Interlaminar fracture of commingled GF/PET composite laminates

The Mode I, Mode II and mixed mode (Mode I:II ratios of 4:1, 1:1 and 1:4) fracture behavior of novel textile glass fiber reinforced polyethylene terephthalate (GF/PET) laminates has been investigated. The laminates were manufactured by compression molding two different fabrics produced by weaving and warp knitting commingled GF/PET yarns. The initiation fracture toughness 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 toughness of the two materials was smaller. An extensive scanning electron microscopy (SEM) investigation of the fracture was conducted to identify any characteristic failure mechanisms. The main fractographic features of the Mode I dominated failures were a brittle matrix failure and large amounts of fiber 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 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.