Why fibers and fiber composites are strong, and why bulk materials usually aren’t.
One may wonder why soft armor materials — and other materials for defense and aerospace applications that exhibit a very high strength-to-weight ratio — all happen to be fiber composites. Why aren’t they bulk materials? Why is graphitic fiber so much stronger than graphite, why is fiberglass so much stronger and tougher than bulk glass, and why is it that basalt fibers are now heralded as a new-age superfiber, whereas basalt itself is almost completely useless? The below account of the origins of fiberglass may prove both interesting and enlightening. It is taken from the excellent “The New Science of Strong Materials,” by James Edward Gordon, one of the founders of materials science as a science:
New Ceramic Armor Materials – From Boron Suboxide to Diamond
At the present time, the only commonly-encountered ceramic armor materials are alumina, boron carbide, and silicon carbide. All of these materials were introduced in the 1960s and saw service in Vietnam-era armor plates. They haven’t changed since. The current state of the art — the monolithic, multi-curved boron carbide armor ceramic tile — was developed by scientists at the US Army Natick Research and Development Laboratories and Picatinny Arsenal in 1965. If there has been any innovation in the many years since, it has been to manufacturing processes; these innovations have brought boron carbide tile production costs down and have facilitated high-volume production, but, pointedly, they have not improved performance. (And, indeed, in many cases these innovations have actually reduced performance — e.g. in reaction-bonded boron carbide — RBB4C — that is softer, denser, and ultimately just less protective than the 1965 product.)
On Graphene Armor
A lot of ink has been spilled on the subject of graphene armor. For example:
Such articles show little understanding of the facts. A deeper reading should indicate that graphene will probably never make for an armor material.
Soft Body Armor Statistics — Level IIIa vs. Level II, Body Armor Cuts, and Rifle Plate Selection
The FBI keeps statistics on police officers murdered or assaulted in the line of duty, and they’ve recently released a number of reports which cover the past ten years, from the beginning of 2008, through the end of 2017.
The Ballistic Event
The Ballistic Event
When an hard projectile impacts a ceramic-faced armor plate at 400-2000 meters per second, there two factors that need to be taken into consideration: The behavior of the penetrator, and the behavior of the ceramic armor plate in a local zone.
The first phase of penetration: Dwell.
(When an AP bullet with a hard core impacts a ceramic armor target, it is stopped for a moment, on the order of microseconds. This is called “dwell.”)
– The Projectile: The nose of the AP core starts to break or erode as it comes into contact with the ceramic plate, which is always harder than the projectile, and which has a very high compressive strength. The penetrator, momentarily arrested, begins lose velocity. Temperatures begin to rise steeply in the area of impact. Thus the kinetic energy of the penetrator is reduced due to the mass loss of its tip, the deceleration of its back part, and the conversion of some of that kinetic energy to heat.
– The Armor: Fractures immediately begin appearing in the ceramic plate. They form in a characteristic inverted cone pattern, which has aptly been named the “fracture conoid.” This conoid typically has a base diameter of approximately two to three projectile diameters from the center of impact, and a semi-angle of 60-70°. (68° is often used for modeling purposes.)
Amorphous metals and bulk metallic glasses for armor applications
Steel has many merits as a body armor material, foremost of which are its durability and its ability to be used in very thin sections. Sadly, we’re all familiar with its drawbacks, too — its high density and its near-total uselessness against armor piercing rounds — and not just those made with tungsten carbide cores, but also those rounds with cores of hardened tool steel. But what if steel’s merits could be enhanced, and its drawbacks reduced or eliminated? Recent advances in amorphous metal alloys, also known as bulk metallic glasses, might be just the thing.
Armor: Future – Boron Nitride Nanotubes
Boron Nitride Nanotubes for Armor Composites
Carbon nanotubes have been of broad engineering interest for 25 years. Practical applications, however, are still very few and far between — due primarily to the fact that CNTs like to form aggregates and clumps, which can dramatically impair the mechanical properties of composite materials. It can be extremely difficult to disentangle the nanotubes, and homogenize them within a matrix or spin them into a yarn, without damaging them. Where their incorporation into armor materials is concerned, there’s another problem: Carbon nanotubes are not oxidatively stable, they’re not chemically stable when used alongside certain elements such as iron, and they’re not thermally stable — in fact, they begin to degrade at just 400°C. For these reasons, it is very difficult, if not impossible, to meaningfully incorporate carbon nanotubes into ceramic and metal matrices. Even if chemical compatibility is assured, processing temperatures are always just too high.
It’s said that the development of ceramic armor can be traced back to 1918, to the final days of WWI, when British Army Major Neville Monroe Hopkins discovered that a plate of rolled homogeneous armor steel was much more resistant to penetration if covered with a thin layer of hard enamel. Over twenty-five years later, as Doron was being studied by the Monsanto Chemical Company in the final days of World War II, it was discovered that a solid plate of glass in front of a hard Doron plate was more effective than an equal weight of Doron plate alone. But the first truly modern ceramic armor system dates back to early 1963, and a patent filed by Richard Cook on behalf of the Goodyear Aerospace Corporation. This patent, entitled “Hard faced ceramic and plastic armor,” describes a piece of armor of essentially modern construction: A plate of alumina, or a mosaic of alumina tiles, over a backing of Doron fiberglass.
The first titanium alloys — including the famous Ti6Al4V, which now accounts for more than 50% of total titanium production — were developed in the USA in the late 40s. Shortly after their development, an assessment from Pitler and Hurlich noted that these new alloys showed promise at defeating small arms projectiles. Despite numerous subsequent investigations, experiments, and studies over the 1950s and 60s — which included the development of extremely hard (62HRc) alloys — titanium body armor was never issued to US or NATO soldiers. There is just one exception: River boat crews in Vietnam were issued a light titanium-nylon flak jacket, which was not intended to stop high-velocity projectiles; it was merely a lightweight analog of WWII’s steel-nylon aircrew flak jacket.
Over the ten-year period from roughly 1996-2006, titanium enjoyed a small resurgence in the US as an armor material: Several private US companies examined monolithic titanium body armor plates, the Army Research Labs investigated hot pressing titanium metal powders in 2005, and the now-defunct DragonSkin’s namesake armor vest made use of titanium-ceramic composite armor tiles. Ultimately, these efforts did not meet with much success.
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: