Armor

The steel helmet and the aramid/UHMWPE helmet are products of two entirely different engineering traditions.

There’s a science fiction story by Harry Turtledove called “The Road Not Taken,” which has two core plot devices:  (1) That faster-than-light and antigravity technologies are simple and obvious, but that humanity had somehow missed them — and, (2), that as soon as any nascent technological culture discovers those technologies, they obsess over them, and scientific progress in all other directions stagnates.  It’s a good story.  To excerpt a little bit from its sequel, which summarizes it:

“Back then, on Terra, they knew FTL travel was impossible forever. It was a rude shock when they found that a couple of simple experiments could have given them the key to contragrav and the hyperdrive three, four, even five centuries earlier.” [Note: In the 1500s.]

“How did they miss them?” Chang asked.

“No idea — in hindsight they’re obvious enough. What’s that race that flew bronze [faster-than-light] ships because they couldn’t smelt iron? And every species we know that reached what the old Terrans would have called a seventeenth-century technological level did what was needed — except us.”

“But trying to explain contragrav and the hyperdrive skews an unsophisticated, developing physics out of shape. With attention focused on them, too, work on other things, like electricity and atomics, never gets started. And those have much broader applications — the others are only really good for moving things from here to there in a hurry.”

With a chuckle, Chang said, “We must have seemed like angry gods when we finally got the hyperdrive and burst off Terra. Radar, radio, computers, fission and fusion — no wonder we spent the next two hundred years conquering.”

(From “Herbig-Haro,” sequel to “The Road Not Taken,” by Harry Turtledove.)

In Turtledove’s stories, those technological societies that have somehow missed the development of FTL are said to be traveling down “the road not taken.”  

Over the past 80 years, steel armor technology has been the road not taken. Hadfield’s manganese steel, though it turned out to be a real workhorse, is 140 years old.  The use of martensitic, high-hardness grades of steel in armor dates back to WWI.  The NeoSteel helmet is the only steel armor product that couldn’t have been issued in the 1940s.  

And yet, even neglected and ignored, steel is by no means inferior to the best modern materials which see use in helmets.  Consider:

– Steel helmets last longer than those made from polyethylene, and are more durable to field conditions.

– Behind-helmet trauma and TBI are less of a concern with steel, for it is incapable of significant plastic deformation.

– Steel helmets offer a superior practical area of coverage, for there are no edge effects with steel. The helmet’s ballistic material won’t roll if it’s hit near the rim.

– As steel helmets don’t deform much upon impact, they can be used with thinner pads — this decreases the profile of the helmet, reducing its silhouette, and thereby presents a smaller target to the enemy.  What’s more, a closer fit improves stability.

– Steel handgun armor is qualitatively different from rifle-rated steel body armor.  Given the lower velocity, different composition, and typically higher diameter of handgun projectiles, there are far fewer issues with respect to bullet fragmentation.

– The NeoSteel helmet offers ballistic performance that is comparable to, if not superior to, the best polyethylene-based helmets.

The polyethlene helmet has, admittedly, a marginally better ballistic-performance-to-weight ratio — but, in most if not all other respects, including blast and blunt impact performance, the modernized steel helmet is superior.  What’s more, there is still considerable room for improvement in steel alloy development and processing for helmet applications, whereas improvements in polyethylene materials are, at this point, likely to be much more modest and incremental.   

Truly, steel helmets and fiber-composite helmets are similar only insofar as they’re both helmets.  They differ markedly in most other respects, from how they are made, to their material characteristics, to how they respond to impact.  The modern steel helmet owes a great deal to Cold War-era steel helmets, and to more recent European and Russian advances in titanium helmets.  The composite helmet is the product of an entirely different technological lineage.  It owes its existence to the tremendous amounts of effort which the US and UK Militaries had spent on fiber-composite body armor systems, dating back to WWII — first with Doron, then with nylon and various nylon laminates, then with Kevlar, and now with UHMWPE.  

Technological products always have traceable lineages. [1]  The steel helmet is the product of one engineering tradition, and the composite helmet is the product of another.

There’s an evolutionary perspective to this.  In nature, there are two closely-related notions: Convergent evolution and recurrent evolution.  In the former, traits that are adaptive from an evolutionary perspective will arise in organisms that aren’t necessarily related.  In the latter, traits that are adaptive will evolve repeatedly over time.  The eye — a light-sensing organ — seems to have evolved independently more than 50 times.  The wing, likewise.  These are evolutionarily favored forms; they are broadly adaptive.  It’s extremely plausible that they will arise wherever in the universe there is multicellular life.

Armor itself is an excellent example of a favored and broadly adaptive trait.  In fact, it is the ur-adaptive trait.  The earliest forms of multicellular life were armored — instead of excreting unwanted trace elements and metabolic byproducts as waste, they secreted them on the outside of their bodies, and let predators break their teeth and blunt their claws on these new hard shells. And our distant ancestors certainly needed strong shells to survive the Cambrian — that they might be ignored by the mighty anomalocaris in favor of flimsier prey.

Since the Cambrian, armor has become a fairly generic feature of life on Earth — but it has had ample time to diverge substantially, and it has taken many different paths.  Clearly, the mollusc, the beetle, and the fish have all taken very obviously different approaches.  

As in nature, with the profusion of armor types in the natural world, the helmet is as old as human civilization, and there is tremendous divergence in how helmets can be made.  Our ancestors armored themselves with metals, with fibers, with animal skins, with stone, with wood, with tree barks and resin, or with “composites” derived from the combination of these materials.  

Today’s options are narrower, consisting solely of metals, fiber composites, and plastics. All metal helmets are broadly similar in how they’re made and used, and in how they respond to ballistic impacts.  All fiber composite helmets, likewise — whether they’re made of glass fibers, aramid fibers, polyethylene fibers, ceramic fibers, or something else.  

Which brings me to a point I had made previously, and which should be re-stated:  Since the 1950s, the US military has heavily prioritized research and development in fiber composite helmets, and metal helmets have not received the same attention.  The fiber composite helmet is a product with a traceable lineage that dates back 80 years.  The steel helmet, neglected for 80 years, has no such lineage, and steel helmet development has become the path less traveled…  But we’re walking that path, and we’re confident that steel helmets will soon exceed fiber-composite helmets in every respect.  In most respects, they’re already there.

[1] – The more complex the product, the more complex the lineage tends to be; your cell-phone is the product of numerous advances in transistor and network technologies that were developed at Bell Labs decades ago, but it also owes much to the development of the OLED, to software that was developed by Nokia in the 90s, and to functional ergonomic and form-factor studies carried out by Apple.  A deep and complicated lineage, that — and one with many great names involved: John Bardeen, Claude Shannon, Steve Jobs.  The German stahlhelm, in contrast, was simply a sallet or salade — made with industrial tools, along industrial lines, and standardized.

Ask an expert why M193 at full velocity will reliably perforate a steel armor plate, and you’ll hear “speed kills” or a variation of that refrain.  In other words, it’s commonly assumed that M193 penetrates better simply because it is faster.

This is untrue. 

In fact, it can’t possibly be true. For one example, we know of a fairly standard 6.5mm-thick steel armor plate that has a V50 against the M855 of 3300 fps, but which has a V50 of only around 3000 fps against the M193.  What this means is that M193 at 3100 fps will more reliably penetrate that steel armor plate than M855 at 3300 fps.  Even if the steel-cored “penetrator” “semi-AP” round is traveling 200 fps faster than the old lead-cored M193, it is the latter which is more likely to penetrate that steel armor plate. 

The table below is from Deutsche Edelstahlwerke (DEW), a supplier of steel armor plates to companies that build high-end armored cars.  DEW claims that a 5.9mm thickness of their very good steel is sufficient to reliably stop M855 at 3116 fps, yet they note that an 8.5mm thickness is required to reliably stop M193 at 3215 fps.  That’s roughly 45% more thickness, to stop a lighter round, with less energy, moving just 100fps faster.  At first glance, this makes no sense.



What’s clear is that it’s not velocity, it’s certainly not total kinetic energy, and that it can’t possibly be either or both of those things — there must be a qualitative difference in how the M193 and M855 interact with steel targets.  And indeed there is.

Take a look at the image below.  (A) is what happens when M855 hits a steel plate at muzzle velocity. 


The M855’s steel “penetrator” is blunted when it comes into contact with the steel armor plate.  It is slowed and momentarily stopped. At the same time, the lead plug behind the penetrator hasn’t slowed down.  It flows around the penetrator, and interacts with a large volume of the steel armor plate’s surface — as large as three projectile diameters.  As the projectile’s kinetic energy is thus spread out, shear failure in the steel plate is averted, and the projectile is stopped.

(B) is M193. 

When M193 impacts a steel plate at high velocity, the entire projectile can be modeled like a high-density fluid.  In some respects it behaves like a high-pressure water-jet.  All of the projectile’s kinetic energy is dumped onto a relatively small surface area.  This typically results in plugging shear failure, which is why the hole in the steel plate will always be slightly larger than the projectile itself. 

Some lead and jacket material will splash up and out — at a roughly 45° angle at first, and then following the rim of the crater formed upon impact.  This ablated material doesn’t interact very much with the steel armor target, has rather insubstantial mass, and typically doesn’t much reduce the kinetic energy load dumped upon the site of impact.  But if the steel plate is strong or thick enough to defeat M193, you’re going to get a very small crater and a lot of lead splash.

This was all discovered largely by modeling in LS-DYNA and similar ballistic simulation programs, but it has also been seen experimentally.

So there you have it.  M855 under-performs against steel plates because the projectile’s steel “penetrator” is decelerated upon contact, forcing the lead plug behind the “penetrator” to flow around it — increasing its contact area on the steel plate surface, and spreading out the projectile’s kinetic energy load.  M193, which is of monolithic all-lead core composition, doesn’t have that problem, and performs substantially better.  It stands to reason that an all-steel or all-copper projectile would also perform better than the M855.  Well, needless to say, there are implications here for projectile design as well as armor design. 

Of course, we’re talking muzzle velocity.  At 300-500 yards out, M855 retains more armor penetration capability.  Famously, it was designed to penetrate thin steel helmets from long distances — which is something that the M193 had problems with.

And all of this is not to say that there are no steel plates that’ll stop M193.  RMA just released a steel armor plate that’s rated to M193 at up to 3250 fps.

Although the first steel helmet is lost to history, it was the Romans who were first to make wide use of steel helmets in battle, and they were certainly the first to make those steel helmets iconic.  Roman helmets, which were used alongside bronze helmets of similar design, were of sophisticated metallurgical composition — their helmets were generally either carburized or made of a reasonably high-quality medium-carbon steel alloy, though not necessarily quenched.  

But to write the history of the steel helmet itself from the Roman era would fill an entire book.  The history of the ballistic steel helmet is a much shorter story.  

We can begin by simply noting that the plate armor and helmets of the 14th-16th centuries were increasingly designed to withstand ballistic impacts, and, on that account, grew thick and heavy over time.  Some of the helmets of that era weighed more than twenty pounds — and, although that’s an admittedly extreme example, and helmets that heavy were very rare, helmets of more than five pounds were downright common. 

Not surprisingly, steel body armor was also growing very thick and very heavy.  In 1587, French military theorist and famed essayist Michel de Montaigne noted that “today the officer is so heavily armed that by the time he becomes thirty-five his shoulders are completely humpbacked.”  In a separate account of roughly the same period, Montaigne noted that “Alexander, the most adventurous captain that ever was, very seldom wore armour, and such amongst us as slight it, do not by that much harm to the main concern; for if we see some killed for want of it, there are few less whom the lumber of arms helps to destroy, either by being overburdened, crushed, and cramped with their weight, by a rude shock, or otherwise. For, in plain truth, to observe the weight and thickness of the armour we have now in use, it seems as if we only sought to defend ourselves, and are rather loaded than secured by it.”  

Soldier and chronicler Pierre de Brantôme, a contemporary of Montaigne, likewise declared that men who wear heavy armor are “spent at thirty years.”  

François de la Noue, a Huguenot captain known as “Iron Arm,” wrote much the same thing in Discours Politiques et Militaires, also published 1587: “Our men of arms in the time of King Henry made a far fairer show … their armor wasn’t very heavy, so that they were well capable of wearing it for 24 hours.  Those armors that are worn now are so weighty, that they will deaden a 35-year old Gentleman’s shoulders.  I myself have seen the late Lord of Eguillie, and the knight of Puigreffier, honorable old men both, remain fully armed for a whole day, marching in the face of their companies — whereas now a young captain can hardly bear to remain fully armed and armored for two hours.”

If armor was then falling out of fashion in France and elsewhere on the continent, things were much the same in England, and at much the same time.  The English poet and adventurer Sir Philip Sidney was shot in the thigh at the Battle of Zutphen in 1584, and died of an infection caused by that wound.  As Sir John Smythe describes it, Sidney’s death was because he wasn’t wearing all of his armor; just before the battle, he had noticed that one of his men was not fully armored, so he removed some of his own armor on the grounds that it would be dishonorable to be better armored than his troops.  

Sidney’s death set off a heated controversy between English military experts of the era.  De la Noue’s Discours was published in London, in English, in late 1587.  In 1590, Smythe, a cousin of Edward VI and a lifelong soldier-of-fortune, wrote a small book on military matters titled Certain Discourses, wherein he came to the conclusion that armor is effective against every threat, except for the increasingly popular musket — “the blows of the bullets of which no armours wearable can resist.”  He disparaged the musket and the arquebus for many other reasons — noting, for instance, that they are slower to fire than the traditional English longbow, less accurate, and can be difficult to use in poor weather — but was unequivocal about their ability to pierce armor. 

Sir Roger Williams and Humphrey Barwick both released books on the same topic later that same year, 1590.  Williams, who like Smythe had seen a great deal of battle on the continent, disagreed with Smythe on many points — but not on the musket, “the best small shot that ever was invented.”  Barwick, a war-hardened mercenary and adventurer like the others, was analytical about it, noting that “The musketes are weapons of great force, and at this day, bothe with leaders and followers, much feared: for fewe or no Armours, will or can defend the force thereof, being neerehand, which is as well a terror to the best armed, as to the meanest: it will kill the armed of proofe at ten score yardes, the common armours at twenty score, and the unarmed at thirty score, being well used in bullet and tried powder.”  

In other words, Barwick claimed that the musket can kill men wearing armor of “proof” — the heavy stuff which, generally, was built to stop firearm rounds — within 200 yards; that it can kill men wearing “common armor” — mass-produced iron armor, called munitions-grade, generally much thinner and of poorer construction than armor of proof — at 400 yards; that it can kill unarmored men at 600 yards.  No surprise, then, that the musket was much feared, by armored and unarmored men alike. 

Although they disagreed on many important points, all of the aforementioned experts bemoaned the fact that the armor and helmets of their era had become very thick, very heavy, and that its aesthetic qualities and its functionality both suffered for it.  Plate armor had reached its zenith — in functionality as well as in artistry — right around the year 1500, but it was evidently falling out of favor by 1580, thanks in large part to the development and increased utilization of the musket.

And indeed, just a few decades after this public debate raged across Europe, the Swedish attained great military victories in the Thirty Years’ War with very lightly armored and highly mobile musketeers and artillery.  Although they didn’t invent the musket, they perfected its use — and they can stake a good claim to having invented mobile artillery with the famous leather cannon and the subsequent development of the light 3-pound gun, which could be pulled by two horses and operated by two men.  As Gustav’s adversary Montecuccoli noted, and later emulated in battle, “one is appointed to load and fire, and the other conducts it by the end of the carriage, and brings it forward at the march tempo of the infantry.”

With respect to armor: Gustav II Adolf personally stopped wearing armor following a bullet wound to the shoulder sustained at the Battle of Dirschau in 1627.  Ever since, he entered battle without armor — even to much larger battles than Dirschau, and to much greater victories.  Gustav’s officers were naturally inclined to follow suit — either because they wanted to emulate their leader, or because they felt that it would be shameful to be better-armored than Gustav himself, or merely to lighten their burden.   

In the ranks of Gustav’s infantry, pikemen were issued light steel helmets and breastplates, but his musketeers and artillerymen — who outnumbered pikemen by at least two to one — were never issued body armor, and rarely wore helmets. 

The cavalry situation was much the same.  The heavy cavalry, the Cuirassiers, were issued very light armor, if any at all, which would at most consist of a helmet and pistol-proof breastplate.  They frequently wore no more armor than a buffcoat — a heavy leather jacket, as much as 5mm (~0.2”) thick, that offered limited protection against slashing attacks, but none at all against firearms or pike-thrusts.  Gustav’s Dragoons — who were, practically speaking, a mounted infantry force — were not issued armor at all, and were armed in the manner of infantry musketeers. “They took us for musketeers, seeing that no animal in the world is more like a musketeer than is a dragoon, and if a dragoon falls from his horse, he rises up a musketeer.”

Gustav II Adolf’s lightly-armored force won key victories with speed and concentrated firepower, and battles like Breitenfeld (1631) marked a turning point in the history of warfare: For the first time, overwhelming firepower, in the form of muskets and light artillery, were utilized as the primary and decisive weapons of war. This presaged the era of maneuver warfare. Also, because it ushered in the demise of expensive steel armor, and because musketeers and gunners were fairly easy to recruit and train, these innovations also enabled, by 1670, a permanent increase in the size of standing armies.  For these reasons, among others beyond the scope of this post, Gustav was rightly called “the father of modern warfare” by 19th and 20th century historians. 

Thus by the late 16th century, soldiers were discarding their heavy “proof” armor and war-hardened military experts were publicly deriding it.  By the mid 17th, the musket and light cannon had attained battlefield supremacy, and breastplates and helmets, once considered essential, were no longer issued to most troops.  Musketeers no longer wore helmets; they wore hats, often with very broad brims, as depicted more or less correctly in most of the artwork inspired by Dumas’ Three Musketeers.  Of course, this doesn’t mean that armor vanished, for there were more than a few holdouts — particularly in the heavy cuirassiers, who fought primarily with sword and pistol into the 19th century.  Ersatz armor was also used for decorative, morale, or aesthetic reasons.  Most famously, the spiked German pickelhaube, which was made of leather with metal fittings.  More to the point, however, are the iron-tin breastplates and helmets of the later cuirassiers and such units as the Jager-zu-Pferde, but these gleaming accoutrements were very thin, and softer than they looked; like the Pickelhaube, they offered very little protection.

So it was that at the outset of World War One, soldiers were issued no protective armor of any kind.  In a perhaps apocryphal story from early in the war, it is said that Intendant-General August-Louis Adrian of the French army was speaking to a wounded soldier, who told him that an inverted mess bowl worn under his cap saved him from a gravely serious head wound.  General Adrian is said to have realized that protective headgear might save the lives of French soldiers, and experimented with a metal skull-cap (calotte métallique, cervelière) to be worn under the standard-issue military hat (kepi). This skull-cap was 0.5mm thick, made of mild steel, and offered limited protection against fragments, shrapnel, and blunt impact — yet its use led to a measurable decrease in the number of head-wound casualties.  Between December 1914 and February 1915, 700,000 of these skull-caps were made, and 200,000 of them were issued to the ranks.  

The skull-cap caught the attention of General Adrien’s superior, Marshal Joseph Joffre, and in April 1915 Joffre ordered General Adrien to set to work on a more sophisticated solution.  The Paris Fire Brigade’s helmet was rapidly adapted for wartime use and designated the “Casque Adrian.”  It was 0.7mm thick, made of mild steel, and weighed roughly 1.8 pounds.  Initially developed only for infantry, it was quickly distributed to all branches of the military.  By the end of 1915, over 3 million Adrian helmets had been issued to the ranks.  

The British and Germans — among many other nations — quickly followed in the footsteps of the French.  

The British issued a helmet designed by John Brodie in 1916.  Initially made of mild steel, it was quickly recognized that Hadfield manganese steel — an austenitic steel containing 13% manganese and 1.2% carbon, which was patented in 1883 — would offer significantly better ballistic protection.  Hadfield manganese steel exhibits very pronounced strain-hardening effects — it rapidly transforms from ductile austenite to strong and hard martensite when it’s impacted or subjected to compressive or tensile loading.  Hadfield steel is also, like all austenitic steels, nonmagnetic, and this was important for soldiers who needed to rely upon compass readings.  The Brodie, as-issued, was 0.9mm thick, and weighed roughly 1.3 pounds in total.  It was primarily designed to protect from falling shrapnel, so it was built with a broad brim.  Overall, it is extremely reminiscent of the medieval kettle hat or chapel de fer.  

The Germans, for their part, began work on a helmet for general issue in late 1915.  (Earlier in 1915, Lt.Col Hesse, the chief of staff of Army Group Gaede, purchased custom-designed helmets for his unit out-of-pocket.  The “Gaede skullcaps” were effective, but were of a peculiar design, and were not adopted by the rest of the German army.) The now-iconic German stahlhelm was developed by Prussian engineering professor Friedrich Schwerd of the Technical Institute of Hanover, and was issued to German troops in early 1916.  Metallurgically, these helmets were made of a superior chromium-nickel alloy, were 1mm thick, and were quenched and tempered to a martensitic microstructure and a hardness of 49-54 Rockwell C.  The design of these helmets was inspired by — and indeed owes much to — the late-medieval sallet, which the Germans simply called “the Gothic helmet.” (The sallet was extremely popular in Germany in the 15th and 16th centuries.)  The old sallet was typically worn with a visor, and a visor was indeed envisioned for the new German steel helmet, but was never put into production.  In terms of performance, the German stahlhelm proved to be the best helmet of the war by a substantial margin — for it was thicker than the rest, it utilized the strongest steel alloy, it was offered in numerous sizes, and it offered a superior area of coverage.   

But it was the Brodie helmet which formed the basis of subsequent American helmet development.  In 1917, the American Expeditionary Forces of WWI were equipped with Brodie helmets; first from a purchase of 400,000 British surplus units, and then from an American-made copy of those helmets, designated the M1917 Kelly.  Nearly 3 million American M1917 helmets had been produced before the end of the war.  

Steel helmets were adopted by the militaries of almost all European nations during WWI and in the interwar period, and the very vast majority of these were substantially similar to the Adrian, Brodie, or stahlhelm.  On the whole, the steel helmet saw little change heading into WWII.  

In 1940, the Germans replaced their helmet alloy, which contained as much as 2.5% nickel, with a silicon-manganese alloy, similar to AISI 5140, which contained no nickel at all.  This was on account of nickel’s scarcity and high cost in wartime — and it proved a wise move, as the Si-Mn alloy had mechanical properties which were effectively identical to the Ni-rich alloy it replaced; it was also tough, predominantly martensitic, and exhibited hardness of around ~51 HRC on average.  

In 1941, prior to America’s entry to the war, the M1917 was replaced by the M-1 helmet, which was larger, bowl-shaped, and eschewed the Brodie’s broad brim in favor of more protection for the sides and back of the head.  The M-1 helmet shell was .94mm thick on average and, like the M1917, was made of the same Hadfield manganese steel.  It weighed approximately 2.25 pounds without the liner and chinstrap; 3 pounds, 2 ounces in total.  Millions of M-1 helmets were produced — over 22 million just between 1941 and 1945 — and they were the standard-issue helmet for US military forces until 1983.  For all those many decades, there were effectively no changes made to the composition, thickness, or shape of the M1.

This is not to say that the US Military didn’t experiment with other helmet alloys or materials between the 1940s and 1980s.  To the contrary, it did quite a lot of work along those lines — mostly in fiber composites, with Doron helmets, nylon helmets, and many other abortive prototypes.  There was some work, also, with polycarbonate as a helmet material.  Most of these experiments were interesting, and some of them performed better than the M1’s Hadfield steel, but it was nevertheless determined that “in essentially all candidate materials, the relative gains afforded by a new material or combination thereof were not considered to be cost-effective.”

The more interesting thing is that the US Military never experimented in a serious and methodical manner with better steel helmet alloys.  They knew very early on, from experiments with captured helmets, that the German helmets performed considerably better than the M1 on a weight basis, and they also knew that the German Si-Mn alloy — a very lean alloy due to wartime constraints — was by no means the optimal alloy for helmet development.  That they made no real efforts to replace Hadfield steel is therefore quite astonishing.  In hindsight, a substantial performance improvement, at an equal weight, could have been gained, had they identified and utilized a better alloy.

This must be due in part to the fact that, from the 1940s on, US Military arsenals and research laboratories seem to have been enamored of the idea of a fiber-composite helmet.  (Doron was one of the Navy’s favorite pet-projects, and the Army was always deeply enamoured with Nylon.)  Thus, when Kevlar was invented, they threw all of their efforts into refining and improving it for use in an entirely new type of military helmet.  With Kevlar, and with what they learned from decades of work with Nylon and Doron, the K-Pot was born.  But technology could have developed in a different direction — we could have seen thicker helmets of improved steel alloys introduced in the 1980s… that’s just not how things played out.  

Diamond Age is now rectifying that: We are reviving the steel helmet.  The NeoSteel helmet — which was designed for the European police market, but which exhibits ballistic performance similar to the best fiber-composite helmets in modern military service — is the natural evolution of the steel helmet.  It’s what the steel helmet should have already become.  

I’ll soon expand on this with a short post on the future of the steel helmet.  What we’re doing with novel alloys and advanced forming methods, with accessories, and where it can all go from here… to the theoretical maximum strength of steel, which we have modeled and experimented with.  That this also has implications for steel body armor should be clear, and we’re making great strides in that direction, as well.

More to come.

Most of the people who work in armor and munitions design are die-hard experimentalists, and for good reason: The best way to test an armor system is to hit it with live rounds and see what happens.

But, true though that may still be, modeling software has become increasingly useful — even quite shockingly useful — and it is beginning to change the way we design products.

Numerical and hydrocode ballistic impact models aren’t new — they have been in regular use since the 1960s, when they were first applied to ceramic armor systems by the pioneering LLNL team led by Mark Wilkins. Initially, these early efforts attained good accuracy in modeling ballistic impacts onto monolithic aluminum armor systems.  

Their use was also broadened to include ceramic and ceramic-aluminum targets, which also led to simulations that matched experiments with reasonably good accuracy.  

Wilkins’ modeling and analysis of ballistic impacts on ceramic-metal tiles were the first of their kind, and strongly influenced all subsequent work in the field.  Most of the ceramic models that Wilkins devised were enthusiastically taken up by other researchers, and, in some form, they remain in use today.

It is unfortunate that, in the 1960s and 70s, it was effectively impossible to model armor systems comprised of a ceramic strike face over a fiber-reinforced composite backing layer.  The computational means available at the time weren’t up to the task.  Progress was nevertheless made, as the Wilkins group did a lot of trailblazing empirical work with ceramic armor systems which utilized a fiberglass backing layer.  They ran hundreds if not thousands of real experiments, over a period of roughly five years.  

Slow computers aren’t the only reason that modeling fiber-composite materials was impossible in those early days.  Another problem is that modeling the impact response of any material accurately requires a complete picture of that material’s static, dynamic, and structural mechanical properties.  For most metal alloys and ceramics, this is relatively trivial.  

Ceramics are always brittle materials, and they typically behave in a well-understood and characteristic manner; the slight differences between, e.g., boron carbide and silicon carbide boils down to sonic velocity, hardness, density, Poisson’s ratio, fracture toughness, and a few other simple parameters. 

Most of the aluminum and steel alloys in military service are simple and have been thoroughly characterized, so good and accurate parameter sets exist.  It’s rather more difficult to model less typical steel alloys, such as duplex steels — because, for instance, how and to what extent they exhibit strain-hardening is sometimes unknown, or poorly-known — but it can nevertheless be done, though sometimes one has to run real-world ballistic experiments alongside simulations to dial things in.     

So ceramics are trivial, commonplace metal alloys are also trivial, unusual metal alloys are just a bit more complicated.

But fiber composites are another matter entirely.  There are two extreme approaches.  

The first extreme: Model fiber composites as a bulk material with more-or-less accurate overall properties, and ignore the fine details.  

The second extreme: Model fiber composites at a “high resolution,” as fine-scale heterogenous materials, and take into account the arrangement and individual properties of fibers, tows, and matrix.   

The first extreme approach is called a “homogenized model.”  These are simple, and will generally give a fair picture of how a system is likely to perform in a penetration/no-penetration test — but won’t be as accurate as any other approach, and won’t give a realistic view of deformation behavior.  

The second extreme approach should be ideal.  It gives much more accurate results, and should also provide an accurate picture of deformation upon impact. The anisotropic properties of fiber composite materials can be fully taken into account, dynamic properties such as fiber strain-hardening and between-fiber friction can show up in the simulations, and different resin systems can be compared in a fairly rigorous and exacting manner.  The trouble is that this approach can cost hundreds of thousands of dollars in compute resources — per experiment.  Modeling anything at this level of detail is computationally very expensive.  But this level of detail is required for good accuracy, isn’t it?

Well, here’s the good news: There are hybrid models that simplify the high-res details, without converting fiber composite materials into monolithic blocks.  These models have been considerably refined over the past five years, and it is now possible to easily, accurately, and inexpensively model fiber composite systems — and ceramic armor systems where ceramic tiles are supported by a fiber composite plate.  

The below image, from the US ARL Report “Modeling Ballistic Response of Ultra-High-Molecular-Weight Polyethylene (UHMWPE)” by Zhang et al., illustrates this to some extent:

The highly-simplified 5-layer model gave results which weren’t substantially different from the 50-layer model.  And the 5-layer ballistic impact simulation took just 1.3 hours of computer runtime, whereas the 50-layer simulation took 55.8 hours. 

The age-old gap between the speed of homogenized models and the accuracy of high-resolution approaches has plagued academic, industrial, and military armor engineers for generations — but it has been bridged, with the advent and easy utilization of fast and accurate hybrid models.  

These hybrid models enable the iteration of design parameters in a truly meaningful sense.  The design process of an armor plate, for instance, generally involves:

1) The quantification of the threat

1. Pass/fail conditions.
2. The selection of appropriate materials  
3. Determination of the optimal arrangement of those materials, and material thicknesses — or, in place of bulk thicknesses, the number of composite layers involved.

Practically speaking, steps three and four are rarely very rigorous.  There is, traditionally, a fairly small palette of material choices, there are only a few ways in which those materials are commonly arranged, and material thicknesses are often tested empirically in fairly rough increments.

…But now, with good material models, fast computers, and software that allows for automation, it’s possible to simulate ballistic impacts on hundreds or thousands of different material and thickness configurations, in 0.1mm increments, until just the right system is found, for any purpose.  Empirical ballistic experiments can proceed from that point.  In the vast majority of cases, it will be found that the simulations exhibit very good (>90-95%) accuracy, and that the system selected by simulation, for any given threat or combination of threats, is an optimally or near-optimally designed system. 

Diamond Age has excellent models for all materials — including very good hybrid models for fiber composites — and we typically have some runtime to spare on our computer systems running LS-DYNA.  If your firm is interested in working on some simulations or iterated design programs with us, please contact us at your convenience.

We’ve found that, in general, the non-linear, explicit, finite element code LS-DYNA — which we also use for metalforming and other such work — is the best software package for modeling ballistic impact events.  Most of the fiber-composite simulations previously-described were run with LS-DYNA.   

As an aside, it should perhaps be mentioned that good models and good software will enable something very interesting in the near future: Armor systems designed by machine learning AI models, in an iterative process where thousands or tens of thousands of simulations are run in sequence.  AI models are already in-use — Diamond Age has developed and is testing an armor system with an AI-designed component… though, in this particular case, thousands of simulations were not involved. 

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.)

All three of these old ceramic armor materials have their shortcomings: Alumina is far too heavy, to such an extent that even steel armor is now giving it a run for its money; silicon carbide is, although much lighter, still heavy enough to be burdensome; the best grades of boron carbide, although nearly 40% lighter than alumina and more than 20% lighter than silicon carbide, are rather expensive and underperform against bullets with cemented carbide (WC-Co) cores.

Yet by no means are these three ceramic materials the only ones suitable for armor applications.  As I’ve mentioned in older posts, many other ceramic materials have been successfully tested towards armor applications, and there is an ongoing search, at Diamond Age and elsewhere, for a better ceramic armor material — something which out-performs boron carbide on a weight-basis, and maintains this high level of performance against extra-potent threats such as WC-Co-cored rounds.

To that end, one material which has captured the DoD’s imagination of late is boron suboxide, B6O.  It mirrors boron carbide in its low density (2.52 gm/cc) and its extremely high hardness (>3000 HV1) and current thinking is that it should perform well against even the toughest threats — unlike boron carbide, which, as mentioned previously, can be less than perfectly reliable.  The trouble with B6O is that it’s even harder to process and sinter than boron carbide, already a very difficult material to work with.  Even worse, B6O is staggeringly expensive to synthesize as a raw material.  For these reasons, its commercial development has never gotten off the ground — and boron suboxide armor may never enter mass production.

The aluminum dodecaboride AlB12 and the aluminium-magnesium-boron compound AlMgB14 are also attractive materials — both as standalone pure ceramic phases for armor plate development, and as candidates for better cermet development.  AlB12 and AlMgB14 are very similar to each other, and to boron carbide:  Their density is very low at roughly 2.5 gm/cc, they are all of similar hardness, and, unless something like the Lanxide process is employed, they’re best densified with the application of moderate pressure, e.g. in a hot-press or SPS/FAST furnace.  AlB12 and AlMgB14 have never been tested against WC-Co cored projectiles, but have shown good efficacy against steel-cored threats.  The Achilles heel of these ceramic materials is the fact that they’re very expensive to synthesize from the commercially-available materials that are the product of boron mining.  (Boron carbide, in contrast, is relatively easy to produce from cheap raw materials such as boric acid.)

Another option, mentioned in a few scientific papers over a decade ago, is a composite material made from roughly 60% boron carbide and 40% calcium hexaboride (CaB6).  This multiphase ceramic material has some inherent ductility, is slightly lighter than boron carbide, is slightly easier to densify, and appears to outperform pure boron carbide in head-to-head ballistic tests.  CaB6, however, is not stable in the presence of water — and degrades severely even upon exposure to moderately humid air — which is a trait that may limit its use in armor systems where, by definition, stability and reliability are of key importance.  Diamond Age has worked with CaB6-B4C composites that, quite literally, turned to dust following several months of exposure to ambient conditions.  These test tiles started exhibited spalling corrosion within days of their densification, which is certainly an extremely undesirable trait in an armor material.  Coatings which prevent the decomposition of CaB6 may, however, eventually enable the utilization of B4C-CaB6 composite plates… but not without some degree of legal liability and reputational risk.

Yet as this CaB6-B4C composite makes apparent, one doesn’t need to replace boron carbide entirely — strengthening it with a dopant or secondary phase is also a viable strategy, and a very promising one.  At Diamond Age, we’ve gotten very impressive results with B4C-TiN composites, which have a slightly higher density than pure boron carbide, but which display enhanced fracture toughness and better ballistic performance.  The addition of very small amounts of silicon or aluminum dopants to boron carbide, or tailoring the material’s stoichiometry so that it’s more boron-rich than usual (> B6.5C), have also been proposed as methods for producing a boron carbide ceramic product that’s more resistant to failure via shear amorphization, and would thus exhibit more predictable performance against WC-Co-cored ballistic threats.

An entirely different approach, which is perhaps the most promising approach, lies with high-pressure chemistry and engineering, for these enable the production of metastable high-pressure bulk materials, thereby opening up new frontiers in armor material development.  Diamond Age has been experimenting with HPHT polycrystalline diamond ceramics for some years and has recently been issued a patent on the technology.  Bulk diamond is much harder, much tougher, and much better-performing than any known ceramic material.  We’re now at a stage where we’re beginning to make large diamond tiles on a large enough scale for commercialization, and we’re actively working on R&D, so that even larger, stronger, and superlatively high-performance diamond tiles are available for next-generation armor applications.  For instance, by utilizing fullerenes as a starting material, instead of carbon black, it seems possible to make bulk diamond tiles that are structured, comprised of aggregated diamond nanorods, and which are so hard and tough that they may be the truly ideal and ultimate armor material — unsurpassable.

Plates made with our current diamond tiles are already capable of stopping threats such as the M855 at as little as 2.2 pounds per 10×12″ shooter’s cut plate.  In the very near future, we believe that diamond-faced armor plates should be able to stop the APM2 at well under 4 pounds per body armor plate.

For more information on our diamond armor plates, please click here.

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.

Metals and alloys are generally considered crystalline, which means that their constituent atoms are arranged in a regular, periodic, granular manner. Granted, some have extremely complex crystal structures, but they are nevertheless regular and structured. Solids which do not exhibit periodicity — that is, which do not exhibit any regular arrangement of their constituent atoms, and which are fundamentally unstructured — are considered “noncrystalline,” “amorphous,” or “glassy.” Broadly speaking, the arrangement of atoms in an amorphous metal is no different from that disordered or chaotic arrangement of atoms observed in an oxide or silicate glass. And, like these ubiquitous glasses, amorphous metals can generally only be prepared via rapid solidification from a melt — which, particularly for metals as opposed to oxide glasses, involves extremely high cooling rates.

The non-crystalline atomic structure of amorphous alloys lends them unique properties; they are a novel class of material, not quite an “alloy,” not quite a glass, and certainly not a ceramic — but with traits that borrow from all of the above. Broadly speaking, their strength and hardness are greatly superior to crystalline metal alloys, their ductility is reduced, their resistance to abrasion and wear is far higher, their resistance to corrosion is higher. Their impact resistance and toughness vary, with some compositions behaving much like ceramics and glasses, whereas others are as tough as maraging steel. Generally speaking, they tend towards brittleness, as their atomic structure prevents them from arresting growing cracks. Work-hardening is impossible in amorphous alloys, and heat-treatment can turn an amorphous alloy into an inferior crystalline one — so, once made, amorphous alloys are not thermally or mechanically treated.

Amorphous alloys are generally complex from a chemical and metallurgical standpoint. One of the most popular commercially-available amorphous alloys, known as Vitreloy 105, has the following composition: Zr52.5Ti5Cu17.9Ni14.6Al10 (In other words: Zr: 52.5% Ti: 5% Cu: 17.9% Ni: 14.6% Al:10%.) It is therefore characterized as a zirconium-based amorphous alloy, as zirconium is the primary element of its composition. Vitreloy’s complexity is typical of commercial bulk amorphous alloys. But they’re not all complex; the simple ferroboron alloy Fe80B20 can be amorphous, and the first discovered amorphous alloy was a similarly simple Au-Si alloy. As these examples demonstrate, the least complex amorphous alloys are generally comprised of a transition metal alloyed with a metalloid.

Although many amorphous alloys are softer and more brittle than tool steel, certain bulk amorphous alloys — particularly those based upon a foundation of nickel, tantalum, or iridium — have displayed incredible hardness values of 1200 to 2000 HV, which means that the very softest of these amorphous alloys is nearly twice as hard as the best ultra-high-hardness armor steel, and the hardest is significantly harder than armor ceramics such as alumina and reaction bonded boron carbide! What’s more, these amorphous alloys display reasonably good elasticity, stiffness, damage tolerance, and compressive strength. Alloys based upon lightweight scandium, in particular, display outstanding elastic properties combined with good hardness and low density.

The mechanical properties of these aforementioned amorphous alloys, taken together, strongly indicate that they should be capable of defeating armor piercing rounds in thin sections, and may be even more durable and damage-resistant than armor steel.

It is worth noting here that the high-performance glassy alloys mentioned above were not optimized for use as light armor materials. Thus it should almost go without saying that a modern research program could identify and develop better materials; the best light amorphous alloys for armor use; characterized by low density and reasonable materials costs in addition to extremely high hardness, good elasticity, and so forth.

It is highly plausible that thin sections of these hard amorphous alloys would out-perform both steel and ceramics as an armor material. Preliminary work has already done much to confirm this assumption.

If there are hurdles, they’re in the development and bulk production of these unusual metallic glasses. The production methods used for steel and other metals are, in general, wholly inappropriate for amorphous alloy production. Presently-available commercial amorphous alloys like Vitreloy are completely inappropriate for use in armor systems, for reasons which go beyond the fact that they were never optimized for use in armor.

Diamond Age is actively researching bulk metallic glasses and amorphous alloys for armor applications, and is working to overcome the issues associated with their production on commercial scales. We anticipate that our light armor amorphous alloy shall find its way into commercial armor products within the next few years. What’s more, we believe that there’s a sizable market for ultra-high-performance bulk metallic glasses in aerospace, optical devices, and jewelry.

Please contact us for more details.