A Potential Mechanism for Defeat of Pure Polyethylene Plates by M-855 Projectiles

In more than a decade and a half of destructive testing of armor, to include both pure ultra-high molecular weight polyethylene (UHMWPE) and UHMWPE-backed rifle plates, it has been interesting to note that standard “Green Tip” (aka M-855) ammunition easily penetrates pure UHMWPE plates, and even causes problems for UHMWPE-backed hybrid/combination rifle armor.

Which lead to the working hypothesis that M-855 is able to defeat this material as a function of two factors:

A) Projectile heat, and

B) Non-deformable core

Regarding the first factor, independent testing by several disparate groups has demonstrated that 5.56 bullets can attain temperatures in excess of 500 degrees F upon leaving the muzzle, and retain those elevated temperatures out to several hundred meters. As readers of this site will recall, UHMWPE has impressive strength (tensile strength 15 times that of steel), but is extremely sensitive to elevated temperatures. Above 183 degrees F, it will experience “de-naturation,” and revert back to essentially the same material found in polymer milk-jugs. Hence, a projectile at three times this critical temperature would likely cause heat-denaturation (and outright melting @ 260-277 degrees F) of the fibers in the vicinity of the impact zone.

This would likely be far more noticeable in a pure polyethylene plate, but would also be a factor in ceramic and metal-faced hybrid plates (albeit much less of a factor due to projectile-fracture induced by these hard face materials causing a reduction in projectile thermal mass). M-855 projectiles, having a moderately hard (~45-50 RC) steel insert near the tip, would act as a “hot knife” system (which is how material such as UHMWPE is cut in the manufacturing process). The (ideal scenario) impressive tensile strength for UHMWPE material would be irrelevant due to heat-induced loss of strength.

Regarding the second factor, it offers a possible explanation as to why rounds such as M-193 and M-80 lead core ball are dealt with more easily by pure polyethylene plates. Even though their residual temperatures would be similar (~500 degrees F), their cores are easily de-formable (more so due to heat-softening of the core alloy). It is reasonable to conclude that though similar localized heat-denaturation is taking place, impact forces would cause enlargement of the frontal area of the projectile through deformation of the soft lead-alloy core of these threats, allowing engagement of a greater number of fibers, more rapid cooling, and commensurate increase in projectile-defeat efficiency of the plate.

Comparitively, UHMWPE is roughly 15 times stronger than steel on a per-weight basis. Aramid is roughly 7 times stronger than steel on a per-weight basis. However, UHMWPE loses 100% of its strength (de-naturation and melting) upon instantaneous exposure to temperatures above 277 degrees F. Whereas aramid fibers lose 10% of their strength -over the course of 500 hours continuous exposure- to temperatures above 320 degrees F, and 50% reduction -over the course of 70 hours continuous exposure- to temperatures of 500 degrees F.

So, in an apples to apples comparison, a bullet @ 500 degrees will reduce UHMWPE to essentially zero percent of its prior impressive tensile strength, while aramids will lose a small percentage (instantaneous vs. continuous exposure to 500 degrees F). Even assuming full strength loss similar to 70 hours continuous exposure, aramids will still be 3.5 times stronger than steel vs. UHMWPE (which functionally reverts to standard PE).

So, in an apples to apples comparison, a bullet @ 500 degrees will reduce UHMWPE to essentially zero percent of its prior impressive tensile strength within the impact zone, while a similar aramid matrix will lose a small percentage of its on-paper tensile strength (instantaneous vs. continuous exposure to 500 degrees F). Even assuming full strength loss similar to the worst-case 70 hours continuous exposure, aramid will still be 3.5 times stronger than steel vs. UHMWPE (which functionally reverts to standard PE structure).

It is therefore suggested that aramid unidirectional bias-ply materials be utilized in armors not containing a hardened strike face as a matter of course, specifically in the first third of the ballistic structure. Being far more tolerant of heat, aramid unidirectional fibers would serve to slow and cool the projectile before “hand-off” to the UHMWPE fibers making up the remainder of the plate. This suggestion also pertains to hybrid plates with a metallic or ceramic strike face. It is postulated that up to 50% of the total fiber mass of the pure fiber plate could be constructed with aramid, with a concurrent non-linear increase in functional efficiency.

Conclusions and suggestions for further research: It is reasonable to hypothesize that due to the unique characteristics of UHMWPE used in armor systems, projectiles that include a non-deformable core or sub-core (such as M-855, and the newer M-855A1 and M-80A1) will be more likely to defeat pure UHMWPE plates, and cause decreased real-world efficiency in plates utilizing this material as a backing matrix.

Further research is suggested, in particular post-impact measurement of the frontal cross section of both M-855 and M-193 prejectiles recovered from pure-UHMWPE plates. For more sophisticated labs, microscopic and mechanical evaluation of impact-adjacent UHMWPE fibers can be performed to evaluate potential temperature-induced loss of strength, and/or structural changes that would indicate compromised mechanical characteristics. Similar testing of unidirectional aramid used in rifle backing can also be performed to evaluate the amount of strength loss in each material. It is hypothesized that aramid will be found to lose far less strength as a percentage of its starting strength vis-a-vis UHMWPE.

It is reasonable to suggest that a pure aramid backing matrix for rifle plates would achieve similar or even superior performance vs. UHMWPE backing matrices when confronted by centerfire rifle projectiles due to the greater heat tolerance inherent in aramid. This in spite of aramid’s “on-paper” tensile strength difference compared to UHMWPE. It is further postulated that through optimization of the backing matrix (either with judicious combination of aramid with UHMWPE or use of a pure aramid backing matrix), existing designs might be incrementally improved in excess of what would be expected if simply looking at the “ideal scenario” mechanical strength numbers. Until such time as new materials (such as DuPont’s pending M5 fiber) are made widely available, it is suggested that aramid remains the “best practices” ballistic fiber for broad usage scenarios.

As always, it is my hope that this information will be used to improve the efficacy and safety of life-protecting products.

Copyright 2022, fair-use notice permitted with attribution.

Gear Review: Midwest Armor Sigma III+ Rifle Plate

The legendary Tactical Armor Products GAMMA III+ series of plates was one of the most highly regarded rifle plates of its time. With decent weight, good quality build materials, and excellent stopping power, the now extinct TAP’s plates command a premium on the secondary market.

To fill the vacuum left by the discontinuation of the GAMMA, Midwest Armor stepped up to the plate. Like pretty much everything else in their linup, their continuation of the lineage, the SIGMA, was a grand slam.

I ordered my sample of this plate from Appalachian Training. Though they did not even have it officially in stock at the time, Mike was more than happy to get the ball rolling. The plate was a little over $449 shipped, thanks to Appalachian’s enviable flat rate shipping rate.

Due to an unfortunate (albiet very minor) issue with the plate’s PU finish, I got to experience the superhuman customer service that both Appalachian, and Midwest, strive for. I alerted Mike to the issue, and from that moment, it was as if a small army of extremely efficient Armor Ninjas were set in motion. I had an RMA number and label in my hands within mere hours, and the plate was back to Midwest, but NOT BEFORE they had already initiated shipping on a replacement. The box showed up two days later, full of extra goodies (some edible, some not). Now that, folks, is how CS is done.

The replacement plate arrived in good order, and the unboxing commenced. Even though this is armor, and is rugged as heck, the packing job done by Midwest would have kept a dozen eggs from getting so much as a single crack. The extra little details really matter.

Midwest Sigma 3+ front strike face.

The first impression of the plate was “tough.” The tan PU coating is really grippy and resilient. Curvature is not too extreme, and fit my plate carrier very nicely (SKD PIG). Weight is right at 5.45 lb. and thickness is 1.27″. The weight puts this into the “just under medium” category. Much lighter than steel, it is still heavier than the pure UHMWPE plates. However, this is due to the welcome addition of a thin ceramic strike face (in keeping with the GAMMA theme), which allows it to defeat the Green Tip M855, the bane of all UHMWPE plates.

Rear face with label

Thickness is right at 1.27″

Overall, I give this plate 4.5 out of 5 stars. The only reason it was not 5 stars is that I generally don’t give 5 star ratings to anything. This plate is rugged, relatively affordable, and comes with a 5 year warranty (and take it from me, if you need the warranty, you will be well taken care of).

Kudos to Midwest Armor and Appalachian Training. They make and sell excellent products.

http://www.midwestarmor.com/body-armor/sigma-iii

http://store.appalachiantraining.com/SearchResults.asp?Search=sigma&Submit=

Body Armor: The Good, The Bad, and The Ugly, Part VIII- Rigid UHMWPE

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.

The Trouble With Front Face Spall/Splash

Steel rifle plates have several advantages, especially for certain applications (PSD, concealed). they are the thinnest profile of all rifle plate styles, are generally very inexpensive, do not suffer from fragility issues inherent with ceramic plates, and handle multiple hits extremely well (some plates have been documented with hundreds of rounds while still retaining their protective qualities).

Interior of 6X6 splash guard after M193 3100 fps impact @ 10 feet on AR500 steel side plate. Non penetrating strike.

With all these advantages, there are several drawbacks, the largest and most pressing being that of front face spall (also known as splash). Spall refers to any ejecta resulting from an impact event. Rear face spall has been an issue mainly with tanks/AFVs for as long as they have been used. In WWI, British and German tank crews suffered casualties from rounds impacting the armor and kicking off shards of the plates at high velocity. Even though the rounds may not have penetrated, damage was still done. The British took advantage of the principle of spall with their development of the HESH round after WWII. This was an explosive shell with a soft head, which, when it hit the enemy vehicle, exploded on the outside. The shock wave blew large chunks of spall off the interior of the vehicle.

Front face spall, or splash, is the result of the projectile disintegrating into a cloud of high velocity fragments. The cloud usually manifests as a primary and secondary conoid of splash, the first being made up of the larger jacket pieces. This usually bounces back at an angle very close to the initial strike. The secondary conoid, which makes up about 80% of the total splash threat, travels primarily along the face of the plate, to exit at close to 90 degrees from the original strike. It is this conoid that presents the greatest threat to life and limb.

Because of how steel rifle plates function, this unfortunate property occurs every time a bullet is successfully stopped. Stopping the round is the purpose of the plate, but up until recently, no consideration was given to keeping the face, throat, and extremities safe from the cloud of cutting fragments. Generally, higher velocity rounds produce more spall (the M193 is a more dangerous round for splash than M80 ball, for instance). The core composition will also play a large role in splash/spall. The M855 produces less spall, due to a larger volume of the core being taken up by a mild steel penetrator. The penetrator is fairly easy to trap with spall mitigation technologies. Close range impacts will obviously pose a greater spall/splash risk than longer distance shots.

Early solutions (still used today) involved plastic or elastomer coatings (in some cases, the same materials used as truck bed liners). These coatings range in thickness from fractions of a millimeter, to nearly half an inch. While effective (at least in the thicker iterations), the material is removed by each successive hit, and in the thinner versions, the spall mitigation is very limited. The main drawbacks involve the thickness of effective coating depth, which mitigates the main point of steel plates (their thin profile), the fact that the coating is permanently removed (which is difficult or impossible to replace). Coated plates often have an offensive, strong chemical odor as well. Coating steel plates is not a bad rust-preventative, but the thinnest coating is recommended.

Other, more recent solutions involve adding metallic components to the plate. While undoubtedly effective at catching splash exiting the sides of the plate, adding MORE metal, MORE weight, and MORE thickness to already heavy plates is not an optimal solution.

Some users have attempted various DIY “solutions,” which usually include haphazard combinations of duct tape, epoxy, gorilla glue, woven aramid or UHMWPE, or brush-on truck bed liner. These solutions are a poor choice, tending to turn steel plates into a messy, ugly, smelly, and above all, ineffective mess. The plates will still stop the rounds as usual, but the splash mitigation is negligible. Because the attempts do not take into account the dynamic nature of an impact, the strong adhesion will couple the facing materials to the plate, resulting in very quick erosion of the homemade guards. This, combined with poor choice of materials (woven Kevlar fabric, or UHMWPE laminate) guarantees failure.

Specially designed slip-on guards utilizing custom engineered/manufactured aramid blend fibers have so far proven to be the best combination of attributes for capturing front face splash/spall. These guards can be removed, swapped between plates, or replaced once used, and are thin, light, and above all, COMFORTABLE. They can be built for ANY steel (or ceramic, for that matter) plate currently extant.

Steel plates can be an excellent choice, but it is vital for end users to be aware of potential safety issues. There are several options for mitigating splash, each with their unique benefits and weaknesses. Above all, ensure you are using adequate splash protection if you choose to run steel rifle plates, and keep your face and extremities safe!

https://drmorgear.wordpress.com/products/spall-guards/

Version 4.2.2 Splash/Spall Guards Now Available!

Steel protective plate are a great option if you are looking for both thin profile and affordable costs, but do have a well known issue that can be hazardous to your health: bullet splash (also known as front-face spall).

Splash is the result of projectiles breaking up on impact with the steel plate (and in fact, is part of the plate doing its job to save your life). This cloud of razor sharp lead and copper poses its own threat to life and limb when exiting the top, bottom, and sides of your plate carrier. This should be taken into consideration when purchasing/using steel plates (and even many ceramic plates).

Other solutions negate the advantages of steel plates, by making them thicker and even heavier. They are also difficult to repair or replace, negating another of steel’s advantages (its durability/multi-hit properties).

Look no further than the D-Rmor Gear Splash guards, custom made and continually improved since 2007.

The D-Rmor Gear Splash/Front Face Spall guards work WITH the steel plate strengths, offering the following notable advantages:

-Thin
-Light
-100% all-fabric construction
-End-user replaceable
-Comfortable
-Effective

In addition, they carry a 100% lifetime guarantee: if they are ever used in a documented duty/self defense situation, they will be repaired or replaced, for the life of the product, fully transferable.

Available in Coyote Brown, and custom made to fit your plates! Check out the link for more information and to get your set today!

https://drmorgear.wordpress.com/products/spall-guards/