The Application of Dilatant Fluids in Body Armour
Current body armour
Most body armour is made of a weave of a material called Kevlar. Kevlar is an aramidic fibre, which forms hydrogen bonds between its chains of molecules and thus has a very high tensile strength and high toughness[8]. Most body armour consists of multiple layers of woven Kevlar, sometimes with ceramic plates to give extra protection[10].
Disadvantages
Although this body armour does offer increased protection for the wearer, it does have some drawbacks. Firstly, many layers of Kevlar can be very bulky and stiff, meaning that the wearer cannot move around as easily. Secondly, with many layers of Kevlar and/or ceramic plates, the body armour can become very heavy[11]. Thirdly, body armour does not offer protection for extremities, such as arms, legs or the neck. This is because the number of layers of Kevlar needed to offer sufficient protection would be too stiff and bulky for use as sleeves, trousers etc[9][10].
Improving Kevlar with dilatant fluids
The way dilatant fluids are currently being used in body armour is by being impregnated in Kevlar and thus strengthening it. In one case silica particles in ethylene glycol form the dilatant fluid - this is then impregnated into the Kevlar[9]. This results in a material that is just as flexible as normal Kevlar, yet when struck by a projectile it hardens, offering more protection to the individual wearing it.
Most body armour is made of a weave of a material called Kevlar. Kevlar is an aramidic fibre, which forms hydrogen bonds between its chains of molecules and thus has a very high tensile strength and high toughness[8]. Most body armour consists of multiple layers of woven Kevlar, sometimes with ceramic plates to give extra protection[10].
Disadvantages
Although this body armour does offer increased protection for the wearer, it does have some drawbacks. Firstly, many layers of Kevlar can be very bulky and stiff, meaning that the wearer cannot move around as easily. Secondly, with many layers of Kevlar and/or ceramic plates, the body armour can become very heavy[11]. Thirdly, body armour does not offer protection for extremities, such as arms, legs or the neck. This is because the number of layers of Kevlar needed to offer sufficient protection would be too stiff and bulky for use as sleeves, trousers etc[9][10].
Improving Kevlar with dilatant fluids
The way dilatant fluids are currently being used in body armour is by being impregnated in Kevlar and thus strengthening it. In one case silica particles in ethylene glycol form the dilatant fluid - this is then impregnated into the Kevlar[9]. This results in a material that is just as flexible as normal Kevlar, yet when struck by a projectile it hardens, offering more protection to the individual wearing it.
The following table from Lee et al 2002 [9] shows a number of configurations of Kevlar and the dilatant fluid and how far a projectile travelled into it. I have produced the below graph to better illustrate the penetration depths for each sample.
The graph shows that the 4 layers of Kevlar impregnated with the dilatant fluid is the strongest and has the least penetration. The graph shows that sample G (just the 4 layers of Kevlar) was penetrated 3 times deeper than F (the fully impregnated Kevlar).
This large increase in strength is very useful in the application of body armour as it allows increased protection as well as making the protective gear lighter.
Impregnated Kevlar
As the above table, diagrams and graph show, the impregnation of the Kevlar layers by the dilatant fluid gives the strongest result. The reason for this lies in the interaction between the dilatant fluid and the Kevlar fibres. To prevent penetration by a projectile or other sharp object the energy of it has to be dissipated into forms other than the kinetic energy driving it towards the body. When a projectile hits the dilatant fluid-impregnated Kevlar, the dilatant fluid undergoes shear thickening and thereby increases the coefficient of friction between the aramidic fibres of the Kevlar [12]. This allows the energy to be dissipated by increasing the amount of fibres fracturing instead of the fibres being pulled outwards (towards the body). In untreated Kevlar the main mechanisms of energy dissipation are fibre pullout, fibre plastic deformation and fibre fracture. When treated with dilatant fluids, there is significantly less fibre pullout as the dilatant fluid becomes rigid and holds the Kevlar fibres together [10]. This is important as it means that the projectile will not travel as far into the body armour and the body,thereby resulting in less injury.
Advantages of dilatant fluid impregnated body armour
Dilatant fluid impregnated body armour has a number of advantages over untreated Kevlar-based body armour.
Disadvantages
The graph shows that the 4 layers of Kevlar impregnated with the dilatant fluid is the strongest and has the least penetration. The graph shows that sample G (just the 4 layers of Kevlar) was penetrated 3 times deeper than F (the fully impregnated Kevlar).
This large increase in strength is very useful in the application of body armour as it allows increased protection as well as making the protective gear lighter.
Impregnated Kevlar
As the above table, diagrams and graph show, the impregnation of the Kevlar layers by the dilatant fluid gives the strongest result. The reason for this lies in the interaction between the dilatant fluid and the Kevlar fibres. To prevent penetration by a projectile or other sharp object the energy of it has to be dissipated into forms other than the kinetic energy driving it towards the body. When a projectile hits the dilatant fluid-impregnated Kevlar, the dilatant fluid undergoes shear thickening and thereby increases the coefficient of friction between the aramidic fibres of the Kevlar [12]. This allows the energy to be dissipated by increasing the amount of fibres fracturing instead of the fibres being pulled outwards (towards the body). In untreated Kevlar the main mechanisms of energy dissipation are fibre pullout, fibre plastic deformation and fibre fracture. When treated with dilatant fluids, there is significantly less fibre pullout as the dilatant fluid becomes rigid and holds the Kevlar fibres together [10]. This is important as it means that the projectile will not travel as far into the body armour and the body,thereby resulting in less injury.
Advantages of dilatant fluid impregnated body armour
Dilatant fluid impregnated body armour has a number of advantages over untreated Kevlar-based body armour.
- Firstly, when struck, it offers more protection for the wearer, as bullets will not travel as far through the armour into the body, because the solidified dilatant fluid holds the Kevlar fibres together.
- Secondly, it has roughly the same flexibility as the same amount of untreated Kevlar would, as when not stressed the dilatant fluid is in the liquid state and thus does not impede movement. Yet when it is struck, the dilatant fluid solidifies, becomes rigid and increases the strength of the Kevlar weave.
- Thirdly, it could be used for armour for extremities, as it is strong yet relatively thin and flexible.
Disadvantages
- The dilatant fluid impregnated Kevlar is heavier than untreated Kevlar; if a lot of it is used in the body armour, then it will have an impact on the weight of the armour.
- It will not provide much more protection than untreated Kevlar when slowly pierced. Slow moving objects will not provide enough stress to make the dilatant fluid harden.
Comparison
This video clip from BAE Systems [11], shows the difference between the impact of a projectile on 10 layers of the dilatant fluid impregnated Kevlar and 31 layers of untreated Kevlar respectively. It shows just how much effect the dilatant fluid has in dissipating the energy across the surface and preventing fibre pullout, and how much less material is needed to offer superior protection.