On the effects of stitching in CFRPs—II. Mode II delamination toughness

The fibre geometry resulting from the addition of throughthickness fibres in the form of stitching generates considerable improvement in the mode I delamination toughness of laminated carbon fibre composites. However, interlaminar shear, caused either by direct loading or from eccentricities in the structural load path resulting in out-of-plane loading, is a critical loading condition in many composite structures. This paper investigates the effect of stitching on the mode II delamination toughness. The mode II delamination toughness of a carbon/epoxy composite, both unstitched and stitched, was experimentally determined using an end-notched flexure (ENF) specimen. Specimens with varying stitch density, thread diameter and thread type were studied. The steady-state toughness, G IIRs, of unstitched and stitched specimens were compared. The addition of stitching was found to create an improvement in toughness of up to 4 times. In addition, experimental results were compared with theoretical predictions from a micromechanics-based model developed previously.