Polymer composite armor
The first truly modern woven armor material — a laminate comprised of layers of woven glass fabric, impregnated with an ethyl cellulose resin, and bonded under high pressure — was invented by the Dow Chemical Company in May 1943. Initial reports were promising, and the Military Planning Division, Office of the Quartermaster General, under the command of Colonel Georges Doriot, launched the “Doron project” to find suitable applications for this new material. (“Dor” from the Colonel’s name, +”on”, from the generic ending for fibers such as rayon, nylon, cotton, etc.)
Doron was first used in battle in August 1944. The T37 flak vest for airmen was a straightforward design where the steel plates of the ubiquitous M1 flak jackets were replaced by flat hot-pressed Doron plates approximately 50mm square and 3.3mm thick. Later modifications utilized thicker, curved Doron plates. The T37 and its derivatives remained experimental, however; they were not adopted by the Air Force, as aluminum and nylon-based solutions were deemed more suitable for use at that time. The Army, following the Air Force’s lead, did not engage in further experiments with Doron over the last days of WWII.
Which is not to say that nobody was interested. The US Navy remained keenly interested in Doron, and pioneered its research and development in armor systems.
Two Naval officers, Lt. Commander Edward Corey and Lt. Commander Andrew Paul Webster, were the first to test the personal protective ability of Doron — and they did this in an unusual and brave, if not foolhardy, way: Lt.Cdr Corey wore Doron panels, with various backing materials, over his arms and held them in his hands, as Lt.Cdr Webster fired at him with a .45 pistol. A short account of their tests follows:
“Dr Corey, with a great deal of physical courage, volunteered to be the subject of this test. The strain of this first test is understood when one considers that we did not know whether his arm would be torn off by the blow from the bullet or whether I would even hit the armor on his hand. The shots proceeded as follows. When Doron was backed with [a 1 inch layer of] sponge rubber or a heavy layer of kapok, the impact of the bullet on the hand resulted in no discomfort, and even with a few thicknesses of duck cloth no injury resulted. The bullets were literally picked out of the air and caught as if catching marbles flipped at the hand. When the backing was reduced to a single thickness of duck cloth, severe bruising resulted, with hematoma, pain and edema, but without fracture. Deep sensation did not return in the hand for a period of about six weeks. It was concluded from these experiments that since our problem of armouring ground troops and shipboard personnel was one involving maximum protection and comfort for minimum weight, no backing should be used.”
The folks who today obsess over BFD and behind-armor trauma should read that last part again.
The Navy subsequently utilized Doron in two forms: By sewing Doron panels into pockets on the outside of the Kapok lifejacket, and by sewing Doron panels into the standard-issue Marine Corps utility jacket. Roughly a thousand such jackets were prepared for use in the Battle of Okinawa, but, unfortunately for the development of body armor if not for the men involved, the Marine division outfitted with the armored jackets was not involved in the fighting.
Doron, ballistic nylon, and doron-nylon vests were again tested during the Korean War. For the first time in modern history, these body armor experiments were judged a complete operational success. The Doron vests were popular with Army troops, commanders, and battlefield physicians alike, to the extent that the body armor evaluation team wrote in its final report:
“The effect of body armor on confidence is probably best expressed in the results of the post-use interviews where over 85 percent of the men stated that they felt safer and more confident when wearing body armor.”
“Interviews with commanders, who have led troops wearing body armor in combat, have repeatedly emphasized that aggressiveness is increased and that there is more of a desire and willingness to engage the enemy at close quarters.”
“A poll of over 100 front line physicians and surgeons has resulted in the almost unanimous expression of opinion that the use of body armor would result in an increase in morale among combat troops.”
“Under certain conditions the effect of body armor on morale may not be good. For example, during the last month of the test period there were several instances where soldiers who had previously used body armor expressed a reluctance to their unit leaders to go out on patrols when body armor was not available.”
More than anything prior, this glowing evaluation of Doron and nylon armor in Korea cemented body armor’s place in the modern soldier’s kit.
Given the Navy’s interest in Doron vests during WWII, it should come as no surprise that the Marine Corps were early adopters of body armor – and, throughout the course of the Korean war, all Marine Corps frontline personnel were armored in Doron-Nylon flak vests. The Army, at the same time, was still evaluating Doron and nylon vests on a very limited trial basis. As positive feedback from these trials started to come in, and as positive reports from the Marines reached Army HQ, the Army decided that they needed to hurry up and issue body armor to all of their frontline personnel as well, so they requested 13,020 Marine vests on August 11, 1952. By the end of the war, the Army had procured well over 25,000 Marine Corps vests. It should go without saying, therefore, that the Marine vest, the M-1951, was the Korean War’s predominant form of body armor. It weighed 7.75 pounds and was a sleeveless vest with a zipper closure in front. It was made of 1050 denier ballistic nylon fabric and hard Doron fiberglass plates. The shoulder girdle area was protected by 12 layers of ballistic nylon, which was flexible and reportedly comfortable to wear; the chest, abdomen, and back were covered in sixteen curved, 3.175mm thick, overlapping Doron plates.
The Army, following further evaluations, ultimately decided to adopt a different design. The Army personnel assigned to the development of armor projects believed that ballistic nylon was just as effective as Doron, but easier to wear on account of its flexibility and ability to drape. Thus the all-nylon M-1952 was issued to troops over the last months of the war, starting in February 1953. Like the M-1951, it was a sleeveless vest with a zipper in front. It had two front and back panels composed of ballistic nylon bonded with laminating resin, but this was still reasonably flexible, and the vest was reportedly comfortable to wear. It weighed 8.25 pounds, so was slightly heavier than the M-1951, but its flexibility made up the difference.
The M-1951 and M-1952 remained essentially unchanged into, and over the entire course of, the Vietnam war. There were minor, mostly cosmetic, upgrades — but the weights and materials did not differ to any meaningful extent. The vest of the Marine Corps in Vietnam was designated the M-1955, which was, like the M-1951 it replaced, made of ballistic nylon with Doron plates. The Army vest, the M-1969, was of all-nylon construction. It was identical to the M-1952, save for the addition of a high collar, which added roughly half a pound to the vest’s weight, and which soldiers generally disliked, as it interfered with the wearing of the M-1 helmet.
Enter Kevlar. Just as the Vietnam War was ramping up, in 1965, DuPont scientist Stephanie Kwolek was working on a research project tasked to find high-tensile-strength fibers for use in reinforcing automobile tires. To this end she was experimenting with the stiff polymer precursors poly-p-phenylene terephthalate and polybenzamide, when she noticed that the reaction product of the two precursors displayed some unusual properties. As she later explained:
“The solution was unusually (low viscosity), turbid, stir-opalescent and buttermilk in appearance. Conventional polymer solutions are usually clear or translucent and have the viscosity of molasses, more or less. The solution that I prepared looked like a dispersion but was totally filterable through a fine pore filter. This was a liquid crystalline solution, but I did not know it at the time.”
Unusual-looking chemical solutions of that sort were not often pursued; they were usually considered failed or abortive experiments. Kwolek went on to mention that “I think someone who wasn’t thinking very much or just wasn’t aware or took less interest in it, would have thrown it out.” But she persisted, so the solution was spun on a spinneret, and the mechanical properties of the resulting fibers were tested. It quickly became apparent that this new “aramid” fiber was three times as strong as nylon and possessed a far higher modulus and heat resistance. And, despite its high modulus, even these first fibers were so fine that woven fabrics derived from them were flexible, and were able to drape like nylon.
The military quickly realized the potential of this new material, though they didn’t rush to implement it in armor systems and issue it to troops in Vietnam. Military-grade aramid fabrics were first designated as PRD-29, and PRD-49 — and later as Kevlar® 29, which was previously PRD 49-IV, and Kevlar® 49, which was PRD 49-III. Kevlar 29 was judged the superior grade for ballistic protection, and prototype flak vests were made which offered significantly greater protective ability than those made of nylon and Doron, at the same weight.
In June 1978, vests comprised of 13 layers of Kevlar 29 ballistic fabric within an envelope of water-resistant ballistic nylon were issued as the “Body Armor Fragmentation Protective Vest, Ground Troops” for United States soldiers. In the early 80s, this type of vest was standardized as part of the PASGT armor system, which also included a helmet made of resin-bonded laminate Kevlar 29. The PASGT system was quite long-lived — it remained in use by the US military until the mid-2000s, and still remains extremely common worldwide, as it was extensively exported. The PASGT’s successor, the Interceptor Body Armor system, was also made of aramid — in this case, the improved KM2 fiber. The most recently fielded body armor system, the Improved Outer Tactical Vests, which are a partial replacement for the Interceptor, still incorporate aramid fibers in their soft armor component.
Yet this is not to say that aramid is the only option, nor should it even imply that aramid is the best option.
Scientists at DSM in the Netherlands discovered ultra-high molecular weight polyethylene (UHMWPE) fibers by accident in 1963, during experiments in fractionating out different polyethylene lengths from a solution. It was immediately noted that these very long-chained polyethylene molecules were incredibly strong, but, initially, only small amounts were produced, and it seemed impossible to produce meaningful quantities. This is on account of the fact that the UHMWPE fibers were nearly impossible to orient in solution, so they’d typically form intractable aggregates and clumps, and only a small fraction of each production batch would result in usable UHMWPE fibers.
(Spaghetti is the usual analogy. On a molecular level, strong UHMWPE fibers resemble bundles of stiff, uncooked spaghetti. UHMWPE in solution resembles tangled clumps of soft, overcooked spaghetti. The art of drawing strong Dyneema fibers — and it is an art — resembles turning cooked and tangled spaghetti into uncooked spaghetti!)
In 1978, 15 years after the original discovery, DSM engineers discovered the ingenious gel-spinning method, which enabled the production of commercial quantities. Dr. Piet Lemstra, who worked on the project, described as follows:
“It’s actually very simple. The more you dissolve the long chain molecules in a solvent, the more they become separated. Then you cool it down to a gel state and the molecules are more or less disentangled, so it is easy to stretch them. You can then remove the solvent, or later, during the stretching/drawing process – either way you end up with nearly perfectly aligned molecules.”
Although the development of this clever gel-spinning process enabled the industrial production of UHMWPE fibers, DSM was not an industrial fiber company. It was — and to a large extent remains — a food and agricultural supply company that discovered UHMWPE fiber by accident. The UHMWPE team at DSM was essentially a “skunkworks” crew that did their experiments in spare time and on weekends, with little corporate oversight. In the years since its discovery and the development of a commercial production process, the decision-makers at DSM showed no interest in commercializing UHMWPE fiber, and this was not helped by the fact that the consulting companies they asked to evaluate their potential product did not think very highly of it.
“This [consulting] company said the fiber was just candlewax unable to resist high temperatures and it lacked any good properties. They saw no future in it,” says Lemstra. “And the management at DSM believed them.”
Around that time, in the very early 80s, Allied Signal (now Honeywell) obtained a patent on a very similar UHMWPE fiber product, though they claimed that the melting point of their material was 20-30 degrees higher than that of DSM’s fiber. As it turned out, the two UHMWPE fibers were largely identical, the melting point differences were due to an methodological error on Allied Signal’s part, and, as the DSM patent had priority, Allied was compelled to purchase a license for the original DSM patent.
With the license in hand, and no potential intellectual property conflicts on the horizon, Allied started marketing their own fiber, Spectra, as a competitor to Dupont’s Kevlar. Dupont countered with effective marketing campaigns which showed how Spectra would weaken at moderately high temperatures — from 60-80°C. The military, aided by institutional inertia, essentially said “if it ain’t broke, don’t fix it,” and stuck with aramid armors.
It could be argued, in hindsight, that Allied and DSM didn’t do enough to promote their material. Yet, at the same time, the first grades of UHMWPE fiber were quite poor. We have seen continual improvement since those early days, and today’s grades of UHMWPE fiber are more than 44% superior to the best grades of Kevlar on an equal-weight basis.
Dyneema and Spectra are presently used in high-end body armors, the very newest models of military helmet, and lightweight ballistic shields. Unlike aramid, moderately thick panels of UHMWPE (2cm+) are capable of stopping lead-cored rifle rounds at high velocity, but invariably fail against steel-cored rifle rounds. In fact, they can fail against steel-core pistol rounds; that the SS190 AP 5.7x28mm pistol round can punch through a UHMWPE Level III plate has been documented.
In any case, there’s significant room for improvement in UHMWPE fiber materials, and it’s likely that further gains will be made within the next 10-15 years.
Mechanical properties of selected woven and polymer armor materials
E-glass fibres, manufactured from alumino-borosilicate, are the oldest armor fibers in modern use, as their use dates back to WWII’s Doron. This grade of fiberglass is infrequently used in modern body armor systems, but, on account of its very low cost, is often encountered as a spall mitigation liner in armored vehicles.
S-glass, of magnesium-aluminosilicate, is a superior grade of fiberglass with much-improved mechanical properties: lower density, higher strength, greater stiffness, and a 20% improved strain-to-failure. However, even the best-performing glass fibers perform only roughly half as well as aramid for ballistic applications.
Ultimate Tensile Strength: 3450 MPa
Modulus of elasticity: 72.4GPa
Elongation at break: 4.8%
Ultimate Tensile Strength: 4750 MPa
Modulus of elasticity: 90GPa
Elongation at break: 5.7%
Aramid is, without doubt, the most ubiquitous ballistic fiber in the world at the time of this writing. It is extensively used in standalone body armor systems, spall liners and backings, ballistic helmets and shields, and virtually every other conceivable application — including “armoring” fiber optic lines for extended stays in harsh environments, and armoring satellites from micrometeorite impact. It has excellent resistance to fire and heat, but is sensitive to UV light (photooxidative degradation) and humidity (hydrolysis), so it is typically sealed or coated prior to use, and its characteristic bright yellow color is infrequently seen.
Aramid – Kevlar 29
Ultimate Tensile Strength: 3620 MPa
Modulus of elasticity: 70.3 GPa
Elongation at break: 3.6%
Aramid – Kevlar KM2
Ultimate Tensile Strength: 3880 MPa
Modulus of elasticity: 84 GPa
Elongation at break: 4.52%
Ultrahigh Molecular Weight Polyethylene
Broadly speaking, this class of polymer composite offers the best performance-to-weight ratio. More specifically, there are a number of grades available, ranging from low-cost grades with a weight efficacy roughly equal to aramid, to very highly-optimized grades, such as Dyneema SB-117 and Spectra Shield 5143, that are much more effective than aramid on a per-weight basis.
UHMWPE is stable in the presence of UV light, but has a low melting-point that leads to performance degradation at high temperatures. This becomes a serious consideration at temperatures over 70°C.
UHMWPE – Spectra 1000
Ultimate Tensile Strength: 2570 MPa
Modulus of elasticity: 120 GPa
Elongation at break: 3.5%
Note: The antiballistic properties of Dyneema HB212 are significantly better than Spectra 1000, but its mechanical properties have not been made public in a comprehensive way.
Carbon fiber is a structural composite that’s rarely used as a ballistic material on account of its brittleness, relatively high density, and lack of ductility. It boasts an extremely high yield strength and flexural modulus, however, and its strength and stiffness make it attractive for certain niche applications in armor. It is also highly resistant to UV light, chemical exposure, and temperature extremes. The very best grades of carbon fiber — Toray’s T1100G (ultra-high strength) and M60J (ultra-high stiffness) — represent the current state of the art, and are uncommon, whereas the grades T300 and T800 represent the fibers most frequently used in aerospace and automotive applications. T300, in particular, dominates the carbon fiber industry — 95% of sales are of the “standard modulus” T300 grade.
Carbon Fiber – Toray T300 (Standard modulus)
Ultimate Tensile Strength: 3800 MPa
Modulus of elasticity: 230 GPa
Elongation at break: 1.5 – 1.76%
Carbon Fiber – Toray M60J (Ultra-high modulus)
Ultimate Tensile Strength: 3950 MPa
Modulus of elasticity: 588 GPa
Elongation at break: 0.7%