Investigation on the Thermoelastic Constant of Carbon Fibre Composites

Orthotropic materials show different thermo-elastic constants depending on their fibre orientation. While most materials undergo a positive elongation with increasing temperature, carbon fibres present a heat-shrink behaviour, which in carbon fibre composites has an important consequence on thermoelastic constants. A decrease in thermoelastic constant with frequency has already been observed in glass fibre composites. Experiments made on uniaxial carbon fibre composites showed that the longitudinal thermoelastic constant increases with the frequency, while the transverse constant decreases. Furthermore, due to the opposite signs of the thermoelastic carbon fibre constant and of the surrounding resin matrix, the absolute values of the longitudinal thermoelastic constant were ten times lower in CFRPs than in GFRPs. An analytical model could successfully reproduce the frequency dependence of the longitudinal thermoelastic constant, thus helping to explain the reason for the observed behaviour. Two calibration samples were used to obtain the thermoelastic constants in the longitudinal and transverse directions. The values of the thermoelastic constants were then applied to a test sample with fibres forming an angle of 10° to the load direction. The expected theoretical results were compared with the results experimentally obtained, showing good agreement. A preliminary calibration of the longitudinal and transverse thermoelastic constants proved to be a useful approach to obtaining the correct value of the thermoelastic constant in a generic direction.