How do bulletproof vests protect wearers?

Modern bulletproof vests save thousands of lives every year. Although a rifle shot may cause a few rib fractures, the vest will not allow the bullet to enter your body. An interesting material called kevlar is the real superhero in the modern bulletproof vest, which has amazing tensile strength with good flexibility. In this article, I will explain about this life saving technology in detail.

Invention of kevlar based bulletproof vest

The crowd of michigan got amazed when Mr. Richard Devis, a former US marine, shot himself to prove his newly developed kevlar based bullet proof vest is better than any other vest available at that time. Usage of kevlar in bullet proof vests for the first time is mostly credited to Richard Devis, even though we couldn’t find the word “Kevlar” in his patent. Richard devis was shot twice by robbers while delivering pizzas. He wanted to gift the police a super strong bulletproof vest so that the robber's free run would come to an end. This is how kevlar based bullet proof vests were born. To demonstrate the sheer strength of his vest, devis shot himself 190 times. Later, the National Institute of Justice (NIJ) officially approved this material for use in bulletproof vests. The reason why this flexible material is able to withstand a shot from a bullet, I will explain this at the end of the article.

One kevlar layer is shown in fig 1a. Many such layers are used in bullet proof vests (refer fig 1b).

Fig 1a : Single layer of kevlar
Fig 1b : Multiple layers of kevlar

Is soft armor able to protect against pistol bullets?

A bullet from the pistol generally travels at the speed of 400 meters per second. This bullet hits the first layer of kevlar. The bullet’s energy is so high, so the kevlar fiber’s first layer is not able to withstand it and it breaks. However, after passing a few layers, the speed of the bullet is reduced. In this way, a few layers of kevlar will get damaged. After a significant speed reduction, the kevlar layers will block the incoming bullet as shown in fig 2. Now our kevlar based vest has been able to save a life! The vest, I just explained, is known as soft body armor.

Fig 2 : Bullet is blocked by kevlar layers

Hollow point bullet vs kevlar vest

A type of bullet called hollow point is used to increase the wound area and its effectiveness (refer fig 3a). This hollow point bullet will expand its tip or the bullet will mushroom while it travels through the human body. The interesting thing is that the vest we developed just now will block such bullets quite easily. Here the bullet tip gets flattened out due to the resistance of the kevlar layers. This spreading out of the bullet tip reduces the pressure impact on the next kevlar layers significantly(refer fig 3b). Thus the bullet can be stopped quite easily.

Fig 3a : Hollow point bullet mushroomed due to resistance of kevlar layer
Fig 3b : Hollow point bullet vs kevlar layers

A Bullet from a rifle vs bulletproof vest

However, if the shot is from a rifle, meaning the bullet is traveling at a very high speed (min 800 m/s), all kevlar layers will fail(refer fig 4a). So, what’s the solution? Just add a steel plate above the kevlar layers. A strong steel plate will make sure that the bullet is not even allowed to penetrate it. It will bounce back. You may think this new bullet proof vest is super effective, but it can cause you blunt trauma - an injury where you look fine from the outside, but your internal organs are damaged (refer fig 4b).

Fig 4a : Kevlar layers failed
Fig 4b : Metal based hard body armor

In the soft body armor, we saw the bullet take some time before it came to a rest. However, in this steel plate case, the bullet was stopped almost instantly. This means the human body has to absorb all the energy of the bullet in a very short time span. This translates into a tremendous amount of force and the internal organs of the human body won’t be able to withstand such a high intensity pressure (refer fig 4b). Another issue with the steel body armor is bullet splash (refer fig 4c).

Fig 4c : Bullet is splashing

Ceramic plate based hard body armor

To overcome the above issues, nowadays ceramic alloy based plates are preferred over the steel plates. They will allow bullet penetration and reduce the bullet speed significantly. After this speed reduction the remaining Kevlar layers will be able to stop the bullet quite easily (refer fig 5). Here, the human body is experiencing a much lower force since the whole process takes place during a longer time duration. These kinds of vests are called hard body armors.

Fig 5 : Ceramic alloy based hard body armor

Analogy to understand the bullet force on steel vs ceramic based vest

With the help of an interesting cricket ball catch example, we can understand the difference between a steel plate and ceramic alloy based body armor in a better way. If you catch the ball with the steady hands, you will experience a good amount of pain. However, during the catch completion if you move your hands down you won’t experience much pain at all. In this case, your hands absorb the energy of the ball in a longer time duration.

The difference between steel plate and ceramic plate based vest are as follows :

Hard Body Armor (Ceramic plate) Hard Body Armor (Steel plate)
1. Lightweight 1. Heavier
2. Better at absorbing bullet energy 2. Chance of blunt trauma
3. Can’t handle multiple gunshot 3. Can handle multiple gunshot
4. Expensive 4. Cheaper
5. Bullet penetration 5. Bullet splashing

How is Kevlar material flexible as well as super strength?

Now, the most interesting part of this article. How such a flexible material like Kevlar is able to provide superior strength? Generally strong materials are never flexible. This super strong organic Kevlar material was developed by a woman, Stephanie Kwolek, in the DuPont laboratory. She was in fact developing a material to replace steel cord in tires. If you spin the Kevlar, which is initially in a crystalline liquid form, you’ll see that it transforms into a thicker form. The reason - formation of the hydrogen bonds (refer fig 6). These hydrogen bonds are the reason why kevlar is a super strong material. Using this interesting material for bulletproof vests was simply a Richard Devis idea.

Fig 6 : Formation of hydrogen bonds in kevlar material

What qualities make a bulletproof vest useful?

Three major qualities needed for a bullet proof vest are; flexibility, resistance, and lightweight. Kevlar-based bullet proof vests perfectly meet all these requirements. However, you will be amazed to know that the quality of Kevlar fibers in bulletproof vests degrade with time. That's why it is advised to discontinue use after 5 years of their manufacturing.

We should thank Stephanie Kwolek of DuPont for inventing a flexible and super strong material and Richard Devis for understanding its potential.

I hope you have learned how the kevlar based bulletproof vest saves the human body.

Thanks for reading!


Prerna Gupta

Prerna Gupta, a postgraduate in control and instrumentation. Currently, she is a product manager at Lesics Engineers Pvt. Ltd. Her areas of interest are telecommunication, semiconductor materials and devices, embedded systems, and design. Prerna has done projects such as MOSFET, optical fibre cable, routing, GPS systems, satellites, etc. Check out this link for more information about the author.