An investigation was made to determine the effect of cooling rate on the interphase adhesion and mechanical properties of carbon fibre/PEEK matrix composites. The fibre/matrix interface adhesion was characterised based on the fibre fragmentation and fibre pull-out tests. A series of tensile, flexure, short-beam shear, interlaminar fracture and impact tests were conducted. The interface adhesion was correlated to the degree of crystallinity and crystalline morphology, as well as the bulk mechanical properties of laminates composites. It is shown that the amorphous PEEK rich interphase introduced in fast cooled specimens results in relatively ductile interface failure and low interfacial shear strength (IFSS), whereas the transcrystalline interphase in slow cooled specimens gives rise to...[ Read more ]

An investigation was made to determine the effect of cooling rate on the interphase adhesion and mechanical properties of carbon fibre/PEEK matrix composites. The fibre/matrix interface adhesion was characterised based on the fibre fragmentation and fibre pull-out tests. A series of tensile, flexure, short-beam shear, interlaminar fracture and impact tests were conducted. The interface adhesion was correlated to the degree of crystallinity and crystalline morphology, as well as the bulk mechanical properties of laminates composites. It is shown that the amorphous PEEK rich interphase introduced in fast cooled specimens results in relatively ductile interface failure and low interfacial shear strength (IFSS), whereas the transcrystalline interphase in slow cooled specimens gives rise to a high IPSS with relatively brittle interface debonding. The sensitivity of matrix crystallinity on cooling rate was much higher than the changes in INS. The interlaminar shear strength (ILSS) varied with cooling rate in a manner similar to IFSS. The increases in the interlaminar, tensile and flexural properties with decreasing cooling rate was attributed to the changes in interfacial shear strength, matrix properties, and transition of the failure mechanisms which was observed from adhesive to cohesive failure when the cooling rate was decreased.

The presence of brittle crystalline interphase and matrix material resulting from a low cooling rate reduced the mode I and mode II interlaminar fracture resistance despite the large contribution from the strong interface adhesion. At high cooling rates, however, the high ductility of PEEK matrix with less crystallinity was not fully translated to interlaminar fracture toughness due to the weak interfacial bond. A strong fibre/matrix interface bond is essential to the occurrence of extensive matrix plastic deformation, otherwise the matrix deformation is preempted by interfacial debonding. The fast cooled carbon/PEEK composites had better impact performance in terms of a higher incipient load, a smaller delamination area, and a higher threshold energy and better retentivity of residual strength than the slow-cooled carbon/PEEK and carbon/epoxy laminates.

It is demonstrated in this study that the fibre/matrix interphase adhesion and the bulk mechanical properties of the thermoplastic composite can be optimised, if not totally maximised, by controlling the processing cooling rate. The fundamental knowledge acquired in this study would provide practical processing windows and guidelines that can produce composites possessing balanced mechanical and structural performance.