Thermal mismatch in carbon fibre-reinforced epoxy composites can be overcome using auxetic polymers, according to researchers at the University of Bolton, UK. This could reduce the time and money involved in manufacturing composites.

‘A unidirectional carbon-epoxy ply develops significant residual stresses as it cools down during manufacture. The stresses arise as a result of the mismatch in the coefficient of thermal expansion (CTE) between the carbon fibre and epoxy constituents – these tend to weaken the ply and reduce the mechanical performance of the composite,’ explains Professor Andy Alderson of the Centre for Materials Research and Innovation at Bolton and the Northwest Composites Centre.

Reduced thermal distortion through expansion of the auxetic phase (green)‘The carbon fibre has a low, in fact, slightly negative, CTE along the fibre direction and higher CTE normal to the fibre direction, [while] the epoxy resin has a high CTE (see diagram above, left). The individual ply [therefore] also exhibits anisotropic effective CTE behaviour.’

 Thermal distortion occurs when the top ply contracts horizontally as a result of the resin-dominated large horizontal coefficient of thermal expansion (green), whereas the bottom ply maintains horizontal dimension due to the low horizontal CTE

The team at Bolton proposes the inclusion of an auxetic polymer in each ply to achieve intra-ply balancing, rather than relying on the current expensive and weighty technique of introducing additional balancing plies with complex stacking sequences.

Auxetic (negative Poisson’s ratio) materials expand in width when stretched, rather than get thinner. This means that when the ply cools down, the constraining effect of high stiffness, low longitudinal CTE carbon fibres, causes the auxetic polymer to undergo tensile stress longitudinally and transverse expansion. This counteracts the contraction of the epoxy and of the carbon fibre normal to the fibre direction.

Alderson says, ‘The mismatch in the effective CTE along and normal to the fibre direction can be reduced to zero CTE when the appropriate combinations of constituent volume fraction, Young’s modulus, Poisson’s ratio and CTE are employed.’

Although, unable to comment specifically on the research at Bolton, Quentin Fontana of composites manufacturer Advanced Composites Group Ltd, based in Heanor, UK, says, ‘Thermal expansion mismatch is a problem in accuracy of manufacture and in component dimensional stability in-service.

‘Regarding accuracy of manufacture, Precimould software is being developed as part of the LUSAS finite element code [by Finite Element Analysis Ltd, the Advanced Composites Group, Bombardier Aerospace and BAE Systems], which enables prediction of the required tool geometry [to] yield an accurate component.’

He adds, ‘In terms of in-service behaviour, the primary approach of using balanced lay-ups and quasi-isotropic constructions in laminates [is] not optimal. There is an in-built redundancy that increases material requirements and costs, and has consequent weight penalties, which are of particular importance in transport.’

Alderson explains that the use of auxetic materials could also enable intricate, non-planar components to be machined. ‘Machining to size and shape cannot [usually] be employed since this removes plies and leads to further distortion as the laminate becomes unbalanced once again.’

The team has recently reported the production of auxetic polypropylene, polyester and polyamide in fibre form, and is working to expand the range of polymers employed and incorporated into composites. Researchers are in discussions with end-users in a range of applications areas, such as aerospace and cryogenic tanks for spacecraft, through spin-out company Auxetic Technologies Ltd.

Further information:

Auxetic Materials Network at Bolton University, UK
Auxetic Technologies Ltd
Northwest Composites Centre