How can bulletproof glass be bulletproof?

There was a piece of news: A bird hit the glass on a high-speed train from Hangzhou to Beijing, causing all passengers to change trains. According to the data, the front windshield of the high-speed rail that was hit this time was "super bulletproof deceleration glass, with 6 layers, including 3 layers of inorganic glass, 2 layers of soft plastic, and 1 layer of anti-splash layer with a thickness of 2 to 3 mm. "What do these definitions of glass mean?

Automotive glass: tempered

The current front windshield of ordinary cars is made of tempered glass and organic interlayer. The organic interlayer is used to bond the slag together after the glass is damaged, and will not affect the passengers in the front seat (especially the driver). ) Cause harm.

Be careful not to confuse the concepts of tempered glass and FRP: tempered glass is essentially a kind of glass, while FRP is essentially plastic (fiber reinforced plastic, commonly used glass fiber reinforced unsaturated polyester, epoxy resin, phenolic resin, etc.). .

The biggest difference between tempered glass and ordinary glass is the shape of the broken slag after cracking: almost all the broken slag after cracking of tempered glass becomes honeycomb-like obtuse-angled small particles, which are not easy to cause cuts or scratches to people; The broken slag after the glass is damaged is sharp and sharp, and a little carelessness can cause harm to the human body. Legend has it that a classmate was so sad that he wanted to cut his wrist after being broken in love. After trying his best to smash a piece of toughened glass, he found that this product could not cut blood vessels at all. He silently complained and gave up the idea of committing suicide, and then went to the glass business. No return.

The reason for the huge difference between tempered glass and ordinary glass is the difference in processing technology. The process from raw material to glass is the same, and tempered glass is to cut the formed ordinary flat glass into the required size, and then heat the glass to close to its softening point (about 500~750℃ depending on the composition) and proceed. Rapid cooling (similar to the quenching process of forged metal), so that uniform compressive stress is formed on the surface of the glass, and tensile stress is formed inside. The glass processed by such a "tempering" process has doubled its strength, and the broken slag will not show sharp corners like a blade. Therefore, tempered glass is often used as the original sheet for making other glass, such as insulating glass, laminated glass and so on.

Before the advent of tempered glass, car windshields were mostly ordinary flat glass, but when the glass shattered, the person in the front seat was basically disfigured. It is said that the founder of Ford Company, after suffering such disfigurement, turned his grief and indignation into motivation and invented laminated glass to benefit global drivers. Of course, this is just a legend. Later, when I looked at Henry Ford's Baozhao photos, I didn't find any disarray. Maybe he had been to Korea.

Although laminated glass solves the problem of disfigurement, it has created a new problem: when the vehicle is running, especially at high speed, if it is suddenly hit, the entire windshield will stick to the front of the cockpit like a twisted screen. , Can't see the way at all! Therefore, the clever glass engineer improved the interlayer film inside the laminated glass-made it into a spider web shape, so that even if the glass is cracked, it will appear like a spider web crack, and the driver can also protect the driving sight. Stop the car safely as soon as possible. This kind of glass is called "safety glass". More than 95% of current windshields are made of safety glass that combines laminated and tempered glass and laminated technology to protect the safety of vehicles and drivers to the utmost extent.

As the strength of tempered glass is much higher than that of ordinary glass, after the emergence of the major automobile manufacturers decisively used this kind of thing that sounded quite advanced in their products, instead of the original ordinary laminated glass, until one day, a car accident occurred. The passengers were trapped in the car and failed to break the glass in time and escape due to various reasons. So the engineers also agreed to make a toughened glass that can ensure the strength and break when necessary. It didn't take long for the local tempered glass to be born. It is the glass that can be seen on various buses and subways today. A safety hammer is usually placed next to the window.

Bulletproof glass: laminated

Speaking of laminated glass, the prototype of this product is actually a hamburger—usually a transparent organic film (usually PVB, or polyvinyl butyral) sandwiched between two layers of glass, which is formed by hot pressing and bonding. In the case of greater impact or severe vibration, only cracks will appear, and will not cause crushing. In addition to the common double sandwich, there are three, four or more layers according to special needs, just like Orleans The difference with the Big Mac. . The special needs of some special fields have directly promoted the development of laminated glass, and some derivatives and improvements have emerged, such as the bulletproof glass mentioned at the beginning of this article.

Bulletproof glass is a type of safety glass with bulletproof and explosion-proof characteristics produced by improving the production process of laminated glass. Just like its name, it can protect from BB bullets to grenade and even more powerful weapons depending on its thickness, production process and interlayer.

Simply put, bulletproof glass is usually divided into three layers: bearing layer, transition layer and protective layer. The load-bearing layer is cannon fodder, that is, it is used to deform or destroy the warhead by being broken by the impact, which greatly slows down the speed of the warhead. This layer is mostly made of high-strength tempered glass or plexiglass. The transition layer is mostly polycarbonate fiber, which uses the toughness of the transition layer to absorb part of the kinetic energy of the warhead, and ensures that the front and rear interlayers can still be bonded together without falling off after being broken. The protective layer and the load-bearing layer are commonly used high-strength tempered glass or plexiglass, which requires high strength, impact resistance and shock resistance, and has a certain toughness to ensure that it will not be penetrated by bullets or shrapnel, thereby providing safety protection effect. The following video shows the boss of a certain company sitting behind bullet-proof glass in order to prove the safety of his product and let the employee shoot himself with an AK47:

As stated in the video, don't try this extremely dedicated experiment at home.

With the continuous improvement of security requirements, one-way bullet-proof glass has been developed, which can protect against shooting from the outside, while the defender fires back from the inside, and the bullet will pass through the glass and hit the attacker outside. The principle of this one-way bulletproof glass is actually to use brittle materials for the outer layer and tough materials for the inner layer. The others are similar to ordinary bulletproof glass. In this way, the bullet flying from the outside hits the brittle layer first, the kinetic energy is absorbed, and is intercepted by the tough layer; and the bullet shot from the inside will penetrate the tough layer first, because its energy is concentrated on a small point. When the ductile layer expands, the brittle layer is broken outwards, which does not affect the progress of the bullet. The bullet-proof glass used in the bank has also undergone a wire clamping process, that is, a wire is clamped inside the glass. When it is impacted or damaged, the wire is pulled or broken, and the disturbance of the current triggers the alarm device.

Generally speaking, the protective effect of bulletproof glass is proportional to the total thickness of the glass and the thickness of the interlayer. The protective effect of tempered glass in the original glass is better than that of ordinary flat glass. And the glass used in the high-speed rail in the accident is not only bulletproof but also nominally decelerating. So what is going on with this magical decelerating glass?

Slow glass: actually a legend

In fact, there is no glass that can really slow down. This is a rumor that has been circulating for nearly half a century. After the liberation, the whole country was engaged in construction, and many truck drivers were needed. Later, those truck drivers drove into cars and found that the speed of the two was obviously different. They thought that foreigners had the function of slowing down, so this rumor has spread to this day. In fact, this rumor is easy to crack.

The reason why people think that the current glass is "deceleration" glass is mainly due to the improvement of glass quality. The earliest windshield used flat glass. At that time, float glass had not been introduced into my country (float glass was first invented by Pilkington in the United Kingdom in the late 1950s, and my country started the float industrial test in 1971). There are many defects in the glass. Among them, there are stripes or nodules inside the glass due to uneven composition or impurities, which will produce high optical distortion, and the scenery outside through the glass will be deformed. If the home was installed in that era The glass person must remember that the outside world is distorted. Such glass superimposes a kind of trance caused by deformation on the moving object, which easily makes the driver visually fatigued and makes the driver feel faster. Later, with the improvement of glass production technology, the quality of glass is getting better and better, and the glass used on the trolley is generally better than the glass on the truck, so that the outside scenery through the glass will have a higher degree of authenticity, and it may even feel bad. The presence of glass makes it look more comfortable, and there will be an illusion of slowing down.

Another point that makes people think that the current glass can "slow down" because of visual errors. Usually the driver’s seat of a truck is high, and the driver can see the road very close to the front of the truck; while a small car usually has a front. In order to look beautiful and reduce wind resistance, the front glass is installed at an angle, and the slope is also large. It is relatively rearward, so that the distance between the person and the front of the car is far, and the road in front of the car can only be viewed from a relatively flat angle. The road that the driver can see is a large horizontal distance from where he is sitting. When driving a truck and a small car, the viewpoint height, field of view and angle of view are different, and people judge that the variable of motion perception is not linear velocity but angular velocity, so there will be different sense of speed. Just like the moon moving around the earth, people can run more than one thousand meters per second, but from the earth you can hardly feel it running.

Another thing that makes people think that the current glass can "slow down" because of the smoothness of the road and the comfort of the vehicle. It feels fast when driving at 30km/h in poor road conditions, and it is slow at 100km/h on high speeds. At the time of liberation, many places were still muddy roads. Later, cement and asphalt roads were slowly paved. The road conditions were naturally improved, and the shock-absorbing type of the car was better than that of the truck, so the driver felt more stable, so the speed It doesn't feel so obvious.

In the same way, the glass used on the high-speed rail does not have a deceleration function, it is just that the above factors give passengers and drivers an illusion of speed.

Any kind of material has an applicable range and force limit. For example, the glass that resists pistol bullets cannot withstand grenades or sniper rounds. This is normal, just like you can't expect Iverson to oppose Yao Ming. Even so, as long as it is used within its safe range, it will not cause harm to people, and the glass used as a high-speed rail is the same-when running at high speed, as long as it is driven under the impact of the design upper limit range Even if the safety of personnel and passengers is completed, there is nothing that can be done by changing the concrete if it hits a UFO. Angry bird? It only exists in the game.