Mechanical Behavior Of Rock Research Articles

Study on the Mechanical Behavior and Acoustic Emission Properties of Granite under Triaxial Compression

To study the rock mechanical behaviors and damage process mechanism of granite samples under triaxial stress, conventional triaxial compression tests were carried out on an RMT-150B rock mechanics testing machine and acoustic emission detector. The test results show that the strength of the granite sample has a good linear relationship with the confining pressure, the cohesion force c of the granite samples is 29.37 MPa, and the internal friction angle is 54.23° by calculation based on the Mohr-Coulomb strength criterion. The larger the initial confining pressure of the rock sample is, the larger the crack initiation stress ( σ ci ) and dilatancy stress ( σ cd ) of the granite specimen are, the larger the energy values at the crack initiation point and dilatancy point are, and the larger the peak energy storage and energy release rate at the failure are. In the case of a small initial confining pressure, the AE ringdowning counts and the cumulative AE ringing counts increase to their maximum instantaneously at the peak stress point, and the damage of the sample develops rapidly. While the initial confining pressure is high, the AE ringing counts and the cumulative AE ringing counts of the granite specimens increase evenly, and the deformation damage of the granite specimens is slow. Before the crack initiation point, AE signals are mainly low-energy and low-frequency friction-type AE events, while after the dilatation point, AE signals of samples are mainly high-frequency and high-energy fracture-type AE events. The failure mode of granite samples judged by acoustic emission parameters according to the distribution of characteristic values of AE parameters RA and AF is consistent with the reality. The AE b value of the granite sample is large when the confining pressure is low, and there will be a sudden drop, the decrease time is late, and the decrease rate is large. Under the same stress level, the larger the confining pressure is, the larger the damage variable D is.

Development and Application of a New Triaxial Testing System for Subzero Rocks

Abstract To face the challenges of coal production today, the mechanical behaviors of rock in subzero and disturbed environments are critical in the geomechanics of coal mining and artificial freezing engineering. Disturbances significantly affect the long-term mechanical properties and nonlinear viscoelastic-plastic deformation characteristics of frozen soft rock. This article summarizes current rock triaxial test systems, which cannot concurrently solve the problems of subzero temperatures, creep, disturbance, and high sampling rates. Based on existing equipment limitations and the current research aims, an innovative, applicable, and scalable measurement/control system based on National Instruments hardware is integrated into LabVIEW to design and construct a new triaxial testing system capable of achieving both dynamic disturbances and subzero temperatures. The testing system can produce a basic environment with confining pressures up to 30 MPa, axial forces up to 500 kN, and lowest temperatures of −30°C. The system has a 51.2 kS/s sampling using 5-channel closed-loop control. By changing the transfer valve between the supercharger and the servo valve, both conventional triaxial compression and creep tests can be carried out. The triaxial creep tests are performed on frozen rock specimens under step loading and graded disturbances to analyze the creep deformation characteristics and failure modes of the specimens. The results show that at low stress levels, the disturbance load closes and reorganizes microcracks in the rock specimens, reducing the specimen deformation rate. With increasing stress levels or disturbance intensities, the specimen cracks expand rapidly, the creep rate increases, the disturbance energy stored in the rock specimen releases, and the specimens rapidly fail. Therefore, this disturbance creep phenomenon must be considered, and timely support should be provided for exposed frozen rock wall to guarantee construction safety. Moreover, the test results also verify that the design of the new testing system is reliable, feasible, and useful.

Petroleum rock mechanics: An area worthy of focus in geo-energy research

Petroleum rock mechanics: An area worthy of focus in geo-energy research

Experimental study on the effect of granular backfill with various gradations on the mechanical behavior of rock

Backfilling is a common practice in the mining industry and the backfilling performance plays a significant role in supporting the surrounding rock mass. To evaluate the backfilling performance, an experimental apparatus has been developed to understand how backfill affects the compressive strength of sandstone specimens in the laboratory. Pebbles were selected to model the backfill and divided into six groups with different particle sizes using a set of standard sieves. The backfilling pebbles with three types of particle size compositions were then produced, i.e. single gradation, two adjacent gradations, and increasingly widening gradations. A series of compressive tests were carried out to study the mechanical behavior of the sandstone specimens confined by these pebbles. The effects of the gradations of the filled pebbles on the peak and residual compressive strengths were analyzed. It is found that the increasing amount of the compressive strength is over 10% in most cases, even up to 20%. Based on the experiment data, the increasing amount was also estimated theoretically under some assumptions and it further confirmed the experimental results. The effects are closely related to the gradations of the filled pebbles except for their dense degree.

A Damage Constitutive Model for a Rock under Compression after Freeze-Thaw Cycles Based on the Micromechanics

The freeze-thaw cycles will cause continuous damage to the rock, which is much related to the microcrack length, rock permeability, and frost heaving pressure. However, the failure mechanism of the rock under compression after freeze-thaw cycles is not very clear; therefore, it is studied with the damage theory here. First of all, according to the hydraulic pressure theory, the relationship between the frost heaving pressure and the microcrack propagation length in one single microcrack is established based on the elastoplastic mechanics and fracture theory. Second, by assuming the total strain of the rock under compression is comprised of the initial damage strain, elastic strain, additional damage strain, and plastic damage strain, a constitutive model for a rock based on the deformation and propagation of the microcrack under compression after freeze-thaw cycles is established. Finally, the proposed model is verified with the test result. In all, the proposed model can perfectly reflect the deterioration of the rock mechanical behavior under compression after the freeze-thaw cycles.

Logging evaluation on mechanical-damage characteristics of the vicinity of the wellbore in tight reservoirs

The rock mechanical behavior and damage characteristic is of great importance for in situ stress evaluation, wellbore stability analysis and hydraulic fracturing design. The velocities of elastic waves are usually reduced in the presence of rock damage, it may be used for determining the progressive damage of the rock. Therefore, this paper aims to investigate the damage characteristics of transversely isotropic tight sand formation, the rock mechanical and damage parameters in the vicinity of the wellbore were calculated using acoustic logging data. The results indicated that the Poisson's ratio and damage parameters decrease with increasing in radial distance, while the elastic modulus and Thomsen’s coefficients increase. At the same radial position, the vertical elastic modulus is smaller than that of the horizontal, the degree of anisotropy for P-wave is greater than that of S-wave, and the horizontal damage parameter is greater than that of the vertical, which indicated that the micro-cracks near the wellbore mainly occur in the horizontal direction. The changes in mechanical parameters, Thomsen’s coefficients and damage parameters rapidly changed in the range of 1.0–1.8 times of borehole radius. The variations of Thomsen’s coefficients and damage parameters in mudstone are obviously greater than that of sandstone, which may be due to the induced damage between rocks and drilling fluid of mudstone is much higher than sandstone.

A mechanical model for the circular tunnel considering the interaction between the surrounding rock and support structure based on the unified strength criterion

It is very significant to study the stress and displacement fields in the surrounding rock induced by the tunnel excavation and its support. First of all, in order to take into account the effect of the intermediate principal stress on the tunnel surrounding rock mechanical behavior, the unified strength criterion (USC) is adopted to describe the plastic yield behavior of the surrounding rock. Secondly, the model proposed by Kastner does not consider the effect of the initial elastic displacement of the tunnel surrounding rock before the support structure is constructed and the interaction between the surrounding rock and the support structure. Therefore, a new mechanical model for the circular tunnel, which can overcome these shortcomings, is finally proposed by considering the tunnel practical construction process on basis of Kastner’s method. The effects of the calculation parameters such as the intermediate principal stress coefficient b, the initial elastic displacement u0 at the tunnel wall, the rock internal friction angle and the stiffness ks of the support structure on the surrounding rock plastic zone, and radial displacement of the tunnel wall are studied in detail. The results of the calculation examples show that the tunnel surrounding rock relative plastic zone rp/r0 and ground radial displacement us of the tunnel wall decrease with increasing b. With increasing u0, rp/r0 and us both increase, while with increasing ks and the rock internal friction angle, rp/r0 and us both decrease. In sum, it indicates that the proposed model for the circular tunnel can perfectly reflect the tunnel mechanical behavior.

Mining-Induced Redistribution of the Abnormal Stress under the Close Bearing Coal Pillar for Entry Design

Underground space is vulnerable to large deformation influenced by the abnormal stress induced by the bearing coal pillar. A numerical simulation model was established to determine the redistribution of the abnormal stress induced by the mining activities. The double-yield model, the strain softening model, the interface model, and the Mohr–Coulomb model were determined to simulate the gob compaction effect, the pillar strength reduction effect, the structure plane discontinuity effect, and the rock mechanical behavior, respectively. This numerical simulation model is reliable to predict the abnormal stress under the bearing coal pillar by the comparison of the abutment stress from this model and the existing theoretical model as well as the entry roof surface displacement from this model and the field measuring method. The results from the validated numerical model indicate that the abnormal stress including stress concentration coefficient, stress gradient, and lateral pressure coefficient will redistribute to another state that the stress concentration coefficient and stress gradient increase gradually and then decrease, and the lateral pressure coefficient decreases gradually, then increases, and finally decreases sharply with the approach of the mining working face. Their maximum increasing rates are calculated as 121.05%, 198.56%, and 236.82%, respectively. This predicted mining-induced redistribution of the abnormal stress is available for designing the underground entry layout in the determination of the entry position, determination of the driving operation time, mining disturbing range warning, and the prediction of the strengthening support area.

Quartz types, silica sources and their implications for porosity evolution and rock mechanics in the Paleozoic Longmaxi Formation shale, Sichuan Basin

Quartz is a common mineral in mudrocks, displays variable forms and can have multiple origins. Consequently, the form and source of quartz in mudrocks may have a distinct impact on porosity evolution and rock mechanics such as brittleness. In this study, 51 shale samples from both the Lower and Upper parts of the Longmaxi Formation in the Sichuan Basin of China were collected for geochemical, mineralogical and elemental analyses, microscopic observation, porosity characterization, as well as rock mechanics testing, in order to better understand the quartz types, the sources of the silica, and their impacts on porosity evolution and rock mechanical behavior. Optical microscopy and field emission-scanning electron microscopy (FE-SEM) with energy dispersive spectroscopy (EDS) were applied to describe the crystal morphology and investigate the types and silica sources of quartz and their associated pores. Images from both methods reveal several forms of quartz in the Longmaxi Shale, including terrigenous detrital quartz, organism skeletal quartz, quartz overgrowths, authigenic microquartz and quartz veins. Furthermore, the authigenic microquartz has three subcategories, namely clay matrix-dispersed, euhedral and amorphous. The distinguishing criteria include crystalline morphology, luminescence, grain size and forming stage (including the stage of deposition and different periods of the diagenesis progress). Geochemical proxies, such as an Al–Fe–Mn ternary diagram, negative correlations of Zr vs. SiO2, and positive correlations of TOC vs. excess Si, indicate a substantial contribution of biogenic silica to the total silica in the Lower Longmaxi Formation. All possible pores associated with quartz are systematically described and classified into 3 types and 8 subtypes, notably dissolution pores, organism skeletal cavity pores, crystal defect pores, pores between quartz grains, grains contact micro-fractures, shrinkage micro-fractures and quartz filling micro-fractures. This study shows that the origin of the quartz may have implications for porosity evolution and rock mechanics. Positive correlations of surface area vs. excess Si indicate that biogenically derived forms of microquartz (including euhedral microquartz and amorphous microquartz cements) play a constructive role for micro-scale pore development. The correlation between high brittleness index and excess Si in the Lower Longmaxi Formation suggests that biogenic silica is the major cause of the brittleness.

Analysis of capillary water imbibition in sandstone via a combination of nuclear magnetic resonance imaging and numerical DEM modeling

The physics of water imbibition into initially unsaturated sandstone is critical to the understanding of displacement processes and fluid transport in the vadose zone. The distribution of water within rock is important due to its significant influence on rock mechanical behavior. Here, therefore, we used nuclear magnetic resonance (NMR) technology to visualize and quantify the dynamics of water infiltration and distribution in initially unsaturated sandstone. The progression of water imbibition in sandstone specimens under the following two conditions were analyzed: (1) specimens soaked in water for different durations and (2) specimens soaked in water for the same duration and then held in this state for different durations. An analytic function was developed to estimate the sandstone moisture profile and to determine the unsaturated flow within the sandstone when the water distribution matched laboratory observations. Finally, a three-dimensional discrete element grain-based model was formulated that incorporates the local parallel-plate method, the unsaturated flow function, and the generalized effective stress principle. We used this model to effectively reproduce the process of water imbibition in the laboratory sandstone specimens. The effect of water imbibition on the mechanical properties of the studied sandstone was also evaluated. These results show that the strength of the core of the sample was reduced as water migrated from its surface to its center, resulting in a decrease in bulk sample strength as standing duration (i.e. water distribution uniformity) increased. The results of this study aid in our understanding of the influence of water imbibition on the mechanical behavior of sandstone, which is important for rock slope stability assessments following rainfall.

Coupled Hydrologic-Mechanical-Damage Analysis and Its Application to Diversion Tunnels of Hydropower Station

Since the traditional model cannot sufficiently reflect the multifield coupling problem, this paper established an elastoplastic stress-seepage-damage analysis model considering the seepage field, stress field, and damage field. Simultaneously, the elastoplastic damage model involves many parameters and is difficult to determine. An inverse analysis program is compiled based on the differential evolution algorithm, and the surrounding rock damage parameters are inverted. Finally, the elastoplastic stress-seepage-damage coupling program and the damage parameter displacement back analysis program is compiled using C++ language. Then, the program is used to calculate the coupling problem of tunnel elastoplastic stress-seepage-damage. The results show that the proposed elastoplastic damage constitutive model can well describe the mechanical behavior of rock. The computational procedure can also simulate practical engineering problems, which can provide specific guidance for site construction.

Determination of Scale Effects on Mechanical Properties of Berea Sandstone

The key rock mechanical parameters are strength, elastic modulus, Poisson’s ratio, etc., which are important in reservoir development. The accurate determination of reservoir’s mechanical properties is critical to reduce drilling risk and maximize well productivity. Precisely estimating rock mechanical properties is important in drilling and well completion design, as well as crucial for hydraulic fracturing. Rocks are heterogeneous and anisotropic materials. The mechanical properties vary not only with rock types but also with measurement methods, sample geometric dimensions (sample length to diameter ratio and size), and other factors. To investigate sample scale effects on rock mechanical behaviors, unconfined compression tests were conducted on 41 different geometric dimensions of Berea sandstones; unconfined compressive strength (UCS), Young’s modulus ( E ), Poisson’s ratio ( υ ), bulk modulus ( K ), and shear modulus ( G ) were obtained and compared. The results indicate that sample geometry can significantly affect rock mechanical properties: (1) UCS decreases with the increase of length to diameter ratio (LDR), and the UCS standardize factor is between 0.71 and 1.17, which means -30% to +20% variation of UCS with LDR changing from 1 to 6.7. The test results show UCS exhibits positive relationship with sample size. (2) Young’s modulus slightly increases with LDR increases, while Poisson’s ratio decreases with the increase of LDR. For the tested Berea sandstones, Poisson’s ratio standardizing factor is between 0.57 and 1.11. (3) Bulk modulus of Berea sandstone samples decreases with the increase of LDR, while shear modulus increases with LDR increases. Both bulk modulus and shear modulus increase with the increase of sample size. (4) The principal failure modes were analyzed. The failure modes of the tested Berea sandstones are axial splitting and shear failure. Stocky samples ( LDR < 2 ) tend to go axial splitting, while slender samples ( LDR > 2 ) tend to show shear failure.

Experimental investigation on fracture behaviors and acoustic emission characteristics of sandstone under different strain rates

To further investigate differences of fracture behaviors and acoustic emission characteristics of rock under different strain rates, unconfined uniaxial compressive tests of thirty cylindrical specimens with 50 mm in the diameter and 100 mm in the height were performed within quasi-static strain rates of 10–5–10–2 s−1 by rock material testing system and multi-channel acoustic emission (AE) monitoring system. The uniaxial compressive strength, peak strain and elasticity modulus, failure mode, fractal dimension and equivalent average size of the rock fragments, and the three-dimensional spatial distribution of acoustic emission events were obtained and analyzed. The experimental results verify that the uniaxial compressive strength, elasticity modulus and peak strain are strain rate-dependent significantly. Particularly, with increasing strain rates, macroscopic failure modes present three types of single shear failure, multiple shear failure, and shear and tensile mixed failure, and the rock fragments show a smaller equivalent size and a larger fractal dimension, which resulted in a higher fragmentation degree. Besides, it is noted that the evolution process of acoustic emission ring-down count and spatial distribution of events under different strain rates are closely related to macroscopic failure modes. Furthermore, characteristic stresses based on the evolution process of acoustic emission ring-down count, such as the elastic stress, initiation stress and yield stress, were analyzed deeply at different strain rates. A simplified rock model consisting of discrete fracture network and equivalent elementary volume was proposed to discuss the effect of strain rate on energy accumulation and dissipation. Finally, to better reveal the micro mechanism of the effect of strain rates on rock mechanical behaviors, the microscopic fracture morphology was observed and characterized by means of scanning electron microscopy. The experimental results should be taken into account in underground mining, tunnel excavation and other rock engineering activities.

Experimental and simulation study of the tensile strength of wet carbonate rocks using the Brazilian Test

Mechanical behavior of rocks would have substantial implications for different operations from fluid injection and hydraulic fracturing in hydrocarbon reservoirs to underground carbon dioxide storage in the crust of the earth. In all of these operations, restricting the undesired seismicity events and preserving the integrity of formations is a crucial subject that enforces engineers to evaluate the mechanical behaviors of constituent rocks and predict the formation response. Tensile strength is an important factor for maintaining caprock and reservoir formation from induced tensile cracking which can be influenced by the pore fluid. This paper focuses on the tensile strength of carbonate rock, as an important reservoir rock, in the presence of seawater, as a common injection fluid candidate for enhanced oil recovery objectives. For this purpose, twelve dolomite samples from the Asmari-Jahrum Formation outcrop in Fars province, Iran, were prepared for performing tensile tests. Also for a thorough comparison, sample properties, fluid contents, digital image correlation measurements, and a finite element simulation have been investigated. The experimental results showed a noticeable reduction in the tensile strength of dolomite samples in the presence of distilled water and seawater. In comparison with a dry sample though, a reduction of 16% tensile strength for a seawater saturated sample was observed for similar samples. Among the various mechanisms, which are reported in the open literature, the hydration force seems to be the main reason for the tensile strength reduction and this conclusion was examined by measuring the mass of vaporized adsorbed water on the powder of the same dolomite sample. In addition by testing flooded samples, the impact of flow on the tensile strength was examined. For both cases of seawater and distilled water flooding, the samples which were placed in the outlet zone during the flooding showed lower tensile strength. Moreover, simulation results confirmed the similarity of the weakening trend and impact of seawater.

A new look at rock mechanical behavior from the meso-scale grain

The mechanical behavior of the rock at the macro-scale depends significantly on its granular meso-scale structure as well as properties of each mineral component. Current methods only calibrate the mechanical properties from different macro-scale samples, which certainly introduces limitations to recognize the realistic physics behind rock failure. To address this problem, theoretical analyses of meso-scale mechanics for mineral grains representing the inhomogeneity are studied based on the overall mechanical behavior of the aggregated rock material. Measured porosity and permeability as well as quantitative mineral content from X-Ray Diffraction yields peak patterns are used to build this meso-scale models which are then calibrated with typical laboratory mechanical tests. Through a number of numerical simulations for the uniaxial compression test, the influence of size effect and spatial arrangement of mineral grains on the overall mechanical behavior and failure at the macro-scale sample are analyzed. The macro failure could be attributed to the accumulation and interconnection of the damaged grains. Compared with the results of uniaxial compression tests, numerical results show good agreement, which demonstrates the rationality and accuracy of that novel approach. The proposed methodology may help the operator to estimate rock compressibility and its failure mode under different loadings by merely using micro CT-scan images rather than direct testing in the lab.

Experimental study on rock mechanical behavior retaining the in situ geological conditions at different depths

Understanding the real physical and mechanical characteristics of deep rocks under in situ geological conditions is fundamental for deep rock engineering. Rock mechanics that retains the in situ geological conditions is defined to study the rock behaviors under the in situ geological conditions (geostress, temperature, pore pressure, etc.). A new experimental methodology called the in situ stress restoration test is proposed to recover the in situ geostress conditions of a core as closely as possible. Cores obtained from 10 different burial depths are used to carry out in situ stress restoration tests and triaxial compression tests. The results of the two kinds of tests indicate that in situ stress restoration would heal the damage induced by removing the in situ geostress during the coring process to a certain extent. Compared with the rock mechanical behaviors observed during the triaxial compression tests, the peak strength, residual strength and elastic modulus are higher during the in situ stress restoration tests. In situ stress restoration can make a core less brittle and enhance its inelastic deformation, including strain hardening and strain softening, especially at deeper burial depths. The elastic energy density, dissipated energy density and total energy density are all higher during the in situ stress restoration tests than those during the triaxial compression tests for the different rock types and burial depths. The proposed experimental method can consider the effects of the in situ geostress to only a certain extent. Rock experimental techniques that can comprehensively reflect all the in situ geological conditions at different depths should be further studied.

Experimental study on acoustic emission frequency characteristics of granite under different cyclic loading and unloading

In Granite acoustic emission tests were performed during three different cyclic loading and unloading processes. The relationship between the acoustic emission frequency distribution pattern and the loading and unloading mode of granite during the cyclic loading and unloading process was discussed. The distribution pattern of the peak frequency with the stress evolution is analyzed qualitatively and quantitatively. Studies have shown that: in the first cycle of loading and unloading at each stage of the three cyclic loading and unloading processes, the acoustic emission ringing count and peak frequency are significantly increased, which has a significant Kaiser effect. It proves that the Kaiser effect essentially remembers the extent of the damage inside the rock previously. In the process of loading and unloading in the fourth stage cycle, the AE ring count showed a “U”-shaped change law, showing a significant Felicity effect. Comparing the three different cyclic loading and unloading modes, with the increase of the lower limit stress, the stress amplitude decreases, and the AE ringing count corresponding to the lower limit stress value of the unloading gradually decreases, and the peak frequency distribution shows more dense or concentrated characteristics. At the beginning of the third stage cycle, the proportion of high frequency band (140∼162kHz) gradually decreased, and the proportion of lower frequency band (11-18kHz) gradually increased. With the increase of the lower limit of cyclic loading and unloading, the lower frequency band (11-18kHz) in the fourth-stage cyclic loading and unloading process takes up a higher proportion, even as high as 95%. It indicates that the rock samples at this stage mainly undergo large-scale failure, and the rock samples are in a critical failure state. These provide the basis for exploring the evolution mechanism of rock mechanical behavior and the prediction method of instability and damage.

Experimental Study on Rock Strength and Deformation Characteristics Under Triaxial Cyclic Loading and Unloading Conditions

The mechanical behavior of rock under cyclic loading is quite complicated compared to monotonic loading or unloading conditions. The triaxial cyclic loading and unloading testing of rock specimens under 6 confining pressures (σ3) was carried out through the MTS 815 rock mechanics testing system, to explore the strength, deformation, and expansion characteristics of the rock specimens. The stress–strain curves of the rock specimens in the triaxial cyclic loading and unloading testing presented the hysteresis effect. Besides, as σ3 increased, the rock specimen strength increased, while the failure form brittle to ductile. The elasticity modulus (El) increased first and consequently decreased as the cycle index increased, while it increased as σ3 increased. However, the generalized Poisson’s ratio (μl) increased as the cycle index increased, whereas it decreased as the σ3 increased. Based on the Mohr–Coulomb strength criterion and plastic shear strain (γp) as the plastic parameter, the subsequent yield plane model of the loaded rock was characterized by generalized cohesion (c´) and generalized internal friction angle (φ´). Ultimately, the evolution rules of c´, φ´ and Ψ (dilatancy angle), with σ3 and γp were revealed. Moreover, the post-peak dilatancy angle models with regard to the influence of σ3 and γp on the volume dilatancy of the rock specimen were established, which indicated that Ψ increased first and consequently decreased along with the γp increase, whereas it decreased as the σ3 increased.

Effects of flaw width on cracking behavior of single-flawed rock specimens

Cracking behavior of flawed samples has been extensively studied in rocks and rock-like materials using experimental and numerical approaches. However, the existing research mainly focuses on the effects of flaw length l and inclination angle α on rock mechanical behavior, a limited attention has been given to understand the role of flaw width b on crack initiation and propagation in relation to mechanical parameters of rocks. In this paper, a two-dimensional particle flow code (PFC2D)-based synthetic rock model, which is calibrated by the laboratory sandstone properties, is used to investigate the effects of flaw width on cracking behavior of single-flawed rock samples under uniaxial compression tests. Numerical simulation results show that the flaw width b has significant effects on the crack geometrical parameters (crack initiation location d and crack initiation angle θ). The values of θ and d significantly decrease with the increase of flaw width when b < 1.0 mm and their values tend to be stable with the further increase of flaw widths. It is also found that the mechanical properties (uniaxial compressive strength UCS and deformation modulus Es) of flawed rock samples are related to the crack geometrical parameters (θ and d) and the effects of flaw width on the values of UCS and Es of single-flawed rock specimens is significant when b < 1.0 mm for lower crack inclination angle (α < 60°).

Study of Damage Constitutive Model of Brittle Rocks considering Stress Dropping Characteristics

Deep brittle rock exhibits characteristics of rapid stress dropping rate and large stress dropping degree after peak failure. To simulate the whole process of deformation and failure of the deep brittle rock under load, the Lemaitre strain equivalent theory is modified to make the damaged part of the rock has residual stress. Based on the damage constitutive model considering residual strength characteristics, a correction factor reflecting stress dropping rate is added, the Weibull distribution is used to describe the inhomogeneity of rock materials, and Drucker–Prager criterion is used to quantitatively describe the influence of stress on damage; a damage constitutive model of deep brittle rock considering stress dropping characteristics is established. According to the geometric features of the rock stress-strain curve, the theoretical expressions of model parameters are derived. To verify the rationality of the model, triaxial compression experiments of deep brittle rock under different confining pressures are conducted. And the influence of model parameters on rock mechanical behaviour is analysed. The results show that the model reflects the stress dropping characteristics of deep brittle rock and the theoretical curve is in good agreement with the experimental results, which indicates that the proposed constitutive model is scientific and feasible.