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The separation and investigation of plastic dissipation energy and damage dissipation energy, and the exploration of the association between energy dissipation and crack development, contribute to revealing the essence of rock failure. In this study, triaxial cyclic loading and unloading tests were performed on porous siltstone widely distributed in mining areas in western China. According to the characteristic stresses and energy evolution characteristics, the failure process of siltstone was divided into five stages. The cumulative energy dissipation ratio shows a spoon-shaped evolution characterized by a significant decline, slight decline, slow increase, significant increase, and stable trend. Importantly, a method for separating plastic dissipation energy and damage dissipation energy was proposed and applied to investigate the evolution characteristics of plastic dissipation energy and damage dissipation energy. Corresponding to the energy characteristics, the acoustic emission activity exhibits significant phased characteristics in five stages. Furthermore, the analysis of crack evolution based on acoustic emission parameters reveals that the proportion of tensile cracks increased significantly near peak failure, and the development of tensile cracks substantially contributed to the damage of load-bearing structures. In particular, the evolution of shear cracks exhibited a strong correlation with the cumulative plastic dissipation energy, while the evolution of tensile cracks showed a strong correlation with the cumulative damage dissipation energy. Finally, the energy dissipation mechanisms of tensile cracks and shear cracks were revealed. Highlight Highlights: A characteristic stress determination method based on evolution characteristics of multiphase volumetric strain Spoon-shaped evolution characteristics of the cumulative energy dissipation ratio A determination method of plastic energy dissipation and damage energy dissipation Association mechanism between the development of shear and tensile cracks and the evolution of plastic and damage dissipation energy [ABSTRACT FROM AUTHOR]

Mechanical characteristics, instability precursors, and fatigue life prediction of rocks under fatigue loading are significant for the prediction and prevention of mining disasters. Taking the porous siltstone widely distributed in the mining area of western China as the research object, a characteristic stress determination method based on the evolution characteristics of the multiphase volumetric strain of porous rocks was established. Referring to the characteristic stress interval, bivariate fatigue damage-controlled tests with different upper limit loads and different numbers of cycles were conducted. The results indicate that the strengthening and the weakening of the macroscopic strength of siltstone under fatigue loading are caused by the competition of two mesostructure effects, namely the "weak structure failure effect" and the "compaction embedment effect". The dominant effect varies with different upper limit loads and different numbers of cycles. Importantly, the transition of the bulk compliance from positive to negative is an early precursor of fatigue failure, while the simultaneous sudden increase in cumulative acoustic emission count, acoustic emission bk value, and energy dissipation ratio is an imminent precursor of fatigue failure. The evolution characteristics of the strain, acoustic emission, and energy dissipation indicate that the crack damage stress of siltstone is a reliable indicator of its fatigue strength. Furthermore, a nonlinear viscoplastic damage element was introduced to construct a nonlinear fatigue rheological damage model, and model validation and quantitative analysis of parameter effects were conducted. Finally, by determining the accelerated rheological start threshold and the accelerated rheological failure threshold, a rapid prediction method for the fatigue life was established. Highlights: A characteristic stress determination method suitable for porous rock. Mesoscopic mechanism of strength strengthening and weakening. Early and imminent precursors of fatigue rheological failure. A nonlinear fatigue rheological damage model. [ABSTRACT FROM AUTHOR]

During underground engineering projects, siltstone—a porous, weakly cemented rock—commonly undergoes cyclical states of creep and fatigue, raising questions about its mechanical stability under such conditions. To investigate, this study employed laboratory experiments, featuring a creep-fatigue alternation action test and uniaxial reloading test, with the absolute expansion stress serving as the upper load limit. Results reveal a counterintuitive synergy between creep and fatigue actions. Fatigue processes reduced the viscoplastic deformation capacity of siltstone, prolonging its creep rupture life. Besides, creep actions amplified the fatigue life of siltstone by over 168%, attributable to the filling and embedding of rock debris particles in the rock matrix. Additionally, siltstone’s elastic modulus and Poisson’s ratio displayed a periodic “rise-drop” oscillatory pattern under the creep-fatigue regime. Plastic strain values emerged as a robust metric for quantifying damage. Intriguingly, creep-fatigue alternation increased the absolute expansion stress by 11.7%, bolstering the rock’s damage resistance. However, this cyclical regime exacerbated the postpeak fracture behavior of siltstone, underscoring the complexity of its mechanical response. [ABSTRACT FROM AUTHOR]

A graded cyclic loading test was designed and conducted. The mechanical characteristics, acoustic emission (AE), and energy evolution law of granite under cyclic loading were demonstrated. Meanwhile, a multifactor damage model of granite under graded cyclic loading was constructed. Results indicated that hysteresis evidently existed in the stress–strain curve of granite. The envelope curve consisted of compaction, elastic, yield, failure, and residual strength stages. The Poisson's ratio of granite was minimally affected by cyclic loading and confining pressure. Conversely, the elastic modulus of granite was highly sensitive to cyclic loading and confining pressure. Considerable AE phenomena occurred in the yield and failure stages. After entering the residual strength stage, the AE intensity of granite decreased until it disappeared. In the cycle process, the proportion of elastic energy is higher than that of dissipated energy. As the number of cycles increased, the elastic energy proportion and dissipated energy proportion increased and decreased, respectively; finally, both tended to be stable. The damage state of granite during the cyclic loading test was quantitatively described by the newly established multifactor damage model. The damage degree of granite under graded cyclic loading was mainly determined by loading stress level. [ABSTRACT FROM AUTHOR]