The research shows that there are five ways for soft body armor to absorb energy: (1) fabric deformation, including the deformation in the direction of bullet incidence and the tensile deformation 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 in the form of thermal energy through friction; (4) Acoustic energy: the energy consumed by the sound emitted by the bullet after hitting the bulletproof layer; (5) Deformation of projectile. The bulletproof mechanism of the soft-hard composite bulletproof vest developed to improve its bulletproof ability can be summarized by "combining soft and hard".
When the bullet hits the bulletproof vest, the first thing that affects it is the hard bulletproof material such as steel plate or reinforced ceramic material. During this instant contact, both the bullet and the hard bulletproof material may be deformed or broken, which consumes most of the energy of the bullet. As the cushion and the second line of defense of bulletproof vest, high-strength fiber fabric absorbs and diffuses the remaining energy of bullets and plays a buffering role, thus minimizing non-penetrating injuries. In these two bullet-proof processes, the former plays a major role in energy absorption, greatly reducing the penetration of projectiles, which is the key to bullet-proof.
The factors affecting the bulletproof efficiency of bulletproof vests can be considered from two aspects: projectiles (bullets or shrapnel) and interactive bulletproof materials. As far as the projectile is concerned, its kinetic energy, shape and material are important factors that determine its penetration. Ordinary warheads, especially lead-core or ordinary steel-core bullets, will be deformed when they contact bulletproof materials. In this process, the bullet consumes a considerable part of kinetic energy, thus effectively reducing the penetration of the bullet, which is an important aspect of the bullet energy absorption mechanism.
However, the situation is obviously different for shrapnel produced by bombs, grenades and other explosions or secondary fragments formed by bullets. These shrapnel are irregular in shape, sharp in edge, light in weight and small in volume, and do not deform after hitting bulletproof materials, especially soft bulletproof materials. Generally speaking, the speed of this kind of debris is not high, but it is large and dense. The key for soft bullet-proof clothing to absorb the energy of such debris lies in that the debris cuts, stretches and breaks the yarns of bullet-proof fabric, which causes the interaction between yarns in the fabric and between different layers of the fabric, leading to the overall deformation of the fabric. In these processes, the debris does external work, thus consuming its own energy. In the above two kinds of human energy absorption process, a small part of energy is also converted into heat energy through friction (fiber/fiber, fiber/bullet) and into sound energy through impact.
In terms of bulletproof materials, bulletproof materials must have high strength, good toughness and strong energy absorption ability in order to meet the requirements of bulletproof clothing to absorb the kinetic energy of bullets and other projectiles to the maximum extent. The materials used in bulletproof vests, especially soft bulletproof vests, are mainly high-performance fibers. These high-performance fibers are characterized by high strength and high modulus. Some high-performance fibers, such as carbon fiber or boron fiber, have high strength, but they are basically not suitable for human armor because of poor flexibility, low fracture work, difficult textile processing and high price.
Specifically, the bulletproof effect of bulletproof fabric mainly depends on the following aspects: tensile strength, elongation at break and work of fiber, modulus of fiber, fiber orientation and stress wave propagation speed, fiber fineness, fiber assembly mode, fiber weight per unit area, yarn structure and surface characteristics, fabric structure, fiber mesh layer thickness, mesh layer or fabric layer number, etc. The performance of fiber materials used for impact resistance depends on the fracture energy of fiber and the speed of stress wave propagation. Stress wave needs to spread as soon as possible, and the fracture energy of fiber under high-speed impact should be improved as much as possible. The tensile fracture work of materials is the energy that materials have to resist external damage, and it is a function related to tensile strength and elongation deformation.
Therefore, theoretically, the higher the tensile strength, the stronger the elongation and deformation capacity and the greater the energy absorption potential. However, in practical application, the materials used in body armor are not allowed to deform too much, so the fibers used in body armor must have high deformation resistance, that is, high modulus. The influence of yarn structure on bullet-proof ability is due to the difference of single fiber strength utilization rate and yarn overall elongation and deformation ability caused by different yarn fabrics. The fracture process of yarn depends on the fracture process of fiber at first, but the fracture mechanism is very different because it is an aggregate. The finer the fibers are, the closer they are to each other in the yarn and the more uniform the force is, thus improving the strength of the yarn.
In addition, the straightness and parallelism of fiber arrangement in yarn, the number of inner and outer layer transfers, yarn twist and so on. All these have an important influence on the mechanical properties of yarns, especially the tensile strength and elongation at break. In addition, due to the interaction between yarn and yarn, yarn and elastomer, the surface characteristics of yarn will produce or strengthen or weaken the above two effects. The existence of grease and moisture on the yarn surface will reduce the resistance of bullets or shrapnel to penetrate the material, so people often need to clean and dry the material to find 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 friction coefficient is low. When these fibers are used in bulletproof fabrics, the ability of energy transfer between fibers is poor after being hit by bullets, and the stress wave cannot spread quickly, which also reduces the ability of fabrics to stop bullets. Ordinary methods to improve the surface friction coefficient, such as napping and corona finishing, will reduce the strength of fibers, while the method of fabric coating will easily lead to "welding" between fibers, resulting in the reflection of bullet shock waves in the transverse direction of yarns, which will lead to premature fiber breakage. In order to solve this contradiction, people have come up with various methods.
The United States Lianxin Company introduced an air-wound fiber to the market, which increased the contact between the bullet and the fiber through the fiber winding inside the yarn. In US patent 5035 1 1 1, a method to improve the friction coefficient of yarns by using sheath-core structural fibers is introduced. The "core" of this fiber is high strength fiber, and the "skin" is fiber with slightly lower strength and higher friction coefficient, and the latter accounts for 5% ~ 25%. Another American patent, 525524 1, invented a similar method, that is, coating a thin layer of high-friction polymer on the surface of high-strength fiber to improve the resistance of fabric to metal penetration. The invention emphasizes that the coating polymer should have strong adhesion with the surface of high-strength fiber, otherwise the coating material peeled off in the impact process will be used as a solid lubricant between fibers, thus reducing the friction coefficient of the fiber surface.
In addition to fiber properties and yarn characteristics, fabric structure is also an important factor affecting the bulletproof ability of bulletproof vests. Fabric structure types used in software bulletproof vests include knitted fabrics, woven fabrics, weft-free fabrics and needle-punched nonwovens. Knitted fabrics have high elongation, which is beneficial to improve wearing comfort. However, when used for impact resistance, this high elongation will lead to great non-penetrating damage. In addition, due to the anisotropic properties of knitted fabrics, they have different degrees of impact resistance in different directions. Therefore, although knitted fabrics have advantages in production cost and efficiency, they are generally only suitable for making stab-resistant gloves and fencing clothes. , and can't be completely used in bulletproof vests.
Woven fabrics, weft-free fabrics and needle-punched nonwovens are widely used in bulletproof vests. Ballistics cannot give a complete explanation because of the different structures and bulletproof mechanisms of these three fabrics. Generally speaking, after a bullet hits the fabric, it will generate a radial vibration wave in the impact point area and propagate through the yarn at high speed. When the vibration wave reaches the interweaving point of the yarn, part of the wave will be transmitted to the other side of the interweaving point along the original yarn, the other part will be transmitted to the yarn interwoven with it, and the other part will be reflected back along the original yarn to form a reflected wave.
Among the above three fabrics, woven fabrics have the most interweaving points. After being hit by a bullet, the kinetic energy of the bullet can be transferred through the interaction of 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 virtually plays the role of a fixed end. The reflected wave formed at the fixed end and the original incident wave are superimposed in the same direction, which greatly enhances the tension of the yarn and breaks after exceeding its breaking strength. In addition, some small shrapnel may push away a single yarn in the woven fabric, thus reducing the penetration resistance of shrapnel. In a certain range, if the fabric density is increased, the possibility of the above situation can be reduced and the strength of woven fabric can be improved, but the negative effect of stress wave reflection superposition will be enhanced.
Theoretically, to obtain the best impact resistance, it is to use unidirectional materials without interweaving points. This is also the starting point of "shield" technology. "Shielding" technology, that is, "unidirectional arrangement" technology, is a method for producing high-performance non-woven bullet-proof composite materials, which was introduced and patented by United Signal Company of the United States in 1988. The right to use this patented technology has also been awarded to DSM in the Netherlands. The fabric made by this technology is a weft-free fabric. Weft-free cloth is made by arranging fibers in parallel along one direction and bonding them with thermoplastic resin, while crossing the fibers between layers and pressing them with thermoplastic resin. Most of the energy of bullets or shrapnel is absorbed by stretching and breaking fibers at or near the impact point. "Shielding" fabric can maintain the original strength of fiber to the greatest extent, and quickly disperse energy to a larger range, and the processing procedure is relatively simple.
Single-layer weft-free cloth can be used as the skeleton structure of soft bulletproof vest, and multi-layer pressing can be used as hard bulletproof materials such as bulletproof reinforced insert plates. If most of the elastic energy in the above two kinds of fabrics is absorbed on the fibers at or near the impact point, and the fibers are excessively stretched or punctured and broken, then the bulletproof mechanism of needled nonwovens can't be explained. Because experiments show that needle-punched nonwovens hardly break fibers. Needle-punched nonwovens are composed of a large number of short fibers, with no interweaving points and almost no fixed-point reflection of strain waves. Its bulletproof effect depends on the diffusion speed of bullet impact energy in the felt.
It is observed that after being hit by shrapnel, there is a roll of fibrous material on the top of the fragment simulation bomb (FSP). Therefore, it is predicted that the projectile or shrapnel will become dull at the initial stage of impact and it is difficult to penetrate the fabric. Many research data point out that the modulus of fiber and the density of felt are the main factors affecting the bulletproof effect of the whole fabric. Needle-punched nonwoven felt is mainly used for bulletproof sheets of military bulletproof vests.