Rock-like Specimens Research Articles

Uniaxial compression test and numerical simulation of rock-like specimen with T-Shaped cracks

Uniaxial compression test and numerical simulation of rock-like specimen with T-Shaped cracks

Experimental study on acoustic emission stress memory function of rock-like specimens under uniaxial compression

The Kaiser effect in rock acoustic emission (AE) test is the most direct manifestation of rock memory function. This article focuses on the influence of different deformation stages and different historical stress conditions on stress memory function, and conducts AE testing of rock-like specimens. It explained the stress memory function in AE testing from the perspectives of crack propagation and damage accumulation. The crack initiation stress σci and crack damage stress σcd of specimens were obtained based on the stress-strain curve method, and the different deformation stages were divided. The damage evolution coefficient [Formula: see text] was proposed to measure the size of the stable development range of damage based on the normalized crack initiation and crack damage stress. The historical stress in the elastic stage could be easily identified from the Kaiser effect during the reloading process, even if the time interval reached 120 hours. The Felicity effect appeared during the reloading process when the historical stress was in the stage of stable crack propagation, and the FR value showed a decreasing trend with the extension of the time interval between loading tests. The loading history in the elastic stage was a training for the AE stress memory function under complex historical stress conditions, which restored the Kaiser effect in the stage of stable crack propagation. The distribution of AE events and CT scanning results were also analyzed in the article, and the damage accumulation information characterized by both are basically consistent. The double Kaiser effect phenomenon appeared in the AE test under complex historical stress conditions, although the criterion for discriminating the AE signal at the Kaiser effect point corresponding to the lower stress remained to be further studied and verified.

Experimental study on mechanical and fracture characteristics of inclined weak-filled rough joint rock-like specimens

The uniaxial compression experiments with acoustic emission (AE) system are conducted to explore the coupling effect of the JRC and dip angle (α) on the mechanical and fracture characteristics of rock-like specimens containing weak-filled rough joints (WFRJ). With the increasing α, the variation of mechanical properties and the percentage of tensile cracks shows a trend of decreasing and then increasing; the variation of accumulated AE count shows a trend of increasing first, then decreasing, and then increasing again. The increasing JRC weakens the influence of α on mechanical properties but strengthens AE characteristics and the percentage of tensile cracks. The action mechanism of JRC for mechanical properties and AE characteristics is mainly reflected in the waviness and unevenness for α = 27˚ − 60˚ but only in the waviness for α = 0˚ − 90˚. The fracture pattern of WFRJ specimens is strongly related to the waviness of the joint profile. The weak-filled layer is usually broken near the inflection point. Then, the fracture of WFL induces the tensile fracture for α = 0˚ − 45˚ but shear fracture for α = 90˚ in the hard matrix from the inflection point. For α = 67˚, the fracture pattern is almost unaffected by the variation of JRC. Finally, the action mechanism of unevenness and waviness of joint profile on mechanical properties, AE characteristics and fracture pattern is analyzed.

Strain field evolution and crack coalescence mechanism of composite strength rock-like specimens with sawtooth interface

The mechanical behaviors of composite rock masses containing a complex interface widely distributed in nature are significantly different from those of monolithic rock masses. The objective of this study is to perform a series of uniaxial compression tests to study the influence of sawtooth interface parameters on the fracture behavior of rock-like composite specimens. Meantimes, the acoustic emission (AE) and digital image correlation (DIC) technique were used to classify damage stages and record strain field evolution, respectively. The results of this study demonstrate that stress–strain curves of composite rock-like specimens with different sawtooth interfaces display characteristics of plastic failure, friction failure characteristics and two different brittle failure characteristics in the post-peak phase. The peak strength of the specimen first decreases and then increases as the inclination of the interface increases, achieving the maximum value of 28.29 MPa at the inclined interface of 90°. The strain concentration zone is mainly generated and expanded around the interface. The parameters of the sawtooth have a significant influence on the type and location of cracks developed along the interface. In addition, there is a strong relationship between the mode of failure of the specimen and the strength of the rock layer and the inclination of the interface. According to the relative positional relationship between the coalescence path of the specimen and the interface, the failure modes of the specimen are classified as single weak rock layer failure (S-AB-0, S-AB-15), oblique failure through rock layer interface (S-AB-30, S-AB-45), slip failure along the interface (S-AB-60, S-AB-75) and axisymmetric failure along the interface (S-AB-90). This result is of great importance for a thorough understanding of the crack coalescence mechanism and strain field evolution of rock-like composite specimens.

Numerical investigation on fracture characteristic and failure mechanism of rock-like materials with intermittent flaws under compressive-shear loading

An extended non-ordinary state-based peridynamic (NOSB-PD) theory is utilized to investigate the fracture characteristics of rock-like materials containing intermittent flaws under compressive-shear loading. The stress-based failure criteria, i.e., the maximum tensile stress criterion and the Mohr-Coulomb criterion, are integrated into the NOSB-PD theory to respectively determine the tensile failure and shear failure of flawed rock-like materials. The distribution characteristics of maximum principal stress and shear stress acquired by the NOSB-PD theory are employed to analyze the fracture failure mechanism of flawed rock-like materials under compressive-shear loading. The effects of inclination angle of the central flaw on the initiation and coalescence modes of flawed rock-like specimens are investigated. The numerical results of flawed rock-like materials are in good agreement with the experimental results. The extended NOSB-PD has the ability to simulate the fracture processes under compressive-shear loading.

A novel DIC-based methodology for crack identification in a jointed rock mass

Crack identification in brittle rocks is a challenging problem in rock mechanics. To represent the details of jointed rock masses, rock-like specimens containing inclined joint sets were generally prepared by three-dimensional (3D) printing using sand. Digital image correlation (DIC) was employed for full-field deformation measurement during loading. The results show that the sand 3D-printed specimen has distinct advantages in the preparation of jointed rock masses. DIC was extended to measure the displacement vector field around newly propagated cracks. Two types of cracks are recognized: tensile and shear cracks. Two different coalescence patterns are classified: step-path failure and planar failure. A covariance matrix-based multivariate measure, the rate of effective variance change (REVC), was proposed to quantify the dispersion of strain data. The variation in the strain dispersion measure was found to be closely related to the crack type. For the first occurrence of tensile and shear cracks, the rates of mutation (RMs) are within the ranges of 1.56–3.42 and 14.54–86.44, respectively, which can be considered a new crack identification criterion. A virtual extensometer method based on DIC was established to interpret the variation mechanism of the REVC associated with different crack types.

The mechanical evolution behaviors and failure mechanism of rock-like specimen containing complex shape goaf

A iron mine in Xinjiang has left a huge goaf after years of mining. The stability of rock mass around goaf is very important to the safe mining of the whole mine. In order to study the structural stability of rock mass around goaf, and 10 sections of the goaf are selected and its fractal dimensions are calculated. Combined with 3D printing technology, rock-like specimens containing goaf (RCG) are made, tests and PFC numerical simulation are carried out, and the mechanical properties, energy consumption characteristics, and damage evolution of RCG are analyzed. The results show that: (1) The mechanical properties of RCG are closely related to the fractal dimension (FD) of goaf. The peak stress, peak strain, and elastic modulus are linearly negatively related to the FD. (2) The absorbed total energy and the stored elastic energy have strong correlation with FD. The energy evolution process of RCG is as follows: RCG slowly absorbs energy to compact the specimen; RCG quickly absorbs energy for storage; RCG quickly releases elastic energy stored inside. (3) The concentrated stress of RCG is 4–6 times of the axial stress, and the crack initiation and propagation are driven by the concentrated stress. The RCG show the through-shear failure starting from the stress concentration zone and developing along the diagonal. (4) The damage evolution process of RCG can be divided into four stages: the stress rises slowly, the damage accumulates slowly, and the damage rate is at a low level; the stress rises rapidly, and the damage accumulates rapidly and the cumulative speed continues to increase; the stress decreases rapidly, and the damage continues to grow at a high level, but the damage accumulation speed decreases rapidly; the damage increases slowly at a high level, and the damage rate decreases slowly and tends to zero.

Strength Properties and Damage Evolution Mechanism of Single-Flawed Brazilian Discs: An Experimental Study and Particle Flow Simulation

Understanding the tensile strength properties and damage evolution mechanism in fissured rock is very important to fundamental research and engineering design. The effects of flaw dip angle on the tensile strength, macroscopic crack propagation and failure mode of symmetrical Brazilian discs of rock-like materials were investigated. A parallel bonding model was proposed to examine the damage of pre-flawed discs under splitting the load. The microscopic parameters of particles and bonds in the model that can characterize rock-like materials’ mechanical and deformation properties were obtained by calibrating against the laboratory test results. The crack development, energy evolution and damage characteristics of Brazil discs containing a single pre-existing flaw were studied at the microscopic scale. The results show that the flaw significantly weakens the strength of the Brazilian disc, and both the peak load and the initial cracking load decrease with increasing flaw angle. The failure modes of the rock-like specimens are mainly divided into three types: wing crack penetration damage mode, tensile-shear penetration damage mode and radial penetration failure mode. Except for the flaw dip angle 0°, the wing cracks generally sprouted at the tip of the pre-flaw, and the wing cracks at both tips of the pre-flaw are centrosymmetric. Crack coalescence was concentrated in the post-peak stage. Based on the particle flow code (PFC) energy partitions, the damage variables characterized by dissipation energy were proposed. The disc specimen’s pre-peak damage variables and peak damage variables decreased with increasing flaw angle, and the damage was concentrated in the post-peak phase.

Study on mechanical and fracture characteristics of rock-like specimens with rough non-persistent joints by YADE DEM simulation

Rough non-persistent joints exist widely in natural rock mass. The roughness of joints influences its mechanical and fracture characteristics. However, the research on rough non-persistent jointed rock mass is inadequate. To investigate the effects of joint angle, rock bridge angle and roughness of rough non-persistent joints, the rough non-persistent joints were truncated and their JRC was calibrated based on the Barton typical roughness profiles. A series of DEM numerical uniaxial compression tests were conducted based on the laboratory tests of rock-like specimens with 3D printed non-persistent joints. By the investigation of mechanical behaviour, crack coalescence types, crack evolution law, fracture modes presented by the strain field, and micro-fracture mechanism revealed by the displacement vectors, it was verified that YADE is suitable to simulate the fracture of rough non-persistent jointed specimens; the six types of coalescence of rough non-persistent joints were concluded, and the relation between micro-crack and macro-fracture was established by the four types of micro-cracks causing macro-fractures.

Failure mechanism of circlular tunnel supported by concrete layers under uniaxial compression: Numerical simulation and experimental test

Under this study, particle flow code modeling and experimental testing were used to examine the failure behavior of a pre-holed rocklike specimen including concrete interlayers in uniaxial compression. Rectangular pre-holed gypsum samples with interlayer concrete have been made specifically for this purpose. The sample's measurements were 150 mm*150 mm*50 mm, and the hole's diameter was 20 mm. Changes were made to the concrete's length, location, and number. There were 0 and 20 mm between the concrete layer and the hole. The lengths of the concrete were 50, 100, and 150 mm. 20 mm thick concrete was used. This complex was quantitatively tested and simulated under uniaxial loading circumstances. 13 experiments in all have been performed. The findings show that concrete length, concrete layer number, and concrete circumstance all have an impact on the crack growth trajectory. Two tensile fractures started in the hole wall while it was in the middle of the concrete layer and spread parallel to the loading axis until coalescing with the sample edges. By increasing the distance between the concrete layer and the hole, more tensile cracks and oblique fractures occurred. By lengthening the concrete, the model's crack count was reduced. The shear bands at the hole were expanded by extending the distance between the concrete layer and the hole when the hole was encircled by two concrete layers. Additionally, by shortening the concrete, more shear bands were formed close to the hole. Longer concrete had higher young modulus, fracture initiation stress, ultimate strength, and absorbed energy. Additionally, the concrete layer's distance from the hole was increased, which diminished these mechanical qualities. By increasing the number of concrete layers, the concrete's young modulus, fracture initiation stress, ultimate strength, and absorbed energy were all raised. The results of the numerical simulation agree well with those of the experiments. Based on the results of this article, an innovative method and a unique technique can be proposed to install the concrete lining layers at different dimensions around the tunnel in such a way that the circular cross-section of the tunnel remains constant.

Influence of Prefabricated Fissure Combinations on Strength and Failure Characteristics of Rock-Like Specimens under Uniaxial Compression

Owing to the uncertainty in the rock formation process, there are different forms of fissure combinations that significantly influence the stability of engineering. In this paper, the peak strength and failure characteristics of rock-like materials with four types of double prefabricated fissure combinations was investigated by combining similar material tests of the digital image correlation (DIC) techniques and discrete-element numerical method (PFC2D). With reference to the experimental observation of crack initiation and propagation, the prefabricated fissure combination effect was analyzed by varying the fissure angle and its combination type. The results indicated that the mechanical properties of the specimen with prefabricated fissures were affected by the fissure inclination angle. When the double fissure inclination angles were 45°, the total fracture length was longest and the peak strength and the proportion of dissipated energy were smallest. It can be obtained that when the fissure inclination angle reached 45°, the effect of the prefabricated fissure combination was maximum. Except for the specimens whose fissure inclination angles had 90° cracks that processed along a perpendicular direction, the cracks of other types propagated and coalesced near the lower side of the middle specimen. The research can provide guidance for controlling engineering disasters involving complex fissure combinations under natural geological conditions.

Excavation unloading response of cylindrical rock-like specimen with axial joints: laboratory experiment and numerical simulation

Abstract Joints have a significant influence on the deformation and failure mechanism of the surrounding rock. To reveal the influence of axial joint on mechanical response of the roadway surrounding rock after excavation unloading, the deformation and failure characteristics of cylindrical rock-like specimen (CRLS) with an axial joint was studied through laboratory tests and numerical simulation. Also, the influence of joint size and position on strain variation, secondary stress evolution and plastic zone distribution of the CRLS were analyzed. Results show that the axial joint can promote the deformation of the surrounding rock on the inner side of the joint, while hindering the deformation of the surrounding rock of the roadway on the outer side of the joint. In addition, the size and position of the axial joint have a significant influence on the distribution of secondary stress and plastic zone of the surrounding rock. The stress relief zone is mainly located between the joint and the excavation profile, whereas the plastic zone is mainly distributed at both ends of the joint and between the joint and the excavation profile. Finally, the tangential stress concentration can be alleviated by choosing proper distance between the joints and the roadway, and the failure mode of the surrounding rock between the joint and the excavation profile transmitted from tensile failure to shear failure with the axial joint moves away from the roadway. The research results could provide technical reference for roadway support and disaster prevention in a deep jointed rock mass.

Influence of intermittent opening density on mechanical behavior and fracture characteristics of 3D-printed rock

To investigate the influence of intermittent opening density on the mechanical behavior and fracture characteristics of rock, sand 3D-printed specimens containing different opening numbers are prepared using quartz silica sand and furan resin adhesive as printing substrates. Uniaxial compression testing is performed on these rock-like specimens. Based on digital image correlation (DIC) technology, a methodology for identifying crack types and evaluating rock damage is proposed to quantify the mechanical mechanism of crack evolution during loading. The experimental results are verified and explained using the rock failure process analysis (RFPA) code. The results show that with increasing intermittent opening density, the stress–strain curves of the specimens are similar. Meanwhile, the increase in intermittent opening density causes the stress field distribution of the specimen to change, which in turn leads to a decrease in the uniaxial compressive strength and elastic modulus. The types of cracks around the opening can be grouped into tensile cracks (T) and shear cracks (S), and the coalescence modes of the rock bridge between the openings can be grouped into shear coalescence (C1) and traction coalescence (C2). As the intermittent opening density increases, the coalescence type between openings changes from C1 to C2, and the overall failure pattern is converted from tensile-shear failure to tensile failure. The damage factor (Df) evolution is closely related to the intermittent opening density of the specimens. A one-way exponential decay correlation between the stress ratio and the damage factor is found. The above study is expected to deepen the understanding of the deformation and fracture mechanism of rock masses with openings and to provide a reference for the application of 3D printing technology in the rock mechanics field.

Research on the Fracture Properties and Mechanism of Carbon Dioxide Blasting Based on Rock-like Materials

Liquid carbon dioxide blasting technology has a wide range of applications and is characterized by sound fracturing effects, low vibration hazards, and high safety. In order to investigate the characteristics and mechanism of CO2 phase change rock breaking, liquid CO2 blasting tests on rock-like specimens were carried out in this paper. The results show that 130 MPa is the threshold value at which a CO2 blasting system moves from dynamic tensile stress damage to dynamic pressure stress damage. When blasting pressures of 100 MPa and 70 MPa are used, the lumpiness ratio of the fragments does not change much as the strength of the rock changes, so a suitable blasting pressure should be chosen to improve the blasting effect. Under the impact of blast stress and high-pressure gas flow, cracks develop to form a rough failure surface.

Experimental and Numerical Investigation on Crack Propagation and Coalescence in Rock-Like Specimens with Fluid-Infiltrated Parallel Flaws

Experimental and Numerical Investigation on Crack Propagation and Coalescence in Rock-Like Specimens with Fluid-Infiltrated Parallel Flaws

Effect of bedding plane on mechanical properties, failure mode, and crack evolution characteristic of bedded rock-like specimen

To study the influence of bedding planes on mechanical properties and failure characteristics of specimens, an experimental test was conducted first on bedded rock-like specimens by uniaxial compression test with acoustic emission and digital image correlation. Experimental results show that both uniaxial compression strength (UCS) and failure strain (εf) present a tendency to first decrease and then increase with the increase in bedding angle (α). Moreover, the existence of bedding planes changes failure mode and fracture process of bedded specimen. Based on experimental results, three-dimensional numerical models containing bedding planes were used to determine the effect of orientation angle (α), spacing (s), and strength (λ) of bedding planes. Numerical results are as follows: (1) For α = 0° and 30°, the increase in s leads to the decrease in translaminar tensile crack and increase in translaminar shear crack, and the trend is reversed for α = 45° and 60°. Besides, (2) the variation of s leads to the variation of the percentage of the crack in bedding and rock matrix. However, (3) with the increase in s, the UCS and εf of bedded specimen at the same α remain almost unchanged. Unlike the influence of s, (4) the UCS and εf of bedded rock for α = 30°–60° are positively correlated with λ. (5) Moreover, with the increase in λ, the BP-crack percentage decreases dramatically, leading to the variation of the failure mode of bedded rock.

Study on mechanical and fracture characteristics of inclined weak-filled rough joint rock-like specimens

Weak-filled rough joint (WFRJ) strongly influence the mechanical and fracture properties of the rock mass. A uniaxial compression test with acoustic emission (AE) and digital image correlation (DIC) system was first conducted on the WFRJ rock-like specimens with a fixed inclination and filling thickness. The test results showed that the increasing JRC of WFRJ caused an increase and then a decrease in peak stress (σp), a steady increase in failure strain (εf) and a steady decrease in elastic modulus (E). And the increasing roughness of WFRJ changes the fracture pattern from sliding fracture along the hard-weak interface to mixed tensile-shear fracture. The AE results demonstrate that the increasing roughness in WFRJ caused the quantity increase in total cracks and the percentage decrease in shear cracks. Then, mechanical models considering local roughness and global structure of the weak filling layer were established to analyze the force characteristic of WFRJ rock-like specimens. Finally, ANSYS/LS-DYNA was used to numerical study the roughness effect of WFRJ specimens. The stress evolution in numerical results is consistent with the X-direction strain field evolution in DIC results. And the quantity and percentage variation laws of cracks in each part were verified by the numerical study results.

Crack Coalescence Behavior of Rock-Like Specimens Containing Two Circular Embedded Flaws

Abstract Experimental research on the growth of internal flaws has rarely been reported due to the fact that it is difficult to cut internal flaws in specimens and cannot capture the initiation and propagation processes of internal flaws through direct observations. This paper proposed a method for creating internal flaws in specimens by utilizing the volatilization of camphor. A series of compression tests were performed on rock-like specimens including two embedded circular flaws, and CT techniques were used to investigate the internal damage behavior of flawed specimens. Experimental results illustrate that the strength and deformation properties of flawed specimens increase nonlinearly with the confining pressure as well as flaw inclination angle. Crack coalescence patterns and failure modes of flawed specimens depend on not only the confining pressure but also the flaw inclination angle. The crack coalescence pattern varies from wing crack coalescence to mixed tension-shear crack coalescence and then to the shear crack coalescence as the crack inclination angle increases. Confining pressure contributes to shear crack growth and has an inhibiting effect on the propagation of tension cracks. For specimens with the same flaw inclination angle, the failure mode changed from tension failure to mixed shear-tension failure or from mixed shear-tension failure to pure shear failure with the increase of confining pressure.

Three-dimensional cracking and coalescence of two spatial-deflection joints in rock-like specimens under uniaxial compression

Aiming at the fact that joints in rock mass exhibit different spatial extension directions, uniaxial compression tests of rock-like specimens were carried out to investigate three-dimensional cracking and coalescence of two spatial-deflection joints. Based on the 3D reconstructions of specimen CT images, 3D cracking behaviour and ligament fracture pattern were investigated. The fracture evolution of each fracture type was analysed. And the rock block instability and bearing capacity for a jointed coal pillar reflected by the experimental results is revealed. The results indicate that wrapping wing and anti-wing cracks are dominant inducement of ligament fracture, and the helical cracks tend to occur when the joint angle and spatial-deflection angle of specimens are relatively large. There were five fracture types of ligaments, two of which are complete crack coalescence, and the failure surface presents convex curve and twisted surface, respectively. The other three types are partial crack coalescence, the coalesced region may be the front and back region of ligaments, and the fracture surface presents trapezoid or inverted trapezoid. For specimens containing a pair of joints with no spatial deflection or small spatial deflection, when joint angle is less than 60°, the 3D rock fracture can be simplified into a 2D problem. With the increase of spatial-deflection angle, the ligament changes from crack coalescence completely to crack coalescence partially. The results also implied that when the joint is gentle and two sides of discontinuities cut the block with a trumpet shape, rock block instability is easy to occur.

A multifunctional rock testing system for rock failure analysis under different stress states: Development and application

The stress state in a rock mass is complex. Stress redistribution around underground excavation may lead to various failure modes, including compressive-shear, tensile-shear, and tensile failures. The ability to perform laboratory tests with these complex stress states is significant for establishing new strength criteria. The present paper introduces a new rock testing system with “tensile-compressive-shear” loading functions. The device includes bi-directional and double-range hydraulic cylinders, auxiliary loading equipment, and roller rows that can perform direct compressive-shear tests, direct tensile tests, and direct tensile-shear tests. The testing system provides maximum vertical and lateral loading forces of 2000 kN and allows testing cubical rock specimens with dimensions of 0.5 m × 0.5 m × 0.5 m. The performance of the testing machine was evaluated by testing a rock-like material based on cement mortar under compressive-shear, tensile, and tensile-shear stress states. The failure process and deformation characteristics were monitored during loading using acoustic emission (AE) transient recorder, piezoelectric AE sensors, a high-speed camera, and a thermal infrared camera. The failure mechanism was investigated by analyzing AE counts, AE amplitude, strain, and temperature changes on the rock specimen surface. The test results confirmed that the testing system could successfully simulate the abovementioned stress path. The AE counts and amplitude responses were influenced by different failure modes. The temperature response during the compressive-shear test indicated the development of a high-temperature band on the rock specimen surface. In contrast, a negligible temperature change was observed during the tensile and tensile-shear tests. The newly developed multifunctional rock testing system allows laboratory tests under various failure modes. The monitoring results of multiple variables during rock failure tests provide valuable information on failure characteristics. • Develop a novel rock testing system with tensile-compressive-shear loading function. • Investigate the failure behavior of rock-like specimens under different stress states. • Analyze the various attributes response characteristics in different failure modes.