The dependence of the fracture toughness of thermoplastic composite laminates on interfacial interaction

The paper investigates the influence of microstructure of the matrix resin and polymer/reinforcement interface on the fracture performance of thermopl astic composites in relation to processing conditions. Solution-pre-impregn ated, unidirectional, carbon-fiber-reinforced poly(ether ether ketone) lami nates prepared at different melt residual times were used as the experiment al materials to obtain different interfacial effects and matrix morphologie s. Composite crystalline structures were analyzed by X-ray diffraction and infrared spectroscopy. The stress-intensity factor was measured by means of the single-edge-notched bending method along the two orthogonal directions of the laminates. The results indicate that the Mode-I fracture toughness along the fiber direction, K-IC", increases with a rise in melt residual ti me, while the stress-intensity factor at failure perpendicular to the fiber direction, K-IC(perpendicular to), changes with the melt residual time in a more complicated way. Through a careful comparison of the two stress-inte nsity factors with composite microstructure, the dependence of the fracture toughness on the crystalline morphology of the matrix resin has been found . That is, K-IC(perpendicular to) increases with increasing matrix crystall inity and K-IC" increases with either a decrease in the heterogeneous cryst allization or an increase in the orientation of matrix crystallites. It is, thus, proved that the fracture toughness of the laminates depends on cryst allinity and crystalline morphology of the matrix as well as the interfacia l interaction, but not merely on matrix crystallinity. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.