Evaluation of tensile properties in in-plane auxetic carbon/epoxy and carbon-glass/epoxy composite laminates

Abstract

Auxetic laminates, i.e. with a negative Poisson’s ratio (NPR), show great engineering promise, with enhancements in shear resistance, fracture toughness, energy absorption, and delamination and damage extension limitation. This study focuses on the tensile properties of in-plane (IP) NPR fibre-reinforced polymer (FRP) laminates, namely carbon/epoxy (C/E) and carbon-glass/epoxy (C-G/E) laminates, with lay-up sequences which maximised the effect. An analytical study was conducted to achieve maximum NPR sequences. Laminates were manufactured via hand lay-up and hot compression moulding techniques. Poisson’s ratios of −0.33 and −0.18 were achieved for C/E and C-G/E IP auxetic lay-ups, respectively. A 72% reduction in longitudinal stiffness between the maximum IP NPR and a unidirectional (UD) lay-up sequence was verified, with a further reduction of 24% from the C/E to the C-G/E auxetic configurations due to less anisotropy from G/E plies. Regarding longitudinal strength, maximum IP NPR sequences showed low failure strengths, with a reduction from 1213 MPa in the UD case, to 209 MPa in the studied C/E laminates. An alternative design sequence was studied analytically with the incorporation of 0° layers in the auxetic sequences, which improved stiffness, albeit at a reduction of IP auxeticity. Further research is recommended to explore alternative auxetic lay-up sequences to study the influence on stiffness, strength, and auxetic enhancements.