Investigation on the Thermoelastic Constant of Carbon Fibre Composites

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