# Impact of Soil on the Penetration of Prefabricated Spherical Fragments and Its Protective Effect
## 1 Introduction  
As a natural medium in nature, soil constitutes various environmental types on land and has played an important role in human society from ancient times to the present. In earlier times, human shelters were mostly constructed from soil, and many containers were also fired from the soil. In modern life, soil is mainly used as an auxiliary material for reinforced concrete buildings. Even though humans built a large number of roads and hardened the roads in modern times [1], natural soil still covers a vast majority of the land's surface, such as suburbs, hills, forests, etc. In the design and application of protective structures, the focus is generally placed on the design and testing of new materials and structures, and soil is mainly used for protection against hazards such as large-caliber projectiles and explosive shock waves. There is little research on the damage of spherical, small-size penetrators similar to fragments. Fragments usually have a higher speed compared to large projectiles [2], so they pose a greater threat to organisms. In the wild or a relatively primitive natural environment, soil can usually be constructed by digging tunnels or stacking to construct simple protective structures. These facilities can be used for simple protection in experiments or to reduce the additional damage caused by high-speed fragments. In addition, due to the fact that soil is relatively easy to obtain andcan be quickly backfilled after damage, the overall damage to the structure is also relatively small, so it can be reused. Based on the above points, studying the protective ability of natural soil media against small-scale fragments is of practical value. With the development and progress of science and the continuous development of metal materials, in order to deal with hidden fortifications and important facilities below the ground, countries have developed weapons and equipment represented by ground-penetrating bombs to penetrate deep soil and concrete protection. Therefore, studying the penetration of earth-penetrating projectiles into soil is quite important. Based on numerical simulation calculations, [PERSON] et al. [3] studied the effect of initial attack angle on the oblique penetration of earth-penetrating projectiles into soil, conducted numerical analysis of the penetration process at different angles, and studied the motion trajectory, penetration depth, and acceleration curve of the penetration process of the projectile. In response to the ballistic changes in projectile penetration into natural soil, [PERSON] et al. [4] conducted ballistic tests and obtained trajectory images of projectile penetration into soil. It was found that penetration into soil would have a significant impact on the attitude of the projectile and further affect the depth of penetration. For different penetration depth calculation formulas, [PERSON] et al. [5] conducted numerical simulations on the penetration of a 57 mm-diameter kinetic energy projectile into a finite-thickness soil medium and concluded that the selected model is consistent with some soil types described by [PERSON] and [PERSON]. Based on this type of research achievement, [PERSON] et al. [6] conducted experiments and numerical simulations on the penetration situation under low projectile velocity conditions and obtained a calculation method for the penetration depth and trajectory of ground-penetrating projectiles. From the above research results, it can be seen that the method of numerical simulation combined with experiments is more often applied to the process of projectiles penetrating the soil, with the diameter of the projectile studied by the above scholars being generally large and the projectile velocity being relatively low.