Performance of tufted carbin fibre/epoxy composites

摘要

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