The study aims to solve the problem of calculating the thickness limit of high-strength steel-concrete composite structures under the impact of slender thin-walled projectiles, a key consideration for protective engineering design. A series of impact tests on composite targets were carried out. These targets were composed of different high-strength steel plates and concrete backplates. Slender thin-walled projectiles were launched with a gas gun at controlled velocities, and the impact process were captured by high-speed cameras. The resulting damage to the structures and the failure modes of the projectiles were analyzed using both non-destructive and destructive testing methods. Based on test results, the protective mechanism of the composite structures and the failure modes of projectiles were analyzed. An improved thickness limit calculation model was then developed. Unlike the original model, this new model incorporated the structural strength of slender thin-walled projectiles, considering their wall thickness, material yield strength, and geometric dimensions, and was established based on force equilibrium and energy conservation principles. The results show that the high-strength steel in the composite structures provides material strength to resist penetration, while the concrete backplate offers support stiffness. As slender thin-walled projectiles are prone to compression and expansion cracking during impact, their structural strength must be factored into the calculation model. Moreover, the design of composite structures should consider both the mechanical properties of high-strength steel and the thickness limit. In conclusion, though the proposed model offers a new theoretical approach, it has limitations such as empirical parameters and conservative results. Further research is necessary to refine and enhance the model. The study's findings provide a theoretical basis for the design and application of high-strength steel-concrete composite structures in protective engineering.

To investigate the evolution of phase structure, dislocation distribution, energy absorption capacity, and impact accumulation effect of high-entropy alloys (HEA) under shock loading, molecular dynamics simulations were employed to systematically analyze the dynamic response behavior of Al_0.3CoCrFeNi HEA plate subjected to single and secondary impact load. The results show that under the first impact, the phase structure evolution and energy absorption mode of the plastic region of Al_0.3CoCrFeNi HEA plate exhibits significant velocity dependence. As the velocity increases, the proportion of face-centered cubic structure shows a three-stage downward trend, while the disordered structure increases accordingly. Under low velocity impact (0.5－1.0 km/s), energy is mainly absorbed by dislocation network; at medium velocity impact (1.0－2.0 km/s), both dislocations and disordered atoms contribute; under high velocity impact (2.0－3.0 km/s), disordered atoms dominate energy absorption. Within the velocity range of 0.5－0.8 km/s of the rigid sphere, the dislocation line length increases linearly with the impact velocity. However, at higher impact velocities, the dislocation line length decreases due to the limitation of the plate thickness. The stress analysis shows that when the impact velocity increases, both the maximum stress and the boundary stress of the plastic zone exhibit nonlinear variations characterized by a quadratic relationship. Under the secondary impact, the Al_0.3CoCrFeNi HEA plate forms a damage zone resembling a trapezoidal shape after impact. The radius of the pit within this damage zone exhibits a quadratic relationship with the impact velocity. Additionally, the minimum affected area resulting from the secondary impact also demonstrates a quadratic relationship with the impact velocity. Regarding impact resistance, as the initial impact velocity increases, the residual velocity following the secondary impact also rises, indicating a reduction in the resistance capability of HEA. At a distance of 10 nm from the impact center, the ballistic limit velocity decreases nonlinearly with increasing initial impact velocity. However, an increase in the secondary impact velocity mitigates the effects induced by the initial impact.