- Theme A – fibre reorientation concepts
- Theme B – aligned fibre and interface approaches
- Theme C – ductile constituents
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.
For 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. 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.
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. Some of these approaches are summarised here:
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. The HiPerDiF method is explained in more detail here.
This theme aims to design composite constituents that will enable new mechanisms for ductility, whilst retaining high strength and stiffness. These will ultimately be integrated with novel architectures to produce high performance composites with a ductile response.
One example of this is the work we have undertaken with Cellulose nanocrystals (CNCs). Despite their promise, mechanical properties of CNC/polymer composites, especially in terms of tensile strength and Young’s modulus, have often been limited.
High loadings of water-soluble CNCs were well dispersed in poly vinyl alcohol (PVOH) to form coagulation spinning dopes for composite fibres. The resulting CNC/PVOH composite fibres showed significantly increased yield strength and Young’s modulus for all fibres, compared with PVOH fibres. The 40 wt.% of CNC/PVOH showed the highest Young’s modulus (29.6 ± 4.2 GPa) and tensile strength (0.85 ± 0.06 GPa) with a strain-to-failure of 5.6 ± 0.2 %. The graph shows that incorporation of CNC into the PVOH matrix increased the tensile strength and Young’s modulus, and the PVOH fibre become tougher and more resistant to deformation in the presence of CNC: