Cooling rate influences in carbon fibre/PEEK composites. Part II: interlaminar fracture toughness

A study has been made of the effect of cooling rate on interlaminar fractur e toughness of unidirectional carbon fibre-polyetheretherketone (PEEK) matr ix composites. It is shown that the propagation values of both mode I and I I propagation interlaminar fracture toughness increased with increasing coo ling rate towards a saturation level for the range of cooling rate studied. The cooling rate dependency of the composite interlaminar fracture toughne ss is the result of complex interactions between two important properties, namely the matrix ductility and fibre-matrix interface bond strength. These two properties varied totally in an opposite manner against cooling rate t hrough its effect on crystallinity: matrix ductility varied directly propor tional to cooling rate while the converse is true for interface bond streng th. The extent of plastic deformation of PEEK matrix contributed a predomin ant part to composite toughness, while an adequate interface bond is requir ed to allow matrix deformation to take place to a full degree. A practical implication is that these two properties need to be optimised using an appr opriate cooling rate to produce composites possessing high interlaminar fra cture resistance. (C) 2001 Elsevier Science Ltd. All rights reserved.