stickmaker | JOHT 10B: Gimmie Shelter

The Joy of High Tech

Rodford Edmiston

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.

Material World: Gimmie Shelter

The protective value of armor is closely related to its strength. However, there are other factors involved. For instance, rigidity is also important. It has been said and printed multiple times that soft armors operate differently from hard. This is only true at the low end of the scale. While soft armor will be deformed by even a minor impact, hard armor will bounce a low-energy projectile away, while itself remaining essentially unchanged. However, even "rigid" armor will deform near the upper limit of its protective range. Hit steel hard enough and it will flow like putty. This is plastic deformation, where a material undergoes a permanent change in shape.

Density also plays a role. A thick sheet of lead will stop a projectile, simply because there is not have enough energy in the impactor to push much of the mass of the lead out of the way. However, except for fixed fortifications you don't want a lot of mass, so for the purposes of this discussion armor which depends primarily on the mass effect will be ignored.

Further simplifying the matter, this article will begin by discussing single-layer, uniform armor. Even this isn't really simple enough; 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, at or near the center of the target. Projectiles are uniform, solid and of an ogival shape. There is no backing to the plates, and they are large enough that edge effects can be ignored. 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.

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-125)). 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; in large part because steel tends to be more brittle.) 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.

Additionally, it is easier and probably cheaper to get uniformly good RHA than wrought iron. 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, 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.

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.

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 by the sort of projectile it is designed to stop, 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 a heavy tank. A tank was parked on a firing range and hosed thoroughly. All pieces of external equipment (including the weapons and tracks) 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. 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 and/or return fire. Since most of the outside of a tank is heavily armored, the likelihood of disabling one with a Vulcan before it got away or started shooting back is low.

Another example comes from Naval warfare. After the Iowa-class battleships were most recently recommissioned, a joke started circulating. "What does the captain of an Iowa 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.

Designing a homogenous armor strong enough to do the job 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 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 European knight's armor had layers of other stuff under the solid sheets of shaped steel. If nothing else, padding was essential, even under solid armor. 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 better 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. Composites generally have an added advantage of retaining a significant portion of their strength after failure. While fiberglass, graphite and other typical composite materials have been successfully used as armor, so have steel and ceramics. Indeed, the famed Chobham armors - such as the type used in the M1 Abrams Main Battle Tank - are composites.

Soft armors - at least, those woven from fibers - are also composites, at least in a sense. They combine multiple layers of the same material, and sometimes more than one material. In a sense, they are a composite with space as an ingredient, since several thin layers are more effective than a single layer of the same total thickness. (There have been metal spaced armors, too, used to defeat shaped charges.) Any strong, flexible, moderately elastic fiber would probably make a good soft armor.

Silk has been used for centuries for 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 wrought iron (60,000 N/cm^2) while being much lighter (density of 1.3 grams per cubic centimeter as opposed to 7.15). (Note that these values are typical of silkworm silk; other silks will vary in strength, elasticity and density.) It is also a good electrical and thermal insulator. It absorbs very little water, about 11% by weight. Small wonder that thick layers of silk cloth have been used for thousands of years as 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. 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.

As a final comment, note that nothing is absolutely bullet-proof, because someone always has a bigger bullet. One of my reference books has a photo of an armored vest worn by a Union officer who charged a Confederate artillery position. There are several dents in the armor, where it stopped rifle balls, and one large hole. Seems one cannon crew got off a lucky shot.