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Bulletproof mechanism of body armor material

by:XinXing     2020-09-09
There are many kinds of materials for body armor. Due to the different structure of the fabric, the bulletproof mechanism is also different. Foreign studies have shown that the bulletproof effect of knitted fabrics as bulletproof vests is poor, while woven fabrics, laminates and needle punched non-woven felts have good bulletproof effects.

1. Bulletproof mechanism of woven fabric
When bullets or shrapnel hit the woven fabric, shock waves and strain waves will propagate in the yarn. Because the yarns are interconnected and interact with each other, the strain wave can propagate in a large number of yarns, which is beneficial for the shock wave energy to be absorbed in a relatively large area. The rate of strain wave and energy consumption is directly related to the modulus of the fiber. The connection points of the yarn in the woven fabric will reflect the strain wave. These connection points can be regarded as fixed ends. The amplitude of the reflected wave at the fixed end and the amplitude of the original strain wave are in the same direction. The strain wave is superimposed, which may cause excessive yarn Stretch and break. In addition, some small shrapnel or specific projectiles can also push away the single yarn in the woven fabric. If the solution is simply to increase the density of the fabric, it will increase the number of fixed ends in the fabric and increase the reflected wave. The use of the matrix can fix the yarns, enhance the synergy between the layers, and make the strain wave propagate between more layers, but the matrix itself can also enhance the reflected wave.

2. Bulletproof mechanism of laminate
Bulletproof composite materials are mainly formed by lamination process, and the impact of composite material laminates against bullet impact is a rather complicated process. When the projectile penetrates the laminate, the laminate goes through the following failure stages:
1. Tensile failure stage. When the projectile acts on the fiber of the laminate, the fiber is stretched and deformed, and part of the kinetic energy of the projectile is converted into the breaking energy of the fiber.
2. Shear failure stage. Under the penetration of the projectile, the laminate is penetrated by the projectile and is cut into a 'punch' of fine particles along the thickness direction. This shear failure absorbs most of the kinetic energy of the projectile.
3. The stage of layered destruction. The laminate fiber is impacted by the projectile, and a stress wave is generated at the contact point between the projectile and the fiber. The stress wave propagates outward in two directions: one is the stress wave propagates along the fiber axis in continuous pulses. After the stress wave reaches the boundary, part of the energy is lost, and the rest returns from the boundary to the original impact point, resulting in the superimposition of the energy of the stress wave at the impact point. Due to the superposition, the intensity of the stress wave increases, and within a sufficient period of time, the location is broken; the second is that the stress wave propagates along the thickness of the laminate. The stress wave generates a reflected wave at the fabric interface of the laminate. This reflected wave and the next stress wave produce stress in the opposite direction, which tends to tear at the interface, resulting in delamination of the laminate. The delamination failure of the laminate also absorbs part of the energy of the projectile.
4. Melting failure stage. The melting point of glass fiber and the carbonization point of aramid fiber are relatively high (the melting point of glass fiber is about 1200°C, and the carbonization temperature of aramid fiber is >500°C). When the fiber is damaged by projectile impact, there will be no melting damage. The UHMWPE fiber has a low melting point (about 147°C). When the UHMWPE fiber laminate is penetrated by the projectile, heat is generated due to friction. When the temperature exceeds the melting point of UHMWPE, the fiber is melted, causing fiber breakage.
The laminated board can transform the kinetic energy of the projectile into the strain energy of the laminated board through the above-mentioned 4 kinds of damage methods, and reduce to all the kinetic energy of the projectile, which blocks the progress of the projectile, thereby playing a bulletproof effect.

3. Bulletproof mechanism of needle punched nonwoven felt
This kind of felt is made by needle punching of a large number of short fibers, with few intersections between short fibers, and the fibers contain free ends, so there is almost no fixed point reflection of strain waves. At the free end of the fiber, the amplitude of the reflected wave is opposite to that of the original strain wave, so the two amplitudes cancel each other out, and the fiber will not be excessively elongated. The most important property of fiber in mat is modulus rather than strength. Modulus and density together determine the propagation speed of strain waves in the fiber. This kind of non-woven felt has good bulletproof effect and is most suitable for protecting explosive shrapnel.
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