Fracture and fatigue performance of textile commingled yarn composites

The response to mechanical loads of unidirectional commingled warp knitted and woven glass fibre reinforced polyethylene terephthalate laminates has been characterized. The mechanical properties of the two materials were determined under tension, in-plane shear and flexure. The flexural fatigue properties were determined for the woven laminates by means of three-point bending tests with a loading ratio of R = 0.1 at stress levels of 50-90% of the ultimate static strength. 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 and mixed mode bending tests, respectively. The main fractographic features, as determined by a scanning electron microscopy 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 fibre pull-out was reduced and the features of the matrix appeared to be more sheared. Cusps were found on the fracture surfaces of specimens tested in pure Mode II and mixed mode I : II = 1 : 4. Cusps are normally not found in thermoplastic matrix composites. (C) 1998 Kluwer Academic Publishers.