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.