The response to mechanical loads of commingled warp knitted and woven glass fibre reinforced polyethylene terephthalate (GF/PET) laminates has been characterised. The Mode I, Mode II and mixed mode (Mode I:II ratios 4:1, 1:1 and 1:4) interlaminar fracture toughnesses of the laminates were determined by means of the double cantilever beam (DCB) and mixed mode bending (MMB) tests respectively. The initiation fracture toughnesses of the woven laminates in pure Mode I and Mode II were slightly higher than those of the warp knitted laminates. For the mixed modes, the difference between the fracture toughnesses of the two materials was smaller. The main fractographic features, as determined by a scanning electron microscopy (SEM) examination, of the Mode I dominated failures were a brittle matrix failure and larger amounts of fibre pull-out. As the Mode II loading component increased, the amount of fiber pull-out was reduced and the matrix had a more sheared appearance. A relatively large amount of shear cusps were found in pure Mode II and mixed mode I:II = 1:4; such features are seldom seen in thermoplastic matrix composites. A general mixed mode failure criterion, which accounts for the appearance of the fracture surface, was evaluated and was seen to give a good fit to the experimental fracture toughness values.
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