Theme B: Aligned fibre and interface approaches
The theme aims to design and evaluate aligned fibre approaches that achieve a ductile or pseudo-ductile response, while maintaining strength and stiffness. This theme explores hybridisation, aligned discontinuous fibre architectures and fibre surface modifications that promote slip at the interface.
Approaches
Ductility via fragmentation
One strategy in this theme involves fragmentation in thin-ply hybrids. Again there is a trade-off between pseudo-ductility and yield stress. A range of different glass-carbon hybrid configurations has been evaluated, and pseudo-ductile strains of up to 2.66% have been obtained with a plateau stress of 520 MPa, or 0.86% pseudo-ductile strain with a plateau stress of over 1300 MPa.
Similar responses have also been achieved with carbon-carbon hybrids. However in both cases, loading-unloading-reloading tests show a reduction in initial modulus due to the damage, and so these laminates show pseudo-ductility rather than true ductility.
Ductility via fragmentation in thin angle plies with 0˚ plies
The two previous mechanisms can be combined by replacing the lower modulus glass plies in a glass/carbon hybrid with carbon fibre angle plies. This allows the fragmentation mechanism exhibited by the unidirectional hybrid composites to occur in the 0˚ plies together with the fibre rotation of the angle plies. For example [±265/0]S laminates of Skyflex thin carbon/epoxy gave a pseudo-ductile strain of 2.2%.
Discontinuous fibre composites
Another mechanism for pseudo-ductility is slip at the interfaces between discontinuous fibres or plies. This has been demonstrated in model systems of discontinuous carbon/epoxy prepreg where the plies have been cut through the thickness prior to layup.
The effect of ply thickness, cut spacing and alignment on the response have been investigated both numerically and experimentally. For example specimens of IM7/8552 carbon/epoxy with 0.25 mm thick discontinuous ply blocks (0.125 mm for surface plies) and overlap length of about 8 mm were tested and showed significant non-linearity, providing a clear indication of damage, with a modest pseudo-ductile strain of 0.25%.
Non-linear tensile behaviour can also be produced in short fibre composites. The HiPerDiF method allows manufacture of high volume fraction well aligned unidirectional short fibre composites. When high-modulus carbon and glass fibres are mixed, non-linear response and pseudo-ductility can be obtained similar to that achieved with thin plies.
Final reports
Hybrids – Intermingled fibre types
A gas-flow-assisted process was developed to spread and co-mingle fibres and successfully produce hybrid composites.
Intermingled Fibre Composites Report (PDF, 250kB)
Email: a.bismarck@imperial.ac.uk
Thin-ply Pseudo-ductile Hybrids
A new failure mechanism of fragmentation in hybrid thin-ply laminates has been demonstrated which gives a pseudo-ductile stress-strain response. This allows load redistribution at stress concentrations, greatly reducing notch sensitivity, and can also be used as a visual overload indicator.
Thin-ply Pseudo-ductile Hybrids Summary Report (PDF, 492kB)
Email: m.wisnom@bristol.ac.uk
Discontinuous pre-preg
The insertion of tough thermoplastic partial interleaves in a unidirectional discontinuous carbon fibre reinforced composite has been explored.
Discontinuous Prepreg Report (PDF, 138kB)
Email: a.bismarck@imperial.ac.uk
Discontinuous fibre manufacturing
The High Performance Discontinuous Fibre (HiPerDiF) technology allows to produce aligned discontinuous fibre reinforced composites with mechanical properties comparable to those of continuous fibre composite, by exploiting the sudden momentum change of a jet of fibres suspended in water directed in a narrow gap between parallel plates.
Aligned discontinuous fibres report (PDF, 219kB)
Email: k.potter@bristol.ac.uk
Laser modified fibres
Unduloid-reinforcing carbon fibres were produced using controlled laser irradiation which resulted in expanded and ablated regions along the fibre axis. When embedded into a compliant matrix, extensive ploughing from the tapered ends contributed to a ~7 fold increase in work of pull-out.
Laser Modified Fibres Summary Report (PDF, 392kB)
Email: a.bismarck@imperial.ac.uk
Hierarchical bundle composites
Natural composites often present hierarchical microstructures, which make them stronger and tougher. We are developing bio-inspired composites which, instead of being based on an individual bundle of fibres, are based on a hierarchical “bundles-of-bundles” microstructure.
Interlaminar and ply weakening
Controllable pseudo-ductile tensile behavior can be achieved by tailored ply weakening. Multi-directional laminates show notch-insensitive open-hole tensile behaviour.
Interlaminar and ply weakening Summary Report (PDF, 530kB)
Email: p.robinson@imperial.ac.uk
Discontinuous composites across the scales
A semi-analytical virtual testing framework was developed to predict the response of aligned discontinuous composites with single and hybrid fibre-types, as produced by the HiPerDiF manufacturing process. This framework can be used to model full specimens (with millions of fibres) within minutes, and its predictions have been compared against experiments.