Effect of stitching on Mode I strain energy release rate

Stitching advanced graphite/epoxy composites in the through-thickness direction improves the interlaminar fracture toughness. The Mode I energy release rate of a graphite/epoxy composite, with and without stitching fibers, was experimentally determined using double cantilever beam specimens. The fibers employed in this research are not densely stitched. Three different fibers, namely Para-Aramid fiber, Glass fiber, and PAN-based carbon tow fiber, were used to study the effect of fiber stiffness on the strain energy release rate. The 3-D finite element model combined with the crack closure integral method is employed to obtain the GI and GIC values. Experimental results were compared with finite element analytical calculations and good agreement was found. The first mode of strain energy release rate of stitched specimens is about three to six times that of unstitched specimens depending on the stiffness of the stitched fibers.