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Liquid filled body armour relies on shear thickening fluids

BAE Systems : 23 January, 2013  (Special Report)
A new type of body armor is fortified with an innovative liquid that becomes solid when something hits it. This could offer a much greater level of protection for soldiers.
Liquid filled body armour relies on shear thickening fluids

A new type of body armour is fortified with an innovative liquid that becomes solid when something hits it. This could offer a much greater level of protection for soldiers. It is also lighter than today's bulky body armour, so troops won't be slowed down by their protective gear.

The liquid, made by BAE Systems, is inserted between layers of Kevlar, which results in a "super armour" that, compared to a typical bullet-proof vest, is both thinner and lighter.

How does it work? A so-called Shear Thickening Liquid has special particles that are freely suspended. The particles collide when the fluid is disturbed, creating a resistance to the disturbance. When the force of the disturbance is large enough, the particles actually "lock" together. So, when a bullet hits the material at high
speed, the liquid armour absorbs the energy of the impact and hardens extremely quickly.

Conventional body armour tends to bend inward when a bullet hits it. That's enough to keep vital organs from being pierced, but the victim can still suffer painful injuries such as broken ribs.

In addition to protecting soldiers, the liquid could be used to create a product for protecting police officers, Secret Service agents, FBI personnel, and other law enforcement professionals.

How Shear-thickening Fluids work

The term "liquid body armour" can be a little misleading. For some people, it brings to mind the idea of moving fluid sandwiched between two layers of solid material. However, both types of liquid armour in development work without a visible liquid layer. Instead, they use Kevlar that has been soaked in one of two fluids.

The first is a shear-thickening fluid (STF), which behaves like a solid when it encounters mechanical stress or shear. In other words, it moves like a liquid until an object strikes or agitates it forcefully. Then, it hardens in a few milliseconds. This is the opposite of a shear-thinning fluid, like paint, which becomes thinner when it is agitated or shaken.

The fluid is a colloid, made of tiny particles suspended in a liquid. The particles repel each other slightly, so they float easily throughout the liquid without clumping together or settling to the bottom. But the energy of a sudden impact overwhelms the repulsive forces between the particles -  they stick together, forming masses called hydroclusters. When the energy from the impact dissipates, the particles begin to repel one another again. The hydroclusters fall apart, and the apparently solid substance reverts to a liquid.
Before impact, the particles in shear-thickening fluid are in a state of equilibrium. After impact, they clump together, forming solid structures.

The fluid used in body armour is made of silica particles suspended in polyethylene glycol. Silica is a component of sand and quartz, and polyethylene glycol is a polymer commonly used in laxatives and lubricants. The silica particles are only a few nanometers in diameter, so many reports describe this fluid as a form of nanotechnology.

To make liquid body armour using shear-thickening fluid, researchers first dilute the fluid in ethanol. They saturate the Kevlar with the diluted fluid and place it in an oven to evaporate the ethanol. The STF then permeates the Kevlar, and the Kevlar strands hold the particle-filled fluid in place. When an object strikes or stabs the Kevlar, the fluid immediately hardens, making the Kevlar stronger. The hardening process happens in mere milliseconds, and the armour becomes flexible again afterward.

In laboratory tests, STF-treated Kevlar is as flexible as plain, or neat, Kevlar. The difference is that it's stronger, so armour using STF requires fewer layers of material. Four layers of STF-treated Kevlar can dissipate the same amount of energy as 14 layers of neat Kevlar. In addition, STF-treated fibers don't stretch as far on impact as ordinary fibres, meaning that bullets don't penetrate as deeply into the armour or a person's tissue underneath. The researchers theorize that this is because it takes more energy for the bullet to stretch the STF-treated fibres.

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