In cold regions' engineering applications, cement stabilized soils are susceptible to strength degradation under freeze-thaw (F-T) cycles, posing significant challenges to infrastructure durability. While metakaolin (MK) modification has shown potential in enhancing static mechanical properties, its dynamic response under simultaneous F-T cycling and impact loading remains poorly understood. This study investigates the dynamic mechanical behavior of cement-MK stabilized soil through split Hopkinson pressure bar (SHPB) tests under varying F-T cycles. The effects of strain rate and F-T cycles on the dynamic failure process and mechanical properties of cement-MK stabilized soil were investigated. Pore characteristics were analyzed using a nuclear magnetic resonance (NMR) system, providing an experimental basis for revealing the degradation mechanism of F-T cycles on the strength of cement-MK stabilized soil. Based on the Lemaitre's strain equivalence principle, a composite damage variable was derived to comprehensively characterize the coupled effects of F-T cycles and strain rate. A dynamic constitutive model is established based on damage mechanics theory and the Z-W-T model. The results indicate that under the effect of F-T cycles induce progressive porosity increase and aggravated specimen damage. At varying strain rates, the strength of cement-MK stabilized soil decreases with increasing F-T cycles, while the rate of strength reduction gradually diminishes. Under impact loading, both strain rate and the number of F-T cycles significantly reduce the average fragment size of fractured specimens. The modified Z-W-T model effectively predicts the stress-strain relationship of the cement-MK stabilized soil under impact loading.

Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation. Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production. In this study, using the well-known notched semi-circular bend (NSCB) sample and the novel double-edge notched flattened Brazilian disc (DNFBD) sample combined with a split Hopkinson pressure bar (SHPB), various dynamic anisotropic fracture properties of Lushan shale, including failure characteristics, fracture toughness, energy dissipation and crack propagation velocity, are comprehensively compared and discussed under mode I and mode II fracture scenarios. First, using a newly modified fracture criterion considering the strength anisotropy of shale, the DNFBD specimen is predicted to be a robust method for true mode II fracture of anisotropic shale rocks. Our experimental results show that the dynamic mode II fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode I fracture. The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation, while the maximal ones occur in the Divider orientation. In addition, it is interesting to find that the mode II fracture toughness anisotropy index decreases more slowly than that in the mode I fracture scenario. These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

In this work, a novel multi-layer composite structure has been proposed for its application in the design of protective shelters. The proposed work investigates the dynamic behaviour of a multi-layer composite under the action of single and multiple projectile impact loading. The target consists of soil, reinforced concrete, steel plate, high-density polyethylene and boulder-mixed cement mortar. It has been subjected to the projectile impact (single and multiple) of a 2.3 kg ogive-nose hard cylindrical projectile having a 50.80 mm diameter. The mechanical performance in terms of the velocity profile of the projectile, residual velocity, penetration depth, ballistic limit velocity, energy absorption and damage of the target has been quantified through a numerical framework. Further, equations have been formulated to determine the fracture material parameters associated with the Riedel-Hiermaier-Thoma (RHT) model to cater for varying strengths of concrete or similar materials. The proposed composite target has demonstrated enhanced penetration resistance and lesser damage compared to its reinforced concrete monolayer counterpart. The interaction between shock waves and the target's material characteristics lead to projectile ricocheting in multiple projectile impacts. Further, an analytical model has been developed to predict the forces transmitted to the lowest layer, which are essential for design purposes. The model has been proposed from the fundamentals using the concepts of impedance. The result derived from the theoretical formulation is in good agreement with the numerical results. Finally, the damage has been quantified to assess the extent of structural deterioration in multi-layer targets, emphasizing the relationship between energy absorption ratio and damage.