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"Design, Manufacturing and Impact Testing of Advanced Composite Materials"

Final Report Summary - CARHAY2011 (Design, Manufacturing and Impact Testing of Advanced Composite Materials)

Executive Summary:
The environmental impact of commercial air travel is currently of great concern and stringent future targets are in place that require significant reductions in harmful emissions. Reducing the weight of aircraft is a crucial step in meeting these targets, because carrying less weight requires less fuel burn. Existing composite materials have already allowed aircraft designers to make weight savings in the production of the latest generation of aircraft (i.e. Boeing 787 and Airbus A350). However, they are very expensive to manufacture and their poor damage tolerance requires the use of additional material to ensure safe operation; therefore limiting weight saving.

This work has explored two key areas 1) the use of carbon nanomaterials (carbon nanotubes and graphene) to reinforce carbon fibre composite materials. 2) the use of a liquid infusion manufacturing technique to manufacture nano reinforced composites at a component scale. The main challenges lie in ensuring good dispersion, deagglomeration and adhesion of the nanocarbons within a polymer matrix. This is difficult because the surface of a nanocarbon is chemically inert and does not interact well with other materials. They also form large agglomerates due to the relatively high Van der Waals forces. To promote dispersion and adhesion the surfaces must be chemically altered to enhance their interaction. This has previously been achieved using strong acid solutions, ultrasonication and solvents. This approach is not easily controlled, is not practicably scalable and can damage the nanocarbons. In this work a novel plasma treatment process was adopted that allows the type and amount of chemically alteration to be controlled and is a scalable, dry and energy efficient process.

A manufacturing approach was developed and successfully demonstrated for the manufacture of component scale nano reinforced stiffened panels (0.9x0.6m). Large batches of nanofilled resins were produced (up to 10kg) with carbon nanotube and graphene reinforcement. A liquid infusion technique was developed that allowed the manufacture large composite components using a 0.5wt% filled resin and a ‘single shot’ manufacturing process was demonstrated for I-section stiffened panel sections. A 50% improvement in compression after impact (CAI) strength was demonstrated for the nano reinforced composites compared with the standard composite material: CAI is one of the main limiting design cases that requires additional material (conservative design) for safe operation. Significant improvements in compression properties, in-plane shear and fracture toughness were also observed.
Project Context and Objectives:
Polymer nanocomposites represent a fairly new class of polymeric composite, with promising mechanical, thermal, optical and physio-chemical properties obtained at low filler loadings. The filler employed in the production of nanocomposite resins are typically clays (layered silicates), nanospheres (silica), nanoscale metal and metal oxides, carbon nanotubes, fullerenes (buckyballs) and more recently graphene. The exceptionally high strength and stiffness properties of carbon nanotubes and graphene could offer significant mechanical enhancements to polymer matrices at relatively low loadings. Enhancing the properties of a polymer matrix material in this way has the potential to increase the strength and damage tolerance of a fibre reinforced composite. The effective dispersion of nanofillers and ensuring good adhesion is critical to realising the maximum properties from a nanofilled resin. In the case of nanocarbons (which are the focus of this work) dispersion and adhesion is challenging because the particles are chemically inert, with low surface energy and form large agglomerates due to van der Waals forces. Generally nanocarbons are chemically treated with organic modifications to improve their compatibility with polymers. This is commonly achieved using strong acids and ultrasonication, which is not scalable, can damage the nanocarbons and adds processes to the manufacture.

In this work a novel plasma treatment is being utilised to functionalise and de-agglomerate nanocarbons to promote high quality dispersion. The process allows far greater controllability of the type and extent of chemical functionalization and is dry, energy efficient and highly scalable. The main focus of the research undertaken is to demonstrate improved compression after impact performance and scalability of manufacturing procedures to extend manufacturing capabilities beyond coupon scale. Specific objectives included:

Developing, optimising and characterising a nanofilled resin that can be readily produced in industrially viable quantities.

Developing, optimising and characterising a liquid infusion manufacturing approach for the manufacture of multi-scale composites.

A detailed assessment of the mechanical performance of filled resins and multi-scale composite materials.

To manufacture component scale aerospace stiffened panels by a liquid infusion technique using nanofilled resins and to demonstrate the potential of a ‘single shot’ infusion process for stiffened skin sections.

To assess the impact, fatigue and compression performance of the produced panels.
Project Results:
The project involved the development of methods of fabricating carbon fibre - epoxy composite by resin infusion, incorporating graphene and other nanofillers into the epoxy resin. The materials used were commercially available and the manufacturing techniques were conventional although considerable optimisation and development were required to make them work for this combination of materials.
Using graphene or carbon nanotubes with suitable surface functionalisation, it was possible to obtain satisfactory dispersion in the resins. Some nanofiller pretreatments led to modest but significant increases in the strength of the unreinforced resin and to small increases in the strength of composites manufactured using the nanofilled resins. Using graphene with selected pretreatments, an increase in Compression After Impact (CAI) strength of up to 50% was demonstrated at the coupon scale. This is very important because CAI is one of the main limiting factors in design, so an increase in CAI could allow a corresponding reduction in weight.
Full scale wing panel sections were manufactured and tested to failure without cyclic loading and after realistic lifetime fatigue loading simulating two inspection intervals after visible impact damage and also a full service life after barely visible impact damage. These large scale tests corroborated the coupon CAI results although a larger number of tests would be needed to draw any definitive conclusions.
A smaller sample panel incorporating I-section stiffeners was made by a single shot process, demonstrating that a complete stiffened panel could be made in a single step without the need to manufacture stiffeners separately and bond them to the panel. Combined with the improved CAI strength this demonstrates the potential for a lightweight structure that can be economically produced.
Potential Impact:
The work undertaken has demonstrated significant improvements in the strength of composite materials through the addition of MWCNT and FLG. In particular the compression after impact strength at coupon scale was increased by up to 50%. This is a very important result because compression after impact (CAI) loading is commonly the limiting design case for composite structures and poor performance often leads to conservative design. Improving the properties of liquid infused composites has the potential to offer significant impact on a number of levels. Improvements in properties, such as CAI strength, can reduce the requirements for conservative designs leading to weight saving. Reduced airframe weight means less fuel burn which reduces running costs for operators and importantly reduces the environmental impact of air travel. The improved damage resistance and fatigue performance demonstrated by the coupon tests and main panel tests also offers the potential to extend inspection and service intervals, which will have a significant effect on running costs for aircraft operators.

The use of liquid infusion techniques in manufacture is significantly cheaper than the ‘gold standard’ of autoclave curing. However, lower quality of manufactured materials has limited its adoption by the aerospace industry. In this work we have demonstrated the ability to produce component scale FLG reinforced parts, with the potential to improve damage and fatigue tolerance. This is a significant step forwards beyond the lab-scale coupon manufacture that has dominated previous research. Potential exists to significantly lower manufacturing costs of composite structures through the use of liquid infusion, whilst achieving properties that are consistent with autoclave cured components.

Publications resulting from the undertaken work are listed below. A press release detailing project results was presented at the UK Composites Engineering Show (National Exhibition Centre, Birmingham, UK) November 2014. Results have been reported in popular press (Western Mail newspaper, UK). In addition to this a poster publicising project results was presented at the Electromechanical Characterisation of Carbon Composites Network, Cardiff University, Cardiff, UK, June, 2014.

Published, accepted or in-press
1. M.J. Eaton. W. Ayre, M. Williams, R. Pullin and S.L. Evans. Nano-reinforcement of Resin Infused Carbon Fibre Laminates using Carbon Nano-tubes and Graphene. 16th International Conference on Experimental Mechanics, Cambridge, UK, 7-11th July, 2014.
2. M.J. Eaton, W. Ayre, M. Williams, R. Pullin and S.L. Evans. Developing Component-scale Heirarchical Composites using Nanocarbons. 20th International Conference on Composite Materials, Copenhagen, 19-24th July, 2015.

In preparation
3. M.J. Eaton. W. Ayre, M. Williams, R. Pullin and S.L. Evans. Improving compression after impact performance of composites using MWCNT and FLG. – Target Journal: Composite Science and Technology (IF: 3.633).
4. M.J. Eaton. W. Ayre, M. Williams, R. Pullin and S.L. Evans. Towards Component-scale Hierarchical Composite Structures. – Target Journal: Composite Structures (IF: 3.120).

The project results have been used to support the following funding applications.

1. Efficient Composite Curing by Intelligent Microwave Processing (68k EUR) - Centre for Innovative Manufacturing of Composites (an EPSRC funded centre).

Under review
2. Multi-scale Composites for Enhanced Multi-functional Materials (1.49M EUR) – European Research Council Starting Grant.
3. Development of Multi-scale and Multi-functional Composites (123k EUR) – Ser Cymru National Research Network.
List of Websites:
Prof Sam Evans, School of Engineering, Cardiff University, The Parade, Cardiff, CF24 3AA, UK