Full knowledge about Bulletproof Vest

Body armor is "a kind of clothing that can absorb and dissipate the kinetic energy of bullets and fragments, prevent penetration, and effectively protect the protected parts of the human body." From the perspective of use, body armor can be divided into two types: police and military. From the material point of view, body armor can be divided into three types: soft, hard and soft and hard composite. The material of soft body armor is mainly made of high-performance textile fibers. These high-performance fibers have much higher energy absorption capacity than general materials and endow the body armor with bullet-proof function. And because this kind of body armor generally adopts the structure of textiles, it has considerable The softness is called soft body armor. Hard body armor is made of special steel, super aluminum alloy and other metal materials or aluminum oxide, silicon carbide and other hard non-metallic materials as the main bulletproof materials. The body armor made from this generally does not have flexibility.


Introduction

Bulletproof vests are protective clothing used to reduce bullet damage like armor. They are worn by the police and the army.

The above name is more or less a misnomer, because most of these protective clothing have little or no protection against large-caliber rifles or rifles, regardless of the type, style, material, or caliber of the rifle bullet (this exception cannot In general. 22 LR type, usually can protect large-caliber rifles and rifle bullets.) These vests provide a broad defense against pistol ammunition fired from pistols-regardless of type, style, material, and caliber of pistol ammunition .

Some types of body armor have metal extensions (steel or titanium), and important parts of the body can be additionally filled with ceramic or polyethylene plates to increase protection. If the bullet hits the filler, these protectors can effectively protect all pistols and some rifles. This type of vest has become the standard for military use, as the advanced ballistic technology makes the "Kevlar-only" vest invalid-The CRISAT NATO standard for vests includes titanium backing. Some vests are also designed to protect against knife attacks.


Body protection equipment

It is used to protect the human body from the damage of bullets or shrapnel. The bulletproof vest is mainly composed of two parts: a jacket and a bulletproof layer. Clothing covers are often made of chemical fiber fabrics. The bulletproof layer is made of metal (special steel, aluminum alloy, titanium alloy), ceramics (corundum, boron carbide, silicon carbide), glass steel, nylon, Kevlar and other materials to form a single or composite protective structure. The bulletproof layer can absorb the kinetic energy of the bullet or shrapnel, has obvious protective effect on the low-speed bullet or shrapnel, and can reduce the damage to the human chest and abdomen. Body armor includes body armor for infantry, flight personnel, and artillery.


Overview

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


Ballistic performance

The bulletproof performance of body armor mainly reflects the following three aspects:

(1) Anti-pistol and rifle bullets: Many soft body armors can protect against pistol bullets, but to protect against rifle bullets or higher energy bullets, ceramic or steel bullet-proof plates are required.

(2) Bulletproof fragments: High-speed shrapnel produced by the explosion of various explosives such as bombs, landmines, artillery shells and grenades is one of the main threats on the battlefield. According to the investigation, the order of the threats faced by soldiers in a battlefield is: shrapnel, bullets, blast wave and heat. Therefore, the function of the bulletproof piece must be emphasized.

(3) Prevention of non-penetrating damage: The bullet will produce a great impact after hitting the target. The damage produced by this impact on the human body is often fatal. This kind of injury does not show penetration, but it can cause internal injuries, and severe cases can be life-threatening. Therefore, preventing non-penetrating injuries is also an important feature of body armor.


Taking performance

On the one hand, the wearing performance requirements of body armor means that the body armor should be as light and comfortable as possible without affecting the bulletproof ability, and people can still perform various actions more flexibly after wearing. The other is the ability of clothing to adjust to the microclimate environment of the "clothing-human body" system. For body armor, it is hoped that after the body wears the body armor, it can still maintain the basic heat and moisture exchange state of "man-clothes", and try to avoid the accumulation of moisture on the inner surface of the body armor, which may cause uncomfortable heat and humidity to the human body. Sense, reduce physical consumption. In addition, due to its special use environment, the suitability of the body armor with other weapons and equipment should also be considered.


development path

As an important personal protective equipment, body armor has undergone the transition from metal armored protective panels to non-metallic synthetic materials, and also from pure synthetic materials.

The process of development of materials to composite systems such as synthetic materials and metal armor plates, ceramic protective sheets. The prototype of human armor can be traced back to ancient times. In order to prevent the body from being injured, primitive peoples used natural fiber braided belt as the material of the chest protector. The development of weapons has forced corresponding advances in human armor. As early as the end of the 19th century, the silk used in Japanese medieval armor was also used in body armor produced in the United States. After the assassination of President William McKinley in 1901, the body armor attracted the attention of the US Congress. Although this body armor can protect against low-speed pistol bullets (with a bullet speed of 122 m/s), it cannot protect against rifle bullets. Thus, in the First World War, a body armor made of natural fiber fabrics and steel plates appeared. Thick silk clothing was once the main component of body armor. However, silk deteriorates quickly in the trenches. This defect, coupled with the limited bulletproof capability and the high cost of silk, made silk body armors neglected by the U.S. Department of Ordnance during World War I and failed to become popular. In World War II, the lethality of shrapnel increased by 80%, and 70% of the wounded died from torso injuries. The 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 that were trial-produced and officially adopted in the United States. The bulletproof vests of this period used special steel as the main bulletproof material. In June 1945, the U.S. Army successfully developed a bulletproof vest made of aluminum alloy and high-strength nylon. The model was M12 infantry body armor. Among them, nylon 66 (scientific name polyamide 66 fiber) is a synthetic fiber that was recently invented at that time. Its breaking strength (gf/d: gram-force/denier) is 5.9-9.5, and its initial modulus (gf/d) is 21-58. , The specific gravity is 1.14 g/(cm)3, and its strength is almost twice that of cotton fiber. During the Korean War, the US Army was equipped with a T52 full nylon body armor made of 12 layers of bullet-proof nylon, while the Marine Corps was equipped with a M1951 hard "Doron" fiberglass bulletproof vest, which weighs 2.7 to 3.6 kg. between. Body armor made of nylon can provide soldiers with a certain degree of protection, but it is larger in size 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 DuPont of the United States, and it was quickly applied in the field of bulletproof. The emergence of this high-performance fiber greatly improves the performance of soft textile body armor, and at the same time improves the comfort of the body armor to a large extent. The US military took the lead in using Kevlar to make body armor, and developed two models of light and heavy.

The new body armor uses Kevlar fiber fabric as the main material, and bulletproof nylon cloth as the envelope. The lightweight 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 performance has soon been widely used in the body armor of the military of various countries. The success of Kevlar and the subsequent appearance of Twaron and Spectra and their application in body armor made soft body armor characterized by high-performance textile fibers become more popular, and its application range is not limited to The military, and gradually expanded to the police and political circles. However, for high-speed bullets, especially bullets fired by rifles, pure soft body armor is still incompetent. For this reason, people have developed soft and hard composite body armor, using fiber composite material as a reinforced panel or insert plate to improve the bulletproof ability of the overall body armor. In summary, there have been three generations of modern body armor development: the first generation is a hard body armor, mainly using special steel, aluminum alloy and other metals as bulletproof materials. The characteristics of this type of bulletproof vest are: the clothing is thick and heavy, usually about 20 kilograms, it is not comfortable to wear, it has a large restriction on human activities, has a certain degree of elasticity, but is easy to produce secondary fragments. The second-generation body armor is a soft body armor, usually made of high-performance fiber fabrics such as multi-layer Kevlar. It is light in weight, usually only 2 to 3 kilograms, and has a soft texture, good fit, and comfortable wearing. It has good concealment when worn inside, and is especially suitable for daily wear by police and security personnel or political officials. use. In terms of bulletproof capability, it can generally prevent bullets fired from a pistol 5 meters away, and will not produce secondary shrapnel, but it deforms greatly after being hit by a bullet, which can cause certain non-penetrating damage. In addition, for bullets fired from rifles or machine guns, soft body armor of general thickness is difficult to resist. The third generation body armor is a composite body armor. Generally, lightweight ceramic sheets are used as the outer layer, and high-performance fiber fabrics such as Kevlar as the inner layer are the main development directions of body armor.


Bulletproof mechanismedit

There are basically two bulletproof mechanisms of body armor: one is to bounce off the fragments formed after fragmentation of the projectile; the other is to dissipate the kinetic energy of the warhead through the bulletproof material. The first batch of body armor developed by the United States in the 1920s and 1930s was protected by overlapping steel plates attached to strong clothing. This body armor and later similar hard body armors play a bulletproof role by popping off the bullets or shrapnel, or breaking the bullet to consume and decompose its energy. For soft body armor using high-performance fiber as the main bulletproof material, its bulletproof mechanism is mainly the latter, that is, the use of high-strength fiber as the raw material to "catch" bullets or shrapnel to achieve the purpose of bulletproof. Studies have shown that there are five ways in which soft bulletproof vests absorb energy:

(1) The deformation of the fabric: including the deformation of the bullet incident direction and the stretching deformation of the area near the incident point;

(2) Fabric destruction: including fiber fibrillation, fiber breakage, yarn structure disintegration and fabric structure disintegration;

(3) Thermal energy: energy is dissipated as heat energy through friction;

(4) Acoustic energy: the energy consumed by the sound made by the bullet hitting the bulletproof layer;

(5) Deformation of the projectile. The bulletproof mechanism of the soft and hard composite body armor developed to improve the bulletproof capability can be summarized by "soft and hard". When a bullet hits the body armor, the first effect is the hard bulletproof material such as steel plate or reinforced ceramic material. During this moment of contact, both bullets and hard bulletproof materials may deform or break, consuming most of the bullet's energy. The high-strength fiber fabric serves as the liner and second line of defense of the bulletproof vest, absorbs and diffuses the energy of the remaining part of the bullet, and acts as a buffer, thereby reducing non-penetrating damage as much as possible. In the two bulletproof processes, the previous one played the main energy absorption function, which greatly reduced the penetration of the projectile, which is the key to bulletproof.


Influencing factors

The factors that affect the bulletproof performance of body armor can be considered from two aspects: the interacting projectile (bullet or shrapnel) and the bulletproof material. As far as the projectile is concerned, its kinetic energy, shape and material are important factors that determine its penetration.

Bulletproof vest

Bulletproof vest

Ordinary bullets, especially lead-cored or ordinary steel-cored bullets, will deform when they come into contact with bulletproof materials. In this process, a considerable part of the kinetic energy of the bullet is consumed, thereby effectively reducing the penetration force of the bullet, which is an important aspect of the energy absorption mechanism of the bullet. For bombs, grenades, and other shrapnels or secondary fragments formed by bullets, the situation is significantly different. These shrapnels have irregular shapes, sharp edges, light weight and small size, and will not deform after hitting bulletproof materials, especially soft bulletproof materials. Generally speaking, the speed of such fragments is not high, but the amount is large and dense.

The key to the energy absorption of such fragments by soft body armor lies in the fact that the fragments cut, stretch and break the yarns of the bulletproof fabric, and cause the interaction between the yarns in the fabric and the different layers of the fabric to cause the overall deformation of the fabric. In the above-mentioned processes, the fragments do work to the outside, thereby consuming their own energy. In the above two types of body energy absorption process, a small part of the energy is converted into heat energy through friction (fiber/fiber, fiber/bullet), and converted into sound energy through impact. In terms of bulletproof materials, in order to meet the requirements of body armor to absorb the kinetic energy of bullets and other projectiles to the greatest extent, the bulletproof materials must have high strength, good toughness, and strong energy absorption capabilities. The materials used in body armor, especially soft body armor, are mainly high-performance fibers. These high-performance fibers are characterized by high strength and high modulus. Although some high-performance fibers such as carbon fiber or boron fiber have high strength, they are basically not suitable for body armor due to poor flexibility, low breaking power, difficulty in spinning and processing, and high price.

Specifically, for ballistic fabrics, its bulletproof effect mainly depends on the following aspects: fiber tensile strength, fiber elongation at break and work at break, fiber modulus, fiber orientation and stress wave transmission speed, fiber The fineness of the fiber, the way of fiber assembly, the fiber weight per unit area, the structure and surface characteristics of the yarn, the structure of the fabric, the thickness of the fiber mesh layer, the number of layers of the mesh layer or the fabric layer, etc. The performance of fiber materials used for impact resistance depends on the breaking energy of the fiber and the speed of stress wave transmission. The stress wave is required to spread as soon as possible, and the fracture energy of the fiber under high-speed impact should be as high as possible. The tensile rupture work of a material is the energy that the material has to resist damage by external forces, and it is a function related to tensile strength and elongation deformation. Therefore, theoretically, the higher the tensile strength and the stronger the elongation deformation capacity of the material, the greater the potential for energy absorption.

However, in practice, the material used for body armor is not allowed to have excessive deformation, so the fiber used for body armor must also have a higher resistance to deformation, that is, a high modulus. The influence of the structure of the yarn on the ballistic resistance is due to the difference in the single fiber strength utilization rate and the overall elongation deformation ability of the yarn due to different yarn fabrics. The breaking process of the yarn firstly depends on the breaking process of the fiber, but because it is an aggregate, there is a big difference in the breaking mechanism. If the fineness of the fiber is fine, the yarn is tightly entangled with each other, and the force is more uniform at the same time, which improves the strength of the yarn. In addition, the straightness and parallelism of the fiber arrangement in the yarn, the number of transfers of the inner and outer layers, and the twist of the yarn all have an important influence on the mechanical properties of the yarn, especially the tensile strength and elongation at break. In addition, due to the interaction between the yarn and the yarn and the yarn and the elastic body during the bombardment process, the surface characteristics of the yarn will have the effect of strengthening or weakening the above two effects. The presence of oil and moisture on the surface of the yarn will reduce the resistance of bullets or shrapnel to penetrate the material. Therefore, people often need to clean and dry the material and seek ways to improve the penetration resistance. Synthetic fibers with high tensile strength and high modulus are usually highly oriented, so the fiber surface is smooth and the coefficient of friction is low. When these fibers are used in bullet-proof fabrics, the ability to transfer energy between the fibers after bombardment is poor, and the stress wave cannot spread quickly, thereby reducing the ability of the fabric to block bullets. Ordinary methods to increase the surface friction coefficient, such as raising and corona finishing, will reduce the strength of the fiber, while the method of fabric coating is likely to cause the "welding" between the fiber and the fiber, resulting in the bullet shock wave in the yarn The reflection occurs laterally, causing the fiber to break prematurely. In order to solve this contradiction, people have come up with various methods. AlliedSignal (AlliedSignal) has introduced an air-wound treatment fiber to the market, which increases the contact between the bullet and the fiber by entanglement of the fiber inside the yarn.

In US Patent No. 5,035,111, a method for improving the friction coefficient of yarns by using sheath-core structure fibers is introduced. The "core" of this fiber is a high-strength fiber, and the "skin" uses a fiber with a slightly lower strength and a higher coefficient of friction, the latter accounting for 5% to 25%. The method invented by another US patent 5,255,241 is similar. It coats the surface of the high-strength fiber with a thin layer of high-friction polymer to improve the fabric's ability to resist metal penetration. This invention emphasizes that the coating polymer should have strong adhesion to the surface of the high-strength fiber, otherwise the coating material that peels off when bombarded will act as a solid lubricant between the fibers, thereby reducing the fiber surface Coefficient of friction. In addition to fiber properties and yarn characteristics, an important factor affecting the bulletproof ability of body armor is the structure of the fabric. The fabric structure types used on the software body armor include knitted fabrics, woven fabrics, non-weft fabrics, needle punched non-woven felts, etc. Knitted fabrics have high elongation, which is beneficial to improve the comfort of wearing. But this kind of high elongation used for impact resistance will produce great non-penetrating damage. In addition, because knitted fabrics have anisotropic characteristics, they have different degrees of impact resistance in different directions. Therefore, although knitted fabrics have advantages in terms of production cost and production efficiency, they are generally only suitable for manufacturing stab-resistant gloves, fencing suits, etc., and cannot be completely used for body armor. The more widely used body armors are woven fabrics, non-weft fabrics and needle punched non-woven felts. Due to their different structures, these three types of fabrics have different bulletproof mechanisms, and ballistics cannot yet give a sufficient explanation. Generally speaking, after the bullet hits the fabric, it will generate a radial vibration wave in the impact area and spread through the yarn at high speed.

When the vibration wave reaches the interweaving point of the yarn, a part of the wave will be transmitted along the original yarn to the other side of the interweaving point, another part will be transferred to the inside of the interlaced yarn, and some will be reflected along the original yarn. Go back and form a reflected wave. Among the above three types of fabrics, the woven fabric has the most interweaving points. After being hit by the bullet, the kinetic energy of the bullet can be transmitted through the interaction of the yarns at the interweaving point, so that the impact force of the bullet or shrapnel can be absorbed in a larger area . But at the same time, the interweaving point acts as a fixed end invisibly. The reflected wave formed at the fixed end and the original incident wave will be superimposed in the same direction, which greatly increases the stretching effect of the yarn, and breaks after exceeding its breaking strength. In addition, some small shrapnel may push a single yarn 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 improved, but the negative effect of stress wave reflection and superposition will be enhanced. Theoretically, to obtain the best impact resistance is to use unidirectional materials without interlacing points. This is also the starting point of the "Shield" technology. "Shield" technology, or "unidirectional array" technology, is a method of producing high-performance non-woven bulletproof composite materials launched and patented by United Signal Corporation in 1988. The right to use this patented technology was also granted to the Dutch company DSM. The fabric made by this technology is a weftless fabric. The non-weft fabric is made by arranging the fibers in parallel in one direction and bonding them with a thermoplastic resin. At the same time, the fibers are crossed between layers and pressed with a thermoplastic resin.

Most of the energy of a bullet or shrapnel is absorbed by stretching and breaking the fibers at or near the impact point. The "Shield" fabric can keep the original strength of the fiber to the greatest extent, and quickly disperse the energy to a larger area, and the processing procedure is relatively simple. The single-layer non-weft fabric can be used as the backbone structure of the soft body armor after being laminated, and the multi-layer can be used as hard bulletproof materials such as bulletproof reinforced inserts. If in the above two types of fabrics, most of the projectile energy is absorbed at the impact point or the fiber near the impact point through excessive stretching or piercing to break the fiber, then the needle punched nonwoven felt is The bulletproof mechanism of the fabric of the structure cannot be explained.

Because experiments have shown that fiber breakage hardly occurs in the needle punched nonwoven felt. Needle-punched nonwoven felt is composed of a large number of short fibers, there is no interweaving point, and there is almost no fixed point reflection of strain wave. The bulletproof effect depends on the diffusion speed of the bullet impact energy in the felt. It was observed that after being hit by shrapnel, there was a roll of fibrous material on the tip of the Fragment Simulation Projectile (FSP). Therefore, it is predicted that the projectile body or shrapnel becomes blunt at the initial stage of impact, making it difficult to penetrate the fabric. Many research materials have pointed out that the modulus of fiber and the density of felt are the main factors that affect the ballistic effect of the entire fabric. Needle-punched non-woven felts are mainly used in military bullet-proof vests mainly made of bulletproof pieces.