Bullet-proof test

The bullet-proof vests used to protect the lives of police officers could be further improved with advanced fibre weaves.

Manchester University is leading a project that will attempt to discover how ballistic fabric assemblies react to a piercing bullet at the micron scale.

The university’s materials experts will then engineer a synthetic fabric with novel fibre structures that match the strains caused by the bullet. The EPSRC-funded project, which is receiving support from the Metropolitan Police, will conclude in 2013.

‘At the moment bullet-proof fabrics are made from high-performance fibre such as Kevlar, Twaron and aramid fibres or high-density polyethylene with one particular weave structure,’ said investigator Ian Kinloch, an advanced engineering materials researcher at Man- chester University. ‘By understanding the way a bullet penetrates the armour and the way it puts stresses in that material, we can create models and improve the design of those fabrics in the future.’

The academic research team is being led by technical textile lecturer Xiaogang Chen, who is assisted by Kinloch and Robert Young, the head of the university’s school of materials.

The team will first model various fibre assemblies and put them through simulations in a computer software program before fabricating sample fabrics and firing bullets at them. The ballistic testing will be conducted at the university’s firing range.

When a high-velocity projectile such as a bullet strikes an assembly of many layers of ballistic fabric, the action on the layers and the reaction from the layers vary significantly depending on the position within the assembly.

The researchers will use high-speed photography and Raman spectroscopy to study the differences in strain distribution in each and every layer of the assembly when the armour is impacted with a bullet.

Kinloch said high-speed photography has been used to study impact on bullet-proof materials before, but Raman spectroscopy is something entirely new for this application.

Raman spectroscopy works by shining a laser light onto a molecule and then collecting and analysing the wavelength and intensity of the resulting scattered light.

He said: ‘It basically measures the bond vibration between atoms. As you stretch that bond, the vibration changes frequency. It’s a bit like tuning a guitar string — the pitch changes.

‘So when you use Raman spectroscopy you can look at the change of the vibrational energy of the bond and calibrate that to the stress on that particular bond.’

Kinloch added that Raman spectroscopy will allow the team to measure the stress in the bonds between the atoms at a resolution of about one micron. However, it still remains to be seen whether the researchers will be able to perform Raman spectroscopy measurements at the speed of a bullet.

‘It is questionable, but we can at least try and improve the speed of the measurements so we can look at strain rates higher than what can be normally done,’ he said.

Normally, Kinloch said, Raman spectroscopy is used for static objects. For example, it is commonly used to identify molecules in chemistry because vibrational information is specific for the chemical bonds in molecules.

Kinloch said his group will first use Raman spectroscopy to study stress distribution at slow speeds by pushing a bullet, little by little, through the engineered fabric. This will, at the very least, verify or invalidate the group’s original computer simulation models.

‘Slow speeds will tell us whether our model is right in terms of how the stresses defer from the object onto the surrounding material and how fibres part as the projectile tries to pierce it,’ he said. ‘When we understand what is happening at slow speeds, we can go to higher speeds.’

The researchers hope to have a demonstrative prototype of an advanced engineered ballistic fibre vest in four years’ time. The team will test its vest against standards set by the National Institute of Justice, the US Department of Justice’s research-and-development agency, and the UK’s Home Office and Scientific Development Branch. It will also be compared to existing versions of body armours.

Siobhan Wagner