Influence of 3D Preform Construction on Composite Damage Tolerance: Stitching and 3D Weaving

Abstract

The benefits of introducing through thickness tows to produce 3D fibrous structured composites have long been studied. These 3D structures are known to typically improve interlaminar shear strengths, reducing the occurrence of delamination under complex out-of-plane loading situations such as an impact event. Weaving and stitching technologies currently exist that are able to create such 3D fibrous preforms. Current weaving and stitching technologies have limitations that prevent industrial uptake. This research details the design and development of a novel stitching technology capable of replicating the orthogonal woven binder profile in cross-ply composite preforms. The two robotic stitching heads utilise the transfer of a double-pointed needle between the heads to produce a disjointed stitching pattern on the preform's surface, known as orthogonal stitching. Analysis of the produced stitched composite shows the capability of the developed stitch heads to create a comparable stitched cross-section profile to that of a produced 3D orthogonally woven specimen. Crimp levels are found to be only marginally higher in the stitched composite, although with higher tow damage visible. At the high binder density both the woven and stitched preforms experience a resistance to compression during infusion. This results in resin rich zones and a low fibre volume fraction. In comparison at a low binder density, both show an improved compressibility and therefore a higher fibre volume fraction. This lower binder density enables more in-plane tow shifting during infusion and therefore results in higher crimp levels. Under low velocity impact loading orthogonal stitching was shown to be comparable with orthogonal weaving for improving damage resistance. This resulted in improved compressive strength retention after impact for the high binder density structures due to lower in-plane crimping, reduced damage extent under impact and impaired delamination growth under compression loading. In lightweighting applications requiring an improved damage resistance, low density stitching has been shown to be a suitable alternative to low density weaving, reducing the damage area under impact and maintaining a higher fibre volume fraction. Where lightweighting is less critical and a lower fibre volume fraction can be acceptable, the designed stitch head has been shown to be a viable alternative to 3D orthogonal weaving for improving damage tolerance through reduced damage area and maintaining low crimp levels.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Venkata Potluri (Supervisor)