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• The effects of high temperature and cyclic load on red sandstone were investigated. • The failure process of the specimen was analyzed in depth by AE and DIC technologies. • A crack classification model based on K-medoids and SVM algorithms was proposed. • The failure precursor characteristics were discussed by critical slowing down theory. This paper investigated red sandstone's acoustic emission (AE) and fracture characteristics under high temperature and cyclic load. The specimen's behavior throughout the process from stabilization to failure was systematically analyzed in terms of the distribution of AE activities, rupture scale, crack type, crack evolution, and failure mode. A crack classification model based on K-medoids and support vector machine (SVM) algorithms was proposed, and the optimal dividing line for tensile-shear cracks at different temperatures was determined as AF = 0.335 × R A + 76.890. The specimen's failure precursor information was recognized based on the critical slowing down theory. [ABSTRACT FROM AUTHOR]

Rocks in deep coal mines are usually in varying degrees of damage state before they are destabilized by impact loads such as rock bursts. For the problem of the mechanical properties and energy evolution of damaged rocks under impact loads, the authors use static loads with different cyclic load thresholds to act on sandstone specimens to make them in distinct degrees of damage. Then, the rock mechanics system (MTS-816) and the Split Hopkinson pressure bar (SHPB) are employed to perform uniaxial compression and impact dynamics tests on sandstones with different degrees of damage. The results show that, from the perspective of mechanical properties, the uniaxial compressive strength and dynamic compressive strength of the damaged sandstone gradually decrease with the increase of the upper limit of the cycle threshold and both obey the growth law of the quadratic function, and the dynamic strength increase factor (DIF) also decreases with the increase of the cyclic load threshold. In terms of energy, with the increment of the cyclic load threshold, the number of cracks in the damaged sandstone is large and the scale is enormous. Due to the effect of cracks, when the incident energy is a fixed value, the transmission energy decreases with the increase of the damage degree and the change law of the reflection energy is the opposite. The systematic study of the dynamic mechanical properties and energy evolution law of the damaged sandstone provides some reference for the prevention and mechanism research of rock bursts. [ABSTRACT FROM AUTHOR]

• The red sandstone was tested under high temperature-cyclic load coupling. • The factors of limiting energy storage and dissipation were defined. • Coupling damage was determined based on Lemaitre strain equivalence hypothesis. • The thermal damage mechanism was revealed by XRD and TG-DSC techniques. In this paper, the fatigue characteristics of red sandstone after heat treatment were analyzed from the aspects of stress-strain curve, deformation, energy and damage. Furthermore, the thermal damage mechanism was discussed by X-ray diffraction and thermogravimetric-differential scanning calorimetry synchronous analysis tests. The results showed that 400 °C was the threshold temperature. Below this temperature, the brittleness, failure stress and stiffness of the specimen were enhanced. Otherwise, they were weakened. Mineral decomposition, dehydration and crystal expansion occurred during the heat treatment process. The thermal damage mechanism of red sandstone can be clarified by a competitive mechanism of strengthening and weakening effects. [ABSTRACT FROM AUTHOR]

• The test of red sandstone under the coupling of freeze–thaw and cyclic loading. • Calculation of freeze–thaw deterioration factor correlations by Pearson's method. • Image integration & least squares methods analyze energy evolution law of rocks. • Energy storage and dissipation of sample under coupling effect follow linear laws. • A conceptual model describes the damage mechanism of rocks under coupling. In order to improve the safety of rock mass engineering in cold regions, this paper discusses the fatigue characteristics and energy evolution of red sandstone under the coupling of freeze–thaw and cyclic loading. Firstly, the correlation between the fatigue life, failure stress, and the number of freeze–thaw cycles of the red sandstone samples was calculated using the Pearson correlation coefficient method. Secondly, the energy evolution of the samples was analyzed using the image integration method and the least squares method. Finally, the damage mechanism of the samples under the coupled effect of freeze–thaw and cyclic loading was discussed using a conceptualized model. The results show that freeze–thaw cycles are negatively correlated with fatigue life and failure stress. The increase in freeze–thaw and fatigue damage leads to an increase in ductility and a decrease in the stiffness of the samples. The total energy density, elastic energy density, and dissipated energy density of the samples all show a quadratic polynomial relationship with the stress level. The energy storage and dissipation coefficients are negatively and positively correlated with the number of freeze–thaw cycles, respectively. [ABSTRACT FROM AUTHOR]

• The red sandstone is tested under coupled dry–wet cycle-fatigue loading tests. • Coupling damage is determined based on Lemaitre strain equivalence hypothesis. • The damage characteristics are discussed with the help of DIC technology. Dry-wet cycle and fatigue loading both severely affect the stability of hydraulic engineering. In this paper, the deterioration process of red sandstone under coupled dry–wet cycle-fatigue loading was investigated from three aspects of deformation, energy, and damage. And the variation law of crack propagation mode of the samples was revealed. The results showed that the elastic modulus and the deformation modulus had a negative correlation with the number of dry–wet cycles, while the maximum strain and the residual strain were the opposite. Coupled damage variable was defined based on porosity and dissipated energy density, and a damage evolution model was established. With the increase of dry–wet cycles, the failure characteristics of the samples evolved from predominantly wing cracks to anti-wing cracks. [ABSTRACT FROM AUTHOR]