Failure behavior of pillars in deep mines is affected by various cyclic loads that cause initial pre-damage. Pillars will be further damaged and developed in the long-term compressive stress until they are destroyed. To reveal the strength characteristics and crack damage fracture laws after rock pre-damage, uniaxial compression tests were carried out on granite specimens damaged by cyclic loading using the digital speckle correlation method. The experimental results indicate that the mechanical properties of pre-damaged specimens show large damage differences for different cycles. The damage variable of the pre-damaged specimens increases with the increase of cycle number and confining pressure. The damage of specimens is primarily due to the strength weakening effect caused by cycle numbers, and the confining pressure restriction effect is not obvious. The evolution laws of uniaxial compression damage propagation in the pre-damaged specimens show differences and obvious localization phenomenon. Pre-damaged specimens experienced three failure modes in the uniaxial compression test, namely tensile shear failure (Mode I), quasi-coplanar shear failure (Mode II), and stepped path failure (Mode III), and under different pre-damage stress environments with high confining pressures, the failure modes are dominated by Mode II and Mode III, respectively.
Deep rock structures are often subjected to complex cyclic disturbances generated by earthquakes and blasting vibrations. The rocks will resist disturbance with multiple stress levels, and the research on mechanical response is still insufficient under such conditions. A series of multi-level cyclic loading experiments were subjected to limestone specimens to obtain the stress–strain relation and fracture behavior. This study explored the effect of amplitude and cycle times on rocks. A Discrete Element Method model of rock specimens was established in Particle Flow Code 2D (PFC2D). The simulation results are coincidental with the experiment results. The results show that loading with low cycles can strengthen the rock, but loading with high cycles will present deteriorated effect on the rock. In the numerical simulation test, the initial crack will appear earlier with the amplitude increase. More micro cracks will be induced as the number of cycles per level increases. Moreover, tensile cracks are mainly distributed around the specimen when shear cracks widely appear in the central area. With the increase of amplitude, failure modes with mixed shear and tensile cracks will become universal.
Considering the recent developments of deep mining, investigating the rock properties under high ground stress periodic load is highly demanded. Studies show that these characteristics are important factors affecting the long-term steadiness of rock. However, the mechanical properties of rock mass without macro failure after cyclic load should be studied. In the present study, granite in a mine is considered as the research object. A rock pre-damage experiment is conducted with the same cycles under different confining pressures and constant cycle upper and lower limit loads. The pre-damaged rock sample is subjected to a uniaxial compression test, and a high-speed charge couple device camera is used to record the speckle field image of the sample surface during the whole loading process. The digital speckle techniques are used to analyse the image of the pre-damaged sample, the deformation field of the specimen surface, the displacement dislocation value of the localized deformation area and the deformation energy value of the specimen surface. The results show that for the same cycle times, the confining pressure is less than 80 MPa, which has a weakening effect on the rock's axial strength. As the confining pressure approaches 120 MPa, the pre-damaged rock uniaxial peak strength increases. The characteristics of displacement dislocation energy evolution of the localized deformation bound are divided into three stages (pre-peak stage, peak point and post-peak stage). After pre-damage under the same cycle times and different confining pressure conditions, the deformation field evolution of rock is relatively consistent.
Deep rock structures are often subjected to complex cyclic disturbances generated by earthquakes and blasting vibrations. The rocks will resist disturbance with multiple stress levels, and the research on mechanical response is still insufficient under such conditions. A series of multi-level cyclic loading experiments were subjected to limestone specimens to obtain the stress-strain relation and fracture behavior. This study explored the effect of amplitude and cycle times on rocks. A Discrete Element Method model of rock specimens was established in Particle Flow Code 2D (PFC2D). The simulation results are coincidental with the experiment results. The results show that loading with low cycles can strengthen the rock, but loading with high cycles will present deteriorated effect on the rock. In the numerical simulation test, the initial crack will appear earlier with the amplitude increase. More micro cracks will be induced as the number of cycles per level increases. Moreover, tensile cracks are mainly distributed around the specimen when shear cracks widely appear in the central area. With the increase of amplitude, failure modes with mixed shear and tensile cracks will become universal.
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