Body Armor Plates Guide
The proper use of body armor – Practical considerations in the selection, wear, and maintenance of hard body armor plates.
Body Armor Cuts:
Body armor torso plates generally come in four different styles.
Full cut plates
The full cut is rectangular, often with clipped or rounded corners, and is usually intended for use as a rear plate. (More on this later.) It is the largest of the plate cuts. A full cut 10×12” armor plate has a surface area of very nearly 120 square inches. If it has an areal density of 6 pounds per square foot, weighs 5 pounds.
SAPI cut plates
The SAPI cut, which is the most popular and widespread body armor cut, is a design modeled after the US military’s SAPI body armor plates. In SAPI cut plates, the top corners have been cut at 45 degrees from the diagonal. To enable better shoulder mobility and facilitate the use of small arms. A 10×12” SAPI cut plate should have a surface area of approximately 108 square inches. If made from the same 6 psf material as our full-cut plate, would weigh 4.5 pounds.
Shooter’s cut plates
The shooter’s cut is very similar to the SAPI cut, but the top corners are cut away at a slightly steeper angle — often at 60° — which makes the plate lighter and enables more shoulder mobility, though it sacrifices some coverage. This cut is not standardized, and varies from manufacturer to manufacturer. Generally, a 10×12” shooter’s cut plate will have a surface area of around 100 square inches, and would weigh slightly less than an otherwise identical SAPI cut plate. In this case, at the 6 psf of our previous examples, around 4.2 pounds.
Swimmer’s cut plate
The swimmer’s cut is generally along the same lines as the shooter’s cut, but the top corners are cut at an even steeper angle which extends further down the plate. This is a fairly uncommon cut, and can usually be considered a variant of the shooter’s cut. I used the word “generally” in the first sentence here because there’s really no clear definition of what this term means; sometimes “shooter’s cut” and “swimmer’s cut” are perfectly interchangeable.
What body armor cut you select comes down largely to personal preference. Rectangular plates should not, however, be used in front. Note that swimmer’s cut plates often sacrifice a lot of protective coverage. They don’t provide much better shoulder mobility than SAPI cut or shooter’s cut plates, so the trade-off might be a poor one.
Proper Positioning of Body Armor:
Ideal fit and positioning is indicated in the image below.
The top edge of the front armor plate should reach the clavicle. The sides of the plate should cover each nipple. The bottom edge of the plate should terminate at or slightly above the bottom of the ribcage, and approximately 3 to 4.5 inches above the belly-button. The plate should not hinder or interfere with the shouldering of a weapon.
The rear plate should be positioned exactly opposite the front plate, and no lower. Some recommend that, ideally, the back plate should be one size larger than the front plate. Because larger back plates, unlike oversized front plates, are far less likely to hinder arm, shoulder, and trunk mobility. For the same reason, it’s common for European military and police operatives to wear SAPI cut or Shooter’s cut plates in front, and rectangular plates in back.
But as you’re far less likely to be shot through the back than through the front, and as there’s clearly an additional weight burden associated with larger or rectangular armor plates. We don’t believe that these rear plate practices are very well thought out. They may also be difficult to put into practice, for modern plate carriers may not accept rectangular plates. Carriers are sized in such a way that they may not accept a size L rear plate alongside a size M front plate.
Though not indicated in the image, the armor carrier’s shoulder straps should ideally be positioned over the shoulder midline. So that the clavicle can provide mechanical protection and support to underlying soft tissues. This can reduce shoulder discomfort and injury risk.
Side plates are also not indicated in the image. These come in two standard sizes: 6×8” and 6×6”. Only people exceedingly small in stature should select 6×6” side plates. The function of the side plate is to protect the vitals, the heart, lungs, and diaphragm. Not to protect the side of the gut. The taller the plate is, the more vital coverage it will offer, and the better it will perform in its intended role. Side plates should also, for this very same reason, be worn as high as comfortably possible.
Needless to say, an ideal fit isn’t always possible. For while human builds vary widely, body armor plates are only available in a handful of standard sizes. What’s necessary is that the armor plate’s top edge reaches the lowest part of the clavicle. What’s also necessary is that the plate is secured to such an extent that it doesn’t move when its wearer is in motion. But, at the same time, isn’t so tight that it restricts its wearer’s breathing.
Ceramic armor is highly effective, but the ceramic layer can be damaged if dropped or handled carelessly.
There’s not much data on how cracks affect the performance of ceramic armor systems. But there’s enough to confirm that cracks decrease ballistic performance. In “The Effect of Through-Thickness Cracks on the Ballistic Performance of Ceramic Armour Systems,” published in 1995 by Horsfall et al., a cracked system performed worse than an intact ceramic system by 3%.
“[T}he introduction of a full width, through-thickness, pre-crack reduces the V50 ballistic limit velocity from 764 m/s to 740 m/s, a drop of 3% … The Students t-test was then used to determine the significance of the difference in the [V50s] of the standard and pre-crack samples. This difference was determined to be statistically significant to a level of better than 1%. Therefore it is possible to conclude that the pre-crack does significantly reduce the V50 ballistic limit.”
Ballistic and physical properties of highly fractured alumina
In 2010, Horsfall revisited the subject in a paper titled “Ballistic and physical properties of highly fractured alumina,” where intact undamaged alumina, explosively shattered alumina, and pressed Al2O3 powder were compared in ballistic experiments. The ballistic performance of the shattered alumina was roughly 30% worse than that of the intact alumina. The pressed powder was >40% worse. (To such an extent that it really had very little ballistic resistance at all.)
So it appears that through-thickness cracks can decrease performance by as little as 3% to as much as 30%. Or perhaps even more than that, depending on the threat and the extent of the damage. In any case, it is clear that the ballistic performance of damaged ceramics is degraded. So non-destructive methods of ceramic plate evaluation are critically important.
The first to see widespread use was the “torque test” described by Haynes et al. in the 2009 paper “Automated Non-Destructive Evaluation System for Hard Armor Inserts of Body Armor,” which could hardly be any simpler. To quote:
“[The torque test] is a process which an individual grabs opposing corners of a plate and tries to twist the plate listening for crunching or cracking. When this test works, adjoining surfaces of a crack rub and create the sound that reveals the crack.”
Though the torque test has a high false-positive and false-negative rate, it can easily be done in any quiet location, at any time, without any equipment whatsoever. It has become quite popular, and should be employed whenever ceramic armor needs to be inspected.
There is, however, an important exception. If a ceramic armor plate is mosaic-style, the torque test will not work and should not be performed. It’s far more likely to lead to a false positive than a valid result, and it may even damage the armor plate. If you do not know whether your ceramic armor plate is monolithic or mosaic-style in construction, contact the plate’s manufacturer.
Similar to the torque test is the “tap test”, which was developed by the US military and is used to evaluate whether or not certain types of ceramic plate have had their ceramic components delaminate from their backers. As a general rule, the tap test is only useful in the evaluation of military SAPI/ESAPI and SPEAR armor plates.
The test is performed by holding a ceramic plate on the fingertips of one’s non-dominant hand, and then striking the edge of the ceramic strike-face with a metal implement like a bolt or folding knife. If the plate rings or chimes, much like a bell, then it’s good to go. If the plate makes a dull thudding sound, delamination should be suspected.
The tap test won’t work at all on modern armor plates built to comply with the NIJ .06 and forthcoming .07 standards, because those plates generally have thick foam layers on their surfaces. But if you have, or are issued, SAPI or ESAPI plates — or if you have ceramic armor plates with a minimal foam layer or no foam layer at all (such as the Hesco 4800LV and many older plates built to the NIJ .04 specification) — the tap test should be performed whenever your plate requires inspection.
Ceramic plates can, of course, also be tested via more sophisticated non-destructive means. Methods proposed in the scientific literature include digital X-ray evaluation, ultrasound, X-ray computed tomography (XCT), microwave interference scanning, and infrared thermography. Of the lot, XCT is the most established and well-validated method. XCT requires dedicated and sensitive instrumentation and cannot be performed in the field or individually by end users.
The primary advantage of XCT is that it provides 3-dimensional structural information. Which makes results very easy to interpret and fast to read. Furthermore, XCT images are digital, so the test results can be enhanced, superimposed over digital data from other test sources, and rapidly transmitted to the end-users. Large police departments and certain military units may find it worthwhile to run XCT analyses of in-service armor plates on a regular, though necessarily infrequent, basis.