Through the Armored Glass
Written by Brendan P. Rivers
GCT 2010 Volume: 1 Issue: 2 (August)
You Want to See Out, But You Don't Want Things Getting In.
Over the past decade, the U.S. military has made a huge push to enhance what is traditionally thought of as armor on its ground vehicles—not just combat vehicles, but tactical and other vehicles as well. The U.S. military has added metal plating, along with newer materials like ceramics, on all sides of many of its vehicles, even the undercarriage. But really, what does all this additional armor accomplish if you leave drivers and crew exposed to enemy fire or shrapnel from an improvised explosive device (IED) though a vehicle’s windows? This is where transparent armor comes into play.
Used primarily on windshields and other windows on military vehicles, but also increasingly as part of blast shields and as protection for turret gunners on vehicles like tanks, transparent armor (sometimes called bulletproof, bullet-resistant, or ballistic glass) is a laminated system. In other words, it consists of layers of glass and other materials compressed together under high temperature and pressure to form a transparent means of protection.
As explained by Dave Hubert, transparentarmor lead for Ibis Tek, traditional transparent armor, the type that is currently in widespread use across the military services, consists of harder strike-face materials, traditionally glass, and softer spall-liner materials, traditionally polycarbonate plies bonded together with inner layer materials, usually polyvinyl butyral or urethane.
According to Daniel Cohen, president and CEO of Oran Safety Glass (OSG), when a threat such as a bullet, fragment or blast overpressure waves strikes the transparent armor, the harder materials tend to shatter, dissipating energy. The softer materials tend to deform, absorbing energy. The complex characteristics of materials used in the lamination result in the transparent armor’s ability to defeat a fairly broad range of threats. In order to be effective, Cohen said, transparent armor must exhibit a range of characteristics: “It must provide exceptional optical qualities; it must be clear, and free of distortion or defects. As an armor material, it must of course defeat many different threats, including various calibers of small-arms fire at varying velocities and in multiple-hit shot patterns, as well as fragments, overpressure waves from IEDs, for instance—and even RPGs [rocketpropelled grenades] and now EFPs [explosively formed penetrators], along with other threats commonly used by the enemy.”
Of course, the specific ordering of the layers and, even in many cases, the materials themselves used in these layers—indeed, even the particulars of the laminating process itself—are carefully guarded, for both proprietary and security reasons. (In fact, when contacted for this article, the U.S. Army Tank- Automotive Command declined to comment on the subject of transparent armor entirely, citing security reasons.) But when taking a broad view, most of the basic materials used in traditional transparent armor are available through open sources. But those materials themselves really aren’t the key. Most of the producers of traditional transparent armor use the same ingredients in their armor “recipes,” if you will, but the lamination process itself is what makes or breaks any given recipe. As Lawrence Shaffer, general manager of Armor- Line (part of the Defense Venture Group) put it, “The secret to lamination isn’t those ingredients, but how you bake the cake.”
Generally speaking, though, transparent armor has the hard, thick glass strike face on the outer surface, with softer materials placed, as OSG’s Cohen put it, closer to the safe side of the armor (i.e., the interior of the vehicle). By virtue of its pliability, this softer layer serves as final protection against projectiles about to penetrate the transparency. This final layer actually balloons inward, capturing and holding not only the projectile but also the fragments of shattered glass created by the projectile. “As you can imagine,” Cohen said, “it doesn’t do the warrior much good to survive the bullet if only to be killed by flying glass shards.”
STICKS AND STONES…WELL, OKAY, STONES MAYBE
But it’s not just bullets—or even IEDs, RPGs and EFPs—with which transparent armor must contend. As simple as it may sound, rocks are a serious problem as well. “From my understanding,” said Shaffer, “There’s a significant number of windows being replaced just because there’s a rock chip in the window.”
How does this happen? Picture a convoy of vehicles traveling down a road in Iraq or Afghanistan, one not particularly well paved. There’s going to be plenty of loose material— rocks, usually—that a vehicle in front of another might kick back into the one following it, and that material could strike the latter vehicle’s windshield, causing it to chip or crack. Because the strike face is typically made of annealed glass, much like the windshields of the typical civilian vehicle, it cracks in a spiderweb pattern (unlike the tempered glass used in the side windows of civilian automobiles, which shatters completely into tiny pieces when struck with sufficient force).
Does that single chip or crack significantly degrade the protection offered by the transparent armor? Probably not, and most likely not at all, said most sources. As Shaffer put it, though: “If you’re driving around in a Humvee and it has a chip in the windshield, a break in the outer ply, and you expect that window to protect your life, you’re more likely to want that windshield replaced than you are to depend upon the performance of armor that has been compromised.”
Here the U.S. military runs into a pretty big, and rather pricey, issue: sustainment costs. A single rock strike will require windshield replacement, resulting in substantial sustainment costs.
The issue of rock strikes sounds like it would be simple to solve: Just use a thicker layer on the strike face of the transparent armor. But this issue is not easily solved, according to Hubert. Several design criteria must be considered—thickness, weight, defrost/de-ice and threat protection. Many of the advanced materials offer superior threat protection, but perform poorly against rock strike.
There’s the weight concern, of course, but as Hubert noted, there is also the question of defrost/de-icing—clearly not a concern in theaters like Iraq, but one that must be taken into account nonetheless (who knows where these vehicles may be needed next). Once you get to a certain thickness, a windshield, for instance, will not defrost simply on the vehicle’s forcedair system. In many transparent armor windshields, Hubert explained, a heating element is buried inside, and the thicker the strike face, the more power is required to meet de-icing needs. And here we run into the classic “Iron Triangle:” How much do you trade off in each of the three areas of payload, performance and protection in order to meet mission requirements?
“In order to defeat the tremendous energy from small-arms fire, fragments, blast overpressure waves from IEDs, RPGs, EFPs and other threats directed against our troops, the transparency must provide strength and mass,” said OSG’s Cohen. “When armor is engineered onto vehicles, mass or weight, presents problems. Weight shortens the effective life of the vehicle, wearing down the drive train and suspension. Weight also reduces the vehicle’s payload capacity and it slows the vehicle, which increases risk by reducing the vehicle’s ability to get out of harm’s way, thereby compromising survivability. This is a key driver in the development of advanced materials and designs.”
Of course, Cohen was referring to the military threats in the combat environment currently faced by U.S. forces, but when you consider that rock strikes also pose a threat, even only a perceived one, to the integrity of a transparent-armor system, how far do you go? Do you use that thicker and heavier strike face and slow down the vehicle, thereby impeding its performance by making it less maneuverable? Do you draw power for de-icing over other systems on the vehicle? These are just some of the questions ground vehicle designers must look at when considering what type of transparent armor to incorporate into their platforms.
COMING APART
If rocks sound ordinary, what about basic environmental elements, like moisture? Moisture and other environmental issues have proved to be a serious thorn in the U.S. military’s side when it comes to the transparent armor on its ground vehicles. It’s not just moisture either. Changes in temperature, exposure to ultraviolet (UV) light and other environmental elements can have a very serious effect on transparent armor. They can lead to delamination—essentially a reversal of the process by which the different layers of the transparent armor are put together.
Traditional transparent armor is a combination of harder and softer (i.e., dissimilar) materials that are laminated together under high temperature and pressure, explained Cohen. “The dissimilarity of the components that comprise transparent armor—and the delicate interlayers that bond sequential layers together—cause those various materials to behave differently when exposed to varying environmental conditions,” he said. Particularly when exposed to the UV rays of direct sunlight, and when subjected to extreme temperatures and rapid temperature shifts, the materials literally begin to work against each other. Each layer pulls at the next, creating tension within the transparency. “What I’ve seen over the past 30 years, the sophistication of transparent armor products that allows a mere window to defeat the kinetic energy and deflagration of an incoming RPG, multiple strikes of a 14.5 mm armor-piercing round, or the tremendous blast pressure and fragmentation from, say, 15 pounds of TNT in an IED, is the same sophistication that may cause transparent armor to eventually begin to work itself apart, allowing air and other contaminants to permeate between laminations— this is delamination.”
At first, delamination shows itself as cloudiness in the transparency; essentially, pockets of air forming between the layers of the armor, obstructing vision usually around the edges of the armor. This in itself isn’t that much of an issue, but it can become one as visibility becomes more obstructed. Moreover, the cloudiness spreading across the armor means that more delamination has occurred.
Delamination will happen eventually, according to Ibis Tek’s Hubert. It’s just the nature of the lamination process: It’s bound to come apart at some point, but water and some other environmental elements, such as UV light and temperature changes, can accelerate the process. Even sitting mothballed in the continental U.S., he said, transparent armor on military vehicles eventually will deteriorate to the point where the protection it offers is compromised.
Is there a solution? Not really, said most sources. Most said it all comes down to the individual manufacturers of the transparent armor: What materials do they use, but more importantly, what process do they use for lamination? It goes back to Shaffer’s comment that most producers of traditional transparent armor are largely using the same ingredients, but it comes down to how they actually put those ingredients together. But those ingredients may be changing.
LOOKING AHEAD
While traditional glass and PVBs and polycarbonates are the current norm, a revolution in transparent armor is on the horizon. Recent work on borosilicate glass (as opposed to the traditional soda-lime glass) and glass ceramics, such as spinel, being developed by companies like OSG, ArmorLine, Schott Defense and CoorsTek, is said to be showing great promise in terms of capability, though not in terms of scalability for most of industry (notable examples being OSG’s work on a variety of Marine vehicles and Schott’s work on the MRAP)—at least not yet anyway.
According to OSG’s Cohen, transparent armor usually ranks among the heaviest of all materials on tactical vehicles per size. For this reason, transparent armor is one of the first places engineers look to reduce overall weight. Over the past decade, he claimed, advances in molecular science have yielded a number of advanced materials and designs that possess exceptional ballistic characteristics—in other words, the ability to defeat high-level threats with far less weight than materials used in traditional transparent armor.
What makes these new materials more effective? One characteristic cited by Armor- Line’s Shaffer is that ceramics like spinel break into larger chunks than traditional glass. As noted earlier, glass tends to break into a spiderweb pattern. Spinel breaks into larger pieces. “The advantage of the larger-sized pieces,” Shaffer said, “is that there’s more material left for that second shot, third shot or fourth shot. It improves the multi-hit capability of a laminate.”
Roy McCallum, Schott’s defense manager for protection, noted that these newer materials are able to offer the same or better protection while dramatically cutting down on the weight that must be added to a vehicle. He claimed his company is able to “typically draw numbers 20 percent to 40 percent lighter than the types of windows that were already being fielded.” He also said that as Schott has developed these new materials, it was found that they offer other benefits as well, such as greater resistance to environmental factors and greater night vision compatibility.
However, Cohen does admit that these advanced materials are often cutting-edge technologies that are expensive to produce. “Although science has allowed the creation of these advanced materials,” he said, “they are clearly cost-prohibitive when applied to military vehicles.”
McCallum agreed, saying that one of the challenges in getting these newer armor technologies into the field is that they are more expensive to produce, because the underlying materials are a little bit more costly. What is needed are large-volume programs to reduce the system cost, as the mine resistant ambush protected vehicle program did. McCallum called the MRAP program Schott’s “big-launch program” for its new transparent armor technology. However, most industry sources noted that the real peak for the transparent armor market came during the U.S. military’s procurement of the MRAP vehicles, and that it’s been on the decline since then.
Developments in the state of the art are pushing the envelope though, according to Cohen, reducing overall weight, improving ballistic performance, and furthering a host of features that are required of transparent armor. ♦