In order to study the microscopic damage evolution of rock under cyclic dynamic impact, red sandstone was selected as samples, and Split Hopkinson Pressure Bar(SHPB) loaded with impact pressure of 0.3MPa was adopted to exert cyclic dynamic impact on samples. After each dynamic impact, Nuclear Magnetic Resonance system (NMR) was then applied to test the microscopic structure of samples, then parameters of red sandstone including porosity, transverse relaxation time (T2) spectrum, permeability, and magnetic resonance images (MRI) after each dynamic impact were obtained. Results show that both porosity and permeability increase with the number of dynamic impacts that red sandstone undertake, and the increase range of them also increases gradually. The T2 cutoff value gradually decreases with the number of dynamic impacts increases, and their decrease range increase gradually, which is consistent with the trend of microscopic damage represented by porosity and permeability. Also, it is found through analyzing the T2 spectrum that dynamic impact produced by impact pressure of 0.3Mpa leads to the gradual increase of primary pore size, increase of macro pores, and enhance of samples’ connectivity, rather than the generation of new microspores. MRI images reveal that cyclic dynamic impact leads to the generation of micro cracks in red sandstone, which coalesce and extend with the number of dynamic impact increase, and form larger micro crack zones finally.

To gain a further understanding of Freeze-Thaw weathering effect on the damage of sandstone from the perspective of energy analysis, uniaxial static compression tests were conducted on sandstone samples that suffered 0, 20, 60, 100 and 140 freezing-thawing cycles. Then total input strain energy, releasable elastic strain energy, dissipated energy and freezing-thawing induced damage of samples under uniaxial static compression tests were calculated and analyzed. In the mean time, the energy absorption of sandstone under dynamic loading tests (SHPB (Split-Hopkinson pressure bar) experiments) were cited and made a contrast with the dissipated energy of samples under uniaxial static compression tests. The results show that the tangent modulus, total input strain energy, releasable elastic strain energy and dissipated energy of samples decrease with Freezing-Thawing cycles go on, while the freezing-thawing induced damage grow with freezing-thawing cycles increase. In term of energy used for destroying rock samples in different test modes, less energy is needed for destroying samples in uniaxial static compression tests and it decreases with F-T cycles increase, while more energy is needed for destroying samples in dynamic impact loading and it increases with F-T cycles increase. In addition, the number of fragments of broken sample increase with the F-T cycles both in uniaxial static compression tests and dynamic loading tests, and sample under dynamic loading tests is more broken than sample under uniaxial static compression tests.

In order to figure out the damage characteristics and mechanisms of sandstone under coupled effect of acid erosion and recurrent freezing‐thawing cycles, sulfuric acid is chosen as acid solution, and sandstone, representative stone in Sichuan province, is chosen as a sample to conduct the freezing‐thawing cycling test. In the meantime, chemical component of the solution is also tested and recorded in the progress of freezing‐thawing cycling. Then, the nuclear magnetic resonance (NMR) test and magnetic resonance images (MRI) test on samples are conducted with the help of the AniMR‐150 NMR imaging system. After a series of tests, the sample’s appearance, dry mass, porosity, T2 spectrum, solution’s pH, solution’s metallic ion concentration, and magnetic resonance images are obtained and analyzed. The results show that dry mass loss and porosity grow with F‐T cycles increase and pH value decrease. Montmorillonite, illite, and clay in sandstone react with sulfuric acid solution; as a consequence, K⁺, Mg²⁺, Fe³⁺, and Al³⁺ separate out, and the solution’s pH and concentration of K⁺, Mg²⁺, Fe³⁺, and Al³⁺ increase with F‐T cycles. Acid erosion and F‐T cycling lead to the generation of new micropores and expansion of micropores at the beginning; when the acid solution is exhausted, new micropores generate under the effect of freezing‐thawing cycling, and micropores in samples keep developing and expanding with the increase of freezing‐thawing cycles. Coupled effect of acid corrosion and recurrent freezing‐thawing cycling causes much more serious deterioration to sandstone samples than acid corrosion.

Considered as an important option to enhance power distribution capacity, improve operation flexibility and increase power quality in distribution grids, the medium-voltage DC (MVDC) technology has gained increasing interests in recent years. In this paper, the state of the art of researches and applications of MVDC distribution system is presented. Main topics and progresses are explored including planning and evaluation, main circuits, key equipment, control and protection. The most recent pilot projects around the world are introduced among which the Tangjia Bay pilot project with the largest capacity is illustrated in detail, together with the practical issues during the design, experiment, and commissioning.

Freeze-thaw weathering changes the pore structure, permeability, and groundwater transportation of rock material. Meanwhile, the change in rock material structure deduced by frost heaving deteriorates mechanical properties of rock material, leading to instability and insecurity of mine slopes in cold regions. In this paper, rock-like specimens containing prefabricated cracks at different angles and having undergone various freeze-thaw cycles are used as the object. Their pore structure, compressive mechanical properties, strain energies, failure characteristics, and the connection between pore structure and mechanical properties are analyzed. Results show that the porosity, spectrum area of mesopores, and spectrum area of macropores increase with the increase in freeze-thaw cycles, while crack angle shows no obvious influence on pore structure. Peak stress and elastic modulus drop with the increase in freeze-thaw cycles, while peak strain shows an increasing trend. Peak stress and elastic modulus decrease in the beginning, and then increase with the increase in crack angle, while peak strain shows a reverse trend. Elastic strain energy and pre-peak strain energy drop with the increase in freeze-thaw cycles. Elastic strain energy decreases first, and then increases with the increase in crack angle. The correlation between the spectrum area of macropores and elastic modulus is the strongest among different pores. Elastic modulus and peak stress decrease with the increase in macropore spectrum area, and peak strain increases with the increase in macropore spectrum area.