Theme A: Fibre reorientation concepts
The aim of work in this theme is to design and evaluate composite architectures that create ductility via geometrical rearrangement of fibre orientation, matched to tailored matrix characteristics.
Example
It has been successfully demonstrated that using Skyflex™ carbon/epoxy plies of only 0.03 mm thickness, matrix cracking and delamination can be completely suppressed in angle-ply laminates, allowing the fibres to rotate under tensile loading, creating additional strain and pseudo-ductility.
Figures
Angle plies of (±45) layup can produce strains of over 20% and necking behaviour despite the brittle nature of both the fibres and matrix. There is a trade off in the stresses and strains, but modelling has allowed us to achieve a good balance of properties with thin ply (±25) carbon/epoxy laminates that gave a pseudo-ductile strain of 1.23% and a maximum stress of 927 MPa.
Tests involving loading, unloading and then reloading have shown that the initial modulus is fully recovered, and so these laminates may be considered as ductile rather than pseudo-ductile.
Final reports
Angle plies
Thin-ply angle plies allow additional strain via fibre reorientation. This can be combined with fragmentation to produce large pseudo-ductile strains and reduced notch sensitivity.
Angle Ply Summary Report (PDF, 339kB)
Email: m.wisnom.@bristol.ac.uk
Microbraided ropes
Hybrid microbraids have been made by braiding UHMwPE fibres over a unidirectional core of carbon fibres, and their direct use as the reinforcing phase within polymer composites investigated.
Microbraids Summary Report (PDF, 500kB)
Email: lo.iannucci@imperial.ac.uk
Wavy ply sandwich
A sandwich structure has been designed, simulated and tested which exhibits large deformations (through unfolding of the wavy composite skins) and high energy-absorption (through crushing of the foam core cells) under tensile loading.
Wavy ply sandwich Report (PDF, 214kB)
Email: p.robinson@imperial.ac.uk
Wavy ply discontinuities
‘Brick and mortar’ composite architectures have been investigated to produce a ductile failure process from either complete yielding of the matrix in the overlap region, or from cracks initiating at the ends and growing in towards the centre of the overlap regions.
Unidirectional composite with in-plane waviness Report (PDF, 142kB)
Email: p.robinson@imperial.ac.uk
Ductile micro-braided helical structures
An analytical investigation of helical lattice structures demonstrated that pseudo-ductile responses can be obtained.
Helical Lattice Report (PDF, 77kB)
Email: paul.weaver@bristol.ac.uk
Friction mechanisms
Interlocking structures have been investigated composed of jigsaw like pieces made of bow-tie shaped elements connected by infill regions. When subjected to tension, the blocks start to slide, generating frictional forces and a non-linear response.
Friction Mechanisms Summary Report (PDF, 145kB)
Email: p.robinson@imperial.ac.uk
Triaxial braided composites
Triaxial braided composites can show a highly non-linear stress-strain response with a pseudo-ductile plateau due to the complex damage behaviour. A meso-scale modelling framework was developed, which showed good correlation with the measured stress-strain curves and damage mechanisms.
Micro-wrapped hybrid tows
Uni-directional woven fabric laminates consisting of micro-wrapped tows have been produced. While conventional hybrids exhibited a significant load-drop after the first failure, micro-wrapped laminates exhibited pseudo-ductility through stable fibre fragmentation/ pull-out.