Bulletproof glass

Bulletproof glass,ballistic glass,transparent armor,orbullet-resistant glassis a strong and optically transparent material that is particularly resistant to penetration by projectiles, although, like any other material, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft.The softer layer makes the glass more elastic, so that it can flex instead of shatter. Theindex of refractionfor all of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from3⁄4to3+1⁄2inches (19 to 89 mm).

Bulletproof glass of a jeweler's window after a burglary attempt.

TheMona Lisabehind bulletproof glass at theLouvre Museum

Bulletproof glass is used in windows of buildings that require such security, such as jewelry stores and embassies, and of military and private vehicles.

Construction

A rough visualisation of bulletproof glass, composed of layers of plastic sheeting (grey) and layers of glass (blue)

Bullet-resistant glass is constructed using layers oflaminated glass.The more layers there are, the more protection the glass offers. When a weight reduction is needed,polycarbonate(athermoplastic) is laminated onto the safe side to stopspall.The aim is to make a material with the appearance and clarity of standard glass but with effective protection from small arms. Polycarbonate designs usually consist of products such as Armormax, Makroclear, Cyrolon: a soft coating that heals after being scratched (such as elastomeric carbon-based polymers) or a hard coating that prevents scratching (such as silicon-based polymers).

The plastic in laminate designs also provides resistance to impact from physical assault from blunt and sharp objects. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet, and the plastic deforms, with the aim of absorbing the rest of the energy and preventing penetration. The ability of the polycarbonate layer to stop projectiles with varying energy is directly proportional to its thickness,and bulletproof glass of this design may be up to 3.5 inches thick.

Laminated glass layers are built from glass sheets bonded together with polyvinyl butyral, polyurethane, Sentryglas, or ethylene-vinyl acetate. When treated with chemical processes, the glass becomes much stronger. This design has been in regular use on combat vehicles since World War II. It is typically thick and is usually extremely heavy.

Sample thickness and weight for bullet-resistant glass materials
	Threat Stopped	Glass Laminate				Polycarbonate				Acrylic				Glass-Clad Polycarbonate				Aluminum oxynitride
Protection Level	(example)	Thickness		Weight		Thickness		Weight		Thickness		Weight		Thickness		Weight		Thickness		Weight
		in.	mm	lb/sq. ft.	kg/m2	in.	mm	lb/sq. ft.	kg/m2	in.	mm	lb/sq. ft.	kg/m2	in.	mm	lb/sq. ft.	kg/m2	in.	mm	lb/sq. ft.	kg/m2
UL 752 Level 1	9 mm3 shots	1.185	30.09	15.25	74.46	0.75	19.05	4.6	22.46	1.25	31.75	7.7	37.6	0.818	20.78	8.99	43.9
UL 752 Level 2	.357 Magnum3 shots	1.4	35.56	17.94	87.6	1.03	26.16	6.4	31.25	1.375	34.92	8.5	41.50	1.075	27.3	11.68	57.02
UL 752 Level 3 (approximately NIJ IIIA)	.44 Magnum3 shots (5 shots for NIJ IIIa)	1.59	40.38	20.94	102.24	1.25	31.75	7.7	37.6					1.288	32.71	14.23	69.47
UL 752 Level 4	.30-061 shot													1.338	35.25	14.43	69.47
UL 752 Level 5	7.62 mm1 shot
UL 752 Level 6	.357 Magnumunderloaded 5 shots
UL 752 Level 7	5.56x455 shots
UL 752 Level 8 (approximately NIJ III)	7.62 mm NATO5 shots													2.374	60.3	26.01	126.99			18.25
UL 752 Level 9	.30-06M2 AP 1 shot
UL 752 Level 10	.50 BMG1 shot																	1.6	40.6	30.76	150.1

9mm 124gr @ 1175-1293fps (1400-1530fps for Level 6), 357M 158gr @ 1250-1375fps, 44M 240gr @ 1350-1485fps, 30-06 180gr @ 2540-2794fps, 5.56NATO 55gr @ 3080-3388fps, 7.62NATO 150gr @ 2750-3025fps. For all ratings in the above chart; all copper-jacketed lead FMJ, except 44 mg is lead semi-wadcutter gas-check, and 30-06 is LEAD core soft point.

Test standards

Ballistic test of a bullet-resistant glass panel

Bullet-resistant materials are tested using a gun to fire a projectile from a set distance into the material, in a specific pattern. Levels of protection are based on the ability of the target to stop a specific type of projectile traveling at a specific speed. Experiments suggest that polycarbonate fails at lower velocities with regular shaped projectiles compared to irregular ones (like fragments), meaning that testing with regular shaped projectiles gives a conservative estimate of its resistance.When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material.Some researchers have developed mathematical models based on results of this kind of testing to help them design bulletproof glass to resist specific anticipated threats.

Environmental effects

The properties of bullet-resistant glass can be affected by temperature and by exposure to solvents orUV radiation,usually from sunlight. If the polycarbonate layer is below a glass layer, it has some protection from UV radiation due to the glass and bonding layer. Over time the polycarbonate becomes more brittle because it is anamorphous polymer(which is necessary for it to be transparent) that moves toward thermodynamic equilibrium.

An impact on polycarbonate by a projectile at temperatures below −7 °C sometimes createsspall,pieces of polycarbonate that are broken off and become projectiles themselves. Experiments have demonstrated that the size of the spall is related to the thickness of the laminate rather than the size of the projectile. The spall starts in surface flaws caused by bending of the inner, polycarbonate layer and the cracks move “backwards” through to the impact surface. It has been suggested that a second inner layer of polycarbonate may effectively resist penetration by the spall.

2000s advances

In 2005, it was reported that U.S. military researchers were developing a class of transparent armor incorporating aluminum oxynitride(ALON) as the outside "strike plate" layer. Traditional glass/polymer was demonstrated by ALON's manufacturer to require 2.3 times more thickness than ALON's, to guard against a.50 BMGprojectile.ALON is much lighter and performs much better than traditional glass/polymer laminates. Aluminum oxynitride "glass" can defeat threats like the.50 caliberarmor-piercing rounds using material that is not prohibitively heavy.

Spinel ceramics

Certain types ofceramicscan also be used for transparent armor due to their properties of increased density and hardness when compared to traditional glass. These types of synthetic ceramic transparent armors can allow for thinner armor with equivalent stopping power to traditional laminated glass.

See also

• Transparent Armor Gun Shield
• Prince Rupert's Drop