Failure Process Of Rock Specimen Research Articles

Compressive Fracture Behavior and Acoustic Emission Characteristics of Sandstone under Constant Crack Water Pressure

Abstract Engineering rock containing flaws or defects under a large water source is frequently subject to the couple influence of constant crack water pressure and geostress. To investigate the fracture behavior of precracked rock under hydromechanical coupling with constant crack water pressure, compression tests were conducted on red sandstone specimens containing a single crack of different angles using a device to realize the constant crack water pressure during loading, and the failure process of rock specimens was monitored by acoustic emission (AE) technique. The results show that the presence of constant crack water pressure has a significant promotion effect on the development of shear wing cracks, and the promotion effect is influenced by the prefabricated crack angle and water pressure. As the constant crack water pressure increases, the failure mode of the 0° precrack specimen changes from “X”- shear failure to the single oblique shear failure along the shear wing crack direction, the main failure crack of the inclined precracked specimens (precrack angles of 15°, 45°, and 60°) changes from a small acute angle with the prefabricated crack to a direction along the shear wing crack, and irregular cracks occur at the chipped prefabricated crack in the 90° precracked specimen. With an increase in the constant crack water pressure, the average energy for a single hit, cumulative AE energy, and cumulative AE hits decrease, and the proportion of the tensile cracks increases and that of the shear cracks decreases.

Experimental study on compression mechanical characteristics of filled rock joints after multiple pre-impacts

The prefabricated artificial filled jointed rock specimens are impacted by a self-made drop hammer impact device for many times, and the specimens with different degrees of cumulative damage characteristics are obtained. Then, the static and dynamic compression mechanical properties are studied by using universal testing machine and SHPB device. Through the static compression test, the strength and deformation characteristics of jointed rock specimens after multiple impacts are obtained, and the influence of the damage degree of jointed rock specimens characterized by wave velocity on the compressive strength of filled joints is analysed. Based on the results of SHPB impact test, the dynamic strength and deformation evolution, wave propagation law and energy dissipation law of filled joints after multiple impacts are analysed. During the SHPB test, the impact failure process of rock specimens is recorded by a high-speed camera. The experimental results show that the damage degree of jointed rock samples increases nonlinearly after multiple impacts. The attenuation laws of static strength and dynamic strength of rock samples under the same damage evolution conditions are different. With the increase of impact times, the failure mode of jointed rock samples after damage is simpler and tends to compression failure.

Investigation of the crack and acoustic emission behavior evolution of red sandstone subjected to water

To explore the influence of water on the crack and acoustic emission (AE) behavior evolution of rock specimen during the deformation and failure, uniaxial compression tests with variable water contents (dry, natural, and saturated state) on red sandstone specimens were systematically conducted in the laboratory. Based on real-time AE monitoring technology and scanning electron microscope (SEM) technology, the crack evolution mechanism in the rock specimen during the deformation and failure was studied. The results indicate that the peak strength of red sandstone in the natural and saturated state is 26.13% and 41.75% lower than that of red sandstone in the dry state. With the increase of water content, the dominant failure pattern of rock specimen changes from tension failure to shear failure. According to the variable characteristics of the AE events rate, the deformation and failure process of rock specimens with the change of water contents can be divided into four stress stages. Moreover, the relationships among the AE events rate, peak frequency, water content and different stress stages were discussed. Approaching to the peak stress, there is an obvious trend of relative decrease of the Ib-value and the phenomenon of “sharp increase” and “sharp drop” in the average AE energy rate. The above-mentioned variable characteristics of AE parameters can be used as a precursor for the instability and failure of rock specimens with different water contents.

Experimental Study on Uniaxial Compression Fracture Process of Marble Specimens with Circular Holes with Different Diameters

After a long period of geological tectonic action, the rock mass develops defects such as pores and fissures of different sizes and scales. The mechanical properties of the rock are largely affected by these discontinuous structures, and there will be obvious stress concentration at the edge of the hole inside the rock. Under the action of concentrated stress, cracks initiate, expand, and penetrate from the edge of the hole to form the final fracture surface and cause instability and failure of the rock mass. In practical engineering, due to the limitation of various factors, it is impossible to conduct uniaxial compressive strength test for each geotechnical engineering. Therefore, the rock in the weak part of the project can be selected as the test specimen before the indoor physical tests. In this paper, the static uniaxial compression test of specimens with circular holes with different diameters is carried out. Digital Image Correlation (DIC) monitoring system is used to observe the test process in real time; then, the data from uniaxial compression test and DIC monitoring test are obtained. At the same time, the macroscopic failure characteristics and crack propagation law of rock specimens are obtained, and the evolution law of strain field during the failure process of rock specimens is also obtained. Meanwhile, Realistic Failure Process Analysis (RFPA) is carried out to verify the accuracy of static uniaxial compression physical tests numerically. Physical tests show that the uniaxial compressive strength of marble specimens with circular holes and the dispersion of measured peak stress data decrease with the increase of hole diameter. The main failure type of marble specimens with circular holes is tension shear failure; the macroscopic failure zone of the sample, the strain field localization zone, and the maximum shear stress field of the numerical test are distributed in a semi-“X” shape with the hole as the center. The localization zone of the strain field on the specimen is usually the region of crack initiation, propagation, and penetration on the surface of the marble specimens. It also shows that specimen breaks in the radial direction along the y -axis of the circular hole, and cracks connecting both ends of the circular hole and the specimen are generated on the specimen. It is of great theoretical value and practical significance to the influence of the cavities in the rock specimens on the stress and strain characteristics and the initiation, propagation, and penetration modes.

Experimental Study on the Influence of Specimen Shape on Rockburst Proneness of Red Sandstone

To investigate the specimen shape effect on rockburst proneness of rock materials, a string of conventional and single-cycle loading-unloading uniaxial compression tests was performed with cylindrical and cuboid red sandstone specimens. Despite similar development paths on stress-strain curves for the specimens with two shapes, the cuboid specimens generally show a higher uniaxial compressive strength than the cylindrical specimens. The energy evolution laws inside the two shaped specimens were explored. The results show that the input energy density (IED), elastic energy density (EED), and dissipated energy density (DED) of the two shaped specimens increased in a quadratic relationship with the increment of unloading level. Moreover, the linear relationships between the EED, DED, and IED were further confirmed for two shaped specimens, which were defined as the linear energy storage and dissipation laws, respectively. The energy storage coefficients and energy dissipation coefficients (the slopes of the linear relationships between the EED, DED, and IED, respectively) are almost independent of the specimen shape. According to the linear energy storage and dissipation laws, the peak EED and peak DED of every specimen can be calculated accurately. Finally, combining the failure process of rock specimens recorded by a high-speed camera, the elastic energy index (WET), the residual elastic energy index (AEF), and the far-field ejection mass ratio (MF) of each specimen were adopted to assess the rockburst proneness of the red sandstone sampled in cylindrical and cuboid. The results show that cuboid specimens exhibited stronger rockburst proneness than cylindrical ones, which favorably agreed with the actual failure phenomena.

Dynamic response of pre-stressed rock with a circular cavity subject to transient loading

Scientists have attempted to investigate deep underground rock failure. A key challenge to study underground rock failure is the difficulty to survey its process. Therefore, a large number of important and insightful laboratory investigations of underground rock failure experiments are conducted. In this paper, an experimental method is proposed to explore dynamic failure process of pre-stressed rock specimen with a circular hole. The failure process of a rock specimen under different initial static stress coupled with dynamic loading is clearly illustrated by a high speed camera. The experimental results indicated that high static pre-stress coupled with dynamic loading induces rock debris to be ejected at the surrounding circular hole; however, lower static pre-stress coupled dynamic loading cannot induce rock failure. The dynamic stress concentration surrounding the circular hole by transient wave incidence was further demonstrated at the condition of half-sine wave loading. In this condition, the results demonstrated that combined action of static and dynamic stress concentrations induces the primary fractures of rock specimen.

New models to predict rock failure by linking the volume dilatant point with the peak stress point

The rock mass failure process can be divided into several distinct deformation stages: the compaction stage, elastic stage, stable failure stage, accelerated failure stage, and post-peak stage. Although each stage has been well studied, the relationship among the stages has not been established. Here, we establish two models which are the Strain model Q and Energy density model S by using the renormalization group theory and investigate the mechanical relationship between the volume dilatant point and peak stress point on the rock stress-strain curve. Our models show that the strain ratio (e f /e c ) and energy ratio (E f /E c ) at the volume dilatant point and peak stress point are solely functions of the shape parameter m. To verify our models, we further studied the failure process of rock specimens through several uniaxial compression experiments and found that the relationship between e f /e c or E f /E c and m shares a notably similar pattern to that from our theoretical model. However, the e f /e c and E f /E c values in our experiments are slightly smaller than those predicted by the models. In brief, we demonstrate that our models can be used to predict the failure process of the laboratory-scale hard brittle rock samples.

3D numerical analysis of crack propagation of heterogeneous notched rock under uniaxial tension

Macroscopic notches play an important role in evaluating the fracture process zone (FPZ) and the strengths of a heterogeneous rock mass. Crack initiation, propagation and coalescence for unnotched, single-notched and double-notched rock specimens are numerically simulated in a 3-D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. For the unnotched case, special attention is given to the complete stress–strain curve and the corresponding AE events for the failure process of rock specimen. By comparing with published experimental results, the simulation results from RFPA3D are found to be satisfactory. For the single-notched case, the effect of the length and the depth of the single notch and the thickness of the specimen on the failure mode and peak stress are evaluated. The 3D FPZ is very different from that in two dimensions. For the double-notched case, the effects of the separation distance and overlap distance of the double notches, as well as influence of the homogeneity index (m) are also investigated. As the overlap distance increases, the direction of the principal tensile stress at each notch-end changes from a perpendicular direction (tensile stress field) to a nearly parallel direction (compressive stress field), which affects the evolution of the cracks from the two notches.

Experimental Study on the Process of the Three-Dimensional Rock Crack Growth Influenced by the Pre-Crack Profile Geometry

The difference of the pre-crack profile geometry could have a certain influence on the surface crack growth in rock materials. In this paper, an experimental study on the failure process of rock specimen with a non-penetrated crack in two different profiles is performed. Profile of the pre-crack on the specimen type I is trapezoidal and profile of the pre-crack on the specimen type II is arc. The surface crack growth of the two types of specimen is analyzed and compared in detail by using the digital speckle correlation measurement (DSCM) techniques and the virtual extensometer (VE) based on DSCM. The results indicate that the extension mode of the main crack on the two types of specimen depends on the pre-crack profiles of different geometry. The cross-section with continuous profile (specimen I) has a fixed propagation direction and a much higher growth speed compared to the cross-section with discontinuous profile (specimen II).

Fractal analysis on the spatial distribution of acoustic emission in the failure process of rock specimens

The spatial distribution of acoustic emission (AE) events in the failure process of several rock specimens was acquired using an advanced AE acquiring and analyzing system. The box counting method (BCM) was employed to calculate the fractal dimension (FD) of AE spatial distribution. There is a similar correlation between the fractal dimension and the load strength for different rock specimens. The fractal dimension presents a decreasing trend with the increase of load strength. For the same kind of specimens, their FD values will decrease to the level below a relatively same value when they reach failure. This value can be regarded as the critical value, which implies that the specimen will reach failure soon. The results reflect that it is possible to correlate the damage of rock with a macroscopic parameter, the FD value of AE signals. Furthermore, the FD value can be also used to forecast the final failure of rock. This conclusion allows identifying or predicting the damage in rock with a great advantage over the classic theory and is very crucial for forecasting rockburst or other dynamic disasters in mines.

可視化ベッセルによる田下凝灰岩の三軸圧縮クリープの観察

It is essential for underground development to understand the failure process of rock subjected to tri-axial compression. Okubo et al. developed a transparent tri-axial vessel made of an acrylic acid resin, and succeeded in observing the failure process of rock specimens in tri-axial compression test. In this study, tri-axial creep tests of Tage tuff were conducted within the transparent vessel.A creep test usually requires longer time than a compression test and it is not feasible to take a photograph continuously at every predetermined time interval. Therefore, in this study, the photographing system was developed which cooperated with the existing loading and measuring apparatuses. This system made it possible to take a series of photographs of a specimen in the tertiary creep region. Photographs taken in a tri-axial test were processed to obtain axial and lateral deformations by the computer program developed in this study.After careful examination of axial and lateral deformations, it was found that the creep strain (axial strain) just before failure was nearly equal to the cross sectional width of the stress-strain curve at the creep stress level, and that not only axial but also lateral strain-rates in the tertiary creep region were inversely proportional to the residual time up to the failure. And it was also found that ratio of the lateral and axial strain increases in the tertiary creep region were similar to those of the tri-axial compression test just after the strength failure point.

強度のばらつきを考慮したー軸引張試験の計算機シミュレーション

In the previous studies, authors carried out the computer simulation by FEM program and discussed the failure process and the size effect of rock specimen model subjected to compressive stress.In this study, behaviour of rock specimen under uniaxial tensile stress is examined by a similar computer simulation and some experimental work. For the computer simulation, FEM program implemented with a non-linear visco-elastic constitutive equation is used. In the experimental work, two kinds of rocks are tested under uniaxial tension to obtain the complete stress-strain curves. Surface of a rock specimen is carefully examined with the aid of CCD camera.The calculated results indicate that failure process of rock specimen is considerably complex, that is, cracks initiate and propagate at many separated locations in the specimen and a macro crack which leads to final failure is not a straight line but having many sharp turns. This calculated result conforms to the experimental result.The computer simulation also indicates that strength of specimen decreases with increase of the number of elements in FEM mesh. However, number of elements exceed 90, then strength dose not change apparently. It is also found that the loading rate dependency of strength decreases with number of elements. These results quantitatively coincide with the results obtained in the previous study for compression stress.