How to relate compressive strengths of multi-directional laminates to fundamental unidirectional material strength?

Over recent years, fibre-reinforced polymers (FRP) have demonstrated excellent in-plane tensile properties and are used in aerospace, automotive, and energy applications due to their significant weight reduction. However, in-plane compressive properties are 40% lower on average than that of tensile properties. The compressive performance of FRP hasn't been well understood due to the challenges of identifying the materials' true compressive strengths. Fundamental compressive tests of composites show inconsistency of results as fibre instability can lead to structural failure. Further research on the compressive performance of composites is required to utilize FRP materials in further applications.
This research project is supported by EPSRC and contributes to the objectives of NextCOMP for the next generation of fibre-reinforced composites to develop novel composite materials that can endure higher compressive load-carrying capability. The development of composite compressive properties can lead to further industrial utilization of composites in compressive applications. A review of naturally occurring composite structures will be used to inspire new techniques to design and manufacture advanced composites with novel architecture.

The aim of this PhD project is to improve the compressive performance of composite materials.
- Experimental procedures must be reviewed and improved to obtain a higher compressive failure strain of FRP materials than those reported.
- Development of finite element analysis (FEA) models to validate experimental results. The model results would be used to predict the compressive performance of FRP materials using different architectures.
- Validate new architectures of composite materials to improve the in-plane compressive performance of composite structures.
- Design, manufacture, test, and analyse complex hierarchical architecture that results in improved compressive properties of composite materials.

The key objectives for this project can be summarised as follows:
- Review the effects of stacking sequence to identify new techniques that can support longitudinally loaded fibres in compression.
- Assess current compressive test methods to identify suitable experimental procedures for compressive failure within composite laminates.
- Investigate methods for fibre stability in compression through the reduction of fibre misalignment.
- Overwound / overbraid architectures to delay kink band initiation.
- Use of pultruded rod architecture to embed into current laminate architecture.
- Monitor uni-directional material stability and its improvement with the use of hybrid composites.

The research would support the purpose of identifying and improving the compressive performance of uni-directional FRP materials. This would increase the use of FRP structures in further engineering applications, in particular the aerospace and civil engineering sector. The traditional materials used are expensive to transport and can cause further risks to employees. FRP structures would re-innovate the current structures to allow for the manufacturing of portable lightweight structures with improved corrosion resistance properties. The development of novel composite architectures from natural composite architectures and current compressive enduring structures will lead to identifying techniques to improve the compressive properties of FRP materials.

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.