While there are many different standards for ballistics protection levels, the two most commonly recognized standards for body armor come from the US National Institute of Justice (NIJ) and the UK Centre for Applied Science and Technology (CAST, formerly HOSDB). These are widely held to be the world leaders in standardizing body armor. Testing methods are often shared between the two, so armor that meets the standards of one will meet the requirements of the equivalent.

These standards outline exactly what threats each level of body armor will protect against, as well as the strength of the attack it will stop. This means that a bullet proof vest at Level II for example will not protect against a Level IIIa threat. But what do these levels mean?

Ballistic Levels

Bullet proof vests are designed to ‘trap’ and slow bullets down as they attempt to pass through the vest. Bullet Proof Vests come in two different types; hard and soft. The ‘soft’ armors are designed to protect against most handgun and small arms ammunition, whereas ‘hard’ armors are designed to stop high caliber and armor piercing rounds. The NIJ classifies all armors in the following levels: 

		NIJ Level IIa	NIJ Level II	NIJ Level IIIa	NIJ Level III	NIJ Level IV
	Areal Density	3.5 kg/m	4.2 kg/m	5.9 kg/m	25.9 kg/m	32.5 kg/m
	Thickness	4mm	5mm	6mm	15mm	20mm
.22mm short		x	x	x	x	x
.9mm		x	x	x	x	x
.45mm		x	x	x	x	x
.380mm		x	x	x	x	x
.38mm		x	x	x	x	x
.22mm long			x	x	x	x
.44 Magnum				x	x	x
.30 Carbine					x	x
5.56mm					x	x
7.62mm NATO					x	x
.30-06					x	x
.30 Armour Piercing (M2 AP)						x

 

Edged Blade Levels

While the NIJ is considered the world standard for ballistics testing, stab-proof vests are standardized by the CAST. Many assume that if a piece of body armor can stop a bullet, it can stop a knife. This is not the case however, and stab proof vests are different in make up to bullet proof vests. It is possible to find multi-threat vests that can protect against both, but these still need to be graded according to the protection they offer against both bullets and edged/spiked weapons. The threat levels for stab proof vests are standardised by the CAST as follows:

 

	KR1		KR2
	Knife Resistant Level 1		Knife Resistant Level 2
Energy Level	E1	E2	E1	E2
Energy (joules)	24	36	33	50
Velocity	5 m/sec	6.2 m/sec	5.9 m/sec	7.3 m/sec
Total Missile Mass	1.9 kg	1.9 kg	1.9 kg	1.9 kg
Maximum Penetration	7mm	20mm	7mm	20mm

 

Spiked Weapon Levels

There is also spike protection to be considered, which was first introduced into CAST’s standards to address the additional threat faced by Prison Officers. Spike protection can be added to stab and ballistic vests, neither of which will ordinarily protect against spiked weapons. CAST will only certify spike protection in addition to stab protection. The threat levels for spike proof vests are standardized by the CAST as follows:

 

 

	KR1 & SP1		KR2 & SP2
	Knife Resistant Level 1 & Spike Protection Level 1		Knife Resistant Level 2 & Spike Protection Level 2
Energy Level	E1	E2	E1	E2
Energy (joules)	24	N/A	33	N/A
Velocity	5.0 m/sec	N/A	5.9 m/sec	N/A
Total Missile Mass	1.9 kg	N/A	1.9 kg	N/A
Maximum Penetration	KR1=7mm, SP1=0mm	N/A	KR2=7mm, SP2=0mm	N/A

 

Materials

Kevlar:

Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, this high-strength material was first commercially used in the early 1970s as a replacement for steel in racing tires. Typically it is spun into ropes or fabric sheets that can be used as such or as an ingredient in composite material components.

Currently, Kevlar has many applications, ranging from bicycle tires and racing sails to body armor, because of its high tensile strength-to-weight ratio; by this measure it is 5 times stronger than steel. It is also used to make modern drum heads that withstand high impact. When used as a woven material, it is suitable for mooring lines and other underwater applications.

Source: https://en.wikipedia.org/wiki/Kevlar

UHMWPE:

Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), or high-performance polyethylene (HPPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material, with the highest impact strength of any thermoplastic presently made.

UHMWPE is odorless, tasteless, and nontoxic. It embodies all the characteristics of high-density polyethylene (HDPE) with the added traits of being resistant to concentrated acids and alkalis as well as numerous organic solvents. It is highly resistant to corrosive chemicals except oxidizing acids; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating (boundary lubrication); and is highly resistant to abrasion, in some forms being 15 times more resistant to abrasion than carbon steel. Its coefficient of friction is significantly lower than that of nylon and acetal, and is comparable to that of polytetrafluoroethylene (PTFE, Teflon), but UHMWPE has better abrasion resistance than PTFE.

Source: https://en.wikipedia.org/wiki/Ultra-high-molecular-weight_polyethylene

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Source: https://www.nij.gov/