Fiber composite armor I: Fiberglass
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.
Both E-Glass and S-Glass are typically used as woven fiber sheets held in place by rigid resins. The resin is typically epoxy. Armor systems are typically comprised of >20 layers of woven sheets.
Applications in armor systems:
Low-cost backing layer for ceramic armor plates.
Spall liner in combat vehicles, crew side.
Military flak jackets. (Obsolete.)
E-glass fiber (alumino-borosilicate, Doron)
Ultimate Tensile Strength: 3450 MPa
Modulus of elasticity: 72.4GPa
Elongation at break: 4.8%
S-glass fiber (magnesium aluminosilicate)
Ultimate Tensile Strength: 4750 MPa
Modulus of elasticity: 90GPa
Elongation at break: 5.7%
As described previously, glass fibers were recognized as a very strong material almost immediately after they were produced. Research then began on its utilization and commercialization as a composite or reinforcing material.
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.”
Those 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 for better feedback from the troops it was issued to.
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, particularly when in or moving to prone position.
Doron-style fiberglass was also used in the hard body armor plates of the time, as a backing material for alumina, silicon carbide, or boron carbide ceramic tiles. Fiberglass-backed plates, which also utilize those same ceramic materials, are still in-use and in-production today, though they have largely been superseded by UHMWPE fiber composites in this role, as in so many others.