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Bond, Michael David (2013) Multi-scale modelling of discontinuous carbon fibre reinforced composites. PhD thesis, University of Nottingham.

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Abstract

Discontinuous carbon fibre composites are becoming increasingly popular in the automotive and aerospace sectors, as an alternative to textile-based fibre reinforced composites for both semi-structural and structural components. Materials are highly heterogeneous, with the random architecture leading to uncertainties when modelling and predicting mechanical performance. The microscopic characteristics are known to dominate the strength of the composite, which need to be correctly represented to improve mechanical property predictions at the macroscale.

This thesis presents a multi-scale modelling approach that captures the effects of microstructural (filament level) parameters at the macro scale (component level) to predict the mechanical properties of discontinuous composite materials. In the present work, a continuum damage approach has been used to initiate and monitor failure in the models at all scales, via a user defined material (UMAT), allowing strength predictions to be made for the discontinuous material within the ABAQUS solver. Experimental testing of the material constituents (fibre bundle and matrix materials) has been performed to provide input data for the finite element analyses.

Micromechanical models have been developed to calculate the properties of fibre bundles, which are used directly at the meso and macroscale. Debonding criterion has also been established and validated which has been used to demonstrate that a small interface, with a thickness of only 1% of the fibre radius, can strongly influence the stress transfer between fibre and matrix materials. Interactions between multiple fibre bundles have been considered at the mesoscale, at a range of bundle orientations and separation distances. As the separation distance between the fibre bundles decreased there was an increase in stiffness 0 f the unit cell (~1.9%) across the bundle orientations considered, however, this also coincided with greater stress concentrations (up to 9.6%) being found in the bundles aligned to the direction of loading. These stress concentrations have been used to produce a comprehensive stiffness reduction scheme at the macro scale to account for the 3D nature of the bundle interactions.

A 2D macro scale model is presented for generating discontinuous random fibre architectures consisting of high filament count bundles, with interfacial debonding permitted between the bundle and matrix materials. The fibre bundles are deposited randomly in a 2D plane to provide a representative material. The model has shown that the interface between the bundle and matrix material is critical at short fibre bundle lengths (~5mm) when determining the mechanical properties of the material, with reductions in strength of up to 40% observed at low interfacial shear strengths. The results from the macro scale analysis for discontinuous materials provide predictions within ~10% for tensile stiffness and ~18% for tensile strength when compared with experimental validations.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Harper, L.T. Warrior, N.A.
Keywords:	Fibrous composites, Mechanical performance, Discontinuous composite materials, Strength predictions, Fibre bundles
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	28879
Depositing User:	Blore, Mrs Kathryn
Date Deposited:	21 May 2015 08:02
Last Modified:	13 Oct 2017 19:28
URI:	https://eprints.nottingham.ac.uk/id/eprint/28879

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