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.
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