Effects of sandstone mineral composition heterogeneity on crack initiation and propagation through a microscopic analysis technique

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Abstract

Crack initiation and propagation are key scientific questions influencing rock failure. To study crack initiation and propagation during rock failure, CT scanning tests were performed on the sandstone samples. The initiation and propagation stages of sandstone cracks were analysed based on a reconstructed three-dimensional crack structure model. The types of crack initiation and propagation in minerals and the effects of mineral composition heterogeneity on crack initiation and propagation were studied using microscopic analytical techniques. The effect of mineral granularity on crack initiation and propagation was analysed using statistical methods. The results show that crack initiation and propagation can be divided into four stages: microcrack inoculation, mesocrack initiation, mesocrack evolution to macrocrack, and macrocrack penetration. Four cracks are formed at different locations, extending along the mineral edge, deflecting at the mineral edge, bifurcating at the mineral edge, and penetrating through the mineral. The poikilitic texture formed by quartz debris and cement makes it easy for cracks to extend along the mineral edges. In quartz debris with high crystallinity, the crack tends to deflect and bifurcate at the edge of quartz debris. The symbiotic structure of altered quartz and clay minerals easily causes cracks to propagate through the minerals. Furthermore, the distribution of mineral granularity affects crack initiation and propagation. The inhomogeneity of mineral granularity can easily cause the initiation and propagation of cracks. The size of the mineral granularity affects the width of the secondary crack. These research results can provide a basis and reference for predicting and evaluating the stability of rock masses.

Introduction

The exploitation and utilization of natural resources have always been important issues in the research community. With the exhaustion of shallow resources, deep mining has become a strategic alternative with scientific and technological problems that must be solved.1,2 The initiation and propagation of rock cracks are related to the safety, efficient exploitation, and utilization of resources. In the mining process, the initiation and propagation of cracks is a common phenomenon that has attracted extensive attention from researchers.3 Many engineering disasters are caused by the unstable propagation of cracks, such as goaf collapse, slope instability, and rock burst.4,5 The initiation and propagation of cracks result from the interaction between rock materials, load, and environmental factors. At the same time, different mineral components, uneven microstructures, and preexisting defects are the source problems causing the initiation and propagation of cracks.6,7 Therefore, from the perspective of rock composition, the study of crack initiation and propagation provides a theoretical basis and technical reference towards assessing strength and stability in safe deep resource production and engineering disaster prevention.

As a heterogeneous material, rocks can be described by their material composition and structure. The structure of rock materials is closely related to the initiation and propagation of cracks. When rocks with defects fail under loading, the formation and evolution of cracks are influenced by the heterogeneity and anisotropy of rocks. Many scholars have attempted to study the factors affecting crack initiation and propagation in rocks or rock-like materials.8,9 Zhang et al. studied the initiation and propagation of cracks in shale through a supercritical carbon dioxide fracturing experiment and found that crack initiation occurred around boreholes. The carbon dioxide-induced cracks were more likely to form complex crack networks.10 Kim et al. studied the initiation and propagation of cracks in layered sandstone. The results showed that cracks always initiated in loose bedding, and the direction of the bedding plane affected the crack propagation path.11 Sivakumar et al. observed the initiation of prefabricated cracks under different inclination angles by combining laboratory experiments and numerical simulations. The results showed that crack initiation did not increase with the increasing inclination angle of the prefabricated crack. Nevertheless, the initiation occurred when the inclination angle of the prefabricated cracks and the direction of the loading stress were 45°.12 The structure of rock materials affects the initiation and propagation of cracks from the microscopic scale. Different rock structures cause different macroscopic fracture patterns affecting the mechanism of crack initiation and propagation.13 The above studies mainly consider the effect of rock structure on the initiation and evolution of cracks. While both the structure and mineral composition of rock together affect the initiation and propagation of cracks, the effect of mineral composition, in particular, needs further study.

Rocks are aggregates of various minerals found in nature, and different mineral components can induce and hinder the initiation and propagation of cracks. Ghasemi et al. studied the influence of different minerals on crack propagation and evolution under uniaxial loading. The study showed that microcracks first appeared in biotite and finally in quartz, and cracks expanded along the direction with more biotite.14 In the process of rock formation, the existence of mineral defects is inevitable. Under load and other environmental conditions, cracks easily initiate at the location of defective minerals and generally propagate in their direction. For example, cracks tend to form at locations with the highest density of mineral defects, and cracks propagate rapidly after the mineral loses strength.15 Rigopoulos et al. studied the influence of mafic ophiolite on microcrack initiation and propagation during uniaxial compression. The results showed that many intragranular and transgranular microcracks led to the destruction of the diorite minerals.16

Recently, with the development of digital image analysis techniques, digital image correlation (DIC) technology and X-ray CT nondestructive testing techniques have been widely used to study the cracking characteristics and failure process of rock under various stress loading conditions. Aliabadian et al. studied the initiation and propagation of cracks in isotropic sandstone using the digital image correlation method.17 Zhou et al. used the digital image correlation method to conduct compression‒shear tests on sandstone with a single crack. They systematically studied the influence of crack geometric parameters on crack initiation and propagation. The experimental results showed that the initiation of anti-wing and secondary cracks lagged behind the wing cracks.18 Liu et al. identified cracks with three different initiation mechanisms and analysed the effect of the crack inclination angle on the crack initiation mechanism using the digital image correlation method.19 Fakhimi et al. used digital image correlation to obtain the displacement patterns of crack initiation and propagation and clearly showed the development of the damage zone and displacement discontinuity under loading.20

In the above research, the digital image correlation analysis method is limited to measuring an image of the rock surface. Consequently, the crack initiation and propagation behaviour within the rock cannot be detected. As a nondestructive method of detection, X-ray CT technology can provide useful information on the dynamic changes of cracks in the rock and has unique advantages in analysing the dynamic characteristics of crack propagation. Duan et al. qualitatively studied the propagation of cracks in shale and the spatial variation of cracks in horizontal and vertical directions based on consecutive CT images.21 Wang et al. described the damage characteristics of rock and soil aggregates under uniaxial compression based on an X-ray CT experiment. They studied the relationship between the crack parameters and average CT values.22 Wang et al. observed the variation in the three-dimensional tomography and the location of the crack distribution in a bimsoil sample using X-ray computed tomography. A three-dimensional model of the crack initiation and propagation was established based on the reconstructed CT volume data.23 Ren et al. identified the X-ray CT technique as a convenient tool for obtaining the spatial distributions of voids and cracks in porous concrete. The results revealed internal damage within the multiphasic composites.24 Although the X-ray CT imaging technique has been widely regarded as a positive approach to studying the behaviour of crack initiation and propagation, it is still limited in accurately determining the mineral composition and the mineral granularity at the crack initiation location and along the crack propagation path.

Most scholars discuss crack initiation and propagation according to traditional theories and methods. However, the intuitive description of the crack initiation and propagation process does not sufficiently characterize cracking in rocks. From the perspective of rock composition, the effect of mineral composition and particle size on crack initiation and propagation is still in the preliminary stage of exploration. Moreover, it is of great academic significance to discuss the influence of mineral composition and mineral size distribution on crack initiation and propagation using microscopic analytical techniques.

In this study, to investigate the mechanism of crack initiation and propagation and the effect of the composition and granularity of minerals on crack initiation and propagation, the spatial distribution of cracks was determined based on three-dimensional CT images and two-dimensional CT sections. The characteristics of crack initiation and propagation were qualitatively described. Polishing slices with cracks were selected from the different stages of crack initiation and propagation for polarized microscopic analysis. The formation of cracks in sandstone and the distribution of minerals around cracks were analysed. The effects of mineral components on the initiation and propagation of cracks were discussed by combining polarized microscopy, electron probe, and scanning electron microscopy. Additionally, the distribution curves of mineral granularity and crack width were obtained by statistical calculation. The effects of mineral granularity and distribution on crack initiation and propagation were further discussed. The research results presented herein provide a new idea and method for predicting and evaluating the stability of rock masses.

Section snippets

Specimen preparation

The sandstone samples were collected from the open quarry in Qingyuan district, Jian city, Jiangxi Province, China. The quarry is composed of clastic particles and interstitial materials. The clastic particles are quartz, feldspar, and lithic debris, primarily subcircular and subangular. Feldspar includes plagioclase and potash feldspar. The lithic debris includes quartz, phyllite, silica, sandstone, and argil. The interstitial materials are iron oxide, pelite, mica, carbonate minerals, and

Fracture process analysis of sandstone

The sandstone specimen in the cyclic loading and unloading test was selected for analysis. Three-dimensional crack structures and typical two-dimensional CT slice images were obtained. Fig. 5 shows the three-dimensional crack structure diagram and typical two-dimensional CT section diagram. The CT scanning images directly reflect the whole process of crack initiation, propagation, and coalescence under different loading conditions. The crack was visualized based on VG software, and the volume

Effect of the bedding structure on crack initiation and propagation

After the cyclic loading and unloading tests, the sandstone samples were destroyed, and crack propagation stopped. Fig. 11 shows the CT scan images and failure morphology before and after sandstone failure. The main crack runs through the whole sandstone and dominates the failure of the sandstone. Many secondary cracks are generated around the main crack, and they are interconnected to form a complex crack network. Comparing the sandstone samples before and after failure, the influence of the

Conclusion

In this paper, CT scanning tests and microscopic analysis tests were conducted on sandstone samples under different applied loads. The process of crack initiation and propagation was analysed, and four different forms of crack initiation and propagation were determined. The effects of sandstone bedding structure, quartz debris, debris and mineral size on crack initiation and propagation and their internal mechanism were studied. The conclusions are as follows:

  • (1)

    For sandstones cemented together by

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China, (Grant Nos. 51574060, and 52074123); The Tangshan Science and Technology Project, (Grant No. 22130224H). The authors are grateful to all study participants.

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