Postgraduate Research – Page 3 – Bristol Composites Institute Blog

Testing the Mechanical Performance of Nature Fibre Composites.

Head shot of Owen Tyley by Owen Tyley owen.tyley.2019@bristol.ac.uk; Tobias Laux tobi.laux@bristol.ac.uk; Neha Chandarana neha.chandarana@bristol.ac.uk

Manufacturers are increasingly looking to develop new natural fibre composites (NFCs) to lower the environmental impact of structures such as wind turbine blades and automotive panelling. For these to be brought to market, their mechanical performance must be understood throughout their operating temperature range. This is ordinarily conducted using strain gauges, though the cost of purchasing and installing strain gauges makes this a relatively expensive undertaking. By contrast, digital image correlation (DIC) is an optically-based imaging technique which can determine the strains on an object such as a standardised testing coupon, at much lower cost. However, the reliability of DIC for composite coupons at elevated temperatures is not well-understood.

Black and white photo of natural fibre composites

As part of a summer internship project supported by the Henry Royce Institute for Advanced Materials, the tensile moduli of flax- and carbon-fibre reinforced polymers using both DIC and strain gauges at temperatures up to 120°C were compared. Preliminary results suggest that the modulus as determined through DIC is the same as for strain gauges, but with greater uncertainties. It is therefore suggested that DIC could be a suitable method for determining the mechanical properties of NFCs for non-safety-critical applications, and as part of early-stage research and development for new natural-fibre composites.

Flax: A sustainable alternative to glass fibres in wind turbines?

by Abdirahman Sheik Hassan a.sh.2019@bristol.ac.uk; Neha Chandarana neha.chandarana@bristol.ac.uk ; Terence Macquart

Flax-fibre composites have been widely praised as a high-performance sustainable alternative to synthetic fibres in the composites industry. However, as with many natural fibre composites, the mechanical properties (strength, stiffness) of flax-fibre composites do not match up to their synthetic counterparts. This study assesses the suitability of flax-fibre reinforced composites as a replacement for glass-fibre composites in the context of a wind turbine blade using a life cycle engineering approach. Research on a computer screen Finite element analysis (FEA) was used to determine the design alterations required for comparable performance, followed by a cradle-to-grave life cycle assessment to ascertain the subsequent environmental impact of these alterations. The preliminary results show a significantly greater volume of material is required in a flax-fibre blade to match reserve factor and deflection requirements; however, these models do show reduced environmental impact compared with the glass-fibre composite blades. End-of-life options assessed include landfill and incineration, with and without energy reclamation.

Amphiphilic Cellulous for Emulsion Stabilisation and Thermoplastic Composites.

Headshot of Amaka Onyianta Headshot of Steve Eichhorn by Amaka Onyianta a.j.onyianta@bristol.ac.uk; Steve Eichhorn s.j.eichhorn@bristol.ac.uk

Biobased polymers, commonly referred to as bioplastics, are made from plant or other biological material instead of petroleum. They, therefore, present opportunities for the development of sustainable plastics from a wide range of pre-cursors including corn, vegetable oil and cellulose. Cellulose, the most abundant polymer on earth, is also renewable material available from vast resources such as wood, plant, bacteria and even sea animal tunicates. Considerable research efforts have been put into developing cellulose-based biopolymers. However, despite all its advantages, cellulose due to its hydrophilic (water-loving) nature presents a significant challenge with respect to blending with other polymers which are often hydrophobic (water-repelling). 

Diagram showing amphiphilic cellulose coated polypropylene composites

To address this challenge, our group is exploring surface modification of cellulose to make it hydrophobic. One such modification we have investigated results in a material that is not only hydrophobic, but largely retains the inherent hydrophilicity of cellulose, leading to an all-new class of material: amphiphilic cellulose. Due to this amphiphilic nature, the cellulose can stabilise oil-in-water emulsions, making it attractive for various applications including in the personal care products sector where consumer desire for nature-based products is increasingly driving demand.   

It is also recognised that while material sources can be sustainable, processing techniques also need to be sustainable for this credential to hold for the final product. Work within our group is therefore also looking into aqueous processing of amphiphilic cellulose with thermoplastics to yield biobased sustainable composite materials with improved tensile modulus. Moreover, the melting profile of the thermoplastic is not affected by the process, neither is a pre-step of compounding needed as seen in the traditional process for incorporation of fillers in thermoplastic composites. 

We interviewed Bristol Composites Institute PhD student Michael O’Leary about his PhD project and the mutual benefits of working with Airbus on a cutting-edge research project.

How did you end up studying at the Bristol Composites Institute?

man looking at the camera smiling — Michael O’Leary

On leaving school, I realised I wanted to pursue a degree in Engineering, eventually specialising in Aeronautical Engineering and graduating with my bachelors from the University of Limerick. I had my final year project examined by Professor Paul Weaver, who recommended applying to the Bristol Composites Institute for PhD.

I decided that the Centre for Doctoral Training would be a great fit for me as I had enjoyed the research aspect of my final year. The collaborative environment of the CDT, being surrounded by people with similar research interests and skills, was a great selling point for me.

What are you working on?

My project is focused on integrated structures with semi-cured elements.

For future wing structure, we are moving towards more highly integrated and larger structures. As we make these integrated structures, we start to encounter some of the manufacturing challenges associated with the scale, such as element alignment and complexity.

The objective of my project is to break the integrated structure back down into smaller pieces and use them semi-cured as a building block to bring them back together. For instance, the state of the art for current structures manufacturing is using a skin, bonded stringers, and bolted ribs. Why not semi-cure each individual part and integrate them all together for a final cure?

How do you manufacture integrated structures with semi-cured elements?

The manufacturing process is a two-step curing process. The initial step is to create the semi-cured elements with a pre-designated degree of curing, somewhere between uncured and fully cured, hence the name. After the curing process, the semi-cured element can be stored, trimmed, and inspected. If they are of acceptable quality, they can then be integrated.

What are some of the manufacturing challenges when using semi-cured elements?

The main manufacturing challenges that we are facing are about determining the degree of curing and scaling, especially for more complex geometries as there are tooling requirements that can complicate the process.

Regardless of specimen manufacturing method fibre bridging was witnessed during DCB testing — Double Cantilever Beam tests were carried out on initially semi-cured and normal, single step, fully cured laminates, with both sample sets displaying similar failure patterns, and failure loads.

What are the next steps for this work?

The next steps are to continue to determine the optimal degree of cure for semi-curing along with better understanding how semi-cured interfaces are forming. Outside of this, we will continue to prove the feasibility of semi-curing by starting to produce parts at a scale greater than coupon level.

How will the project results benefit the academic and industrial project partners?

Proving the feasibility of this work will provide the industry with an additional manufacturing tool that they can use when designing future structures. Hopefully, my work will lead to further questions which can be posed to incoming PhD candidates.

How has your cooperation with industrial partners supported the development of this project or your skills?

My primary industrial partner is Airbus. The industrial supervisors have been very helpful and supportive providing important technical knowledge and ideas which have made their way into my work. Having an industrial project gives a different perspective, it really helps me to see how my work can be applied in the real world.

Through this industrial project, I had the opportunity to interact with one of the world’s largest aerospace manufacturers. It has helped foster relationships which would not have been possible outside of this project.

Category: Postgraduate Research

Natural Fibre Composites Research