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

The fibre geometry resulting from the addition of through-thickness 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. (C) 1998 Elsevier Science Ltd. All rights reserved.