Here's an experiment: take the small coil springs from a dozen or so retractable pens and roll them together in a heap until they are thoroughly tangled and entwined. Now try to pull them apart from end to end. You should find them extremely difficult to pull apart this way, as anyone who has ever tried to untangle a "Slinky" toy will know. Individually, those little coil springs offer only little resistance and can be completely stretched out very easily. But together they seem to acquire extra strength from each other, and it becomes increasingly difficult to stretch any of them. When they are tangled together, one has to stretch all of them in order to stretch any one of them.
What this experiment gives you is an analogous image of what happens inside a "bullet-proof" vest.
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| This vest was shot with a .357 Magnum bullet at the NIST ballistics facility. The bullet does not penetrate, but it deforms the vest enough to burst the outer nylon material. |
| Image Courtesy NIST |
In point of fact, a bullet-proof vest is not actually bullet-proof. Rather, the material of which the vest is made resists penetration by bullets below a certain kinetic energy threshold; once the structure and/or the velocity of the bullet is changed so that the energy profile of the bullet exceeds the threshold, penetration will certainly occur. Thus we can have bullets fired from a gun that can be stopped by a vest, and bullets fired from exactly the same gun that can not be stopped by the vest.
What is of interest is how the vest stops the bullet; what process occurs inside the vest to neutralize the energy of a bullet?
A bullet fired from a gun has kinetic energy and momentum due to its mass and the velocity at which it travels. That bullet carries out its function by delivering its load of kinetic energy completely to its target. When it strikes the target transfer of energy is achieved as the bullet stops moving; the more quickly the bullet stops, the more rapidly the energy is transferred. This is the principle behind the "knock down power" of any bullet-cartridge combination.
A bullet-proof vest accepts the energy from the bullet and dissipates it so that only a small portion is passed on to the actual target, the person who is wearing the vest. That small portion of energy will probably still be enough to knock the wearer flat on his or her backside, it still hurts a lot, and will almost certainly leave a very unpleasant bruise at the point of impact. But if the vest has done its job, the bullet has not penetrated, and the person wearing it gets to walk away essentially unharmed.
The secret to this is in the material used inside the vest. Believe it or not, a bullet-proof vest is filled with nothing more than several loose layers of a light plastic fabric. But not just any plastic will do the job. This application calls for plastic fibers of exceptionally high tensile strength, fibers that it takes a great deal of energy to stretch even the tiniest amount (not fibers that will stretch a lot before they break...). In this case, those fibers are made of a polyarylamide plastic known familiarly as "Kevlar". Kevlar is the proprietary name for the material; it is becoming more common to refer to the material generally as polyarylamide.
Fibers of Kevlar don't stretch very readily when put under tension. In fact, this material is even harder to stretch than steel! But it weighs a great deal less than an equivalent value of steel fibers would weigh.
Here's how it works: remember how those little coil springs resisted being pulled apart because they were so tangled up in each other? The molecular structure of Kevlar works much the same way. The various segments of the polymer molecules are restricted to a rigid orientation such that each molecule of Kevlar has the form of a long, twisting coil. During polymerization, when many of these form at the same time, they also get twisted around and intertwined with each other, making the resulting material very hard to stretch.
When a bullet strikes the vest, it hits the layers of Kevlar fabric. The momentum of the bullet tries to carry it through the fibers, but to do this it must force them apart. But the fibers are woven and hold their positions very well so the lateral movement of the bullet is translated into a stretching force on the individual fibers. While some of the fibers will break under the strain, most will absorb the energy by stretching a small amount. This serves to dissipate the energy and momentum of the bullet. The bullet then stops very quickly before it actually gets to the intended target, and because it has given up its energy to the nest instead of to the person inside the vest, there is usually a much happier ending to the tale.
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