Being the occasionally interesting ramblings of a major-league technophile.
Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.
The protective value of armor is closely related to its strength, but there are other factors involved as well. For instance, rigidity is also important, as is the density. Moreover, soft (flexible) armors operate differently from hard (or rigid) armors, though this is primarily true at the low end of the scale. While soft armor will be deformed by even a minor impact which would not affect hard armor at all, even "rigid" armor will deform near the upper limit of its protective range. Hit steel hard enough and it will not only flow like putty, but splatter like water. This is plastic deformation, where a material undergoes a permanent change in shape from a load. Those loads which deform a rigid armor but do not create a permanent change cause elastic deformation. While repeated minor hits could affect the tempering of the hard armor, such a situation would be unlikely in war.
Density, as noted above, also plays a role. A thick sheet of lead will stop a projectile, simply because there is not enough kinetic energy to push the mass of the lead out of the way. However, except for fixed fortifications you don't usually want a lot of mass, so for the purposes of this discussion armor which depends primarily on the mass effect will be ignored.
To further simplify the matter, this article will discuss only single-layer armor, and ignore spaced armor. (That is, armor which is made of multiple sheets with gaps or layers of spacing material between them.) Even this narrowing still leaves a broad subject; as with anything relating to material strength and overcoming it, armor and armor penetration are complex subjects. Therefore, unless stated otherwise assume the following: Impacts are at 90 degrees to a flat, even surface, which is large with respect to the cross-section of the projectile (so that edge effects can be ignored), and that there is no backing to the hard armor plates besides air, though they are rigidly supported at the edges. (Soft armor will have backing to prevent excessive deformation.) Also assume that projectiles are solid, uniform and of an ogival shape. Even with these restrictions slight differences in shape of shell, mounting of plate and so forth can cause significant variations in results. Treat the values below as typical, and with a large grain of salt.
Given the above criteria, good quality wrought iron (yield strength in tension, compression and shear of 62,000, 62,000 and 55,000 Newtons per square centimeter, respectively) is about 87% as effective as the same thickness (not mass) of Rolled Homogenous Armor (RHA, or MIL-A-12560, has values of 117,000, 110,000, and 100,000, respectively [note that the protection ratio is actually higher than the average of the strength ratios]). Typical mild steel (46,300, 46,300 and 38,600) has a resistance of about 75% that of RHA. Given the above, you might wonder why we aren't making tanks out of wrought iron. (Actually, most early ironclads literally were clad in iron, even after steel became available.) However, there are factors here other than mere resistance to penetration. For instance, steel has an advantage in hardness, as mentioned above. This means it is less likely to deform or lose strength from low-energy impacts, so it is harder to wear down. Of course, wrought iron is more ductile and therefore less likely to shatter than relatively brittle steel, and is also much less prone to rust.
Additionally, uniformly good RHA is actually easier and cheaper to get than wrought iron these days, given the changes in iron-making technologies over the past century. Typical wrought iron has values of 38,600, 38,600 and 30,900, for a protection level of about 59% that of RHA. On the other hand, ordinary mild steel is much more brittle than even typical wrought iron. An impact which has too little energy for penetration may actually shatter steel, or cause the inner layer to flake off, the pieces flying away at high velocity. This phenomenon is known as spalling, and it can be very bad for people and equipment on the far side of a steel armor plate. RHA is a tempered material, less likely to spall than an ordinary steel of the same strength, but spalling can still happen.
Note, by the way, that the relationship between tensile strength and penetration resistance is not linear in the above examples. Partly this is because different materials are being discussed (steel and iron). However, even for materials that are otherwise very similar the relationship between strength and penetration resistance is exponential. (In this case, the calculation involves taking roots. And that is definitely enough math for today...) Similarly, the penetration of a projectile can be approximated from the kinetic energy divided by the area of contact, with a factor, but this again is a non-linear equation. Moreover, with very large, fast projectiles - such as those from the main guns of battleships - a factor which is trivial for small shells adds significantly to the penetration.
The ability of armor to withstand low levels of impact energy without damage is important in any environment where multiple hits between replacement or repair are expected. Soft body armor has almost no such resilience; after an area is hit the ability of that area to resist penetration is much reduced. Hard armor, though, is a different story, as mentioned above. Shortly after the 20mm Vulcan rotary canon was developed, an experiment was carried out to determine whether it would make an effective weapon against heavy tanks. A tank was parked on a firing range and hosed thoroughly. All pieces of external equipment (including the weapons) were destroyed or rendered inoperative. However, the armor itself was hardly damaged. Now, the combat effectiveness a vehicle hit like this would obviously be much reduced. Indeed, the crew might not even survive the repeated shocks, and would definitely not be happy. In combat situations, though, a tank is unlikely to sit still for such treatment; unless the gunner got a lucky hit on, say, a tread first thing, the target would be able to get away or return fire. And since most of the outside of a tank is heavily armored (which is the idea, after all) the likelihood of hitting a sensitive spot with a Vulcan before the target escaped or started shooting back is low.
Another example of the value of rigid armor's ability to take a mild hit without being damaged comes from Naval warfare. After the Falklands/Malvinas War demonstrated the ineffectiveness of modern, lightweight aluminum armor against modern antiship weapons, a joke started circulating. "What does the captain of an Iowa-class battleship do after his ship is hit with an Exocet missile? He sends two ratings up on deck. One with a broom and a dustpan; the other with a can of gray paint and a brush." While an exaggeration, it wasn't much of one. Battleship armor is designed to withstand impact by a multi-tonne, armor-piercing warheads striking at supersonic speed. The Exocet missile travels much slower and has a smaller warhead. Which may be why in recent years anti-ship missiles have become larger, faster and better able to penetrate armor. Just in time for the last battleships to be retired.
Designing a homogenous armor strong enough to do the job of withstanding serious assault without being too thick and heavy is difficult. Few materials combine strength, resistance to deformation, low density and lack of brittleness. People who create armor have therefore been making armor by combining different materials for centuries. Some suits of Japanese medieval armor were made of lacquered layers of silk, bamboo, wood and cloth. These lightweight, semi-hard armors would stop a blade and hold it, binding it, rather than simply resisting by main force. Even the traditional suit of knight's armor had layers of other stuff under the solid sheets of shaped steel. If nothing else, padding was essential. For iron-clad ships many techniques were used to combine the hardness of steel with the resilience of iron. There was case-hardening (in which a surface layer of the iron was actually turned to steel by heating in the presence of carbon and other materials), as well as welding, fusing and so forth. A lot of effort has been expended on developing and using such materials, and for good reason. There are advantages in economy and ease of manufacture to a homogenous armor, but if you want the best...
These armors are composite materials. Composites use the strength of one substance to offset the weakness of another. They are stronger and tougher for the weight and thickness than either material alone. Many composites also have an added advantage of retaining a significant portion of their strength after failure. While fiberglass/epoxy, graphite/epoxy and other typical composite materials have been successfully used as armor, so have combinations of steel and materials such as ceramics. Indeed, the famed Chobham armors - such as the type used in the M-1 Abrams Main Battle Tank - are composites.
Soft armors - at least, those woven from fibres - are also composites, at least in a sense. They actually are a composite with space as an ingredient. They also combine multiple layers of the same substance, since several thin layers are more effective than a single layer of the same total thickness. (As noted above, there have been metal spaced armors, too, their primary use being to defeat shaped charges.) Any strong, flexible, elastic fibre would probably make a good soft armor.
Silk has been used for centuries to provide protection, not only from the elements but from more direct damage. Silk has a yield strength in tension (50,000 N/cm^2) nearly as great as that of good-quality wrought iron (60,000 N/cm^2) while being much lighter (density of 1.3 grams per cubic centimeter as opposed to 7.15) and is also quite elastic. (Note that these values are typical of silkworm silk; other silks will vary in their properties. Spider silk would be far better than silkworm silk - or kevlar - for ballistic vests, if it could be harvested in quantities.) A free-hanging silk scarf will catch most handgun bullets. It is also a good electrical and thermal insulator. It absorbs very little water, about 11% by weight, so it stays light even when wet. Small wonder that thick layers of silk cloth have been used for soft armor, as well as for the types of semi-hard armor mentioned above. Nylon is about twice as strong as silk with a density of 1.15. Kevlar is over 7 times as strong as silk while having a density of 1.14. Diamond whiskers are about 410 times as strong as silk with a density of 3.56, but are very difficult to produce. (Though new techniques of vapor phase deposition (VPD) of carbon atoms on various substrate wires or fibres show great promise.)
Tensile strength isn't everything in soft armor any more than it is in hard armor, of course. You want some resilience, not only to prevent shattering but to soak up some of the shock. However, you don't want too much give. People have actually died from having part of their soft body armor - with the bullet inside - driven into their bodies. For high-threat situations there are soft body armors with ceramic (or ceramic-faced steel) plates in pockets. This provides excellent penetration resistance, even from high-powered rifle rounds. With some of these soft body armors the plates can be removed for normal wear and kept handy to insert when the need arises.
As a final comment, note that nothing is bullet-proof, because someone always has a bigger bullet. During the War Between the States many officers on both sides - especially those in the cavalry, who could ride instead of walking - purchased steel cuirasses for protection. These were effective, if heavy. One of my reference books has a photo of such an armored steel vest, worn by a Union cavalry officer who charged a Confederate artillery position. There are several dents in the
armor, where it stopped pistol and rifle balls, and one large hole. Seems
a canon crew got off a lucky shot.
This article is Copyright 2002 Rodford Edmiston
Smith. Anyone wishing to reprint or repost it must obtain permission from
the author, who can be reached at: stickmaker@usa.net