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Introduction to Auxetic Materials

Let’s just do an experiment. Take and elastic band and pull it, look carefully and you will see that it gets thinner. Pull it some more and it gets so thin it breaks. This is the way our world works. Intuitively it would seem logical that the volume of material remains constant, which is why a material gets thinner as it is extended. This intuitive view is wrong, the relationship between extension and reduction in area is not the same for all materials, and they are all different. This ratio is known as Poisson’s ratio. Poisson’s ratio is a positive value for almost all the materials we use, except for just a few.

Above: Pull a material with a positive Poisson’s ratio it gets thinner

Now imagine pulling an elastic band with a negative Poisson’s ratio, it will of course get fatter as you pull it.

 Above: Pull an Auxetic material with a negative Poisson’s ratio and against all logic it gets fatter!

There are just a few natural materials which have a negative poisons ratio; these include PTFE, some types of skin at the molecular scale, and the brick structures used for nuclear reactors at the other end of the scale.

 Above: Auxetic materials are rare in nature but they do exist in nature from a molecular to Macro scale  

Manufacture of Auxetic materials has only taken place recently, as a result of work done by researchers Prof Andy Alderson and Dr Kim Alderson at the University of Bolton . Materials now available in small quantities include foams and fibres. Unfortunately these quantities are so small that there are no samples available yet for experimenters.

Auxetic materials occur when the structure is arranged in the classic bow tie shape. The best example of this effect is an Auxetic honeycomb. In the example shown in the diagram it can easily be seen that the y axis walls of the cells rotate as the x axis strain is applied creating the effect that the honeycomb sheet is getting larger as it is stretched.

 Above: The classic example of an Auxetic honeycomb used a bow tie structure.

Auxetic Materials are a major area for research and exploitation. Auxetic Technologies Ltd is already developing significant applications. Much of the underpinning technology is patented and this will quite properly have to be considered when developing more products and processes.

The benefits of auxetic materials can be quite extraordinary, for example it is probable that cracks in auxetic materials will be self healing and there is evidence that this may indeed be the case. There are certain to be many applications which have not yet been invented, and some existing ideas which have not been developed. For instance no-one can make auxetic honeycomb at a commercial price. There are undoubtedly opportunities for inventors to develop further specialist products and processes.

Extract from an article prepared by Product Technik for Inventique, Magazine of the Wessex Round Table of Inventors.

For more information from Auxetic Technologies click here

For information on auxetic applications click here

For the Wessex Round Table of Inventors click here

For the Axetics network auxetnet click here

For an article written by Dr Andy Alderson for Technical Textiles Magazine