## 1 Introduction
[17] studied the nonlinear transient response of a honeycomb sandwich plate with a negative Poisson's ratio under an impact load, derived the partial differential equation of the honeycomb sandwich plate, and obtained the nonlinear ordinary differential equation of the structure by the truncation method. It was found that the negative Poisson's ratio honeycomb sandwich panel is better than the positive Poisson's ratio honeycomb sandwich panel. Similarly, [PERSON] et al. [15] also used the higher-order shear theory to derive the partial differential equation for the critical buckling load of carbon nanotube-reinforced three-phase orthotropic honeycomb sandwich panels. Most of the remaining theoretical studies on honeycomb sandwich structures are basically developed based on the higher-order shear theory of laminates [11; 14; 16; 18]. This research method of fragment penetration can be divided into two parts: high speed and ultra-high speed (greater than 5000 m/s), according to the speed. The analysis of sandwich structure in a high-speed state is common. The ballistic gun is the main launching source. The purpose is to analyze the failure mode, deformation mechanism, and protection performance of the honeycomb sandwich structure in a high-speed state. At present, the penetration of fragments into honeycomb sandwich structures at high speed is mainly based on numerical simulation. [PERSON] et al. [19] used ABAQUS/Explicit to simulate the high-speed penetration of a honeycomb sandwich structure composed of carbon/epoxy resin skin and aluminum honeycomb core and determined the contribution of the failure mechanism of the honeycomb structure to the energy absorption of projectile kinetic energy. This study of the ultra-high-speed state is to use a two-stage hydrogen gun to accelerate the bullet to more than 5000 m/s. High-speed photography technology and computer tomography technology are used to capture and observe the fragmentation of the projectile and the deformation and failure modes of the honeycomb sandwich structure, which provides a strong guarantee for the survivability of the spacecraft [20; 21; 22]. New materials, new structures, and new properties are the main research directions for honeycomb sandwich structures. In 2014, [PERSON] et al. [23] conducted a penetration compression experiment on the sandwich structure composed of NOMEXTM core and aluminum plate and judged the relationship between penetration energy and residual strength. In 2018, [PERSON] et al. [24] designed a sandwich structure composed of carbon/epoxy composite skin and different proportions of magnetorheological elastomer core and carried out structural free vibration and forced vibration experiments under different magnetic field intensities to evaluate the dynamic performance of sandwich beams. In 2019, [PERSON] et al. [25] conducted a three-point bending test on the sandwich structure composed of an epoxy honeycomb core containing eucalyptus sawdust and cement particles and fiber palm fiber layer skin and analyzed the influence of structural parameters on core shear stress, surface stress, bending stiffness, and strength. In 2021, [PERSON] [26] tested the damage performance of the sandwich structure composed of glass fiber (GFRP) and carbon fiber (CFRP) hybrid skin and a NOMEX honeycomb core. It was found that the peak force was related to the penetration energy and the diameter of the punch. As the diameter of the punch increases, the peak force gradually increases. [PERSON] et al. [27] are still based on the NOMEX honeycomb core, but the skin is made of a multi-layer composite material that is bonded by a high-strength adhesive film. The penetration test of the structure was carried out using the drop hammer test machine. When the penetration energy is relatively low, the main energy dissipation of the structure is still dominated by the fiber tensile fracture of the composite skin. [PERSON] et al.