Posts Tagged ‘UHMWPE’

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In several of my posts, I mention that while UHMWPE UD armor is an excellent choice for certain applications, and has material advantages over woven or laminate Aramid ballistic fabrics (higher potential V50, positive buoyancy, UV resistant, waterproof), it suffers from several glaring weaknesses (degrades to complete ineffectiveness above 170F, no breathability, delaminates/curls, and is WEAK AGAINST CONTACT SHOTS).

It is important to reiterate that last weakness: a large number (if not the majority) of self-defense and duty scenarios take place at 0-5 feet, where contact shots are a high likelihood. Woven Kevlar soft armor has shown to provide EXTREMELY good protection against contact shots (defined as the muzzle of the weapon being in physical contact with the vest or armor panel). The point at which Kevlar chars is around 500F, and it will retain its strength below this temperature.

The failure mechanism for UHMWPE in contact shots is the high temperature propellant gases that exit the muzzle microseconds after the bullet. These gases heat the area surrounding the muzzle and bullet path, and cause the laminate to melt/denature. This allows the bullet to penetrate much further than would normally be possible. In the case of large caliber revolvers (with a large muzzle blast footprint), this can allow the round to completely defeat the vest.

Test Round

Test Round

For the sake of the test, the round chosen was the .357 Magnum, rather than a .44 Magnum, as I wanted to see if the (relatively!) more modest caliber would still defeat the level II ballistic panel. The panel consisted of 15 layers of Dyneema SB-38. Round chosen was Hornady Custom 158 gr. XTP @ 1250 fps muzzle velocity, from 6″ barrel. The level II panel is specced to stop an equivalent round.

Test Panel

Test Panel

Backing

Backing

The panel was placed against a backing material consisting of 8 layers of bubble wrap, covered in a dish towel. In retrospect, this was probably a bit too “springy,” giving the panel an advantage by permitting it to move away from the hot muzzle blast faster than if the armor was being worn.

Test Panel Ready To Shoot

Test Panel Ready To Shoot

The test panel and backing were placed upon the ground, and the muzzle pressed firmly (but not forcefully) against the surface. The round was discharged into the center of the panel.

First shot

First shot

First shot, through the backing

First shot, through the backing

First shot, rear of panel...

First shot, rear of panel…

The panel was defeated, showing that the muzzle blast had melted a moderately large area around the point of contact. The round penetrated the backing and buried itself into the dirt (as shown).

To verify, a second round was fired into the lower left area of the panel (away from the heat affected zone of the first round). The second round performed identically, burying itself into the dirt beneath the panel.

Second Contact Shot

Second Contact Shot

Second Round, showing penetration

Second Round, showing penetration

Second shot, layers peeled back

Second shot, layers peeled back

Second shot, showing exit into backing...

Second shot, showing exit into backing…

...And into the dirt

…And into the dirt

Conclusions:

The results of the test shows that UD UHMWPE laminates are at risk vs. contact shots. Heat from the muzzle gases (especially medium to large caliber revolvers) “blazes a trail” so to speak, for the round to penetrate further than it normally would.

It is no big mystery that I am at best a reluctant advocate of UHMWPE in soft armor. While the material itself *does* have incredible properties, these properties come at a steep price if the end user is not aware of the limitations and weaknesses inherent in the material.

These limitations and weaknesses are exacerbated by (in MY OPINION, ahhh, I promised you would see that word hurled around here!) a tendency to “softball” the armor test protocols. Even the current “best practices” protocols (The FBI and DEA tests for soft armor), have this same inherent kid glove treatment when it comes to UHMWPE containing vests.

“How can this be?” you may ask. Well, let’s review:

UHMWPE (“Ultra High Molecular Weight Poly Ethylene”) is an exceedingly strong material made up of long chains of ethylene molecules. The tensile strength is astounding, exceeding para-aramid (Kevlar/Twaron) and steel easily. It is positively buoyant, waterproof and does not degrade with exposure to UV light (three of the Achillies heels of aramids). However, as has been mentioned before, UHMWPE (regardless of brand- both Spectra and Dyneema are at their root the same basic molecule) will denature when exposed to temperatures exceeding ~168 F.

Think of it as exposing hardened steel to it’s normalization (annealing) temperature. The hardness disappears, and it becomes soft again. Unlike steel, it is impossible to change the UHMWPE back to its “super” state. In its denatured state, the material is identical to the stuff used to make milk jugs.

This denaturation temperature is well-known.

In the real world, temperatures often climb to well above this temperature, in both storage and incidental use (especially hotter regions of the world where .Mil users often find themselves). Why then are the test protocols seemingly designed to AVOID this issue?

Take the current NIJ 06 protocol. It incorporates many new rigors that an armor must pass in order to be certified (a GOOD thing, no doubt), including environmental conditioning. However, the temperature does not exceed the KNOWN denaturation/transition temp of the UHMWPE (highest temp in the conditioning phase is 149 F):

http://www.ncjrs.gov/pdffiles1/nij/223054.pdf

Even the FBI protocol, which excels the NIJ 06 standard in many ways, still only exposes the armor to a MAX temp of 140 F:

http://www.bidsync.com/DPXViewer/FBI_TEST_2006.pdf?ac=auction&auc=108034&rndid=338514&docid=748537

Quite frankly, this is a ludicrous state of affairs. Since temperatures can *regularly* reach 200 F in a car trunk or APC on hot days in CONUS or OCONUS, to not expose armor to these realistic circumstances could be perceived as softballing.

Even recent tests done by DSM which supposedly showed that their UHMWPE material did OK in elevated temperatures STILL tiptoed around the transition threshold temp:

http://www.dyneema.com/~/media/Downloads/Research%20papers/PASS%202010%20-%20Meulman%20-%20DSM%20Dyneema%20-%20Ballistic%20performance%20of%20Dyneema%20at%20elevated%20temperatures.ashx

In the paper, “temperatures up to 90 C (194 F)” were used. However, these temps were *only* applied to hard/rigid armor, which does not experience the same sensitivity as soft armor, most likely due to an insulative effect of the material in thicker section. The soft armor samples were still only exposed to 70 C (which is 158 F, 10 degrees BELOW the known transition temp). The test should have applied the same max temp to ALL samples, regardless of whether they were hard or soft.

In conclusion, test protocols should be designed to apply REAL WORLD rigors to life saving equipment. The current protocols SEEM TO incorporate temperature threshold requirements that allow the limitations and weaknesses of a specific material (UHMWPE) to pass. Designing tests with less rigorous standards so as to avoid excluding a material does not live up to the purpose of testing in the first place. Providing the absolute best lifesaving gear, regardless of any other considerations, should always be the goal.

The final class of materials for use in rifle plates is a familiar face- UHMWPE. Despite its drawbacks in soft armor format, in hard/rigid applications, this material does not show as many weaknesses. For reasons that are still not fully understood, the heat tolerance of PE hard armors is much better than soft armors (showing a danger zone of 195-200 degrees F rather than 180 F). This may be due to the typically thicker profile, thus providing a larger thermal mass to heat (taking longer and requiring more ambient heat to achieve irreversible denaturing). Also, contact shots are not as likely to have such a high risk of penetration due to the physical properties of a rigid defense compared to a flexible.

In addition to finding wide application as the backing material in many (if not most) ceramic plates (and a VERY effective steel/UHMWPE hybrid by Armored Mobility, the TAC3S), it is also used as the sole material in a significant number of plates by various manufacturers. In a hard armor format, UHMWPE offers some notable advantages: it is positively buoyant (it floats), is immune to acids, zero spallation/splash, and makes for the lightest rifle plate available. The drawbacks are moderate-high cost, and the thickest profile of any rifle plate (some have likened it to wearing a Wheaties box on your chest). In addition, UHMWPE plates typically do not stop M855 Green Tip ammunition, while having no difficulty with M193 (the opposite of most steel plates).

UHMWPE plates stop rounds by means of frictive braking. The fibers of UHMWPE squeeze and apply compressive braking force to rounds that strike. Generally the projectile is embedded about halfway into the armor when it is stopped. This leads to no splash or spallation, since the round remains mostly intact. M855 is thought to penetrate due to the incompressibility of the steel penetrator. Regardless of the mechanism, it is important to take into account when considering potential threats.

Due to their properties, some applications (maritime, swimmers) tend to benefit from their use. If the thicker profile is not a hindrance, they can be quite effective. As always, assess your needs and potential threats before making a decision.

Up until the 1940’s, steel was one of the primary materials in both offensive and defensive arms. During WWII, a new ceramic material was developed for use in antitank projectiles, called Tungsten Carbide, or WC. These penetrators were heavier, harder, and denser than anything that had been used up until that time. At the close of the war, WC was commonly found in most high velocity anti-tank round cores.

WC was extremely hard (much harder than steel plate), but it was also extremely heavy. For armor, at least that worn by personnel, a lighter, yet still extremely hard material was needed. Silicon Carbide, and later Boron Carbide, were first used on armor during the Vietnam War, in derisively named “Chicken Plates.” These were very heavy, and typically were used as improvised seats by helicopter crews to protect against ground fire.

Another material also started to find use in armor, specifically Alumina (Al2O3, which is very similar to Sapphire).

Each of these ceramics underwent significant improvements over the years, both increasing their purity and density (higher purity and density result in better material properties).

Ceramics stop projectiles by a different mechanism than steel plates. While steel plates rely on their combination of hardness AND toughness to cause the impacting round to mushroom, fracture and subsequently splatter (with attendant danger to the wearer), ceramics use a combination of yawing and eroding to render the projectile ineffective. Since the ceramics used in rifle armor are all up in the high 8’s or low 9’s on the Moh’s hardness scale, there are few projectiles that are harder than they are (certain WC cored rounds come close). When a rifle round hits a ceramic plate, there is localized fracturing (causing the projectile to dump massive amounts of its energy) of the ceramic. The projectile will typically yaw off-axis, increasing its frontal area, as well as being eroded by the very hard ceramic particles. These residual projectile pieces are then caught by the backing material of the plate, typically rigid aramid or UHMWPE laminate.

It is important to note that UHMWPE laminates are wholly inappropriate for use in soft armor, due to their sensitivity to heat, poor resistance to contact shots, and poor breatheability. In a hard armor format, it appears that their heat sensitivity is diminished, and breathability is not an issue. It is still not recommended to leave your hard armor containing UHMWPE (either as part of a ceramic plate, or pure UHMWPE plates, more on those next time) in a high heat environment. Specifically, soft armor is at risk at or above 180 degrees, while hard armor is at risk above 200 degrees.

Due to their nature, and depending on their construction, ceramic plates still may experience front face spall, usually consisting of ejected ceramic particles.

Ceramic plates give wearers the option of upgrading their protection level to stop AP (Armor Piercing) rounds that typically blow right through level III plates. Level IV will stop M2AP (black tip) .30-06 rounds, which contain a VERY hard steel penetrator (often found undamaged after punching through up to 1/2″ mild steel). The level IV ceramics erode and yaw this round, increasing its frontal area, and making it easy for the backing material to catch it.

Ceramic plates are typically built using one of two types of construction: monolithic or mosaic. Monolithic utilizes a single piece of ceramic, while mosaic uses multiple smaller tiles arranged and bonded to the backing layer.

Monolithic plates are typically much more expensive, since sintering (hardening) larger plates requires larger autoclaves. Much more QC expense is also incurred, since if a larger plate has a flaw, the entire piece is discarded. This expense is compensated for, because monolithic plates can be built with complex curvature or shapes, compared to mosaic. Mosaic plates are conversely more affordable, using many smaller tiles arranged together and bonded with an adhesive, typically epoxy. It is typically more difficult to form complex plate shapes with mosaic construction.

There are currently three types of ceramic materials that find general use in rifle plates, in ascending order of cost/effectiveness/weight. They are:

Alumina (Al2O3): A whitish ceramic with a good combination of hardness (mid to high 8’s on the Moh’s hardness scale), toughness, and cost effectiveness. This ceramic is still the most-used for general low- to mid-priced ceramic plates. With good design, Alumina plates can be quite protective and comfortable. Chemical structure very similar to Sapphire.

Silicon Carbide (SiC): A dark grayish ceramic (with a bluish tinge), this ceramic is more expensive, and harder than Alumina (Moh’s 9). Also slightly lighter, it finds use in medium to medium-high price point level IV plates.

Boron Carbide (B4C): The most expensive ceramic used in armor, and the second hardest material on Earth (behind diamond), it is also much lighter than Alumina. This material is used in the highest-end rifle plates, and can be enhanced with new methods (such as sinterless pressing) to achieve nearly 100% theoretical density (meaning there are no gaps between particles). A dark gunmetal colored ceramic.

As with all materials, ceramic has its advantages and drawbacks. It is very effective, on a per-weight basis, compared to steel (can be up to 40% lighter than steel plates). It can be made to withstand much more potent threats, such as black tip AP rounds (which again, blast straight through level III). And finally, ceramic monolithic plates can be shaped into complex curvatures, far exceeding the capabilities of steel, to better conform to the shape of the body. With these advantages come weaknesses- ceramic plates are much thicker than steel, making them more difficult to wear concealed. They are MUCH more expensive, some 6th gen ceramic plates pushing $1.5K PER PLATE. And finally, their major Achilles heel is their fragility. Ceramics, especially monolithic plates, are susceptible to rough handling. Cracking, chipping, or even complete destruction of the strike face can occur if the end user mistreats their armor. It is recommended to floroscope (X-Ray) your armor once per year if at all possible, to detect cracks.

I will be making an effort to review several different options for ceramic rifle plates (as funds permit) over the next few months.

As always, it is imperative that the end-user assess their needs and mission requirements when making a determination of what sort of plates to purchase. Each subset of plate types has their definite advantages and disadvantages. In the next post, we will examine the final plate material. Until then, cheers!

In the late 80’s and early 90’s a relatively new material was making an appearance in concealable body armor. Based on the Ultra High Molecular Weight Polyethylene molecule, this material offered tensile strength 8-15 times that of steel on a weight-to-weight basis. This was up to 40% higher than Aramid fiber. Developed by DSM, this material became known by two different trade names, Dyneema (DSM) and Spectra (Honeywell). Initially, this material was utilized as both a woven panel (which had immediate problems, as will be discussed below), and later in a laminate form (called Shield technology, similar to Gold Shield aramid laminate).

In the same way that aramid laminates utilized a poly-film matrix, so too did Dyneema and Spectra laminates. The UHMWPE fibers in laminate armor materials are unidirectional (all running in the same direction) and offset by 90 degrees in each successive layer. While this material, which is still widely used in soft armor, has impressive performance (much lower AD than aramid based armors, no UV susceptibility, positive buoyancy), there are fatal flaws that an end-user must be made aware of.

In addition to having the drawbacks of aramid laminates (de-lamination/peeling, extremely poor breateability), UHMWPE laminates also suffer from heat sensitivity. The UHMWPE molecule is chemically similar to garden-variety Polyethylene (the same material used in plastic milk jugs). When Spectra or Dyneema is exposed to temperatures above 170 Degrees F, it permanently and irreversibly denatures/reverts to the same milk-jug plastic (which has absolutely NO ballistic properties at all).

Because armor is often exposed to a wide range of temperatures (for example, in many parts of the country, a car trunk/interior can easily reach 180-190 F), this is a major concern. Furthermore, since there is no visible change to the material, there is no way for the end user to know if their armor is still viable, or merely layers of coffee can lid. Originally, woven UHMWPE armors were produced (called Spectraflex), but since the higher surface area of the woven fibers made the armor even more prone to heat degradation (a hot cup of coffee, for instance, would have a greater effect on a woven UHMWPE vest compared to a laminate, due to the vapor/moisture barrier properties of the laminate), they were quickly withdrawn from the market.

In addition, Spectra/Dyneema based armors fare poorly in situations where they may be subjected to contact shots- the hot muzzle blast gasses can melt the armor around the impact area, allowing the bullet to penetrate more layers (sometimes even the entire vest) than would have been possible with a woven aramid based vest. All laminates suffer poor contact shot resistance, but UHMWPE is especially susceptible. I will be dedicating a post on contact shots in the coming months.

What does all this mean for you, the end user? First of all, it is vital to identify armor containing UHMWPE, and to a similar extent, first and second generation laminates (I will be posting a tutorial on armor material identification in the coming months). If you are able to assess your needs prior to purchasing armor, ask yourself if you will be operating in environments that expose the weaknesses of UHMWPE or laminates (potential for elevated temperatures, likelihood of contact shots, requirement for high exertion/perspiration). If none of these circumstances are likely to be encountered, the dangers of UHMWPE/Laminates will be minimized. But if one or more apply, it is strongly recommended you find an armor system consisting of 100% WOVEN ARAMID.

It is important to note that this applies only to soft armor. UHMWPE/Spectra/Dyneema/Aramid Laminates find extensive use in hard/rigid armors (both as a pure defense and as backing material for the strike face. This will be discussed in a later post, but for now, please note that evidence strongly suggests in a rigid configuration, UHMWPE/Laminates do not exhibit the same dangers/weaknesses as when utilized in soft armor.

Next: The most dangerous (to the wearer) soft armor material. Stay tuned.