Loading rate dependence of the fracture toughness and fatigue life of stitched GRP composites

The in-plane fracture toughness (K-I) and failure modes of stitched and unstitched GRP composites were studied over a range of quasi-static loading rates between 0.1 and 11.0 kN/s. The GRP was stitched through-the-thickness with Kevlar in directions either parallel or normal to the crack path, The fracture toughness was improved by stitching in the normal direction because the crack followed a tortuous path as it grew from stitch to stitch. In contrast, the fracture toughness of GRP stitched in the parallel direction was similar or slightly lower than for the unstitched composite with the crack growing straight along the stitch rows. The amount of crack branching by delamination was reduced by stitching due to the higher interlaminar fracture toughness of the stitched composites. Raising the fatigue frequency from 0.25 to 3.0 Hz caused a progressive increase in the fatigue life of GRP stitched in the parallel direction due to creep-dominated failure processes.