Investigation of Braid Geometry for Complex Braided Composite Structures

Abstract

The aims of this study are to investigate the mechanism of wrinkle formation on cylindrical preform during bending and minimise the wrinkle formation to improve the properties of the final products. It was found that changes in braid parameters due to the scissoring of fibre tows was one of the major influencing factors for causing out-of-plane deformation. In order to minimise or potentially eliminate this out of plane deformation that leads to wrinkle formation, the changes in braid angle and braid diameter under uniaxial tension and compression loading have been studied. In this study, biaxial braid with ±45° braid angle was produced using T700 Toray carbon fibre tows on a circular cross-section cylindrical mandrel. Bending of the over-braided preform produced a significant change in braid angle around the circumference as well as along the length as a result of opposing forces (tensile and compressive) at the outer and inner of the bend. Optimisation of braid parameters was achieved by changing the braid angle locally around the circumference of the braid to compensate for distortion during the bending process. Reducing the braid angle during braiding at the compression side of the mandrel can minimise the wrinkle formation during preforming using bending. Two methods were used to accomplish the change in braid angle in this study. One method was braiding using an elliptical braid ring instead of a circular braid ring. Two diferent braid angles were produced on the mandrel major and minor diameter of braid ring. The secondary method was braiding the mandrel/core off-centre of the braid ring. Different braid angles were achieved on the plane of off-set because of the difference in distance from braid ring to braid fell point. The braid angle measured after bending these braids and the change in braid angle were optimised due to changes in braid angle in the tension and compression sides as opposed to the angle along the neutral axis. This change eliminated one of the major factors that give out-of-plane deformation, resulting in wrinkles. Braided structures that have been pre-formed to complex geometrical shapes were then resin infused to produce composites. Two preforms were braided using circular braid ring or an elliptical braid ring respectively. After bending and under pressure during resin infusion, optimised structures showed an improvement in wall thickness as well as wrinkle formation. Wrinkles were formed during resin infusion for the braid produced with a circular braid ring. The braid angle reduction in the preform stage resulted in the elimination of wrinkle formation. The optimised braid structure improved wall thickness of the composite.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	William Kennon (Supervisor) & Venkata Potluri (Supervisor)