What is the principle of bulletproof vest?

What is the mechanism of bulletproof vest? We all know that body protection is mainly achieved by bulletproof lining during on-duty operations, but what is the specific principle? The function of body armor is very similar to armor in ancient times. It is used to resist external attacks and reduce self-injury. There are generally three types of linings for bulletproof vests: all soft, hard and composite. Different linings have different bulletproof principles.
1. Fully soft body armor
The lining of the fully soft body armor is mainly made of polymer PE material, which can deform the direction of the bullet's incidence and the area near the entrance point through the deformation and destruction of the fabric to consume the energy of the bullet.
2. Hard body armor
Full-hard body armor, the inner lining is mainly a metal plate with high hardness, which can directly bounce off bullets and shrapnel, or consume energy by fragmenting bullets.
3. Composite body armor
The inner lining of composite material combines soft and hard materials. In the process of bulletproofing, the hard material acts first and consumes most of the energy of the bullet. The PE material simultaneously absorbs the impact force of hard materials and the remaining energy of the bullet, playing a buffering role.