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.
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 this kind of debris 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 ballistic fabric, and cause the interaction between the yarns in the fabric and the different layers of the fabric, resulting in 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 the fiber is assembled, 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 the fiber material used for impact resistance depends on the breaking energy of the fiber and the transmission speed of the stress wave. The stress wave is required to spread as quickly 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, the stronger the elongation deformation ability 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 entanglement in the yarn is tighter, and the force is more uniform, thus increasing 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 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, so 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 bulletproof fabrics, the ability to transfer energy between the fibers is poor after bombardment, 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 easy to cause the "welding" between the fibers and the fibers, 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 accounts for 5% to 25%. The method invented by another US patent 5255241 is similar to this. 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 surface of the fiber. 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 higher 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 the manufacture of 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 area of the impact point and spread through the yarn at high speed.
When the vibration wave reaches the interweaving point of the yarn, 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 kinds 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 plays the role of 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 enhances 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 density of the fabric 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 the reflection and superposition of the stress wave will be enhanced. Theoretically speaking, 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 using 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 maintain 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 by the fibers at the impact point or near the impact point through excessive stretching or piercing to break the fibers, then the needle punched nonwoven felt is The bulletproof mechanism of the structured fabric cannot be explained.
Because experiments have shown that fiber breakage hardly occurs in the needle punched nonwoven felt. The 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 the 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 Simulating 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 bullet-proof sheets.
