1.   Introduction

As the global population and economy continue to grow, the demand for energy and raw materials is increasing [1], resulting in the deepening of coal mining activities. Consequently, many geological disasters have become prominent because of the complexity, suddenness, and diversity of the influencing factors of these disasters. Although it has been extensive researched, the occurrence mechanism of such disasters has no universally accepted explanation.

Rock residual stress is the self-balanced stress that remains in the Earth crust after rock formation, metamorphism, and tectonic movements [2]. Such residual stress is also referred to as closed stress, “locked-in” stress, or freezing stress [3]. Residual stress is usually concentrated in deep rock mass areas with geological structures, such as joints, faults, and folds [4], which are often subject to rockburst, core fractures, and zonal fragmentation [5–6].

Numerous studies have demonstrated the significance of studying residual stress to explain the mechanical behavior of deep rock masses. Varnes et al. [7] proposed the dynamic hypothesis of rock residual stress, indicating that residual stress in rock is a potential cause of problems in the foundation of engineering structures. Holzhausen et al. [2] systematically summarized the concept of residual stress in rock and clarified the confusion caused by misidentifying residual stress as crustal stress. Müller [8] emphasized the importance of studying residual stress in rocks, whereas Tan et al. [9] proposed the hypothesis of “locked-in” stresses and the concept of stress pockets, discussing the importance of stress pockets in practical engineering. The latter study has laid the foundation for subsequent research on “locked-in” stress. Yue [10] explained the rockburst hysteresis through the pocket theory of “locked-in” stress. An [11] suggested that residual stress and its strain energy are superimposed on stress caused by tectonic movement and released to a high degree when the rock mass fractures, resulting in rapid crack expansion and increasing energy released from rock mass failure, which affects seismic activity. Liu et al. [12] created a rock-like material containing residual stress with nitrile rubber and evaluated the effect of residual stress on the mechanical properties of rock-like materials through rock mechanics tests. Residual stress in rock contains rich information on rock structure as well as tectonic and mechanical evolution [13].

Based on recent studies, residual stress can serve as an indicator of paleostress. Sekine et al. [14] estimated paleostress at the time of vein formation using residual stress magnitude and fluid-inclusion temperature measurement data. Kai et al. [15] and Wenk et al. [16] used a residual lattice strain in quartz as a paleotectonic stress indicator to explain stress field behavior during tectonic deformation. In addition, the release of residual stress could cause several special geological structures. Based on the characteristics of rock joints, Weinberger et al. [17] hypothesized that schist was formed because of the release of residual elastic strain energy. The microstructure of rock plays a vital role in its mechanical properties. Liu et al. [18] developed a novel dual-damage thermal-mechanical model to define the evolution of the thermal and mechanical properties of rock during thermal treatment, indicating that the thermal treatment caused a realignment in the rock microstructure and resulted in a change in the ultimate failure pattern. The existence of residual stress could also change the physical and mechanical properties of rocks to a certain extent. Lu et al. [19] found that closed stress was the most influential factor in the conductivity of undeveloped fractures in shale.

The excavation or tectonic movement of a deep rock mass can lead to changes in its initial stress and energy state, resulting in a state that cannot be maintained at equilibrium. The release of internal residual stress can lead to disasters such as rock bursts and special geological phenomena such as core fractures and zone fragmentation. Research on residual stress measurement indicates that rock residual stress is similar to that in other materials. Therefore, methods for measuring residual stress in materials can be used in rock residual stress measurement. These methods are of two types, namely, destructive and nondestructive. Destructive measurement methods include drilling and ring-core methods. Gentry [20] suggested that the stress direction obtained by destructive measurement had some reference significance. However, the results are not successful in accounting for the generation mechanism of rock residual stress and its corresponding impact on structures and the environment because of the large disparity in measurements.

Nondestructive measurement methods include ultrasonic, magnetic, neutron diffraction, and X-ray diffraction (XRD). The ultrasonic method calculates the change in residual stress by measuring the change in ultrasonic propagation velocity in the material. However, accurately determining rock residual stress from changes in ultrasonic wave velocity can be challenging when the ultrasonic wave velocity is disturbed, and the rock is a heterogeneous material. The magnetic measurement method determines the magnitude and direction of residual stress based on the permeability change of ferromagnetic materials, but this method is often inaccurate and energy-consuming. By contrast, XRD is widely used in quantitative residual stress measurement for metals, polymers, ceramics, and other materials [21–25], which measures variations in crystal plane spacing to obtain strain. Geologist Friedman [26] was the first researchers to apply XRD to perform stress analysis for geological materials. Sekine et al. [14] evaluated the residual stress in quartz veins in metamorphic rock using XRD. Some scholars also utilized synchrotron radiation-based X-ray diffraction [15–16] and neutron diffraction to measure rock residual stress [27], and they proposed the theoretical feasibility of using X-ray neutron diffraction and other nondestructive measurement methods [28]. Despite some exploratory studies conducted on the measurement of rock residual stress, the unified understanding of the test methods and principles of residual stress, the influence of residual stress on rock engineering, and the generation mechanism is still lacking. Therefore, in this work, electron backscatter diffraction (EBSD) and optical microscopes were used to characterize the rock microcosm to account for the generation mechanism of rock residual stress. XRD, a comprehensive method for rock analysis, was used to measure the residual stress of specific rocks.

2.   Testing principles and experimental scheme

2.1   Sample selection

The sample used in this study was obtained from a siliceous slate-containing quartz vein located in Hongling Mine, Inner Mongolia. The sample has a complex matrix composition and random geological conditions, meeting the requirements for residual stress generation. In addition, the quartz vein in the siliceous slate is an intrusion. During formation, the vein body may have been constrained by the surrounding matrix, with rockburst often occurring at the intersection of the veins [29]. Based on the known mechanisms of rock formation, this type of rock is predisposed to residual stress, making it an ideal candidate for processing, testing, and analysis in this study. The sample consists of transverse quartz veins that are roughly parallel to the matrix bedding as well as longitudinal and some small quartz veins that are approximately perpendicular to the matrix bedding. The transverse and longitudinal quartz veins exhibit a vertical crossover state, with the longitudinal quartz veins running through the entire rock with a width of 1–4 mm, whereas the transverse quartz veins are 1–3 mm wide.

2.2   Microstructural information of the sample

Rock is a complex material consisting of multiple phases and crystals. XRD requires knowledge of the calibration of the target mineral; thus, the diffraction pattern of a specific mineral crystal was used to determine the stress state in the test area. Mineral composition, particle size, and preferred orientation of the siliceous slate-containing quartz veins were analyzed using EBSD.

As shown in Fig. 1, the sample surface was mechanically polished and argon ion polished, and the siliceous slate-containing quartz vein was prepared into 20 mm × 20 mm × 10 mm EBSD plate samples.

Figure  1.  Polished EBSD plate sample: (a) experimental sample; (b) arrangement of measuring points; (c) orientation contrast diagram.

下载: 全尺寸图片 幻灯片

Data were collected using a JSM-IT800 field-emission scanning electron microscope (FESEM), Oxford Instrument EDS, and CMOS EBSD Instrument, with the matrix lineation direction as the X axis, vertical foliation direction as the Z axis, and vertical X axis within the foliation as the Y axis, at a low vacuum pressure of 30 Pa, an acceleration voltage of 20.00 kV, and a sample tilt angle of 70°.

A forward scattering detector by Oxford Instrument was used to obtain the orientation contrast diagram shown in Fig. 1. Three measuring points with evident differences in brightness and darkness were selected in the scanning area to determine the potential mineral composition of the matrix.

Based on the matrix energy spectrum, the minerals in the matrix were initially categorized. As shown in Fig. 2, the Kikuchi diffraction pattern measured by EBSD was compared with known Kikuchi diffraction patterns in the crystal library to determine the crystal mineral structure. The matrix minerals were feldspar, quartz, and chlorite, whereas the main gangue mineral of the quartz veins was quartz.

Figure  2.  Matrix Kikuchi diffraction pattern calibration diagram of (a) quartz, (b) feldspar, and (c) chlorite.

下载: 全尺寸图片 幻灯片

Fig. 3(a)–(b) shows the statistical map of grain size in the EBSD test area of the matrix, with an average grain size of 7.4 μm. Fig. 3(c)–(d) displays the statistical graph of grain size in the EBSD test area of the quartz veins, with an average grain size of 225 μm. Fig. 4 shows an orthogonal polarizing microscopic image of the sample. The quartz veins have relatively uniform small grains with a particle size ranging from 30 to 40 μm and relatively nonuniform large grains with a particle size ranging from 70 to 400 μm. The small-grained quartz has enough grains in the 2-mm XRD test area for statistical analysis.

Figure  3.  Grain size statistics of the matrix and quartz veins test area: (a) matrix grain size content; (b) matrix grain size distribution; (c) quartz veins grain size content; (d) quartz veins grain size distribution.

下载: 全尺寸图片 幻灯片

Figure  4.  Orthogonal polarizing microscopic image of the sample.

下载: 全尺寸图片 幻灯片

Fig. 5 depicts the inverse pole figures (IPF) of XYZ in the test area of the matrix and quartz vein, respectively. The colors in the test area of matrix and quartz veins are diverse and similarly distributed, indicating the irregular arrangement of crystal particles. The primary mineral phases in the test areas of the matrix and quartz vein were analyzed with the corresponding pole figures to examine the characteristics of the rocks. Fig. 6 shows the pole figures of the main mineral phases of the matrix and quartz vein, corresponding to <001>, <100>, and <110>, respectively. No distinct preferred orientation of the minerals in the test area of the matrix and quartz veins is observed compared with the standard pole figures. Therefore, the EBSD test area of the rock has no specific preferred orientation.

Figure  5.  Inverse pole figure of (a) matrix and (b) quartz veins at different directions.

下载: 全尺寸图片 幻灯片

Figure  6.  Pole figure of (a) matrix feldspar, (b) matrix quartz, and (c) quartz veins.

下载: 全尺寸图片 幻灯片

The quartz veins contain small and uniform areas that satisfy the statistical requirements for XRD testing, as well as areas without any specifically preferred orientation and homogeneous areas of several millimeters within the rock, which also meet the XRD testing requirements. Therefore, residual stress measurements can be performed.

2.3   Measuring points and samples

Samples were cut along the direction of the matrix and vertical planes to establish the X and Y coordinate systems parallel to the matrix foliation and the Z coordinate system perpendicular to the matrix foliation (Fig. 7). Plate samples measuring 20 mm × 20 mm × 10 mm were prepared using linear cutting, and the measuring points were set at Z-1 and Z-2 in the Y–Z plane of the longitudinal vein, H-1 in the X–Z plane on the transverse vein, and point H-2 in the Y–Z plane.

Figure  7.  Arrangement of measuring points and establishment of the coordinate system.

下载: 全尺寸图片 幻灯片

2.4   Principle of X-ray diffraction stress test

Bragg’s law was applied to obtain XRD patterns. When an electron beam with a wavelength (λ) irradiates a crystal with a regular atomic arrangement and when the crystal plane spacing is d at an angle θ, the scattered electron beams will be superposed and reinforced by one another, resulting in diffraction. Diffraction occurs when the optical path difference is equal to an integer multiple of the wavelength. Bragg’s law [30], as shown in Eq. (1), describes this phenomenon.

where d is the distance between parallel atomic planes, λ is the wavelength of the incident beam, θ is the Bragg angle, which is half of the diffraction angle, and n is the integer representing the reflection series.

Considering that X-rays can penetrate the rock to a depth of approximately 10 μm, the surface of the rock sample is in a state of plane stress, along with zero stress in the thickness direction. The azimuth angle (ϕ) of the diffracted crystal plane is the angle between the normal of the diffracted crystal plane and the normal of the sample surface. Using the sin²ϕ method, the stress is measured using various forces within the crystal at different ϕ. With an increase in ϕ, the internal stress of the crystal increases, the spacing between crystal planes decreases, and compressive stress is generated within the rock. Conversely, a decrease in ϕ leads to a decrease in the internal stress of the crystal and generates tensile stress. If the crystal plane spacing is equal to or within the measurement error range, then no stress occurs in the mineral.

Based on the principle that the strain caused by a change in crystal plane spacing is equal to the macroscopic strain, as well as Bragg’s law and elasticity theory, the stress measurement equation of sin²ϕ was deduced under the assumption of a plane stress state. Eqs. (2)–(4) are the basic equations for macroscopic stress measurement [31]. The value of diffraction angle θ₀ corresponding to the true stress-free rock sample was unknown, making it difficult to determine the error related to X-ray measurements. However, an accurate evaluation of stress depends on the relative position of the center of the diffraction peak (i.e., peak displacement) than on the absolute peak position. Thus, θ₀ was considered as the radius corresponding to ϕ = 0° in each direction.

where σ is the macroscopic stress, K is the stress coefficient, M is the 2θ–sin²ϕ fitting line slope, E is the elastic modulus of the crystal surface of a mineral crystal, ν is the Poisson’s ratio of the crystal surface of a mineral crystal, θ₀ is the diffraction angle without stress, and ϕ is the angle between the normal diffracted crystal plane and the normal sample surface.

2.5   Measuring instruments and procedures

The experiment was conducted using a Bruker D8 Advance X-ray diffractometer equipped with Euler rings and a Cu-target X-ray tube with a print focal spot and one-dimensional detection mode. The working voltage and current were set at 40 kV and 40 mA, respectively, and stress was measured by side inclination. The X-ray generated by the interaction between accelerating electrons and the target passed through a Ni filter to remove K_β radiation and then collimated to a diameter of 2.0 mm to irradiate the sample surface and obtain the diffraction pattern. The experiment was conducted in the following steps:

(1) The samples were measured in full spectrum with a scanning step length of 0.1° and a dwell time of 0.15 s for each step to obtain the diffraction pattern within a diffraction angle range of 10°–158°.

(2) The phase of the obtained diffraction pattern was identified and matched with the PDF card.

(3) The diffraction peak with the highest intensity and without interference from random peaks was selected, and its corresponding diffraction crystal plane was determined as the characteristic crystal plane for residual stress calibration.

(4) The scanning range of the diffraction angle was determined on the basis of the width of the diffraction peak. Step scanning was used, with a scanning step length of 0.1° and a dwell time of 0.6 s for each step.

(5) Stress measurements were performed using side inclination. The angle of ϕ was selected as 0°, 9°, 18°, 27°, 33°, 39°, and 45°.

The variation in crystal plane spacing in the three directions was measured at the same measurement point, and the diffraction patterns in the directions φ = 0, φ = π/4, and φ = π/2 were measured sequentially.

3.   Experimental results

3.1   Results of residual stress tests

The diffraction patterns of the quartz veins are shown in Fig. 8. Phase identification of the test results was conducted on the basis of the XRD spectra obtained from the three measuring points. The results of phase identification indicate that the XRD spectra of the three measuring points matched with the standard PDF No. 99-0088. The (324) crystal plane with the highest diffraction intensity and independence in high-angle diffraction was selected as the stress calibration crystal plane. The scanning range of diffraction angle 2θ ranged from 152° to 156°.

Figure  8.  XRD patterns of full spectrum measurement of the quartz veins.

下载: 全尺寸图片 幻灯片

For the experiment, residual stress measurements were conducted by selecting one coordinate axis and another coordinate axis at 45° with two coordinate axes. The direction of the first measured coordinate axis was recorded as the φ = 0 direction, whereas the measured direction with a 45° interval between the two coordinate axes was recorded as the φ = π/4 direction. The direction of the other measured coordinate axis was marked as the φ = π/2 direction, and the magnitude and direction of the principal stress of the residual stress were calculated.

Fig. 9 shows the diffraction patterns obtained in the φ = 0, φ = π/4, and φ = π/2 directions at measurement points Z-1, Z-2, H-1, and H-2. The diffraction peak measured from 0° to 45° in ϕ shifted gradually to the right.

Figure  9.  Diffraction pattern of measurement points (a) Z-1, (b) Z-2, (c) H-1, and (d) H-2.

下载: 全尺寸图片 幻灯片

3.2   Data processing

For each diffraction pattern measured at each measurement point in the directions of φ = 0, π/4, and π/2, Gaussian fitting was performed to determine the peak position (2θ) corresponding to K_α1 radiation. The scattergram was drawn with 2θ as the ordinate, the data after “±” as the error value, and sin²ϕ as the abscissa, and the least square method was used for linear fitting. Fig. 10 illustrates the 2θ–sin²ϕ fitting line of measurement points Z-1, Z-2, H-1, and H-2 on the (324) crystal plane at φ = 0, φ = π/4, and φ = π/2, respectively. The slope of the lines is denoted by M. When M > 0, the rock stress is compressive, and when M < 0, the rock stress is tensile. R² is the determination coefficient of the linear fitting. When R² ≥ 0.8, 2θ–sin²ϕ is linearly correlated, indicating that the rock has internal residual stress. As shown in Fig. 10, 2θ increases gradually with the increase of ϕ, indicating the presence of compressive stress inside the rock. The fitting determination coefficient R² is higher than 0.8, which indicates that 2θ is linearly correlated with sin²ϕ, thereby confirming the presence of residual stress.

Figure  10.  2θ–sin²ϕ fitting line of measuring points (a) Z-1, (b) Z-2, (c) H-1, and (d) H-2.

下载: 全尺寸图片 幻灯片

3.3   Determination of residual stress

3.3.1   Determination of residual stress in each direction of the measuring point

The 2θ–sin²ϕ linear fit of the internal stress in three directions at the measuring points Z-1, Z-2, H-1, and H-2 is remarkable. Using Eq. (2), the residual stress was calculated. The elastic modulus E of the quartz mineral (324) crystal plane was set to 10,300 MPa, and Poisson’s ratio ν was set to 0.31. The radius corresponding to φ = 0 in each direction is presented as θ₀. The results of the linear fitting of 2θ–sin²ϕ and the corresponding calculated residual stress for each measuring point are shown in Table 1.

Table  1.  Calculated results of residual stress

Points	Direction	φ	Line fitting parameters		Maximum strain	Stress
			R2	M		Stress value / MPa	Stress error / MPa
Z-1	Z	0	0.98451	0.00230 ± 0.00012	−0.000123	−21.2263	1.1805
	—	π/4	0.92609	0.00149 ± 0.00021	−0.000111	−13.8468	1.9406
	Y	π/2	0.84195	0.00133 ± 0.00026	−0.000087	−12.2716	−2.3700
Z-2	Z	0	0.86819	0.00151 ± 0.00026	−0.000105	−13.9286	−2.4344
	—	π/4	0.98327	0.00170 ± 0.00011	−0.000092	−15.6844	−1.0225
	Y	π/2	0.92704	0.00129 ± 0.00016	−0.000079	−11.9942	−1.5614
H-1	Z	0	0.95824	0.00170 ± 0.00016	−0.000127	−15.6934	−1.4674
	—	π/4	0.83076	0.00217 ± 0.00044	−0.000092	−20.0349	−4.0491
	Y	π/2	0.89619	0.00356 ± 0.00054	−0.000079	−32.8748	−4.9996
H-2	X	0	0.84963	0.00218 ± 0.00041	−0.000147	−20.1569	−3.8004
	—	π/4	0.87450	0.00207 ± 0.00035	−0.000113	−18.7669	−3.4519
	Z	π/2	0.93576	0.00196 ± 0.00023	−0.000079	−18.1229	−2.1764

下载: 导出CSV 

| 显示表格

3.3.2   Determination of residual principal stress

In determining the surface stress state of the rock samples, the magnitude and direction of the principal stress at the measured points were calculated for the three stress directions. The principal stress of residual stress is denoted as σ₁ and σ₂. The angle between the direction of σ₁ and directions of φ = 0, φ = π/4, and φ = π/2 are presented as Φ, Φ + π/4, and Φ + π/2, respectively. Therefore, the measured stresses in the directions of φ = 0, φ = π/4, and φ = π/2 were recorded as σ_Φ, σ_{Φ + π/4}, and σ_{Φ + π/2}, respectively.

Therefore, the stress values in three directions and the principal stress at each point were obtained in accordance with the principal stress calculation shown in Eqs. (5)–(7), which are described as follows:

where Φ is the angle between the direction of σ₁ and the direction of φ = 0, σ_Φ is the measured stresses in the direction of φ = 0, σ_{Φ + π/4} is the measured stress in the direction of φ = π/4, σ_{Φ + π/2} is the measured stress in the direction of φ = π/2, σ₁ and σ₂ are the principal stress of residual stress.

The calculated results of the residual stress of the test points are shown in Table 2.

Table  2.  Calculated results of residual principal stress

Points	Plane	Directional stress / MPa				Principal stress / MPa			Angle between σ1 and σΦ (Φ) / (°)
		σΦ	σΦ + π/4	σΦ + π/2		σ1	σ2
Z-1	Z–Y	−21.2263	−13.8468	−12.2716		−22.08	−11.41		16.48
Z-2	Z–Y	−13.9286	−15.6844	−11.9942		−15.85	−10.07		−35.22
H-1	Z–Y	−15.6934	−20.0349	−32.8748		−14.70	−33.87		−13.16
H-2	X–Z	−20.1569	−18.7669	−18.1229		−20.23	−18.05		10.50

下载: 导出CSV 

| 显示表格

4.   Discussion

4.1   Distribution characteristics of residual stress

Fig. 11 illustrates the distribution of the residual stress on the rock sample surface. The principal stresses within the quartz vein are all compressive, with stress values ranging from 10 to 30 MPa at the measuring points. The principal stress along the transverse and longitudinal veins parallel to their respective dikes was significantly higher than that perpendicular to their respective dikes. Moreover, the principal stress in the transverse quartz vein, which is roughly parallel to the bedding direction, is greater than the stress locked within the longitudinal quartz vein, which is roughly perpendicular to the bedding direction.

Figure  11.  Schematic diagram of the principal stress direction and relationship of measuring points: (a) longitudinal vein Z-1; (b) longitudinal vein Z-2; (c) transverse vein H-1; (d) transverse vein H-2.

下载: 全尺寸图片 幻灯片

During the formation of the quartz vein, SiO₂ evaporated from magmatic rock, and water vapor filled the fractures in the matrix. The fractures initially attached to the matrix wall, then coagulated and grew continuously with the quartz, and finally filled the cracks in the bedrock. In places where the raw materials were abundant, they were mutually constrained with siliceous slate to achieve balance, and the internal pressure was locked in. The siliceous slate is a metamorphic rock formed after compression and deformation of the protolith. As the maximum stress of the longitudinal and transverse veins is parallel to the direction of the dike, the metamorphic transformation of the siliceous slate might occur prior to the formation of the quartz vein. Residual stress measurements provide valuable information regarding rock formation.

4.2   Analysis of residual stress generation

The EBSD experiment data were processed to obtain the grain boundary diagram, kernel average misorientation (KAM) diagram, and geometrically necessary dislocation (GND) diagram to analyze the mechanism for residual stress generation in rocks.

High-angle grain boundaries are defined as having an orientation difference of more than 15°, whereas low-angle grain boundaries are defined as having an orientation difference between 2° and 15°. As shown in Fig. 12, the grain boundary diagram of the quartz vein test area is composed of high- (51.9vol%) and low-angle grain boundaries (48.1vol%). The defects formed by dislocation, arrangement, and combination are concentrated in the low-angle boundaries. The content of the low-angle grain boundaries is more prone to recovery and recrystallization in quartz veins.

Figure  12.  Grain boundary map of the quartz vein area.

下载: 全尺寸图片 幻灯片

The maximum KAM value of the quartz pulse is 4.52°, whereas the maximum GND value is 5.36 cm⁻². Combined with the grain boundary map observation, the region with a higher geometric dislocation density is located at the low-angle grain boundaries, indicating that stress concentration likely occurs at low-angle grain boundaries of mineral crystals.

In addition to the low- and high-angle grain boundaries, the quartz vein test area contains twin boundaries of the <001> 60° type, which account for 40.7vol% of the total area. When twins form in the crystal, the material experiences internal stress because the natural change in the shape of the twin zone is limited by the surrounding environment [32]. The <001> 60° twins in α-quartz are Dauphiné twins, which rotate 180° (60° in crystallography) around the C-axis, resulting in the reversal of the positive and negative forms of the crystal structure [33–34]. Twins can switch between the positive and negative <a> directions [35]. During plastic deformation, Dauphiné twins create the main active slip system, and their impact on local strain strongly depends on the deformation time relative to the movement of the evolutionary shear band. Research has shown that Dauphiné twins have a strong ability to adapt to strain [34].

While twin boundaries have resistance to large deformation, low-angle boundaries easily induce strain concentration in quartz veins. The mineral crystals inside the rocks deform with pressure or temperature changes in the Earth crust. The ability of mineral crystals to resist deformation differs, and grains squeeze one another because of different deformation states. This process produces dislocation and plastic deformation as well as changes the internal mechanical properties of the rocks. When the temperature changes or when an external force is applied, the internal dislocation and grain boundary hinder the deformation of the crystal, and the stress is locked inside. Consequently, the external state remains balanced and stable.

5.   Conclusions

In this work, EBSD and optical microscopy were performed to characterize the rock microcosm and explained rock residual stress.

(1) The siliceous slate-containing quartz veins in the Hongling Mine were primarily composed of feldspar, quartz, and small quantities of chlorite. The statistics generated were sufficient to meet the requirements of XRD tests. Moreover, a homogeneous region in the millimeter scale was identified, which met the threshold required for residual stress measurement using XRD.

(2) The results showed that the residual stresses measured in rocks were all compressive stresses, with magnitudes ranging from 10 to 33 MPa. The residual stress in transverse quartz veins was greater than that in longitudinal quartz veins. The maximum stress in the vein was roughly parallel to the direction of the vein, and the minimum stress was roughly perpendicular to the direction of the vein. The main stress direction was closely related to the vein. Therefore, the formation and development of residual stress were closely related to the rock microcosm. Thus, studying residual stress from the microscopic perspective of rocks is of great significance.

(3) Plastic strain analysis of the rock microzone was performed by examining the difference in mineral crystal orientation. The mechanism for generating rock residual stress was analyzed by examining the grain boundaries, orientation difference, and dislocation density obtained from grain boundary maps, KAM maps, and geometric dislocation density maps. The results showed that several low-angle boundaries that are prone to strain concentration and twin boundaries could adapt to large strains in the quartz veins. Furthermore, differences in the ability of mineral crystals in rocks to resist deformation were observed. Temperature changes or the application of external forces resulted in incompatible deformation, and the stress was “locked inside” because of internal dislocations and grain boundaries.