Abstract:
The thesis presents a detailed analysis of the effects of one-sided access z-direction reinforcement,
‘tufting’, on the morphology and mechanical performance of the resulting
MVR-444 epoxy matrix/carbon fibre fabric composites. The dry fabric architectures
used are pseudo-UD, twill woven fabric and non-crimped fabric (NCF). They are tufted
with a range of commercial tufting threads, using KSL KL150 tufting head mounted on
a 6-axis robot arm. The main focus is on the use of a twisted carbon fibre thread, at areal
tufting densities of 0.5% and 2%. The composite plates are prepared via Resin-Transfer-
Moulding (RTM) route, making it possible to control the plate thicknesses.
The morphological features characteristic of tufted, cured composites are described and
categorised. The global and local fibre volume fractions are measured and simple models
proposed that connect local increase with local fibre deviation and presence of resin
rich surface loop layers. It is shown that the balance of in-plane and out-of-plane properties
in tufted composites is highly dependent on the tufting parameters, but also on the
fabric architecture, with the NCF option seeming the most attractive. Overall, the stiffness
of tufted materials is not affected and the drop in in-plane strength of any realistic
geometry combinations is below 20%. ‘Thread-less’ tufting experiments prove that the
drop is not caused by fibre breakage from the passage of the needle alone.
Digital image correlation (DIC) techniques is used to map out the strain field distributions
during mechanical testing, increasing the accuracy of crack tip location in Mode II
delamination cracking studies and confirming the mode mixity changes during deformation
of tufted structures. Single-tuft experiments provide the experimental data that are
required for the development and validation of analytical models. A finite element unit
cell model is developed to predict in-plane elastic and failure behaviour of tufted UD
and NCF composites incorporating the critical meso-structural features of fibre deviation
and increased fibre volume fraction.
The thesis also contains an overview of the tufting technology and some detailed information
on recent manufacturing developments that were required to obtain the controlled
quality specimens used in the study. A demonstration structural element was
produced, in the form of a tufted omega-stiffener. A standard pull-off test demonstrates
the superior load carrying and energy absorbing capacity of this strengthened structure.
Details of robot programming, additional single tuft bridging results, test fixture design,
derivation of the analytical bridging model and additional publications are given in appendices
to the main body of the thesis.