OK, so first of all, how can a fabric possibly get thicker when stretched? Doesn’t that go against the laws of physics? Not, it turns out, when that material is auxetic. Cat skin and shin bones also apparently possess this quality. The University of Exteter, in collaboration with their spin-off company Auxetix Ltd, have developed an auxetic blast-proof curtain. If a bomb were to go off near such a curtain, the pressure wave would stretch the fabric outwards, thus thickening it and making it better able to hold back flying glass and other debris. The curtain is intended to be fitted over windows of buildings that are terrorist targets, or that are subject to events such as hurricanes.
Here’s how it works. Each strand of the curtain’s yarn consists of a central stretchy fiber, with a more rigid fiber wound around the outside of it. When put under strain, the rigid outer fiber straightens, squeezing the stretchy inner fiber and causing it to bulge out sideways – the inner fiber doesn’t actually expand, but it does end up taking up more space. Tiny pores open when the material stretches, which are too small to let debris through, but are sufficient to release some of the force of the blast, and keep the curtain from ripping.
Variables such as the two fibers’ relative stiffness and diameter, along with the angle at which the outer fiber is wound, can have a marked effect on the performance of the material. The researchers are currently tweaking these parameters, to develop different curtains for different applications.
There is a range of widely-available types of fibers that are suitable, and conventional processes can be used for producing the yarn and weaving it into a textile. The material is just 1 to 2 millimeters thick, so it still lets in a reasonable amount of light.