On the effects of stitching in CFRPs - I. Mode I delamination toughness

Stitching advanced carbon-fibre composites in the through-thickness direction enhances the interlaminar fracture toughness. The mode I delamination toughness of a carbon/epoxy composite, both unstitched and stitched, was experimentally determined by using double-cantilever beam specimens. Specimens with varying stitch density, thread diameter and thread type were studied. The steady-state toughnesses, GIRs, of unstitched and stitched specimens have been compared. The addition of stitching was found to create an improvement in toughness of up to 15 times. The energy-dissipating crack propagation processes were determined and critical parameters identified. In addition, experimental results were compared with theoretical predictions from a micromechanics-based model developed previously and good agreement was found.