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#TPRCminds: Linda

Research Engineer

A fierce hailstorm can have a considerable impact on for example the wing of an aircraft. As we look to a future where wings are made of thermoplastic composite, how can we tackle the necessary repairs? What do we already know about restoring this material? Linda Grafen is a pioneer in the field of repair of thermoplastic composites at the TPRC. Every day brings a new discovery.

Read other #TPRCminds interviews.

If anyone at TPRC has come full circle, then it’s Linda. As a young girl, she entered the world of engineering almost by accident. ‘My parents ran a garage in a little town called Goor, here in Twente,’ she says. ‘Not a dealership, but a good old-fashioned workshop where the mechanics were always tinkering with engines. I often used to help out, and when I got a bit older they gave me a Saturday job. I loved getting my hands dirty, especially when the whole engine had to be removed from a car. I was in awe of all that technology.’

When the time came to consider her future, it soon became clear that studying ‘something technical’ was the natural choice. ‘Electrical engineering didn’t really appeal to me, so I opted for the mechanical side of things. I started at Saxion University of Applied Sciences and then progressed to the University of Twente for my master’s degree. The course on composites taught by my current TPRC colleague and lecturer Wouter Grouve really stood out for me. The complexity of the material held a special fascination for me - and still does. With metals, all the characteristics are familiar and you know exactly what to expect. With thermoplastic composites, it’s a different story. We know the material is light and strong, and that makes it very valuable, but we still have so much to learn. It continues to capture my imagination. I graduated at TPRC and then stuck around. I’m back looking at ways to fix a means of transportation. Albeit indirectly and in a completely different sector. But yes, you could definitely say that I’ve come full circle.’

There is another key factor that attracted Linda to this new material and to a job at TPRC: sustainability. ‘It’s a topic that is very important to me, at work and in everyday life. I eat vegetarian whenever I can, I have my own vegetable garden and try to travel by bike. On completing my studies, I had my own business for a while. I took on sustainability assignments. For example, I designed a machine that folds recyclable food trays quickly and efficiently. You could both freeze those trays and put them in the oven. The production process used less energy than previously. A fun job. Of course, by working on a larger scale we can make the most impact, and for me at TPRC, that’s now about making the aircraft industry more sustainable. Lighter aircraft use less fuel. My research is geared towards extending the working life of this lighter aviation material, because it doesn’t need to be replaced. That may sound simple, but the story behind it is complex and that’s what makes it so challenging.’

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Journey of discovery
The complexities that surround the application of thermoplastic composites include the question of how to repair the material. This is Linda’s area of expertise. A journey of discovery, in her own words. ‘It was one of the first things I was told when I joined the company: how little is known about repairing thermoplastic composites. At the same time, TPRC’s partners were beginning to ask more and more on the subject. I have spoken to them extensively, not only at our Technical Advisory Board (TAB) meetings, but in many other settings too. Ultimately, they are the ones who move my research along and put it into practice. I have close contact with Fokker, Spirit and Boeing, for example. They are very involved. Learning more about repair is crucial to the future of this material in the aircraft and automotive industries, that much is clear to me now.’

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Linda explains why these partners are so eager to learn more. “Repair is actually one step before recycling. If an aircraft wing made of thermoset sustains severe structural damage, it sometimes has to be completely replaced. When it is still possible to repair the wing a patch is placed on the material. The problem with patches of thermoset is particularly the addition of an adhesive. This has different material properties than the thermoset itself. This can cause problems and is more polluting. However, you could weld a thermoplastic. This gives you an advantage over thermoset material. Another advantage of thermoplastics is that you can re-consolidate them. You melt the matrix material again under pressure. In the year and a half that I have been working on this, I have already demonstrated that structural impact damage can be repaired with re-consolidation that meets the industry's criteria. This does have its limits: if an extremely large amount of matrix material has been removed, there is not enough left over to re-melt. Then you need a patch after all. The welding of patches is what I am working on at the moment. The advantage of placing a patch is that it is the same repair method as with thermosets. That would mean that you could carry out such a repair using the same kind of techniques. That is also very interesting for companies.”

While it’s a thrill to be working in such a pioneering field, Linda is aware of the drawbacks: uncharted territory can all too easily become an island. ‘It’s exciting but challenging too. I often find myself thinking: this thing that I’m about to do, is it actually going to pay off? I develop my own vision and methods, but I’m also lucky to have both partners and colleagues who are always willing to help solve problems. The atmosphere at TPRC is very open. It’s also a nice mix of cultures. I have colleagues from for example Italy, Israel and India. Come to think of it, I’m one of the few locals around here. I started working here just a few weeks after the first Covid lockdown came into effect. That was a strange situation. Thankfully, we now see each other at the lab more often and that always gives me a real boost.’

‘My dream is for my repair methods to be put into practice,’ Linda says. ‘For two methods, I have now been able to show that this potential is there. It would be so cool to see major companies incorporating my work into their production processes. In an ideal world it would be great if these methods were given a place in the public domain, so that the knowledge could remain widely available. Everyone who works with this material needs to know the best way to repair it. The way things used to be in the workshop at my parents’ garage. If you ask me, that’s sustainability in action.’


Our series #TPRCminds introduces you to the people behind our success. Who are they? What is their background?
What drives them and what do they dream about? How do they look at the future of thermoplastic composites?

Continue reading other #TPRCminds episodes:

Photos ©Gijs van Ouwerkerk

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