Fatigue and damage tolerance of 3D-woven composites

Recently, 3D woven composite materials are seeing wide use in engineering applications such as aerospace, marine, automotive and renewable energy sectors. This category of composites provides several economic and performance advantages over their laminated counterparts and metal alloys. 3D woven composites are usually prepared in near net-shape preforms before being infused by resin and cured to form the final structure. This production method eliminates the costs associated with layup of laminated composites or with use of expensive prepreg materials. Additionally, 3D woven composites provide some key mechanical performance advantages such as higher damage tolerance, as well as better impact and delamination resistance.
Since the main area of application for 3D woven composites involves impact, it is necessary to develop modelling capabilities that can capture behaviour such as fatigue life after impact events. The fatigue behaviour of textile composites is less well understood than for laminated structures. To this end, this project aims to develop a deep understanding of the fatigue and damage tolerance of 3D woven composites. This will be achieved via a combination of experimental characterisation and high-fidelity modelling.
Research has shown that small scale damage (e.g., matrix cracking and debonding) initiates in 3D woven composites at low loads. Due to these materials' high tolerance to damage and the ability to redistribute stresses, this meso-scale damage doesn't affect the 3D woven structure's mechanical performance under static loading conditions and is only detectable using advanced Non-Destructive Testing (NDT) techniques. However, when subjected to increased load levels and cyclic fatigue loading, the meso-scale damage progresses, reduces the stiffness, and will ultimately lead to the material's final failure.
The main interest of the project is the progression of meso-scale damage in these materials under cyclic fatigue loading post-impact. Low-velocity impact tests add a certain level of uncertainty and variability in the level of damage introduced into the specimen prior to cyclic fatigue loading. This makes it difficult for several post-impact fatigue tests to start with the same level of damage. Conversely, the use of a notched specimen ensures a level of damage which is more measurable and repeatable. Therefore, the first part of the experimental testing will focus on notched 3D woven composites.
Fatigue testing of 3D woven composites containing pre-initiated meso-scale damage will then be carried out. Their purpose is to gain a deep understanding of how damage progresses in 3D woven composites under cyclic fatigue loading. To this end, advanced NDT and imaging techniques (e.g., CT scanning, acoustic emission, etc.) will be employed. This understanding will help the development of a high-fidelity model capable of predicting meso-scale damage progression in 3D woven composites under fatigue loading. This model will then be validated against the experimental data.
In this project, the meso-scale level is considered because several researchers have shown that the development and progression of meso-scale damage in textile composites can be modelled with reasonable accuracy. However, the associated high computational cost prohibits the modelling of damage at structural scales. Therefore, this project ultimately aims to combine the developed fatigue model with larger, macro-scale analysis tools, to be able to predict the fatigue life of 3D woven composite components at the structural scale.
In view of the above, the key aims and objectives of this project can be summarised as follows:
- Fatigue testing of 3D woven composites containing pre-initiated meso-scale damage.
- Development of high-fidelity fatigue model at the meso-scale.
- Extension of model capability to predict fatigue life of 3D woven composites at the structural scale.

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.