Search

The acoustic characteristics of the rock failure process can reflect the evolution of crack development and expansion. To study the evolution of different types of internal cracks during the rock failure process, the acoustic emission (AE) signal of the yellow sandstone under uniaxial compression was collected with AE monitoring technology, and the acoustic characteristics, crack types and evolution of the correlation dimension of the rock failure process were studied. The parameter analysis of RA (rise time/amplitude) and AF (counts/duration) for classifying different cracking modes during loading revealed that tensile cracks are dominated in the crack closure stage and linear elastic deformation stage, and the proportion of shear cracks increased in the plastic deformation stage and the post-peak failure stage. High-energy events were more widely distributed in the areas of shear cracks. The average AE energy of shear cracks is higher than that of tensile cracks, and the AE energy ratio for shear cracks and tensile cracks is within the range of 4.29–4.78. The correlation between stress and AE data was calculated using fractal theory methods and Grassberger–Procaccia (G–P) algorithms to study the evolution of different types of internal cracks. The correlation dimension displayed a stable stage, a descending stage and a fluctuating low value stage. The descending stage and the fluctuating low value stage appeared during the plastic deformation stage and the post-peak failure stage, respectively, indicating that the type of rock internal micro-crack changed greatly in the above two stages. The findings in this study contribute to better understanding of the rock failure mechanism. Highlights: The evolution of proportion of tensile and shear cracks during the rock failure process were analyzed. The average AE energy of shear cracks is higher than that of tensile cracks, and the AE energy ratio for within the range of 4.29–4.78. The evolution of the type of micro-cracks in rock failure process is analyzed by Grassberger–Procaccia (G–P) algorithms. [ABSTRACT FROM AUTHOR]

In this study, the influence of cyclic impact loading damage and water saturation on mechanical behavior and Kaiser effect of rock samples was investigated through a series of uniaxial cyclic loading and unloading compression tests. The test results showed that the cyclic loading and unloading strength of rock samples in dry condition gradually decreased with the increase of previous damage, while the cyclic loading and unloading strength of rock samples in saturated condition showed a decrease in cyclic loading strength of 10.95% (3 times), − 5.40% (6 times), and 0.87% (9 times) compared to those not subjected to cyclic impact loading. We have elucidated the mechanism underlying this phenomenon from the perspective of water–rock interaction. Statistical analysis of the Felicity ratio values further revealed that the valid response stress interval of rock acoustic emission (AE) Kaiser effect is negatively affected by previous damage and water saturation. Moreover, the relationship among AE signal energy decay rate and previous damage and water saturation was discussed, negatively impacting the valid response stress interval of rock AE Kaiser effect. The results suggest that drilling of cores from stress-disturbed areas should be avoided as much as possible during the measurement of in situ stress using the rock AE Kaiser effect. The tests should also be conducted with dry rock samples to have a larger response stress interval for the AE Kaiser effect. Highlights: The wave velocity change rate and porosity were used to characterize the previous damage of rock. The influence of previous damage and water saturation on rock failure mode was evaluated. The strengthening mechanism of water-rock strength was discussed. The influence of previous damage and water saturation on Kaiser effect was studied. [ABSTRACT FROM AUTHOR]

The cracked chevron notched Brazilian disc method (CCNBD) recommended by the International Society for Rock Mechanics is a major method for measuring the rock fracture toughness. In the rock fracture mechanics, the micro-crack evolution behavior during Mode I fracture is of great theoretical significance. To fully understand the micro-crack evolution mechanism and fracture process zone evolution information of non-uniform sandstone, numerical simulation of CCNBD specimen under Mode I fracture condition was carried out using PFC3D. Combined with the box fractal dimension theory, the sandstone particle size and mineral composition were considered, and the non-uniform bonded particle model was built. By monitoring the evolution of micro-cracks in the interior and boundary of the cluster, the micro-crack evolution process of the numerical model was obtained. The results showed that the micro-fracture of CCNBD specimen during the cracking process was dominated by weak cementation plane and the contact of interstitial filling particles, while the micro-crack generated between detritus particles mainly concentrated in the CCNBD specimen with the peak stress range of 80–100% and the post-peak failure stage. Combined with the stress field analysis, the Mode I fracture experiment of the bonding between diagenetic mineral particles at the crack tip of CCNBD specimen was dominated by tension, while the bonding between interstitial filling particles is dominated by compression. The micro-crack evolution mechanism of the CCNBD specimen during Mode I fracture was obtained by experiments and numerical simulations. This study is expected to improve the understanding of the unstable fracture mechanism of the fractured rock mass. [ABSTRACT FROM AUTHOR]

Rock creep characteristics play an important role in geological engineering and have important physical implications when evaluating the long-term stability of rock structures. In this study, a series of uniaxial compression and creep experiments were performed on red sandstone specimens from Jiangxi Province, China. During experiments, stress, strain, acoustic emission (AE) parameters, and waveform files were recorded continuously. First, an AE creep model with a damping factor of red sandstone specimen was established based on the theory of statistical mechanics and damage mechanics. The intrinsic relationship between the cumulative AE events and the strain in the accelerating creep stage was revealed by a creep model derived from the inversion of experimental data in the decelerating and steady creep stages. Then, using the singular value decomposition method for cumulative AE events, quantitative identification of the accelerating creep stage of red sandstone was achieved. The evolution characteristics of b value in the creep process of red sandstone specimens are revealed. The AE signals in the decelerating creep and steady creep stages had abrupt waveforms, whereas the AE signals in the accelerating creep stage had mixed abrupt and continuous waveforms. Therefore, quantitative identification of the accelerating creep stage was achieved from the perspective of AE waveform characteristics. Finally, the relationship between the wave characteristics of AE signals and the length of microcracks in the rock was identified, and the wave characteristics of AE signals at different creep stages were considered from the perspective of elastic wave dynamics. [ABSTRACT FROM AUTHOR]