Viscoelastic Effects on Interlaminar Fracture Behaviour of Thermoplastic Matrix Composites. I) Rate and Temperature Dependence in Unidirectional PEI/Carbon Fibre Laminates

The influence of the dependence of matrix toughness on deformation rate and temperature on the interlaminar fracture of polyetherimide (PEI)/carbon-fibre unidirectional laminates has been investigated. Fracture toughness versus crack speed data for the matrix and the composite, respectively, were determined by using the double torsion and the double cantilever beam tests. Tensile tests were also performed on the unreinforced resin. Displacement rates varying between 0.1 and 100 mm min were applied, and testing temperatures were varied between 23 and 170 °C. Fracture data reduction was achieved by means of time-temperature superposition, the shift factors being obtained from tensile tests. The toughness of PEI, by contrast with that of other thermoplastic polymers commonly used as matrices for composite materials (e.g. PEEK), showed a non-monotonic, but still moderate, sensitivity to the test conditions explored here. Interlaminar toughness of the corresponding composite increased steadily with crack speed and was always lower than that of the plain resin. This result indicates that the interlaminar structure of the composite material is not optimised as regards this particular property.