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 thermoplastic composites in relation to processing conditions. Solution-pre-impregnated, unidirectional, carbon-fiber-reinforced poly(ether ether ketone) laminates prepared at different melt residual times were used as the experimental materials to obtain different interfacial effects and matrix morphologies. 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, KIC″, increases with a rise in melt residual time, while the stress-intensity factor at failure perpendicular to the fiber direction, KIC⊥, changes with the melt residual time in a more complicated way. Through a careful comparison of the two stress-intensity factors with composite microstructure, the dependence of the fracture toughness on the crystalline morphology of the matrix resin has been found. That is, KIC⊥ increases with increasing matrix crystallinity and KIC″ increases with either a decrease in the heterogeneous crystallization or an increase in the orientation of matrix crystallites. It is, thus, proved that the fracture toughness of the laminates depends on crystallinity and crystalline morphology of the matrix as well as the interfacial interaction, but not merely on matrix crystallinity.