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.

Buffman R.A.N.G.E.: An Invaluable Resource for Armor Testing and Evaluation

Here at D-Rmor Gear, we appreciate no-nonsense, no-hype evaluation of armor (it is, after all, why we started the site).

It is for that reason that we are continually impressed with the work being done by Mike at Buffman R.A.N.G.E. He has, singlehandedly, done more for the armor testing and design community than just about any other site or organization.

Just to name a few of his contributions:

Testing and evaluation of several popular ceramic rifle plates vs. the extreme threat projectile M995 black tip round.

Testing and evaluation of the U.S. ESAPI plate vs. M995, M2AP, and overpressure M2AP rounds.

Honest evaluation of several new body armors offered by several companies.

We strongly urge you to check out his channel at:

https://www.youtube.com/channel/UCE0hyXJkKD_JyEeoA0OrROw

Our hats are off to you, Mike.

Body Armor Test: Hypervelocity .22 LR vs. Level I and IIA Soft Armor

For several years it has been debated whether or not .22 LR could or could not penetrate soft armor vests.

Originally, in the 1970’s, Kevlar soft armor was developed to protect Officers against common street threats, which typically were .22 LR LRN, .32 ACP LRN, and .38 SPL LRN. Threat level I vests were certified to stop these ubiquitous, low-velocity threats. As time went on, the threats escalated, and more powerful rounds became common, necessitating thicker vests. But, with the advent of the internet, rumors persisted and spread that called into question whether or not the .22 LR could pose a valid threat to lower level soft armor (I and IIA).

Level I soft armor is seldom seen in this day and age. Typically comprising between 6 and 10 layers of woven aramid, it lacks the thickness to provide sufficient protection against backface deformation. Surprisingly, Level I armor will often stop rounds such as .45 ACP hardball @ 850 FPS, or even .40 S&W. However, these rounds leave a very large backface signature, regardless of whether they are stopped by the armor.

.22 LR has the distinction of being a very good penetrator, primarily due to basic physics- it has a very small frontal area, and can achieve relatively high velocities (1400 FPS from a 16-20″ barrel is not unheard of in certain loadings, I.E. CCI Velocitors). However, it is not a jacketed round, and therefore deforms fairly easily.

The NIJ specifications for Level I call for it to be able to stop .22LR LRN at or below 1050 FPS. Now, it is very important to note the velocity threshold- most longer barrels (above 10″ or so) push .22 LR above this velocity, and therefore can be expected to defeat level I armor. The ongoing debate on the Interwebs rages, but without paying much attention to the distinction between .22 out of a short barrel (handgun) vs. a long barrel (rifle). In order to (hopefully) put this debate to rest, I am posting a test.

This test is aimed at settling the longstanding debate on whether .22 LR is a threat to lower rated soft armor (I and IIA) Furthermore, it seeks to establish whether it is a viable threat only in longer barrels, or both long and short barrels, when faster ammo/more pointed rounds are used.

For this test, the ammo used is Remington Viper 36gr. Hypervelocity round, which features a solid copper washed truncated cone lead bullet and a stated MV of 1410 fps (out of a 20″ barrel). This round was chosen as the shape is more conducive to penetration (smaller frontal cross section). This is fired from a 16″ and a 4″ barrel, from 12 inches.

The panels (two level I and two level IIA) are identical, and will each be shot only once to allow for their full ballistic potential to be evaluated vs. each barrel length. The level I panels comprise 7 layers of Kevlar 29, while the level IIA panels comprise 12 layers of the same material. The backing material is Roma Plastalina #1 modeling clay, to allow backface deformation/penetration to be evaluated and observed.

Stay tuned for the results.

Armor Test Standards: A Brief Comparison

Ever since the late 15th century, with the advent of powder-propelled projectile weapons (and indeed, pre-dating that time with crossbows), armor smiths have sought a way to ensure their product will resist (within reasonable limits) the injurious tendencies of fast moving bits of metal.

Smiths making plate armor would often shoot the finished product (with crossbow bolts prior to lead muzzle loader round balls arriving on the scene), and upon visual confirmation of a successful “stop,” would engrave their maker’s mark, showing the armor had passed. This methodology was (and is) referred to as “bench testing” or “proof testing.” An article of protective gear is subjected to one or more ballistic events, and when it either passes or fails, provides the maker, and the end user, with evidence that it is effective, or “proofed.” Many modern firearms bear similar proof marks after being subjected to equivalent testing.

Bench testing is still used today, and while it has certain advantages, it also has some notable drawbacks. For instance, if large numbers of finished articles need to be produced, it becomes ungainly to batch test each lot (a neccesary requirement to verify efficacy of the finished product). It is also subject to the whims of either the maker or the end user. Bench testing is much more suited to custom, or small-batch manufacturers of armor.

Bench testing remained the norm until the 70’s, when it became necessary to certify large numbers of concealable vests. The National Institute of Justice (NIJ) scrambled to come up with a way of testing and certifying large quantities of vests. The “NIJ Rating” methodology was created, which should be familiar to everyone with any exposure to armor. The rating levels go from I to IV, and are directly proportional to what threats are stopped. I-IIIA are for soft armor, while III and IV relate to hard (rifle) armor. The tests, in a nutshell, subject a batch of test armor articles to successive rounds of ballistic testing. This ranges from 2 to up to 240 rounds fired, and newer iterations of the tests have become more stringent. However, there are some issues with NIJ testing, which will be discussed in a later post.

In just the past few years, two new test protocols have appeared on the scene: the FBI and DEA test protocols. Publicly released in 2006, the FBI protocol was a vast improvement over the NIJ protocol, subjecting the armor to much more realistic and useful tests. In addition to simply shooting the armor, the FBI/DEA protocols subject the armor to extreme heat, cold, immersion as well as conditioning the armor and subjecting it to flash/flame. While there are still some issues (one in particular that is shared by the NIJ tests), they are far superior to the earlier protocols.

Moving forward, there is still room for improvement regarding armor test standards. This has been a brief overview of the most typical test protocols for modern body armor. In future posts, more detailed analysis of each protocol will be given. Thanks for reading.