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The Truth Behind Bulletproof Glass
Image source: Tssbulletproof.com
Bulletproof glass has a variety of uses:
And obviously one can’t disregard all of the military applications of the material. But there is a well-kept secret behind bulletproof glass…
“Bulletproof glass is neither bulletproof nor is it a glass. Technically it should be called ‘bullet resistant laminate composite’, however that doesn’t sound anywhere near as cool.” – Dr Michelle Dickinson, University of Auckland
Standard glass vs bulletproof glass
If a bullet is fired at normal glass, the glass will shatter.
Image source: Reference 1
Bullet-resistant glass is a laminate composite consisting of glass and polymer layers which delay the entry of the bullet. The polymer layer is most commonly polycarbonate (PC), although combat vehicles use glass and polyvinyl butyral (PVB). The glass layers may still shatter but they don’t fly everywhere because they are bonded to the PC layers. Bulletproof glass is optically transparent like standard glass, but it has a much lower density due to the PC layers, whilst improving the level of protection. It is thicker than normal glass, too (usually between 7 mm and 75 mm); the thickness of the composite determines its bullet resistance according to:
thickness ∝ ability to withstand force ∝ expense & opacity
How does it work?
The glass layer – which is harder than plastic – absorbs the initial impact energy by shattering, whilst the polymer layer deforms and absorbs the rest of the energy and hopefully prevents penetration. The ability of the glass to withstand shock and absorb more energy may be improved by the process of tempering. The PC layer is more elastic than glass and will act to dissipate the impact energy horizontally. The bullet decelerates as its energy is absorbed upon impact; if it is not stopped by the first layer of glass and PC, then it will travel through to the next layers until it becomes stationary (unless the bulletproof glass is too thin).
Image source: Reference 1
Testing standards
Bulletproof glass is manufactured to meet test criteria applied worldwide by national standards agencies. The criteria requires that the composite must withstand impacts from bullets at particular speeds, from set distances and in a set pattern. In order to pass, bullets must not be able to penetrate the entire sample. For example, Techno Glass tested its ‘Bullet Proof Lamination glass’ of 42mm thick by using an AK-47 shot twice from the set distance of 10m.
Image source: Reference 4
Disadvantages
It may be penetrated
It is only used for protection from small firearms
Thicker glass is more expensive, heavy and opaque
Properties can be affected by temperature (PC breaks off when fired at below -7°C so heatable glass is desirable) whilst prolonged exposure to UV can cause the PC to become more brittle and less able to deform
Recent advances
Spinel structured ceramics have found use in bullet-resistant composites. They can be used for new synthetic transparent armours due to their increased density and hardness as compared to traditional glass. Furthermore, they allow for thinner armour but with the equivalent stopping power to traditional laminated glass. For example, the U.S. military are incorporating aluminium oxynitride (ALON) into their ballistic armour: it is 50-60 % lighter and performs much better than traditional glass/polymer laminates.
Hope you enjoyed this quick bite of materials science! Let me know what you think ☆
References
http://matterchatter.wordpress.com/2012/04/29/the-difference-between-normal-and-bulletproof-glass/
http://en.wikipedia.org/wiki/Bulletproof_glass
http://www.romag.co.uk/architectural-glass/bullet-proof-glass
http://technoglass.pk/bullet-proof-glass-testing-achievement/
http://science.howstuffworks.com/question476.htm
http://www.architecturalarmour.co.uk/security-glass/bulletproof---resistant-glass
