Evaluating the mechanical behaviour of orthotropic 3D woven carbon fibre reinforced composites

Abstract

Laminated composites are increasingly employed in a variety of industries ranging from Aerospace to Wind Power. The high specific stiffness and strength of these materials renders their wider application advantageous in many fields. However, the broader implementation of such composites is frequently restrained by their poor impact resistance and damage tolerance. Even events as innocuous as a dropped tool can impart significant damage to such materials. Such damage can lead to drops in the strength and stiffness that are unacceptable for many applications, particularly those in Aerospace. 3D woven composites present a possible means of improving the impact resistance and damage tolerance of composite materials. The incorporation of out-of-plane fibres, transversely passing through layers of in-plane fibres, is the means by which this improvement in performance is obtained. This body of work presents an investigation into the behaviour of three dimensionally woven carbon fibre composites. Specifically, the subject composite fabric had an orthogonally woven three dimensional structure. The novelty of these materials is such that knowledge and understanding of their mechanical behaviour is very limited. The purpose of this work was to remedy this through experimental and analytical analysis of these composites. The 3D woven materials were characterized experimentally using a variety of techniques. In addition to evaluating the material experimentally, analytical methods were also used. Current analytical methods were found to be deficient in their incapacity to account for in-plane crimp on a micro scale. As a result a new micro scale approach for predicting the stiffness and strength of these 3D woven materials was developed. The composites used for this thesis were tested using a variety of means. The range of test methodologies used subjected the materials to in-plane, out-of-plane, dynamic and quasi- static loadings. Techniques used included; tension, shear, impact, compression after impact, bolt shear out and bearing pull through. Other means implemented included microscopy, C-scanning and Digital Image Correlation. In addition, conventional composites made from unidirectional pre-pregs or Non-Crimp Fabrics (NCFs) were tested to provide a basis for comparison. Analysis and prediction of the behaviour of conventional laminated composites can be performed using a variety of methods. While the range of methods available is broad, they commonly use individual plies of composite as their fundamental building blocks. This is both convenient analytically and experimentally as the properties of such individual lamina may be found with reasonable ease. However, 3D woven composites are integrated laminates due to the out‐of‐plane fibres they possess. As a result, the accuracy of conventional experimental or analytical methods for evaluating these materials is likely to be poor. In order to gain a better understanding of the behaviour of these 3D woven materials a new micromechanics model was developed. In contrast to other available methods, this micromechanics approach examines the effect of crimp at the fibre level. The method proposed is also distinct in its capability of simultaneously accounting for varying crimp across and along a section of composite.