Role of cooling pressure on interlaminar fracture properties of commingled CF/PEEK composites

The effects of applied pressure, when cooling from the processing temperature, on consolidation quality and interlaminar fracture properties have been studied for a commingled carbon fibre/poly(ether ether ketone) (CF/PEEK) composite. A compression moulding system using a hot press was employed, varying the applied pressure during cooling (i.e. ambient, 0.2, 0.4 and 1 MPa) after consolidating the CF/PEEK composites at 1 MPa. Consolidation quality was characterized by measuring the void content and/or the density of the consolidated CF/PEEK composites, while transverse flexural properties were measured and correlated with the processing conditions. Modes I and II interlaminar fracture behaviour were studied using double cantilever beam and end-notch flexure specimens, respectively. The implications of the effect of the cooling pressure and consolidation quality on the resulting mechanical property profiles are discussed. The results indicate that the consolidation quality, measured by specimen density and/or cross-sectional void surface area, of the commingled CF/PEEK composites was very good when the composites were cooled at an applied pressure of 0.2 MPa or above, while deconsolidation, or the increase of void content in the composite, was experienced when the composites were cooled at ambient pressure. It was also found that transverse flexural performance and modes I and II interlaminar fracture properties drop significantly only when the applied pressure was reduced to ambient pressure on cooling.