VISCOELASTIC EFFECTS ON THE INTERLAMINAR FRACTURE-BEHAVIOR OF THERMOPLASTIC MATRIX COMPOSITES .1. RATE AND TEMPERATURE-DEPENDENCE IN UNIDIRECTIONAL PEI CARBON-FIBER LAMINATES/

The influence of the dependence of matrix toughness on deformation rat e and temperature on the interlaminar fracture of polyetherimide (PEI) /carbon-fibre unidirectional laminates has been investigated. Fracture toughness ver sus crack speed data for the matrix and the composite, respectively, were determined by using the double torsion and the doub le cantilever beam tests. Tensile tests were also pei formed on the un reinforced resin. Displacement rates varying between 0.1 and 100 mm/mi n were applied, and testing temperatures Mere varied between 23 and 17 0 degrees C. Fracture data reduction was achieved by means of time-tem perature superposition, the shift factors being obtained from tensile tests. The toughness of PEI, by contrast with that of other thermoplas tic polymers commonly used as matrices for composite materials (e.g. P EEK), showed a non-monotonic, but still moderate, sensitivity to the r est conditions explored here. Interlaminar toughness of the correspond ing composite increased steadily with crack speed and was always lower than that of the plain resin. This result indicates that the interlam inar structure of the composite material is not optimised as regards t his particular property.