Safety net

The police are to test UK-developed anti-blast materials that become thicker when they are stretched, potentially offering protection from the deadly effects of flying glass following a bomb explosion.

The Police Scientific Development Branch (PSDB) will carry out trial explosions on a new design of transparent blast curtains composed of auxetic textiles.

These are materials with a number of unique properties currently under investigation by engineers at Exeter University and a dedicated spinout company, also called Auxetics.

When a conventional material is stretched it tends to get thinner. The opposite is true of auxetic materials which, thanks to the geometry of their structure, become thicker in cross-section when stretched and thinner when compressed.

Exeter University researcher Dr. Luke Savage claimed that auxetic materials could be the key to developing a textile that allows light through, but is also capable of containing the huge forces involved in an explosion.

Currently blast-protection curtains are made from aramid nets designed to billow out in the event of an explosion and capture fragments of glass. But because these curtains must also be thin enough to allow light through, they are frequently ineffective in their blast-protection role, claimed Savage.

‘Blast curtains must let the light through otherwise you might as well brick up the window. But the trouble is if you let the light through you limit the effectiveness of the curtain,’ said Savage.

He added that the material could be produced in fairly large quantities using traditional weaving techniques, and that the group will be calling on the services of Devonbased textile specialist John Heathcoat.

Patrick Hook of Auxetics, which is planning to commercialise the technology, explained how the curtains would work. ‘When you get a blast wave the vacuum behind it picks up all the debris. The idea is to slow down the blast so that the vacuum catches up and you get a selfcancelling effect.’

Hook said that this would be achieved using a layered auxetic textile which will respond to the onset of a blast by opening up arrays of pores in each layer. These pores will let the blast wave pass through from one layer to the next, successively dispersing the energy it carries. However, he stressed that the initiative is a feasibility study and that there are still a number of unknowns.

‘We’ll be able to make a translucent curtain but don’t know what degree of optical clarity we’ll get. Our intention is to get something that stops flying debris and lets through more light than any current alternatives.’

Initial tests will be carried out at Exeter University, and towards the end of the project some time next year full-scale blast testing will be carried out by the PSDB.

Meanwhile, Auxetics is investigating a wide range of other commercial applications for the auxetic phenomenon. For example, Hook’s team is investigating the potential of auxetic dental floss which gets fatter when stretched. Hook said that in the pulled state the open pores in the floss will act like a ‘plaque grater’ and help to trap debris.

The company is also looking at the development of fabrics that change colour when stretched. These would have an auxetic layer with a layer of different-coloured material behind it. When stretched, the pores in the auxetic layer would open and reveal the colour of the material behind. This concept could, said Hook, be used in the design of ropes that change colour to warn users when they have been stretched beyond a certain danger point.