Rocks are essential engineering materials, and understanding their internal crack evolution is crucial for preventing instability and failure in rock engineering projects. In this study, researchers from the China University of Mining & Technology (Beijing) used a combination of acoustic emission technology and rock pressure testing to investigate the internal crack patterns and evolution mechanisms of red sandstone with varying fracture inclination angles. Their findings shed new light on the complex behavior of fractured rocks, which could have important implications for the stability and safety of structures built on or in rock formations. Rock engineering, Acoustic emission, Fractal dimension, Sandstone
Unraveling the Mysteries of Fractured Sandstone
Rocks are ubiquitous in our natural environment and play a critical role in various engineering applications, from the construction of buildings and infrastructure to the extraction of natural resources. However, the internal structure of rocks can be highly complex, with a myriad of cracks, fissures, and tiny pores that can significantly impact their mechanical properties and stability.
In this study, the researchers focused on the behavior of fractured red sandstone, a common rock type used in construction and mining. They conducted a series of uniaxial compression tests and acoustic emission monitoring on red sandstone samples with varying crack inclination angles, ranging from 0° to 90°. By integrating these two complementary techniques, the researchers were able to gain a comprehensive understanding of the internal crack evolution and fractal characteristics of the fractured sandstone.
Unraveling the Mechanical Behavior of Fractured Sandstone
The researchers found that the compressive strength of the fractured red sandstone varied in a “U-shaped” pattern with the crack inclination angle. When the crack angle was close to the internal friction angle of the rock, the resistance to deformation was significantly reduced, leading to a decrease in the elastic modulus and compressive strength.
Interestingly, the researchers observed two distinct stress drops in the stress-strain curves of the 30° and 45° crack samples, which corresponded to a sudden increase in acoustic emission events. This indicated that the rock had entered a stage of irreversible damage, with large-scale cracks forming within the material.
Tracing the Crack Evolution with Acoustic Emission
By analyzing the acoustic emission parameters, such as the rise time-to-amplitude ratio (RA) and the ringing count-to-duration ratio (AF), the researchers were able to gain insights into the microscopic fracture modes within the rock samples. They found that the initial damage was primarily caused by tensile cracks, which then steadily increased in number as the external load was applied.
As the loading continued, the shear cracks also began to increase rapidly, leading the rock to transition from a stage of stable fracture development to one of unstable fracture development. This transition point could serve as a crucial precursor for large-scale fractures within the rock, potentially aiding in the early detection of instability and failure.
Unveiling the Fractal Characteristics of Crack Evolution
The researchers also explored the fractal characteristics of the acoustic emission signals recorded during the loading process. Using the rescaled range (R/S) statistical analysis, they found that the Hurst index, which is related to the fractal dimension, showed a general increasing trend as the crack inclination angle increased.
This pattern corresponded to the complexity and self-similarity of the crack growth and development within the rock. When the crack angle was close to the rock’s internal friction angle, the crack evolution tended to follow a certain direction and maintain a higher level of self-similarity at different observation scales.
Implications and Future Directions
The findings of this study have important implications for the assessment of stability and the implementation of early warning systems in engineering rock masses. By integrating acoustic emission technology and rock pressure testing, the researchers have established a powerful joint monitoring methodology that can provide detailed insights into the internal crack patterns and evolution mechanisms of fractured rocks.
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