Composite overmoulding for complex, multifunctional loaded structures

Thermoplastic semi-crystalline composites have experienced a resurgence in interest and demand due to their attractive properties such as their ability to reshape and fuse at elevated temperatures. This makes them ideal candidates for emerging applications in new structural components, additive manufacturing materials, adhesives, and coatings as well as applications that require reusable, economical, and functional products. The quality of the heat-fused interface in the thermal welding process for thermoplastics is important as the regenerated interface plays a critical role in the bulk performance of the composite as well as plays a part in toughening the composite for aerospace applications. The process where the two thermoplastics are fused, known as interface healing, describes the multi-scale physical process that occurs. The interface healing process controls the subsequent interfacial properties, affecting the bulk mechanical properties.

Currently, no design tool nor definitive method is available to predict these thermoplastic interfaces' interface strength, especially in the context of composite over-moulding, a manufacturing process used to create 3D structural components. This is because most current models looking at interface healing are based on theories initially developed for amorphous polymers and then adapted for semi-crystalline polymers. In addition, these models do not account for crystallisation which is the most important phase transition that needs to be considered in semi-crystalline polymer processing. The crystallisation distribution will affect the interfacial properties and in turn, affect the bulk properties. Additionally, the manufacturing process employed when forming the thermoplastic interface will also affect the crystallisation distribution adding another level of complexity to modelling this process.

This PhD study will aim to:

- Develop a tool to model the crystallisation through-thickness in thermoplastic composites.

- Perform optimisation trials on the cooling cycle in the thermal welding process to improve the crystallinity at the interface

- Manufacture a sample thermoplastic composite with a potential gradient crystallinity structure

This model tool would be able to provide information for the optimisation of the cooling cycle in the manufacturing of thermoplastic composites. It will also provide more information on non-isothermal crystallisation modelling for the development of multi-scale interface healing models as well as allow for novel methods of manufacturing 3D-shaped thermoplastic composites with higher interfacial bond strengths.

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.