Stitch-bonded Sandwich Composites for Improved Performance

Abstract

This paper investigates the influence of through -thickness stitching on damage tolerance of NCF sandwich composites. Stitch-bonded non-crimp fabrics (NCF) are a popular choice for both monolithic and sandwich composite structur es. It appears that the fine binding thread used in the production of NCF fabrics offers insignificant resistance to damage. In this paper, NCF fabrics and sandwich foam were stitched together with a high performance thread using a multi-needle stitching process. Performance of the stitched sandwich panels have been compared with the unstitched panels using indentation, transverse compression and three-point bending tests. Optimization of sewing thread performance has also been briefly reported. I. Introduction andwich structures suffer from skin delaminations or de-bonding between the skin and the sandwich core due to accidental impact. Honeycombs and foams are widely used as sandwich core materials; these materials suffer from core compression failure, buckling instability and face sheet to core de-bonding [1]. Through-the-thickness (TT) reinforcement of sandwich structures is a promising attempt to overcome these problems. Delamination is reduced because of the existence of transverse reinforcement, and hence the impact resistance and the damage tolerance may be improved. Through-thickness reinforcement can be achieved by a variety of techniques including 3D weaving, z-pinning, and stitching/tufting. The limitations for 3D weaving are: relatively slow production rate, limited capacity of the textile machines, formation of a relatively soft open-cell foam core and the problem of keeping the faces apart in order to keep the z-fibres in tension. Z-pinning is a relatively expensive process and, additionally, z-pins do not have anchors to the top and bottom skins and hence are less effective in preventing skin debonding. Stitch-bonding is a relatively cheap and promising approach as through-thickness reinforcement for the sandwich panels. However, the traditional sewing machines are not suitable for stitching through the reinforcement of thick and hard/rigid closed-cell foam based sandwich panels. Over the past few years, there have been a number of developments in stitching. A large, high-speed and multi-needle Advanced Stitching Machine (ASM) was designed and built under the ACT programme by NASA to stitch large, thick, complex wing structures [2]. Altin-Nahtechnik [3] and KSL [4] developed robotic stitching systems using concepts such as one-sided stitching or tufting. These machines are not suitable for hard cellular foams. Additionally, inserting one stitch at a time is not efficient for producing large stitched areas. Potluri et. al. [5,6] developed a single-needle stitching process for close-cellular foam cores in conjunction with woven face sheets. In the present study, a multi-needle stitch-bonding machine has been developed by the authors to reinforce the close-cellular foam core sandwich panels with bias and orthogonal stitches. Stitch-bonded sandwich panels were impregnated using vacuum bagging process and subjected to indentation, compression and three-point bending tests to evaluate their structural performance.