Auxetics
Our auxetics research sits within the Exeter Advanced Technologies (X-AT) research group, and is led by Professor Ken Evans.
An auxetic material is one which exhibits a negative Poisson’s ratio: this is a counter-intuitive physical property whereby the material becomes thicker when stretched.
Our world-leading research into auxetic materials and structures examines this surprising property, which is widely occurring in the natural world and has even been measured in the human body: skin, artery walls and trabecular bone all possess a negative Poisson’s ratio.
Benefits of auxetics
Auxetic materials possess a number of benefits over conventional materials:
- When put into flexure they form a dome shape rather than the conventional saddle shape, which is ideal for the production of doubly curved components such as aircraft and automobile components.
- When indented, the material under the indenter densifies acting to make the material harder, ideal for indentation-resistant foam packaging.
- Poisson’s ratio balancing, allowing the use of dissimilar materials where auxetic content is used to match the lateral expansions of components to prevent internal stresses under loading.
- Auxetic materials have been successfully used to enhance the vibration damping of cellular structures used in sandwich panel cores for use in aerospace structures.
The majority of negative Poisson’s ratio materials are cellular solids such as honeycombs and foams, however work into fibres and recently composites (including carbon fibre composites) has allowed the production of auxetic fibre composites which may be used in conjunction with auxetic honeycombs or foams to create entirely auxetic sandwich structures and to make auxetic fabrics used to develop next generation blast curtains.
Our research projects
Auxetic textiles for blast mitigation
Using an industrial prototyping weaving loom, this research project was seeking to create a 'smart' material that could minimise the injuries inflicted by a terrorist attack. The team aimed to create auxetic blast curtains that would catch glass fragments and debris blown through windows by an explosion.
The curtains were put into a test chamber behind glass panels and subjected to an explosive blast to test their ability to minimise the penetration of glass into the chamber and to absorb the shock wave.
Partners: Auxetix Ltd, Home Office Scientific Development Branch, Monofil Trading Company Ltd, Centre for the Protection of National Infrastructure
Related publications
- 2010 - R.Wright, M.R.Sloan, K.E.Evans, M.K.Burns - "The Helical Auxetic Yarn: Performance and Novel Applications" - Polymer Fibres 2010, 7-10 July, Edinburgh.
- 2010 - M.R.Sloan, J.R. Wright, K.E.Evans, The helical auxetic yarn; a novel textile for blast mitigation, IMechE Engineering structures survival: Blast and impact protection, Nottingham, UK 2010.
- 2010 - J.R.Wright, M.R.Sloan, K.E.Evans – “Tensile properties of helical auxetic structures: A numerical study” – Journal of Applied Physics 108, 044905 (2010)
AUXTRUSION: A novel manufacturing process for the extrusion of helical auxetic yarns
The AUXTRUSION project proposed development of a new extrusion manufacturing technique to produce helical auxetic yarns (HAY). This is a yarn composed of two conventional fibres, one helically wrapped around the other. By appropriate choice of materials and geometry this yarn can be made such that it effectively gets wider when stretched (exhibiting a large negative Poisson’s ratio).
This opens a panorama of interesting possibilities for high-performance or ‘smart’ textiles. Uses include pore-opening fabrics in healthcare or security and defence sectors, filtration, colour change fabrics for healthcare, safety harness or fashion, and disaster mitigation – tornado, hurricane or earthquake protection.
Funded by: Auxetix Limited, Wisla Narrow Fabrics Ltd, Fothergill Engineered Fabrics Ltd
An improved safety, climbing, parachute harness incorporating auxetic yarns to enhance narrow fabric technology
Sling webbings are in everyday use in the construction industry and in areas where a lifting mechanism is required. Currently, there is no indication when a webbing has been overextended and is therefore no longer fit for purpose.
This project evaluated helical auxetic yarns as failure or end-of-life indicators in sling webbings. It was conceived that the ability to cause an auxetic yarn to ‘pop out’ of a fabric when under tension would provide an ideal indicator that the fabric had been deployed beyond its safe working limit. (A similar mechanism could also be used to indicate when an ‘optimum’ load had been applied in alternative uses of fabric.)
The trials were conducted by the University of Exeter in collaboration with Wisla Narrow Fabrics Ltd. Working prototypes were produced for both permanent and temporary overload conditions, and a patent is pending on the technology.
Funded by: Smart.mat (Materials KTN)
