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To ensure the safety of coal mining, lots of coal pillars were reserved in the coal mine. The stability of coal pillar and roof composite structures determines the safety of stope. Under the action of cyclic loading and unloading such as mining face, chamber blasting, and roadway excavation, the coal pillar, and roof composite structure will gradually instability, which will affect the safety production of the coal mine. Therefore, this paper takes the dynamic disaster of coal pillar and roof composite structure as the research background, carries on the cyclic loading and unloading test of the coal-rock composite structure with different coal-rock height ratios, and studies the influence of coal-rock height ratio on the progressive instability characteristics, mechanical properties and damage variables of the coal-rock composite structure. The crack initiation stress, crack propagation time and failure pattern of coal-rock composite structure are analyzed. The improved load-unload response ratio considering the damage variable is established, and the stability of the coal-rock composite structure under cyclic loading and unloading is quantitatively analyzed. It is concluded that the larger the coal-rock height ratio is, the greater the load-unload response ratio of the composite structure is, and the more unstable the composite structure is. This conclusion is consistent with the crack initiation stress and fracture propagation time, and can quantitatively reflect the progressive instability process of coal-rock composite structure, and provide a basis for revealing the dynamic disaster of coal pillar and roof composite structure. [ABSTRACT FROM AUTHOR]

To study the mesodamage of migmatite under cyclic loading, single cycle loading and unloading tests of migmatite were carried out under different confining pressures (σ3) and stress ratios (p), and acoustic emission (AE) technology was used to monitor the evolution of microcracks. Additionally, the mesodamage of migmatite was analyzed from the porosity and T2 spectrum distribution measured by nuclear magnetic resonance (NMR) spectroscopy. The results show that the peak stress ratios (pmax) of migmatite under different σ3 are different, and as the value of σ3 increases, the migmatite gradually undergoes a change from tension to shear failure. Moreover, with the increase in σ3, high-frequency signals gradually appeared, indicating that the increase of σ3 to a certain extent limited the generation of large size microcracks. The variations in damage variables defined by the number of AE hits and porosity are the same, "steep increment type," and increase suddenly when approaching pmax. Furthermore, σ3 slightly affects the pore throat distribution of migmatite. In addition, σ3 has a certain inhibitory effect on the porosity of migmatite and accumulation of mesodamage. However, when σ3 exceeds a critical value, the inhibitory effect becomes a promoting effect. [ABSTRACT FROM AUTHOR]

In order to investigate rock mechanical behavior under cyclic loading and unloading conditions, the clump parallel-bond model (CPBM) combined with the newly-proposed loading procedure was used in numerical simulations and the simulated results were compared with those of the experiment. Then the intact rock under different confining pressures and pre-cracked rock with different flaw inclination angles are also investigated in detail. The related numerical results have demonstrated that: (1) the CPBM, combined with the use of the newly-proposed loading procedure, can be used to modelling the intact rock and pre-cracked rock mechanical behavior under cycle loading and unloading conditions. The simulated results are in good agreement with the experimental results; (2) the number of microscopic cracks increases with the cycle increases during the loading process, while few microscopic cracks are generated during the unloading process. The higher the confining pressure, the less damage the rock has accumulated during the cyclic loading process; (3) the inclination of the pre-existing flaw has a significant effect on the macroscopic fracture pattern. [ABSTRACT FROM AUTHOR]

Engineering research has shown that the surrounding rock of deep roadways experienced many times of post-peak cyclic loading and unloading. So studying on rock mechanical characteristics of post-peak loading and unloading is helpful to control the deep surrounding rocks. The test using RMT-150B rock mechanics testing system, to experiment on the mechanical properties of sandstone of post-peak cyclic loading and unloading. The results show that: the stress-strain curves of post-peak cyclic loading and unloading have significant plastic hysteretic loops. The area of plastic hysteretic loops gloss back. The enveloping outer enclosure of cycle loading curve is the similar as the stress-strain curves of strain softening stage when the samples failure, which shows that post-peak failure of rocks have significant memory. With the increase of cycles, the cumulative deterioration parameters are gradually increased, and the ultimate bearing capacity, elastic of loading section of samples and cumulative deterioration parameters conform with the exponential attenuation function with constant term. With the increase of surrounding pressure, the total energy, dissipated energy and elastic energy of samples are gradually increased. With the increase of cycles, the total energy, dissipated energy and elastic energy of samples all are gradually decreasing, the rate of reduction decreases gradually. The samples exist in vertical splitting and transverse shear combination failure under the post-peak uniaxial cycles, or exist in shear failure under the post-peak triaxial cycles. The shear plane exists slip traces. [ABSTRACT FROM AUTHOR]