Thermal expansion coefficients of unidirectional carbon fiber/PEEK composite made of commingled yarns

Unidirectional carbon fiber (CF)/poly(ether-ether-ketone) (PEEK) composite was manufactured from the commingled yams composed of CF and PEEK filaments (this composite is named “commingled composite”). The thermal expansion coefficient (TEC) of the commingled composite was measured and compared with that of CF/PEEK unidirectional composite made of prepreg sheets (this composite is named “prepreg composite”). The minimum value of TEC’s parallel to the fiber direction was observed at around 150 °C for both specimens which were not thermally treated. This minimum disappeared after the thermal treatment at 230 °C for 30 min or the repeating measurements (0 °C → 230 °C, 0 °C → 230 ^oC,…). While the TEC of the prepreg composite was almost independent of temperature, that of the commingled composite showed a remarkable decrease at temperatures above the glass transition temperature (T_g) of PEEK. This could be interpreted in terms of the orientation distribution of carbon fibers in the commingled composite. The difference of longitudinal TEC’s between heating and cooling experiments was attributable to the temperature difference between the surface and the inner parts of the specimen. The TEC’s transverse to the fiber direction were scarcely influenced by the thermal history. The TEC’s increased slowly below 100 °C and increased rapidly above 100 °C with increasing temperature. Theoretical predictions based on the extended equivalent inclusion method and on the classical lamination theory exhibited good agreement with experimental data. However, it could not be predicted that the TEC’s parallel to the fiber direction were depressed at around 30 °C. The cause of this discrepancy was supposed due to the temperature dependence of TEC’s of the carbon fiber itself.