Effect of Reinforcing Tabs on the Mode I Delamination Toughness of Stitched CFRPs

Abstract

Stitching advanced carbon fibre composites in the through-thickness direction enhances their interlaminar fracture toughness. To study this, composite specimens were fabricated from dry carbon fabric and resin using resin transfer molding (RTM) technique, and also from unidirectional prepreg tape. The stitching was performed with 2-ply (40 tex) Kevlar thread. It was observed during the mode I testing of RTM specimens using a double-cantilever-beam (DCB) test that the cantilever arms failed in bending, instead of crack propagating in a self-similar manner. Therefore, DCB specimens were reinforced with aluminium tabs along the length of the specimens. To understand the effect of tabs on the mode I toughness, prepreg specimens were tested with and without the tabs. The results indicated that the tabs altered the failure mechanism of the stitching thread and the improvement in the mode I delamination toughness was underestimated. Specimens with varying stitch density were studied. The steady-state toughness, GIRS, of unstitched and stitched specimens with and without tabs were compared. The addition of stitching was found to impart a 10-fold improvement in toughness. Experimental results were also compared with theoretical predictions from a micromechanics-based model developed previously, and good agreement was found.