MODERN ARMOUR: THE SCIENCE BEHIND CATCHING BULLETS

Read on to hear about the fibers, materials and science behind preserving troops in modern conflicts

INTRODUCTION:

Since ancient times people have been developing more and more advanced ways to protect themselves from equally advancing weaponry. This has fuelled development of materials with ever increasing 'specific strengths' to keep combatants from harm without impairing mobility. This page will focus on developments from the 60's onwards read on to find out more...

WHAT MAKES GOOD ARMOUR?

Good body armour needs to be: flexible, light and protect the major organs:

The first example is the breast plate from a suit of armour. It definitely protects the major organs however it is neither light nor flexible two qualities which make the concept of full plate armour unsuitable for the modern battlefield.

The second example is a piece of Roman chain mail. Unlike the plate armour it is flexible however it is not particularly light and actually does little to protect the major organs. It's flexibility means that it affords no protection against internal trauma - while it may be able to prevent organs being punctured it does little to dissipate the energy of an attack.

The final example is leather armour. It is composed of layers of thick leather scales stitched together. It allows a decent amount of movement; is light and protects the wearer from organ trauma however unlike the chain mail it is not strong enough to prevent penetrating injuries to internal organs

Osprey armour (left) combines hard and soft armour components with padding in a modular way to produce armour which is light, flexible and able to protect against trauma as well as penetrating attacks

Soft Armour Components:

Soft Armour components form the basis of protection against low velocity penetrating and traumatic injuries. They depend entirely on polymer materials for their ratio of strength to weight known as specific strength. The arrangement of the fibers themselves is also important in creating the desired material specifically for allowing it to dissipate the energy of a weapon/projectile:

How is energy dissipated?

The fibers spread the force of an impact like a goal net. They are interlaced in a similar way to the netting shown (left). This means an impact at any point exerts force across many fibers in multiple plains. Fibers are also tightly woven, tightly spun (center) and resin coated (right). these modifications increase the work done when fibers are stretched which increases the energy dissipated.

Read on for a look at the molecular properties of the materials themselves:

How do fibers achieve the required strength?

The above materials are some examples of the polymers used in body armour, on a molecular level their high strength is due to a number of factors which increase the electrostatic attraction between neighbouring chains including: surface contact, aromatic stacking and hydrogen bonding.

Surface Contact:

polymers are formed of many repeat units e.g. kevlar monomer:

These repeat units align in straight chains. Their shape allows them to interlock allowing the greatest amount of surface contact across which inter-molecular forces can act (for more see: en.wikipedia.org/wiki/Van_der_Waals_force). Similarly the longer the molecule chains are the greater the surface contact, Dyneema for instance has chains of up to 250,000 repeat units. In Dyneema surface contact is increased by using a spinneret which extrudes the compound during formation to align the fibers much like a hairbrush!

Hydrogen Bonding:

in Kevlar and Nomex the repeat units contain oxygen and hydrogen. Oxygen is strongly electronegative so it attracts hydrogens from nearby molecule chains which when bonded are strongly positive. The diagram above shows how these interactions exist along the entire length of the chains resulting in increased attraction between polymer chains and a stronger material

Aromatic Stacking:

both kevlar and nomex are part of a class of compounds known as aramids this is short for 'aromatic polyamide'. Aromatic indicates the presence of flat unsaturated (contains double bonds) rings. aromatic stacking is a special case of neighbouring molecules 'fitting' together - the rings align resulting in an area of high intermolecular attraction between two polymer chains.

Hard Armour Components:

Compared with soft armour components, hard armour is relatively simple! It functions much like medieval plate armour: When an object collides with it the material is simply able to withstand an equally sized reaction force without bending too much causing the object e.g. a knife or High velocity round to crumple or bounce off:

Ceramics are used for this purpose due to their high strength. Shown above is a plate probably composed of alumina (right - also the main constituent of sapphires). Other minerals such as silicon carbide (center) are used for similar reasons. Currently there are plans to use dyneema for plating as well as fibers.

The Future of Armour:

currently the next generation of body armour will be made almost entirely from Dyneema. Dyneema is a different form of the the same poly(ethene) found in shopping bags! Unlike the poly(ethene) in shopping bags dyneema is passed through a spinneret during formation which acts much like a hairbrush - causing the fibres to align much better. This results in chains with huge amounts of surface contact leading to very high tensile strength. Furthermore its molecular simplicity means it has a low density. It is also possible to form Dyneema into both flexible fibers and hard plates with the addition of plasticisers.

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