the generation of high-RA and low-AF AE signals and generate low-RA and high-AF ones. As a result, more tensile microcracks occur in samples under stress levels close to the failure stress level (especially the last stress level), which inhibit the formation of shear microcracks in samples and causes samples to be more prone to macroscopic tensile-shear failure. (6) The microcrack characteristics of samples characterized by RA-AF values are consistent with the macrocrack characteristics of samples. Therefore, RA-AF values can satisfactorily characterize the microcracks' variation characteristics in the sandstone samples' creep process before and after anchoring.

Tensile strength and failure behavior of rock-mortar interfaces: Direct and indirect measurements

The tensile strength at the rock-concrete interface is one of the crucial factors controlling the failure mechanisms of structures, such as concrete gravity dams. Despite the critical importance of the failure mechanism and tensile strength of rock-concrete interfaces, understanding of these factors remains very limited. This study investigated the tensile strength and fracturing processes at rock-mortar interfaces subjected to direct and indirect tensile loadings. Digital image correlation (DIC) and acoustic emission (AE) techniques were used to monitor the failure mechanisms of specimens subjected to direct tension and indirect loading (Brazilian tests). The results indicated that the direct tensile strength of the rock-mortar specimens was lower than their indirect tensile strength, with a direct/indirect tensile strength ratio of 65%. DIC strain field data and moment tensor inversions (MTI) of AE events indicated that a significant number of shear microcracks occurred in the specimens subjected to the Brazilian test. The presence of these shear microcracks, which require more energy to break, resulted in a higher tensile strength during the Brazilian tests. In contrast, microcracks were predominantly tensile in specimens subjected to direct tension, leading to a lower tensile strength. Spatiotemporal monitoring of the cracking processes in the rock-mortar interfaces revealed that they show AE precursors before failure under the Brazilian test, whereas they show a minimal number of AE events before failure under direct tension. Due to different microcracking mechanisms, specimens tested under Brazilian tests showed lower roughness with flatter fracture surfaces than those tested under direct tension with jagged and rough fracture surfaces. The results of this study shed light on better understanding the micromechanics of damage in the rock-concrete interfaces for a safer design of engineering structures.

Characterizing the cracking process of various rock types under Brazilian loading based on coupled Acoustic Emission and high-speed imaging techniques

Tensile strength is one of the important mechanical properties of rock and rock-like materials and its accurate estimation and understanding of its failure mechanism are crucial for efficient design of structures on or within the rock masses. Amongst various approaches developed for estimating this property, Brazilian testing has received considerable attention analytically, numerically and experimentally. The test has been deployed for a wide range of materials such as rock, concrete, and ceramic, however its fracturing process at micro and macro scales has not yet been properly characterized. Thus, in this study, for a full understanding of the fracturing process of different rock types under indirect tensile loading, the coupled Acoustic Emission (AE) and high-speed imaging techniques were used to characterize the generated micro and macrocracks under Brazilian loading. The characteristics of AE parameters and the evolution of micro-macro cracks were investigated for three different rocks including sandstone, coal and granite at different sizes and loading configurations. A systematic framework was developed to exhibit the link between micro and macro fracturing processes under Brazilian loading. It was noted that the tensile microcracks are the first to initiate during the Brazilian failure with high intensity compared to shear microcracks. Transition in microcracking mechanism from tension to shear was evident in the samples with high brittleness. The tensile microcracks showed ascending trends with a decrease in geometrical or volumetric sample size for granite and sandstone, while its trend for coal was inconclusive due to its unique intrinsic fracture network or so-called “cleat network”. At macro scale level, single and multiple cracking mechanisms were observed in the tested samples where those with high shear microcracks mostly revealed failure under multiple cracking. Finally, some insights were provided into the transition from single tensile cracking to multiple tensile/shear cracking based on high-speed image data with high resolution.

Numerical study on the shear behavior of concrete-rock joints with similar triangular asperities

The shear behavior of concrete-rock joints with similar triangular asperities (STAs) was investigated by numerical direct shear tests under constant normal stiffness (CNS) conditions based on particle flow code (PFC), and numerical models were validated by laboratory observations recently published by the authors. The failure patterns and shear load transfer of asperities on the joints were revealed in terms of shear micro-cracks and shear force chains on the micro-scale. For comparison, joints with conventional profile, regular triangular asperities (RTAs), were also examined. The comparisons showed that a prominent ductile behavior at post-peak most likely occurred for the STAs profile owing to asynchronous asperity failure rather than brittle shear caused by a synchronous failure of the RTAs profile. Results also indicated that there would exist a critical inclination to limit the further mobilization of joint dilation and it may be useful to guide the preliminary design of the rock-socketed pile foundation.

Laboratory study of fracture initiation and propagation in Barre granite under fluid pressure at stress state close to failure

Hydraulic fracturing is frequently used to increase the permeability of rock formations in energy extraction scenarios such as unconventional oil and gas extraction and enhanced geothermal systems (EGSs). The present study addresses uncertainties in the hydraulic fracturing process pertaining to EGSs in crystalline rock such as granite. Specifically, there is debate in the literature on the mechanisms (i.e. tensile and/or shear) by which these fractures initiate, propagate, and coalesce. We present experiments on Barre granite with pre-cut flaws where the material is loaded to high far-field stresses close to shear failure, and then the fluid pressure in the flaws is increased to move the Mohr's circle to the left and observe the initiation and propagation of fractures using high-speed imaging and acoustic emissions (AEs). We find that the hydraulic fractures initiate as tensile microcracks at the flaw tips, and then propagate as a combination of tensile and shear microcracks. AE focal mechanisms also show elevated levels of tensional microfracturing near the flaw tips during pressurization and final failure. We then consider a numerical model of the experimental setup, where we find that fractures are indeed likely to initiate at flaw tips in tension even at relatively high far-field stresses of 40 MPa where shear failure is generally expected.

Study on mesoscopic failure mechanism of grout-infilled sandstone under uniaxial compression using an improved AE localization technique

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens.

Hardened fracture characteristics of printed concrete using acoustic emission monitoring technique

This paper presents a study on the fracture behavior of hardened printed concrete. The fracture properties of conventional cast and printed concrete (in three printing types) were compared by conducting the splitting and three-point bending tests. The anisotropic behavior of printed concrete was analyzed through the micro-cracking mechanism monitored by using the acoustic emission (AE) technique. Compared to the cast concrete, the fracture strength of printed concrete is reduced, especially when the load was applied at the interface between the successively printed filaments. The interfaces affect the micro-cracking modes during the fracture process of the printed concrete. When the number of shear micro-cracks increases, the values of splitting strength, flexural strength and fracture energy decrease. Useful criterial indicators for the design of a printed concrete member or a structure are suggested.

Damage evolution of concrete under tensile load using discrete element modeling

The objective of this study is to investigate the damage evolution process in the flattened Brazilian test of concrete, and assess the effect of the maximum aggregate size on the tensile strength of concrete using the discrete element method considering the heterogeneity of concrete. Random irregular coarse aggregates were generated in each gradation according to the theoretical content. Based on this, a series of discrete element numerical models of flattened Brazilian disks with aggregates of different gradations were established, including 5–10, 5–16, and 5–20 mm, and numerical simulations of flattened Brazilian tests were conducted. Before the peak load, the compressive force was concentrated near the flat loading ends, which can be considered as the driving force for the initiation of shear microcracks, and the tensile force distributed away from the loading ends induced the appearance of tensile microcracks in the mortar and the interface transition zones (ITZs). Tortuous macroscopic cracks formed owing to the accumulation of tensile microcracks at the center of the specimen when the peak load was reached. After the peak load, secondary cracks initiated and extended gradually, and shear failure began to occur in the mortar and the ITZs owing to the redistribution of the compressive force. The failure mechanism was correlated with the displacement trends of the particles. Tensile microcracks could be associated with the tensile displacement trend of the particles. Shear and tensile microcracks co-existed in the zones where the particles moved in a mixed-mode displacement trend. The tensile strength of the concrete decreased slightly with an increase in the maximum aggregate size in the flattened Brazilian tests, and the tensile strength of the concrete was overestimated by the flattened Brazilian tests.

Creep Rheology of Antigorite: Experiments at Subduction Zone Conditions

AbstractNovel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10−9 − 10−4 1/s), and high pressure (1 GPa) in a Griggs‐type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, by applying 8–12 stress‐strain steps per temperature. The microstructures of deformed samples share features documented in previous work (e.g., shear microcracks), and highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models.

About the technology of seismic location of oil and gas (SLONG) based on emission waves from productive layers

The article deals with the physical foundations and a set of issues on the technology of seismic location of oil and gas (SLONG) related to the seismic impact on productive formations, their emission of emission waves and the method of their isolation during the processing of seismic profiles of CDP-2D. It is shown that the main effect of seismic impact is the occurrence (growth) of fracturing, in which the radiation of high-frequency vibrations by the formation is associated with the formation of shear microcracks, and low-frequency (relative to the frequency of impact) with plastic deformation of the reservoir sections, accompanied by vibrations from ground-based emitters. The method of processing of seismic materials CDP-2D and the method of isolation of radiation areas by a productive layer of low-frequency emission waves are considered. Examples of processing of six profiles of CDP-2D using the considered SLONG technology are given, which indicate the consistency of the results obtained and the possibility of using them to assess the oil and gas prospects of objects. The possibility of using SLONG to search for non-standard hydrocarbon traps is postulated.

Investigation on the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling

Limestone rock widely exists in various rock engineering, its mechanical behavior and permeability evolution have a great influence on the safety and stability of related engineering projects. To study the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling, triaxial compression coupled with permeability tests were carried out. Experimental results reveal that the failure of limestone still conforms to the Mohr-Coulomb criterion under stress-seepage coupling, and effective confining pressure σ3eff can indicate the effect of gas pressure PG and confining pressure σ3 on the deformation parameters of limestone samples. Peak stress σP and residual stress σR increase with the enhancement of confining pressure and decrease with the ascent of gas pressure. Permeability evolution of limestone under different gas pressures and confining pressures is generally similar, and the trend of initial permeability kI, peak permeability kP and residual permeability kR decreasing with effective confining pressure σ3eff can be expressed by the exponential function. The increase of confining pressure and decrease of gas pressure will retard the acoustic emission (AE) response and inhibit the propagation of microcracks. More importantly, limestone samples always show composite failure modes dominated by tensile cracks under low confining pressure, while under high confining pressure, the limestone samples show shear failure modes. The difference is limestone samples are composed of tensile microcracks on the microscale under stress-seepage coupling, and the proportion of shear microcracks grows as gas pressure PG and confining pressure σ3 increases. This research can provide a basic reference for relevant engineering applications.