Improvements in Mode I interlaminar fracture toughness and in-plane mechanical properties of stitched glass/polyester composites

This paper examines the effect of plain stitching by untwisted fibre rovings in the in-plane mechanical properties and Mode I interlaminar fracture toughness of glass/polyester composites. The traditional FRP laminated composites with two dimensional fibre architecture have poor interlaminar fracture resistance and suffer extensive damage by delamination cracking when subjected to out of plane loading. Through-the-thickness reinforcement is done by stitching with twisted fibre yarns in sewing machine to increase the delamination strength, but, it degrades the in-plane mechanical properties. In this work glass woven roving mats (WRM) are stitched in the thickness direction with untwisted kevlar, glass and carbon fibre rovings of various Tex by the plain stitch. It is observed that in plain stitch, no thread cross is formed and thus no resin rich pockets are seen. Uniform distribution of fibres in the stitch roving, absence of resin rich region and lesser amount of fibre damage result in increased in-plane tensile, lap shear, flexural, transverse shear and impact strengths. Effect of stitching on Mode I delamination toughness (GIc) of glass/polyester laminates has been investigated by performing double cantilever beam (DCB) test. It is observed that stitching increases the Mode I delamination toughness up to 20 times higher than that of unstitched specimen.