Fracture behaviour of poly(ether ether ketone) films with different thicknesses

The effect of orientation on the fracture behavior for poly(ether ether ketone) (PEEK) films with different thicknesses (0.05, 0.10, 0.30 and 0.50 mm) has been studied. The initial objective was to study both the modes I and III (tearing) fracture of the PEEK films. For the mode I fracture testing, the essential work of fracture (EWF) analysis was used to characterize the PEEK films as gross yielding in the outer plastic zones was observed. However, for the tearing (mode III) fracture testing using the trousers tear sample, the outer plastic zone could not be developed, and an elastic fracture analysis was carried out. There are additional complications with the trousers tear testing. When the PEEK films are too thick (0.30 and 0.50 mm), the trousers legs will fracture instead of crack propagation in the ligament. For the thinner films tested in one of the sample orientations, the tearing crack can be initiated but will detour gradually into the direction perpendicular to the ligament. In the mode I EWF measurements, DENT specimens were cut from two directions perpendicular to each other. The thinner PEEK films (i.e., 0.05 and 0.10 mm thick) show higher degree of anisotropy as indicated by the F–δ curves, we and βwp values. For the thicker PEEK films (i.e., 0.30 and 0.50 mm thick), the mode I EWF measurement shows that the degree of anisotropy is very mild. Pole figure measurements also support that the thinner films have higher degree of orientation than the thicker films.