Studies on VARTM Forming Z-Stitched Foam Core Sandwich Composites

Abstract

Z-stitched foam core sandwich composite is a newly developed structure with through-thickness load-bearing pillars embedded by a stitching technique. In this paper, large-tow glass fibre stitches and fire-resistant phenolic resin were creatively used to fabricate this kind of sandwich composite, which possessed outstanding properties including fire resistance, debonding resistance and high flatwise compressive strength. Vacuum-assisted resin transfer moulding (VARTM) was conducted for processing. The effects of process conditions on key structural parameters of the z-stitched foam core sandwich composite were investigated, including the dimensions of the pillar, the fibre volume fraction of the facesheet as well as the porosity of the pillar. The results show that macroscopic void defects tend to be formed at the joints between the pillars and the top facesheet if air is trapped in the system. The cross sectional areas of the pillars are all larger than the suture needles due to damage of foam cells in the vicinity during stitching process, which is caused by the brittleness of the phenolic resin. Moreover, the dimensions of the pillar could be affected by the vacuum pressure and the pressure gradient, even resulting in local resin-rich area around pillar. In order to optimize the manufacturing quality of this sandwich composite, a small linear density of stitches and a long enough action time of the vacuum process are both very helpful.