Hot/wet properties of high temperature composites

The replacement of metals with advanced composite materials within a range of industries has become commonplace. This is due to their excellent specific stiffness and strength, wide range of available constituents and array of manufacturing processes, making them suitable for a multitude of applications, including within the aerospace and energy sectors. As composites are employed in more extreme environments, there has been considerable interest in organic matrices that are able to withstand high temperatures and humidities. Thus, research has been conducted to develop novel thermoset systems with enhanced thermal and moisture durability properties. There has been specific interest in cyanate esters due to their desirable viscosity properties for resin infusion and high glass transition temperatures, making them suitable for a range of applications. Although there is some understanding of the behaviour of the neat resin, there is little knowledge on its performance when integrated within composite laminates.

Delamination, the separation of layers, is the primary failure mechanism within composites. Fracture toughness is a measure of a material's resistance to delamination. Most tentative applications for these novel materials involve cycling components in the presence of heat and moisture which accelerates degradation mechanisms. Therefore, it is imperative to understand the interlaminar fracture toughness properties of these thermoset systems, with a particular focus on how they react in hot-wet conditions, before they can become routinely used within composites.

This project will focus on the experimental testing of a leading aerospace-grade epoxy-based composite material, IM7-8552, in conjunction with composites containing the novel thermoset resins. This will allow a baseline set of properties to be acquired for performance comparison with the newly developed materials. Although regulations for moisture conditioning of test specimens exist, there is no universally accepted testing standard for acquiring fracture properties of composites at elevated temperatures and humidities. For this reason, a key objective of this project is to develop a more robust, reliable method for testing within extreme environments, along with obtaining the fracture properties themselves. Testing will initially be conducted in static and quasi-static conditions, followed by fatigue testing performed to mimic the conditions experienced by materials in-service under continuous cycling and vibration exposure whilst in high temperature environments. Fractographic analyses will be conducted to create qualitative measures of fractured surfaces, developing easily identifiable visual indicators of how the material failed in each set of testing conditions for future comparison.

Once the behaviour of the novel thermoset systems within composites is more thoroughly understood, supported through rigorous testing within this project, the envisaged application for their use is within the turbofan engines developed by Rolls Royce. However, if these materials prove to have enhanced durability within environments involving repeated cycling at elevated temperatures, this will open them up for use within a broad spectrum of applications. These could include conventional gas turbines, advanced hybrid electric propulsion systems, motors, generators, and hydrogen storage tanks.

The development of a vigorous protocol for static, quasi-static and fatigue testing in elevated temperatures and humidities will be a significant contribution to the fracture mechanics research field. The ability to confidently produce a data set of properties tested within these conditions is essential for the progression of high temperature composites as the boundaries for their use continue to be broadened.

There are seven principal groups of beneficiaries for our new EPSRC Centre for Doctoral Training in Composites Science, Engineering, and Manufacturing.

1. Collaborating companies and organisations, who will gain privileged access to the unique concentration of research training and skills available within the CDT, through active participation in doctoral research projects. In the Centre we will explore innovative ideas, in conjunction with industrial partners, international partners, and other associated groups (CLF, Catapults). Showcase events, such as our annual conference, will offer opportunities to a much broader spectrum of potentially collaborating companies and other organisations. The supporting companies will benefit from cross-sector learning opportunities and

- specific innovations within their sponsored project that make a significant impact on the company;
- increased collaboration with academia;
- the development of blue-skies and long-term research at a lowered risk.

2. Early-stage investors, who will gain access to commercial opportunities that have been validated through proof-of-concept, through our NCC-led technology pull-through programme.

3. Academics within Bristol, across a diverse range of disciplines, and at other universities associated with Bristol through the Manufacturing Hub, will benefit from collaborative research and exploitation opportunities in our CDT. International visits made possible by the Centre will undoubtedly lead to a wider spectrum of research training and exploitation collaborations.

4. Research students will establish their reputations as part of the CDT. Training and experiences within the Centre will increase their awareness of wider and contextually important issues, such as IP identification, commercialisation opportunities, and engagement with the public.

5. Students at the partner universities (SFI - Limerick) and other institutions, who will benefit from the collaborative training environment through the technologically relevant feedback from commercial stakeholder organisations.

6. The University of Bristol will enhance their international profile in composites. In addition to the immediate gains such as high quality academic publications and conference presentations during the course of the Centre, the University gains from the collaboration with industry that will continue long after the participants graduate. This is shown by the

a) Follow-on research activities in related areas.
b) Willingness of past graduates to:

i) Act as advocates for the CDT through our alumni association;
ii) Participate in the Advisory Board of our proposed CDT;
iii) Act as mentors to current doctoral students.

7. Citizens of the UK. We have identified key fields in composites science, engineering and manufacturing technology which are of current strategic importance to the country and will demonstrate the route by which these fields will impact our lives. Our current CDTs have shown considerable impact on industry (e.g. Rolls Royce). Our proposed centre will continue to give this benefit. We have built activities into the CDT programme to develop wider competences of the students in:

a) Communication - presentations, videos, journal paper, workshops;
b) Exploitation - business plans and exploitation routes for research;
c) Public Understanding - science ambassador, schools events, website.