Armor: Accidental Discovery Aids In EFP Protection


May 29, 2007: Everyone knows explosively forged projectiles (EFPs) are an effective weapon against vehicles in Iraq. However, troops in the field have noticed that although EFPs go through metal armor, often glass laminate armor (aka glass ballistic laminate armor) will stop them. Troops report that the EFPs would not go through the bullet proof windows, which are made of glass laminate. However, the glass laminate only works once. When an EFP strikes the glass, the glass "spiderwebs" (shatters laterally and vertically) but it stops the penetrator. Of course it only needs to work once�"troops lives are saved and the stuff can be replaced. At least one unit is experimenting with mounting field improvised brackets outside the doors of vehicles to hold the three-inch thick glass laminate armor.

Apparently the glass laminate armor is destabilizing the explosively forged projectile penetrator and redirecting its kinetic energy laterally along the glass armor laminations. This is the principle behind the M-1 tanks chobham armor (a sandwich of metal and ceramic laminates). The ceramic armor is held in a metal armor matrix. As heavy metal "long rod" penetrator or high explosive shaped charge debris streams enter these armor matrixes, they are destabilized. The kinetic energy is diverted laterally from the initial penatrator direction of attack as the ceramics shatter. A plus here is that the chobham ceramics are jostled by the penetrator's or shaped charge stream's passage and keep abrading until the attack runs out of energy. Apparently no one has, officially, tested EFP versus glass laminate ballistic armor, so no one knows, officially, if it can do the same thing.

Glass ballistic laminate is expensive - one windshield costs several thousand dollars. The lamination process has a high scrap rate: it takes several tries to create one good, large piece of the material. The silica/polycarbonate plastic sandwich is hard to heat evenly and if it is not perfectly shaped a "void" will appear in during the curing process. This creates visibility issues, and troops need to be able to see through the glass. However, scrap pieces of glass laminate might be useful as anti-EFP armor.

Could this be more battlefield rumors? Even if it is, the armor and vehicle R&D crowds need to step in and check it out.




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