Why bullet-proof glass can prevent bullets?

Bulletproof glass is usually a transparent material, such as polycarbonate fiber thermoplastic (usually force-sensitive resin). It has the appearance of ordinary glass and the behavior of light transmission, which provides some protection for light weapons shooting.

Bulletproof glass usually consists of polycarbonate fiber layers sandwiched between ordinary glass layers. The bullet can easily penetrate the outer glass, but the strong polycarbonate fiber layer will prevent the bullet from moving before it penetrates the inner glass. The thickness of bulletproof glass is usually 70 mm to 75 mm.

Historically, bullet-proof glass was made of glass sheets bonded together by liquid rubber. These large bulletproof glasses have been used for public purposes during the Second World War, usually as thick as 100 mm to 120 mm, and they are extremely heavy.

Unidirectional bulletproof glass

For example, the use of some bank bullet-proof cars promoted the invention of one-way bullet-proof glass. This kind of glass can resist light weapons coming from the outside of the glass, but it is allowed to shoot from the other side (inside) of the glass, for example, the defender shoots from the inside of the bulletproof car, thus threatening to shoot to the outside through the glass.

One-way bullet-proof glass usually consists of two layers: an outer fragile layer and an inner flexible layer. When the bullet is fired from the outside, it will first hit the fragile layer and break an area. This will absorb the kinetic energy of some bullets and spread it over a larger area. When the decelerated bullet hits the flexible layer, it will be stopped. However, when the bullet shoots from the inside, it will hit the flexible layer first. The bullet can penetrate the flexible layer because its energy is concentrated in a small range. When the flexible layer bounces outward, the brittle layer breaks outward, which does not hinder the bullet's progress.

Bullet-proof glass not only has the light transmission and image transmission performance of glass, but also has the protection ability against bullet shooting. The bulletproof principle of bulletproof glass is that the impact kinetic energy of bullets can be converted into elastic potential energy of glass and surface energy of fragments. Bullet-proof glass is actually made by bonding multiple pieces of glass or high-strength organic plates together with transparent bonding materials. Generally, it has the following three-layer structure:

① Load-bearing layer: This layer is cracked by impact first. Generally, glass with large thickness and high strength is used, which can destroy the warhead or change its shape, making it lose its forward ability.

② Transition layer: Generally, organic adhesive materials are used, which have strong adhesion and good light resistance, and can absorb part of impact energy and change the direction of bullets. Sandwich a very strong and transparent chemical film in laminated glass. This not only can effectively prevent bullets from shooting, but also has the properties of surge impact resistance, explosion prevention, earthquake resistance and no crack after impact.

③ Safety protection layer: This layer is made of high-strength glass or high-strength transparent organic material, which has good elasticity and toughness, and can absorb most of the impact energy to ensure that bullets cannot pass through this layer.

The most important performance index of bulletproof glass is bulletproof ability. The index of bulletproof ability is measured from two aspects, on the one hand, the safety protection ability, on the other hand, the killing ability of the protected guns.

According to the degree of protection to human body, bullet-proof glass can be divided into two types, one is safe and the other is life-safe. After the safety bullet-proof glass is shot, there is no splash on its non-impact surface, which will not cause any harm to human body; After being shot, the life-safe bullet-proof glass splashed on the non-impact surface, but the bullet could not penetrate the glass, which may cause secondary injury to the human body. Bulletproof glass has different requirements according to the bulletproof ability of different gun types. Bulletproof glass is divided into three series, the first is aviation bulletproof glass, the second is vehicle and ship bulletproof glass, and the third is bank bulletproof glass. The thickness is between18mm and 40mm.