Sweet inspiration

A Bristol University-led EU project has developed a material that could provide industry with an alternative to currently available core materials used for building sandwich structures.

Project CHISMACOMB’s (CHIral SMArt honeyCOMB) material can be embedded with sensors and electromagnetic actuators to provide information about the structural integrity of sandwich structures used in civil, naval, aerospace, construction and electromagnetic shield applications.

The optimised chiral cellular core configuration of the auxetic, honeycomb-structured material also becomes thicker when stretched, providing greater flexibility without compromising strength.

The explanation behind the material’s unusual behaviour is a scientific principle called Negative Poisson’s ratio, which can occur in the cellular assemblies of honeycomb composite structures. In honeycombs, the ratio behaviour implies a stiffening geometric effect, which leads to high resistance, stiffness and strength. The auxetic behaviour also leads to the material’s bowl-shaped curve, which the researchers say is useful when manufacturing curved sandwich shells.

Fabrizio Scarpa, of the aerospace engineering department at Bristol, is the leader of the project. He said scientists have talked about the possibility of using a material with a Negative Poisson’s ratio since the 1920s.

‘This was known from the theoretical point of view for years, but the problem was no-one could come up with a working material with these sort of properties,’ he said.

The first close attempt at creating a honeycomb structure displaying this type of behaviour was in the late 1980s, when US materials science engineer Rod Lakes developed thermoplastic foam with Negative Poisson’s ratio.

‘The material was very successful at the academic level,’ said Scarpa. ‘However, rapid prototyping technology was not very advanced at the time, and it was difficult to produce these honeycombs in large bulk volumes.’

As manufacturing technology matured in 2005, Scarpa began co-ordinating the project to design a honeycomb structure that could be taken outside the laboratory.

The result is a chiral structure that is similar to Lakes’ but more suitable for manufacturing techniques such as rapid prototyping or possibly injection moulding. The honeycombs, which can be made of PA sintered powder or ABS plastic, consist of cylinders connected by tangent ligaments. When there is a push or pull on the honeycomb, the ligaments bend and the cylinders rotate.

The aim of the researchers is to dedicate their structures for production in niche markets. ‘We are not talking about huge volume productions, but possibly for applications such as parts for orthopaedic processes,’ he said. One ambitious niche application has been identified in aerospace design.

‘During the last five or six years there has been a huge interest in morphing wings, especially for military applications,’ said Scarpa.

The researchers have demonstrated in wind tunnel tests that their structures could be incorporated in aircraft wing design to allow wings to bend, twist, shrink and expand to optimise their aerodynamic properties during flight.

With the use of embedded sensors and actuators, they showed that they could manipulate individual cylinders in the structure. In doing so, they believe future aircraft that incorporate the material will make less noise and produce fewer greenhouse emissions.

Another application, Scarpa added, is in the maritime industry. The materials could give marine designers the ability to improve the sandwich structures in mine-hunting ships, and in the decks and joints of pleasure boats.

‘These materials offer exciting new possibilities and change the nature of how composite materials, in particular carbon fibre cellular structures, can be used to gain even greater advantages from them,’ said Scarpa.