Conjugate joint is one of the most common joint forms in natural rock mass, which is produced by different tectonic movements. To better understand the preexisting flaws, it is necessary to investigate joint development and its effect on the deformation and strength of the rock. In this study, uniaxial compression tests of granite specimens with different conjugate joints distribution were performed using the GAW-2000 compression-testing machine system. The PCI-2 acoustic emission (AE) testing system was used to monitor the acoustic signal characteristics of the jointed specimens during the entire loading process. At the same time, a 3D digital image correlation (DIC) technique was used to study the evolution of stress field before the peak strength at different loading times. Based on the experimental results, the deformation and strength characteristics, AE parameters, damage evolution processes, and energy accumulation and dissipation properties of the conjugate jointed specimens were analyzed. It is considered that these changes were closely related to the angle between the primary and secondary joints. The results show that the AE counts can be used to characterize the damage and failure of the specimen during uniaxial compression. The local stress field evolution process obtained by the DIC can be used to analyze the crack initiation and propagation in the specimen. As the included angle increases from 0° to 90°, the elastic modulus first decreases and then increases, and the accumulative AE counts of the peak first increase and then decrease, while the peak strength does not change distinctly. The cumulative AE counts of the specimen with an included angle of 45° rise in a ladder-like manner, and the granite retains a certain degree of brittle failure characteristics under the axial loading. The total energy, elastic energy, and dissipation energy of the jointed specimens under uniaxial compression failure were significantly reduced. These findings can be regarded as a reference for future studies on the failure mechanism of granite with conjugate joints.
Rock mass is discontinuous because of the movement and development of the crust. The joint, fissure, and fault fracture are the typical modes of this discontinuation. The most common joint geometries in natural rock mass are parallel joints and conjugate joints (also called
Acoustic emission (AE) testing technology is an effective means to study the propagation of defects in brittle materials, such as rocks. At present, this technology is widely used to study the internal damage and fracture behavior of rock materials [
Digital image correlation (DIC) is an optical and noncontact deformation measurement technique, which can be used to calculate the spatial distribution of the stress and the strain of the object during the deformation process. Recently, the DIC technique has been widely used in the field of experimental rock mechanics. Zhao et al. [
According to the law of thermodynamics, the deformation of rock under loading is essentially a process of energy transformation, including energy absorption, evolution, and dissipation [
Although considerable attention has been paid to the initiation and propagation of preexisting flaws in jointed rocks, the influence of conjugate joints on the overall mechanical properties of jointed rock mass and the underlying energy conversion mechanism remain less well understood. The AE of rock materials is a phenomenon where rock elastic strain energy is released in the deformation or failure process [
The joints that are not fully connected and persistent for the existence of rock bridges are termed as nonpersistent joints. It is an effective and economical method to make jointed specimens by using rock-like materials such as concrete or gypsum for laboratory experiments [
In order to improve these deficiencies, this paper selects natural granite to make rock specimens. The granite used in the experiments is collected from Sanshandao gold mine, which is an underground gold mine located in Laizhou city, Shandong province, China. The granite is firstly processed into rock specimens with width of 50 mm, height of 100 mm, and thickness of 25 mm. The ends of each specimen were ground flat so that the error flatness of both end surfaces did not exceed 0.02 mm to avoid stress concentration during loading. To rigorously screen specimens, perform the following: (1) remove specimens with visible surface damage and visible flaws and (2) remove specimens whose size and flatness do not meet the standard requirement. To improve the precision of the preexisting joints, the specimen cutting and processing equipment were used, including a water jet cutter (WJC) and a wire cutting machine (WCM). Two joints with different lengths were cut at the center of the granite specimens by using the cutting equipment to study the effect of conjugate joints on the mechanical properties of granite specimens. As shown in Figure
Preexisting conjugate joints with different included angles. (a)
The uniaxial compression tests were carried out using a computer-controlled electrohydraulic servo compression-testing machine system (GAW-2000, Chaoyang Test Instrument Co., Ltd., Changchun, China). The GAW-2000 testing system can test the specimens in load or displacement control mode with simultaneous data recording. The maximum axial loading capacity of the servo-controlled system was 2000 kN, and the maximum displacement capacity was 100 mm. During the uniaxial compression tests, mechanical behavior and damage evolution of the preexisting jointed specimens were analyzed by the AE method. The AE instrument employed a PCI-2 AE monitoring system produced by American Physical Acoustics Corporation (PAC), which is composed of cable, amplifier (AMP), AE sensors, and data-acquisition control system. The sampling frequency range of the AE sensor is 1 kHz∼3 MHz, and the A/D conversion resolution is 18 bits. The preamplifier is a 40 dB gain adjustable amplifier, which can amplify the signal 100 times. The AE system can perform real-time or postdata analysis and spectrum analysis. The uniaxial compression-testing system equipped with AE monitoring is presented in Figure
GAW-2000 testing system equipped with AE monitoring.
Using the DIC method, 3D displacements and strains are available at every point on the surface of specimen. The experimental equipment used in this study was the VIC-3D DIC System (Correlated Solutions, South Carolina, USA), which is a system for measuring and visualizing full-field, three-dimensional measurements of shape, displacement, and strain based on the principle of DIC. To achieve the effective correlation of the system, two main steps need to be completed before the experiment, namely, the charged couple discharge (CCD) cameras calibration and the speckle pattern of the specimens. Calibration of the system is essential in order to determine the best possible position of the two cameras, whereas the quality of the calibration also determines the accuracy of the DIC. This work can be done by calibration panel and VIC-3D software. For calculating the displacements with DIC, a reference image and an image after deformation must be recorded. Before the software VIC-3D calculates the displacements between these two images, an area of interest has to be set on the reference image. Therefore, the area of interest on the surface of specimen was coated with a white paint and sprayed with black aerosol to produce the required surface condition for the DIC. A setup of the rock specimens and the main equipment is illustrated in Figure
Rock specimens and main equipment.
During the test, the load and deformation values applied on the granite specimens were recorded simultaneously at a same data collection interval. The tests were conducted on the GAW-2000 compression-testing machine system by imposing a constant displacement speed of 0.03 mm/min until failure occurred. Two rigid steel blocks were placed between the loading frame and rock specimen. Vaseline was used as a coupler to load the specimen and AE sensors, and the AE sensors were attached on the two sides of the specimen by insulating tape to continuously record the AE activity during damage and fracture propagation within the specimen. The CCD cameras were used to take a series of images of the front surface of the specimen at a speed of one frame per second. These images were then analyzed by the VIC-3D software to determine the whole area displacement and stress distribution. The GAW-2000 loading machine, AE system, and the CCD camera were executed simultaneously to obtain the correlation of mechanical behavior, AE damage detection, and optical observation results.
Based on the laws of thermodynamics, the failure of rock material is the result of energy conversion. Assuming that a unit volume of material deforms by outer forces and that this physical process occurs in a closed system, the energy conversion can be defined according to the first law of thermodynamics
Figure
Relationship between dissipated energy and elastic energy.
Substituting equations (
The uniaxial compression test is carried out by the AE method to evaluate the susceptibility of the granite specimens to deformation and failure. The stress-strain curves of the intact specimen and the conjugate jointed specimens under uniaxial compression are shown in Figure
Stress-strain curves of the granite specimens.
For the intact specimen, the elastic deformation stage is the longest among all specimens due to the uniform axial loading. In the stage of plastic deformation, the initiation, propagation, coalescence, and interaction of microcracks will induce the degradation of mechanical properties of specimens. The plastic deformation stage of the intact specimen is shorter than that of the jointed specimen. When the axial compressive stress reaches the peak strength, the specimen will be destroyed rapidly and enter the postpeak failure stage. The whole stress-strain curve shows a typical failure characteristic of elastic-brittle materials.
For the conjugate jointed specimens, the overall trend of the initial compaction stage is similar to that of the intact specimen. In the initial compaction stage, the stress-strain curves of the specimen with
Due to the damage caused by the preexisting joints, the ability of the conjugate jointed specimen to resist external force deformation was reduced, and the elastic deformation stage was shorter than that of the intact specimen. The plastic deformation stage of the specimens with included angles of 0°, 30°, and 90° has significant fluctuation characteristics, which means that, under the axial loading, stress concentration will occur around the preexisting joints and accelerate the microcracks initiation, propagation, and transfixion. With the release and redistribution of stress, the area of stress concentration gradually transfers to other parts of the specimen, which is the internal reason of the multistage drop of stress-strain curve. For the specimen with
Table
UCS and elastic modulus of specimens.
Parameter | Included angle | Intact | ||||
---|---|---|---|---|---|---|
0° | 30° | 45° | 60° | 90° | ||
UCS (MPa) | 43.14 | 40.94 | 42.45 | 46.33 | 36.94 | 131.43 |
Elastic modulus (GPa) | 17.68 | 16.48 | 15.30 | 21.06 | 23.92 | 34.75 |
The normalized UCS was defined as the UCS of jointed specimen (
The changes of two damage indices with the included angle are shown in Figure
The changes of two damage indices with
Each oscillation wave of electrical signal exceeding the threshold is an AE count, which is the external acoustic performance of the change of internal structure of rock, reflecting the intensity of AE activity and the evolution process of internal damage of rock. Figure
AE counts and accumulative AE counts of the jointed specimens with
To reveal the evolution characteristics of the stress field during the loading process of jointed specimen with
Stress field developed at different times for the specimen with
Figure
AE counts and accumulative AE counts of the jointed specimens with
Stress field developed at different times for the jointed specimen with
Figure
AE counts and accumulative AE counts of the jointed specimens with
Stress field developed at different times for the jointed specimen with
Figure
AE counts and accumulative AE counts of the jointed specimens with
Stress field developed at different times for the jointed specimen with
Figure
AE counts and accumulative AE counts of the jointed specimens with
Stress field developed at different times for the jointed specimen with
The energy evolution processes of rock specimens under the uniaxial compression test are shown in Figure
Energy evolution of rock specimen under UCS test. (a) Intact, (b)
At the elastic deformation stage, the total energy and the elastic energy increase linearly with the strain approximately, and the dissipation energy almost remains unchanged or even decreases. At this stage, the dissipative energy of the intact specimen is about 10 kJ/m3. The dissipative energy of the conjugate jointed specimens is generally lower than that of the intact specimen, and the energy evolution is closely related to the included angle. The dissipated energy of the jointed specimens with included angles of 0°, 30°, and 45° in the elastic deformation stage is about 5 kJ/m3, 3 kJ/m3, and 7 kJ/m3, respectively. At this time, the total input energy is almost completely converted into the elastic energy, and the dissipated energy is very little. The peak strain of the specimens with
At the plastic deformation stage, the external loading gradually approaches the UCS of rock specimen. With the rapid initiation and propagation of new cracks in the specimen, the dissipation energy begins to increase. Due to the strong brittleness of the intact specimen, when a large number of new cracks appear, the specimen will quickly reach the peak strength and then be fractured. Therefore, there is almost no plastic deformation stage in the complete specimen. At this stage, there is not only the initiation of new cracks but also the further expansion and transfixion of the preexisting joints, leading to the obvious increase of dissipation energy. For the jointed specimens with the angles of 0°, 30°, 60°, and 90°, the continuous accumulation of deformation leads to the stress concentration at the crack tip, thus accelerating the crack initiation and propagation. In this process, the accumulated elastic energy in rock specimen is released suddenly, and the curves of the elastic energy and the dissipation energy increase in the form of “ladder.” At this stage, the dissipation energy of the jointed specimen with
At the postpeak failure stage, the macrofracture occurs, the elastic energy accumulated in the rock is released quickly, and the internal cracks of the specimen are coalesced and penetrated rapidly. Then, the rock loses carrying capacity and shows obvious brittle characteristics. During this stage, the dissipation energy increases with the increase of strain, while the elastic energy decreases with the increase of strain, and the curves of the dissipation energy and the elastic energy intersect.
The peak energy indexes of the intact and the jointed specimens are shown in Table
Peak energy indexes of the intact and the jointed specimens.
Total energy(kJ/m3) | Elastic energy(kJ/m3) | Dissipation energy(kJ/m3) | Energy accumulation rate(%) | Energy dissipation rate(%) | |
---|---|---|---|---|---|
Intact | 221.05 | 208.37 | 12.68 | 94.26 | 5.74 |
0° | 70.17 | 52.29 | 17.88 | 74.52 | 25.48 |
30° | 62.16 | 50.60 | 11.56 | 81.40 | 18.60 |
45° | 82.45 | 58.87 | 23.58 | 71.40 | 28.60 |
60° | 54.62 | 48.28 | 6.34 | 88.39 | 11.61 |
90° | 30.75 | 25.69 | 5.06 | 83.54 | 16.46 |
Comparison of peak energy and accumulative AE counts.
For the conjugate jointed specimens, because of the initial damage caused by the preexisting joints, the energy storage capacity of the specimen is weaker than that of the intact specimen, indicating that the ability to accumulate elastic energy is closely related to the included angle. Among them, the ability to accumulate elastic energy of the specimen with
As the included angle of conjugate joint increases from 0° to 90°, the accumulative AE counts of the peak increase first and then decrease. The number of peak cumulative AE counts of the specimen with
Based on the uniaxial compression test and the AE test of the granite specimens with conjugate joints, the following points are summarized: The angle between the primary and the secondary joints has a significant effect on the stress-strain curve of the rock specimens. The stress-strain curve of the jointed specimens will enter the plastic deformation stage in advance, and the elastic deformation stage will be shorter, while the plastic deformation stage will be longer. The plastic deformation stage of the jointed specimens with included angles of 0°, 30°, and 90° has obvious stress fluctuation characteristics. The damage and deformation of jointed rock under different included angles can be described by accumulative AE counts. The accumulative AE counts of the specimen with Compared with the intact specimen, the peak strength and the elastic modulus of the jointed specimens are significantly decreased, and the decrease range of the peak strength is more obvious. As the included angle increases from 0° to 90°, the elastic modulus first decreases and then increases, and the accumulative AE counts of the peak increase first and then decrease, while the peak strength does not change distinctly. The peak strain of the other specimens is less than that of the intact specimen except for the specimen with The total energy, elastic energy, and dissipation energy of the uniaxial compression failure of the jointed specimens are significantly reduced compared with the intact specimen, and the ability of the specimens to accumulate the elastic energy is closely related to the included angle. Compared with other conjugated joints, the single joint formed by overlapping of the primary and secondary joints has less damage to rock mass and retains the strength characteristics of the granite materials to a large extent.
The experimental data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The first author gratefully acknowledges financial support from China Scholarship Council. This research was funded by the Fundamental Research Funds for the Central Universities (no. FRF-TP-18-015A3) and the Key Laboratory of Western Mine Exploitation and Hazard Prevention of Ministry of Education (no. SKLCRKF1901).