Ceramics in a bag give extra strength
Tom Shelley reports on a construction that allows ceramics to withstand massive impacts.
By putting a ceramic plate in a pre-tensioned composite containment, it is possible to make light weight human body armour that will withstand repeated impacts by quite large, armour piercing bullets.
It is much lighter and cheaper than conventional body armour, and at the same time shows a new way of working with ceramics in engineering, opening up possible applications that could include higher temperature running engines and gas turbines and longer lasting linings for grinding and crushing equipment.
The development is the invention of Shaun Smithy-Ward, the proprietor of VestGuard, which has been developing ballistic products from more than ten years. Currently, more than 160,000 of his company's individual body army systems are in service with customers who range from church leaders to national armies and police forces.
He told us that researching papers published by universities was key to understanding the possibilities of new materials, but the breakthrough came when undertaking development tests on a range at Shrivenham. "The assisting technician on the range suggested we stop testing and pack up one evening, but I told him I was not a PLC and wanted to carry on, so I tried some unusual combinations of materials that I had an interest in and found that they worked better than what we had been working with."
Traditionally, he explained, body armour plates were made from aluminium oxide, which is hard enough to shatter armour piercing projectiles, "And then many manufacturers moved to boron carbide or silicon carbide, but tiles of such materials typically cost some $250 per tile for a small weight advantage. This didn't add up to me if you can achieve the same ballistic performance by using alternatives with a lighter weight for a third of the cost".
What he has developed is a phased ceramic plate, made of a "Proprietary concoction" developed by VestGuard, which is then put into compression by being bonded to pre-tensioned glass reinforced plastic, using a specially developed adhesive system and autoclaving, followed by encapsulation in another ceramic. This is then backed by impact resistant high density, low weight, unidirectional fibre. The effect of putting the ceramic strike plate into compression prevents crack growth outside the immediate impact area. The high density fibre than absorbs the impact energy of the pieces of shattered projectile.
We were shown plates that had had multiple impacts by 30mm armour piercing ammunition fired from the sort of gun that sits on top of the Warrior armoured personnel carrier. Although the ceramic had been shattered at the impact points, this was only over a limited area and nothing had got through to the other side.
The company makes both this material, which is about the same weight as the standard armour supplied to our soldiers, but stops much heavier ammunition, and also its "Max-1 Nine shot hard armour plate", which weighs only 1.7kg, about half that of armour normally supplied to soldiers and the police, but which will nonetheless stop nine shots of armour piercing rounds from AK47 assault rifles, as well as nine SS109 rounds in current use with Nato forces, all fired from 15m away.
Why these material combinations work as well as they do is not entirely clear. Smithy-Ward told us that even after using high speed video at 250,000 frames per second, they were no wiser as to how the material resists impacts, and the development process had mostly been a matter of trial and error, although they were now doing computer modelling. However, it is well known that surface tension in ceramics enables them to defeat rifle ammunition, and if you can keep the plate within a tensioned containment and localise crack growth, you have a good prospect of a multi-hit plate, but this is normally a difficult thing to accomplish. Key to the current configuration is the adhesive, that has to be able to survive autoclaving, and we were told that they had, "Looked at lots" before something special was developed for them by their supplier.
* A ceramic plate can have its ability to withstand impact greatly enhanced by bonding it to composite in tension and encapsulating it
* An adhesive has been found which can give the required strength and which retains this property during autoclaving
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