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Rock engineering in cold regions is in special environment of freeze–thaw cycle (FTC) and complex stress, and its mechanical properties would be seriously affected. To investigate the effects of FTC and complex dynamics on the engineering properties of rocks, the degradation characteristics of the physical and mechanical properties of Tongchuan sandstone after different FTC have been studied through nuclear magnetic resonance test, scanning electron microscope test and split Hopkinson pressure bar test. The study found that the micropores of the sandstone increased significantly after freeze–thaw (FT), and the variation of aperture uniformity coefficient and permeability showed that FT80 was an inflection point, and the damage degree of FT80 increased sharply. When the freezing temperature is − 20 °C, the liquid water inside the pores with a pore size of not less than 3.38 nm will freeze. Failure to fully consider the effects of unfrozen porosity and water content will underestimate the ability of FT to degrade rocks. The coupling effect of FT, confining pressure (CP) and SR shows the strengthening of rock strength, in which SR has the greatest impact on the dynamic strength of FT sandstone, followed by CP, and FT damage is the least. The triaxial dynamic damage constitutive model of FT rock considering FT damage and impact load damage is established, it is verified that the proposed model is reasonable. Highlights: The microstructure evolution characteristics of sandstone after freeze–thaw damage are discussed in detail. The degradation mechanism of freeze–thaw sandstone is deeply studied and a damage model considering water content and unfrozen pores is established. The triaxial dynamic damage constitutive model of rock considering freeze–thaw damage and impact load damage is established. [ABSTRACT FROM AUTHOR]

It has important guiding significance to study the influence of high temperature on physical and chemical properties and dynamic compression mechanical properties of rocks on the stability of rock mass engineering involving thermal behavior. The damage characteristics and dynamic mechanical characteristics of sandstone after different temperatures (20 °C, 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C) are studied. The mineral composition, mineral content and pore structure of sandstone changed significantly after heat treatment. The content of clay minerals decreases and the content of quartz increases. And all the internal hematite is reduced at 400 °C. The content of macropores increases, and the pore size range decreases. The Split Hopkinson pressure bar is used to perform uniaxial impact compression tests on the heat-treated sandstone specimens with different average impact speeds (8.6 m/s, 14.6 m/s and 18.8 m/s). It is found that the impact speed would weaken the degrading effect of temperature on sandstone, and the temperature would increase the strengthening effect of strain rate. The change of the dynamic elastic modulus of sandstone under the coupling effect of temperature and loading includes the temperature weakening stage, the impact load enhancing stage and the temperature weakening stage. A damage dynamic constitutive model considering the combined effects of temperature initial damage and impact loading damage is established according to the study of the macroscopic and mesoscopic degradation characteristics of sandstone by temperature. The rationality of the model is verified. [ABSTRACT FROM AUTHOR]

The dynamic mechanical properties of basalt fiber reinforced concrete (BFRC) with different fiber contents (0.0%, 0.1%, 0.2%, 0.3%, 0.4%), confining pressures (0 MPa, 5 MPa, 10 MPa, 15 MPa) and exposed to different temperatures (20 °C, 200 °C, 400 °C, 600 °C, 800 °C) were investigated by using a 50 mm split Hopkinson pressure bar (SHPB) apparatus, and the factors such as fiber content, temperature and confining pressure effect on the dynamic mechanical properties were analyzed. The results show that the dynamic peak stress increases first and then decreases with the increase of fiber content. At different temperatures, the peak stress and its corresponding strain correspond to different fiber content, and the optimal fiber content is between 0.1% and 0.3%. When the temperature was from 20 °C to 400 °C, the dynamic peak stress decreased less, while when the temperature reached 600 °C and 800 °C, the dynamic peak stress decreased greatly. The confining pressure can significantly increase the dynamic peak stress and change the crushing morphology of specimens. The damage variable was built based on the Weibull distribution. A dynamic damage constitutive model combining statistical damage and viscoelastic model was established based on component combination model. The fitting curve of this model fitted well with test curve by identifying fewer undetermined parameters compared with Zhu-Wang-Tang (ZWT) model; therefore, this model can well describe the dynamic properties of BFRC under impact load. [ABSTRACT FROM AUTHOR]

• The pore properties of FT sandstone are discussed. • The fractal characteristics and energy evolution process of FT sandstone under impact compression are analyzed. • A damage constitutive model considering various forms of energy dissipation is established. Tongchuan sandstone in northwest China was taken as the research object to study the micro-damage, macro-fracture characteristics and dynamic properties of rocks after freeze–thaw (FT). It is found that almost all of the pores that increased after FT of the sandstone specimens are minipores through the nuclear magnetic resonance test, while the initial pores do not expand further. The Φ50mm split Hopkinson pressure bars (SHPB) equipment was used to test the dynamic compressive properties of sandstone specimens with different FT cycles under different impact speeds, the fractal characteristics, energy evolution and dynamic compressive strength variation laws were discussed. The effect of impact loading on rock is greater than that of FT, and it weakens as the rock softens. A damage constitutive model considering various forms of dissipated energy is proposed, which is closer to the experimental results than the previous damage constitutive model based on energy. [ABSTRACT FROM AUTHOR]

• Dynamic constitutive parameters is considered as random variables in inversion calculation. • Model error is added to improve the ability to predict experimental observations under other conditions. • Peak stress error is added to improve peak stress fitting accuracy. • Fitting accuracy and prediction accuracy are superior to traditional optimization algorithms. The difference in dynamic mechanical properties of different rock samples leads to a random process of deformation and stress. Uncertainties exist in the parameters of the rock material dynamic constitutive model. Unlike traditional inversion analysis methods, this paper treats model parameters as random variables. Bayesian theory and Differential Evolution Adaptive Metropolis algorithm (DREAM) are used to accurately quantify the uncertainty of the dynamic constitutive model parameters. The error function of the DREAM algorithm is divided into prediction error and model error to mitigate the effect of parameter compensation and improve the prediction ability. Peak stress error term is added to the DREAM prediction error to increase the peak stress fitting accuracy. Experiments of shock compression under freezing and high temperature cooling damage are presented to illustrate the proposed method. Different optimization algorithms such as Genetic Algorithm and Ant Colony Optimization are used to compare the accuracy of inversion parameters. The results show that the obtained peak stress fitting accuracy of the dynamic constitutive model is significantly improved. The 95% confidence interval considering model errors almost completely covers the experimental observations, indicating that the parameter probability distribution interval can accurately cover the true value while reflecting the model uncertainty. Using inversion parameters to predict other working conditions, 95% confidence interval considering model errors has high coverage. Thus, stochastic inversion method can accurately fit and predict the deformation and failure law of rock under impact load. [ABSTRACT FROM AUTHOR]

The fractal characteristics and energy dissipation of basalt fiber reinforced concrete (BFRC) with five kinds of fiber volume contents (0.0%, 0.1%, 0.2%, 0.3%, 0.4%) after exposure to different temperatures (20 °C, 200 °C, 400 °C, 600 °C, 800 °C) under impact loading were investigated by using a 50 mm diameter split Hopkinson pressure bar (SHPB) apparatus. Scale-mass distribution rules and fractal dimension characteristics of fragments were studied based on the screening statistical method and the fractal theory. Furthermore, the relationship between the energy consumption density and the fractal dimension of fragments was established, and the effects of fiber content, temperature and impact velocity on fractal dimension and absorption energy were analyzed. The results show that the crushing severity of fragments and fractal dimension increase with the impact velocity under the same fiber content. The energy consumption density increases first and then decreases with increasing fiber content, and also decreases with increasing temperature. When the temperature and fiber content remain unchanged, the energy consumption density increases linearly with the increasing fractal dimension, and under the same impact velocity and temperature, there is no obvious linear relationship between energy consumption density and fractal dimension. [ABSTRACT FROM AUTHOR]

In order to study the fractal characteristics and energy dissipation of concrete suffered freeze-thaw cycle actions and impact loading, C35 concrete was taken as the research object in this paper, and freeze-thaw cycle tests were carried out with a freeze-thaw range of −20 °C~20 °C and a freeze-thaw frequency of 0~50 times. The degradation characteristics of concrete material and the variation rules of basic physical parameters under various freeze-thaw cycle conditions were obtained consequently. By using the SHPB (separated Hopkinson pressure bar) test device, impact compression tests of concrete specimens under different freeze-thaw cycle actions were developed, then the process of impact crushing and the mechanism of damage evolution were analyzed. Based on the screening statistical method and the fractal theory, the scale-mass distribution rules and fractal dimension characteristics of crushing blocks are investigated. Furthermore, the absorption energy, fracture energy and block kinetic energy of concrete under different conditions were calculated according to the energy dissipation principle of SHPB test. The relationship between the energy consumption density and the fractal dimension of fragments was established, and the coupling effect mechanism of freeze-thaw cycle action and strain rate effect on the fractal characteristics and energy consumption was revealed additionally. The research results show that the concrete under different freeze-thaw cycle conditions and impact loading speeds has fractal properties from the microscopic damage to the macroscopic fracture. The energy dissipation is intrinsically related to the fractal characteristics, and the energy consumption density increases with the increase of the fractal dimension under a certain freeze-thaw cycle condition. When at a certain loading speed, with the growth of freeze-thaw cycles, the energy consumption density reduces under the same fractal dimension, while the fractal dimension improves under the same energy consumption density. [ABSTRACT FROM AUTHOR]