Influence of stitching on the interlaminar fracture toughness energy – modes I and II – of unidirectional GFRP

Abstract The influence of stitching on the delamination resistance of unidirectional glass fiber-reinforced plastic laminates depends strongly on the loading mode. The application of a lock stitch with a stitch density of 3.4 cm −1 perpendicular to the fibers results in an increase of the interlaminar fracture toughness energy, mode I, G IC by a factor of 2.7 compared to an unstitched reference laminate. The Kevlar threads are pulled out of one half of the DCB specimen leaving large frayed fiber bundles sticking out of the other half. The same stitching, however, does not improve the interlaminar fracture toughness energy, mode II, G IIC . The Kevlar threads are not able to deform the surrounding matrix thereby expending additional energy. They fail directly at the mid-plane of the ENF specimens. An increased stitch density of 11.6 cm −1 can be expected to lead to a further significant increase in the G IC value. But the tight stitching pattern causes the bonded metal hinges to tear off. The force introduction would have to be changed to enable testing of corresponding DCB and subsequently ENF specimens.