Body Armor Basics: Soft Body Armor
What is body armor?
Modern body armor is an insert, made of a strong material, that fits inside a garment such as a vest or plate carrier. Body armor comes in two varieties: “Hard” and “soft.” Hard armor generally consists of rigid panels which are built to offer protection from common rifle rounds. Soft armor generally consists of flexible, layered composites or textiles, and provides protection from handgun and fragmentation threats. Here we will focus on soft body armor.
Soft armor general:
– As just mentioned above, soft body armor is made of multiple layers of ballistic fabrics  or unidirectional ballistic composites.  Both types of material may be combined in a single soft armor panel.
– At present, soft body armor is generally rated in accordance with the NIJ’s 0101.06 specification for body armor, and then either to Level II or Level IIIa.  In the forthcoming NIJ 0101.07 specification, Levels II and IIIa are re-named Handgun-1 (HG1) and Handgun-2 (HG2). Here suffice it to say that an armor panel rated to Level II/HG1 is built to stop multiple shots from .357 Magnum, and an armor panel rated to Level IIIa/HG2 is built to stop multiple shots from the .44 Magnum. Both at around muzzle velocity from a 6”+ barrel.
– Military-issue soft armor does not carry a handgun or NIJ rating at all, but is generally built to stop fragments at a specified size and velocity range — i.e., a military soft armor panel might be rated for the 17-grain fragment-simulating projectile (FSP) at 2200 feet per second. However, in terms of performance against small arms threats, it is generally accepted that most articles of military-issue soft armor are approximately equivalent to NIJ Level II/HG1 armor panels.
– Because sizes and cuts are variable and can be irregular, the weight of a soft body armor panel is generally given in pounds per square foot (psf) — or, in Europe, kilograms per square meter. As of this writing, the lightest Level II soft armor panels weigh about 0.5 psf, and the lightest Level IIIa soft armor panels tip the scales at roughly 0.7 psf. These ultra-lightweight armor panels are generally comprised of the latest and most exotic UHMWPE composite materials. Average soft armor panels — including the military’s frag-rated systems — are around 1.0 psf ±15%. Low-end systems made from the cheapest woven aramids can weigh as much as 1.6 psf, and are frequently around 1.3 psf.
– On average, all else being equal, a Level II/HG1 soft armor panel is going to be approximately 25% lighter and 25% thinner than its Level IIIa/HG2 counterpart. 
– As a general rule, soft body armor systems are not stab-proof.  Unless the manufacturer indicates otherwise, and has test reports to prove it, don’t count on your soft armor panel stopping a knife.
How does soft armor work?
– The underlying principles by which laminate systems comprised of strong fibers are able to efficiently stop ballistic threats are still not fully understood.
– At or under the ballistic limit of the system — that is, against threats that the armor panel is rated to stop — the kinetic energy of the incoming projectile is converted into axial fiber strain energy, heat via friction, and is expended in the deformation of the projectile and deformation of the armor panel. Most of the energy absorbed by the armor system is carried away from the impact site at the speed of sound in the material. Roughly 50% of the incoming projectile’s energy is converted into heat.
– Past the ballistic limit — and, invariably, on the top several layers of the armor panel — localized inelastic failure mechanisms predominate. These failure mechanisms can resemble the shear-plugging that one might observe in metallic armor systems, i.e. with fiber failure around the impact site, and an ejected plug of material traveling in front of the bullet.
What else is there to know about soft body armor?
– UHMWPE materials can degrade if continually or regularly exposed to temperatures higher than 158°F. Such temperatures can be attained, or exceeded, in closed cars on Summer days. 
– Aramid materials can degrade if continually or regularly exposed to UV radiation. In practice, though, this would only affect the top layers of an armor panel, even if the aramid fabrics are totally exposed. As most aramid panels are made up of at least two dozen layers, this should not be much of a concern.
– Soft armor systems made primarily of UHMWPE — which tend to be the lightest and most expensive armor panels in their category — can be weak to contact shots. This is because contact shots are associated with exceptionally high thermal stresses at the point of impact, often well past UHMWPE’s autoignition temperature of ~750°F. Indeed, when subjected to a contact shot, or a very high velocity impact, “flashes” can be observed in UHMWPE panels, and are associated with the sudden and anomalous failure of the first few layers of material.
– Several new soft armor systems are made up of layers of pressed UHMWPE. So instead of a couple dozen flexible layers, they’re made of three or four much thicker layers. This sort of soft armor offers an improved performance-to-weight ratio, but is significantly less flexible than “traditional” soft armor.
Soft Armor Panels: Bottom Line
There are trade-offs to carefully consider when selecting a hard armor plate, but soft armor is conceptually much simpler: There’s really only one kind of soft armor panel, all soft armor panels perform similarly against the threats they’re rated to stop, and they’re all broadly similar in thickness, weight, flexibility and comfort.
There are small differences that might be worth keeping in mind. Aramid, e.g. Kevlar, is somewhat heavier than UHMWPE — but it’s more thermally stable, so it might be preferred if you live in the West Texas desert, or if you wear armor covertly and expect that you might need protection from contact shots. It is also more flexible, can be somewhat more comfortable to wear, and is much cheaper. For its part, UHMWPE-based soft armor systems exhibit a superior performance-to-weight ratio, and can also be somewhat thinner than aramid-based systems.
Ultimately, however, any soft armor panel that suits your needs and budget should provide adequate protection against all common handgun threats.
– These are typically derived from para-aramid. Para-aramid is rarely sold as a “generic” material; there are many trademarked materials derived from para-aramid, which include Kevlar, Twaron, Artec, Heracron, and others. There are frequently subtle chemical differences between these trademarked materials.
These consist of multiple layers of a fibrous material, laid-up — often with layers at 90° to each other, i.e. at a 0/90° fiber orientation — and held in place with a resin component. A “sheet” may consist of anywhere from two to five layers of fibrous material. UHMWPE composites are the most popular example of this class of material. Dyneema and Spectra-Shield are trade-names for UHMWPE composite materials that consist of unidirectional UHMWPE combined with either a polyurethane or a Kraton elastomer resin.
Detailed information is available on our testing standards page. Practically speaking, Level II/HG1 armor will stop virtually all handgun threats that one is likely to encounter, and Level IIIa/HG2 will do the same — and the latter may also provide a little bit more peace of mind, and may stop those rounds with less residual blunt trauma.
Safariland makes excellent soft armor; their Matrix-II weighs 0.9 psf and is 0.212” thick, whereas the Matrix-IIIa weighs 1.2 psf and is .29” thick — which means that the Matrix-II is exactly 25% lighter and thinner than the Matrix-IIIa. Their other models, e.g. the Summit and SX, exhibit the same relationship between their Level II and IIIa variants. That said, it’s important to note that the Level IIIa version of the SX is lighter and thinner than the Level II version of the Matrix. One is a premium product that utilizes the most advanced materials, and the other is very basic, which is reflected in its price.
This is because knives strike with what is effectively an extremely high sectional density. Also, because they are made of materials that do not easily deform, and they have sharp edges that are capable of simply cutting or shearing ballistic fibers and fragments. The vast majority of Level IIIa armor systems would fail the NIJ’s “Knife-1” test miserably.