Synthesis of New and Multifunctional Materials
The Composites Centre is very active in the conception and synthesis of new and multifunctional composites. These materials offer properties and functionalities which cannot be achieved with conventional systems, but present considerable processing and characterisation challenges. We introduce nanomaterials into fibre reinforced composites, and by having addressed the manufacturing issues, are now are fully exploiting the properties of the nanophase in the resulting composites. We are world leading in the field of multifunctional structural power composites: structural composites which have multiple functionalities, such as electrical energy storage. Although currently a relative immature field, we anticipate multifunctional composites will emerge as vital for the future transportation and mobile electronic Sectors. Finally, we are addressing fundamental aspects of composites, such as their inherent brittleness, through the development of ductile composites.
We are developing novel approaches for the fabrication of graphene-ceramic and graphene polymer composites. These approaches combine techniques such as 3D printing, freeze casting, pre-ceramic polymers and spark plasma sintering. The goal is to take advantage of the 2D nature of graphene in order to design and create a highly interconnected microscopic network of thin, electrically conductive interfaces inside a ceramic or polymer matrix. This network directs crack propagation and promotes stable crack growth increasing fracture resistance. It also provides functional properties such as electrical and thermal conductivity. It can trigger Joule heating or used to follow the formation and propagation of cracks in situ. The combination of self-monitoring and high-fracture resistance will be the basis for the development of intelligent materials able to avoid catastrophic failure in service.
This work is funded by EPSRC, initially through the grant Graphene 3D Networks and now through MAPP (Future Manufacturing Hub in Manufacture using Advanced Powder Processes). We are also investigating an extension to the technology towards the fabrication of self-healing and self-shaping composites funded by DARPA and ONRG.
Structural power materials
We are world-leading in the development of structural supercapacitors – structural composites which have the ability to store/deliver electrical energy. Our work on structural supercapacitors has attracted enormous academic, public and industrial interest. These materials offer considerable performance advantages whilst presenting compelling opportunities for innovative design. We have a large portfolio of research addressing the fundamentals of these materials, manufacturing and design issues, mechanical properties and applications. A video showing an early incarnation of the devices can be seen here whilst a review paper we have written of this field can be found here.
(Pseudo-) Ductile and damage tolerant discontinuous composites
We have developed original models to design discontinuous composites with progressive failure and non-linear response, which have and are still driving the experimental development of pseudo-ductile discontinuous composites at the University of Bristol (UK) and FHNW (Switzerland), and have already been adopted by researchers at McGill University. Our models allowed us to develop and design (pseudo-) ductile discontinuous composites based on:
- cut-prepregs and aligned short fibres, with an initial stiffness similar to that of continuous composites, but allowing for a more gradual failure process;
- hybrid fibre-types (e.g. combining carbon and glass), with tailorable properties and a non-linear metal-like stress-strain response;
- bio-inspired hierarchical microstructure, with ‘bricks-and-mortar’ architectures at different scales, able to diffuse damage, dissipate energy, and undergo stable unloading-loading cycles after visible damage initiates. This work has been recognised by a Best Poster Award from the European Society for Composite Materials (Joel Henry, 2018).
Composites with stiffness control and shape memory capabilities
Two types of hybrid composites exhibiting controllable stiffness have been developed in research in the departments of Aeronautic and Chemical Engineering. One type consists of carbon fibre thermoset laminates containing thermoplastic interleaf layers and another consists of thermoplastic coated carbon fibres in a thermoset matrix. In both types, it is the loss in shear stiffness of the thermoplastic at elevated temperature which results in the loss in flexural stiffness of the composite. Experiments on a polystyrene-interleaved carbon fibre reinforced epoxy hybrid composite have shown that reductions of over 90% in flexural stiffness are possible when the hybrid is heated. The interleaved composite also exhibits a shape memory effect because the carbon epoxy layers remain elastic during the high temperature deformation. Research is being conducted to further develop and investigate these hybrid composites which may find applications in deployable and adaptive structures.
Ductile composites by using a wavy sandwich structure
The EPSRC-funded High Performance Ductile Composite Technologies (HiPerDuCT) programme grant (a collaboration between The Composites Centre at Imperial College London and the Advanced Composites Centre for Innovation and Science at the University of Bristol) aims to develop composite systems with a ductile or pseudo-ductile response, while maintaining good strength and stiffness.
One example of the work underway in the Department of Aeronautics is the development of a wavy ply sandwich panel consisting of composite skins and light crushable foam core. The aim is to achieve large extensions under tensile loading by straightening of the load-carrying skins as the core crushes. Following preliminary assessment of potential configurations using simple analytical equations, the tensile response of wavy-ply sandwich structures was in investigated using detailed Finite Element (FE) simulations.
This configuration was manufactured using machined foam cells and wavy metallic moulds and tests have shown good agreement with the FE predictions and deformed as intended, with a very non-linear response and high strain.
Engineering the fracture response of CFRP to create more damage-tolerant materials
The damage tolerance of composites can be improved via micro-structural design- in fact, their fracture surfaces can be engineered by modifying their micro-structure accordingly. In this work, (i) inspired by wood and bone, we designed composites to create hierarchical pull-out features during fracture; (ii) inspired by nacre, we designed CFRP with tiled micro-structures; and (iii) inspired by the Stombus Gigas shell, we designed CFRP with cross-lamellar microstructures. Using these approaches, we have obtained over 500% increases in translaminar fracture toughness, as well as improved damage diffusion. This work is supported by EPSRC (Engineering Fellowships for Growth: Next generation of lightweight composites - how far can we go?). Recent papers on this research can be found here [1,2,3].
We have been active in the development of hierarchical composites; fibre reinforced polymers which are reinforced at both the micron (carbon or glass fibres) and the nano (carbon nanotubes, carbon aerogel or graphene) scales. These materials present considerable processing challenges, so we have taken two approaches to achieve high loading fractions of the nanophase; via reinforcing the matrix and via growing the CNTs on the fibres. We have achieved extremely high volume fractions of the nanophase in the resulting composites, leading to huge improvements in performance over conventional systems as well as added functionalities, such as enhanced thermal and electrical conductivities.
Nanocarbon reinforced lightweight metal composites
Nanocarbon reinforced lightweight metal composites such as Mg, Al and their alloys are produced in this project via melting process. The dispersion of nanocarbon is studied to achieve a homogeneous dispersion through physical and chemical approaches. The composites have obtained improved mechanical properties and the microstructure of these composites has also been investigated. Analysis has been undertaken to predict the potential of the CNTs additions.
Nanocarbon reinforced polymer composites
In this project, nanocarbon reinforced polymer composites have been produced various methods, including solution casting, hot pressing and 3D printing. Both nanocarbon and carbon nanotube veils are investigated as reinforcements. The objectives are to develop suitable manufacturing methods to process strong and lightweight composites, as well as the improvement of the thermal and electrical properties.