Bulletproof vests and bulletproof principles

Overview of bulletproof vests

Bulletproof vests “can absorb and dissipate the kinetic energy of warheads and fragments, prevent penetration, and effectively protect the human body A garment covering the area to be protected." From the perspective of use, body armor can be divided into two types: police type and military type. From the material point of view, body armor can be divided into three types: software, hardware and soft-hard composite. The materials of soft body armor are mainly high-performance textile fibers. These high-performance fibers have much higher energy absorption capabilities than ordinary materials, giving the body armor the bulletproof function. And because this type of body armor generally adopts a textile structure, it has considerable The softness is called soft body armor. Hard bulletproof vests are made of metal materials such as special steel plates, super-strong aluminum alloys, or hard non-metallic materials such as alumina and silicon carbide as the main bulletproof materials. Bulletproof vests made from them are generally not flexible. The softness of the soft-hard composite body armor is between the above two types. It is lined with soft materials and hard materials as the panels and reinforcement materials. It is a composite body armor.

As a kind of protective equipment, the core performance that body armor should first have is bulletproof performance. At the same time, as a functional garment, it should also have certain wearing properties.

Bulletproof performance

The bulletproof performance of body armor is mainly reflected in the following three aspects: (1) Anti-pistol and rifle bullets. Currently, many soft body armors can protect against pistol bullets, but To protect against rifle bullets or higher energy bullets, reinforced plates made of ceramic or steel are needed. (2) Anti-shrapnel The high-speed fragments produced by the explosion of various explosives such as bombs, landmines, artillery shells and grenades are one of the main threats on the battlefield. According to surveys, the threats faced by soldiers on a battlefield are in order: shrapnel, bullets, explosion shock waves and heat. Therefore, great emphasis should be placed on the function of bulletproof fragments. (3) Anti-non-penetrating damage The bullet will produce a huge impact after hitting the target, and the damage caused by this impact on the human body is often fatal. This kind of injury does not appear to be penetrating, but it can cause internal injuries, which may be life-threatening in severe cases. Therefore, preventing non-penetrating damage is also an important aspect of the bulletproof performance of body armor.

Wearing performance

On the one hand, the wearing performance requirements of bulletproof vests mean that without affecting the bulletproof ability, the bulletproof vest should be as light and comfortable as possible, and people can still feel more comfortable after wearing it. Complete various actions flexibly. On the other hand, clothing has the ability to regulate the microclimate environment of the "clothing-human body" system. For body armor, it is hoped that the human body can still maintain the basic "human-clothing" heat and moisture exchange state after wearing the body armor, and try to avoid the accumulation of moisture on the inner surface of the body armor to cause discomfort to the human body such as stuffy heat and humidity. Feel and reduce physical energy consumption. In addition, due to its special use environment, body armor must also consider its compatibility with other weapons and equipment.

The development history of body armor

As an important personal protective equipment, body armor has experienced the transition from metal armor protection plates to non-metallic synthetic materials, and then from pure synthetic materials The process of developing towards composite systems such as synthetic materials and metal armor plates, ceramic protective plates, etc. The prototype of human armor can be traced back to ancient times. In order to prevent the body from being harmed, primitive peoples used natural fiber woven belts as chest protection materials. The development of weapons forces human body armor to make corresponding progress. As early as the late 19th century, silk used in Japanese medieval armor was also used in body armor produced in the United States. In 1901, after President William McKenley was assassinated, body armor attracted the attention of the U.S. Congress. Although this kind of body armor can protect against low-speed pistol bullets (the bullet speed is 122 meters per second), it cannot protect against rifle bullets. Therefore, during World War I, bulletproof vests made of natural fiber fabrics and steel plates appeared as clothing linings. Thick silk garments were also once an integral part of body armor. However, silk deteriorates quickly in trenches. This flaw, coupled with limited bulletproof capabilities and the high cost of silk, caused silk body armor to be ignored by the U.S. Ordnance Department during World War I and failed to become popular. In World War II, the lethality of shrapnel increased by 80, and 70 of the casualties died from injuries to the torso.

All participating countries, especially the United Kingdom and the United States, began to spare no effort to develop body armor. In October 1942, the British army first successfully developed a bulletproof vest composed of three high-manganese steel plates. In 1943, there were 23 types of body armor trial-produced and officially adopted in the United States. The body armor of this period used special steel as the main bulletproof material. In June 1945, the U.S. military successfully developed a bulletproof vest that was a combination of aluminum alloy and high-strength nylon. The model was the M12 infantry bulletproof vest. Among them, nylon 66 (scientific name: polyamide 66 fiber) is a synthetic fiber that was invented recently. Its breaking strength (gf/d: gram force/denier) is 5.9 to 9.5, and its initial modulus (gf/d) is 21 to 58. , the specific gravity is 1.14 g/(cm)3, and its strength is almost twice that of cotton fiber. During the Korean War, the U.S. Army was equipped with the T52 all-nylon body armor made of 12 layers of ballistic nylon, while the Marine Corps was equipped with the M1951 hard "Dolon" fiberglass bulletproof vest, which weighed 2.7 to 3.6 kilograms. between. Body armor made of nylon can provide soldiers with a certain degree of protection, but it is larger and weighs up to 6 kilograms. In the early 1970s, Kevlar, a synthetic fiber with ultra-high strength, ultra-high modulus, and high temperature resistance, was successfully developed by the American company DuPont and was soon applied in the field of bulletproofing. The emergence of this high-performance fiber has greatly improved the performance of soft textile body armor and also greatly improved the comfort of body armor. The US military took the lead in using Kevlar to make body armor and developed two models, light and heavy. The new body armor uses Kevlar fiber fabric as the main material and bulletproof nylon cloth as the envelope. Among them, the light body armor is composed of 6 layers of Kevlar fabric, and the medium weight is 3.83 kg. With the commercialization of Kevlar, Kevlar's excellent comprehensive properties have quickly made it widely used in body armor of armies of various countries. The success of Kevlar and the subsequent emergence of Twaron and Spectra and their application in body armor have made soft body armor characterized by high-performance textile fibers increasingly popular, and their application scope is no longer limited to military circles, and gradually extended to the police and political circles. However, for high-speed bullets, especially bullets fired from rifles, pure soft body armor is still incapable of doing the job. For this reason, people have developed soft and hard composite body armor, using fiber composite materials as reinforcing panels or inserts to improve the bulletproof capabilities of the overall body armor. To sum up, there have been three generations of modern body armor: the first generation is hard body armor, which mainly uses special steel, aluminum alloy and other metals as bulletproof materials. The characteristics of this type of body armor are: the clothing is thick, usually about 20 kilograms, uncomfortable to wear, and restricts human movement. It has certain bulletproof properties, but is prone to secondary fragmentation. The second generation of body armor is soft body armor, usually made of multi-layer Kevlar and other high-performance fiber fabrics. It is light in weight, usually only 2 to 3 kilograms, has a soft texture, good fit, and is relatively comfortable to wear. It has good concealment when worn inside, and is especially suitable for daily wear by police, security personnel, or political dignitaries. use. In terms of bulletproof ability, it can generally prevent bullets fired from pistols 5 meters away without producing secondary shrapnel. However, it will deform greatly after being hit by a bullet, which can cause certain non-penetrating damage. In addition, soft body armor of average thickness cannot withstand bullets fired from rifles or machine guns. The third generation body armor is a composite body armor. Usually, lightweight ceramic sheets are used as the outer layer, and Kevlar and other high-performance fiber fabrics are used as the inner layer. This is the main development direction of body armor at present.

The bulletproof mechanism of bulletproof vests and its influencing factors

There are basically two bulletproof mechanisms of bulletproof vests: one is to bounce away the fragments formed after the projectile body is fragmented; It dissipates the kinetic energy of the warhead through bulletproof materials. The first body armor developed in the United States in the 1920s and 1930s relied on overlapping steel plates attached to strong clothing to provide protection. This kind of body armor and later similar hard body armor play a bulletproof role by bouncing off bullets or shrapnel, or fragmenting the bullet to consume and decompose its energy.

In addition, some small shrapnel may push individual yarns in the woven fabric away, thereby reducing the penetration resistance of the shrapnel. Within a certain range, if the fabric density is increased, the possibility of the above situation can be reduced and the strength of the woven fabric can be increased, but it will increase the negative effect of stress wave reflection superposition. Theoretically, the best impact resistance is obtained from materials that are unidirectional and have no interweaving points. This is also the starting point of "Shield" technology. "Shield" technology, or "unidirectional arrangement" technology, is a method of producing high-performance non-woven bulletproof composite materials launched and patented by the United Signal Company in 1988. The right to use this patented technology was also awarded to the Dutch company DSM. The fabric made using this technology is weftless fabric. Weftless fabric is made by arranging fibers in parallel in one direction and bonding them with thermoplastic resin. At the same time, the fibers are crossed between layers and pressed with thermoplastic resin. Most of the energy of a bullet or shrapnel is absorbed by stretching and breaking the fibers at or near the point of impact. "Shield" fabric can maintain the original strength of the fiber to the greatest extent, quickly disperse the energy to a larger range, and the processing process is also relatively simple. A single layer of weft-free cloth can be used as the backbone structure of a soft body armor after being laminated, and multi-layer pressing can be used as a hard bulletproof material such as bulletproof reinforcement inserts. If in the above two types of fabrics, most of the elastomer energy is absorbed at the impact point or at the fibers near the impact point, through excessive stretching or puncture to break the fibers, then for needle-punched nonwoven felts The bulletproof mechanism of structural fabrics cannot be explained. Because experiments have shown that almost no fiber breakage occurs in needle-punched nonwoven mats. Needle-punched nonwoven felt is composed of a large number of short fibers, there are no interweaving points, and there is almost no fixed point reflection of strain waves. Its bulletproof effect depends on the diffusion speed of bullet impact energy in the felt. It was observed that after being hit by shrapnel, there was a roll of fibrous material at the tip of the fragmentation simulating projectile (FSP). It is predicted that the projectile or shrapnel will become blunt during the initial stage of impact, making it difficult to penetrate the fabric. Many research materials have pointed out that the modulus of the fiber and the density of the felt are the main factors affecting the bulletproof effect of the entire fabric. Needle-punched nonwoven felt is mainly used in military bulletproof vests that are mainly anti-shrapnel.