Q&A: Aerospace Composites: Invention or Perfection

September 14, 2010
Theo Dingemans-Professor of Chemistry and Aerospace Engineering, TU Delft University

Theo Dingemans-Professor of Chemistry and Aerospace Engineering, TU Delft University

Theo Dingemans began working on high performance polymers as a graduate student at UNC-Chapel Hill. After completing his degree, Dingemans began work at the advanced materials and processing branch at NASA- Langley—one of the world leading institutes doing work in composites. In 2003, Dingemans returned to his native Netherlands, where he teaches at TU Delft as a professor of Aerospace and Engineering, working on polymers and composites.

What is your current research focus at TU Delft?

We try to help find solutions. We’re trying to provide the aerospace industry with new polymer-based materials to use, which can either be resins or fibers. We also research new chemistry make-ups as well as try to fix existing polymer related problems within composites.

As a department, we come up with new concepts such as multifunctional polymers, which are like a structural polymer uses in composites but have an extra function such as a photovoltaic, energy storage capacity, sensor, transistor or self-healing mechanism.

 

What primary applications would the aerospace industry like improved?

There are major issues, such as the poor compressive behaviors of the fibers. Composites do really well under tension in their current design, but under bending or compression, they fail miserably. We are trying to fix the characteristics that cause those problems and work on resin materials that incorporate a higher end-use temperature, more toughness and better solvent resistivity.

Where are these flaws an issue?

If you design an aircraft, money isn’t much of an issue, because the materials aspect is minimal compared to hardware. On a satellite for example, manufacturers can afford to use the best polymers that are out there. When you look at an airplane, however, price becomes an issue. One of the biggest problems right now with airplanes is skydrol resistivity, which is a hydraulic fluid that most components—especially the polymer component—doesn’t like. After prolonged exposure to skydrol, the polymer swells and becomes soft. A famous example of this is the wet wing design where people decided to make an all-composite wing out of polymer fiber reinforced composites. They dumped the fuel in there, and it was an automatic fuel tank. But that didn’t really work because the polymer was swelling and they ended up building a fuel tank in the end.

Are there other big flaws?

Some of the biggest problems the aerospace industry currently faces are solvent resistivity, toughness and fiber-to-resin interface (resin adhesion to fiber). All these problems translate back to design rules. To give you an idea of how complex these problems are, the current tests used to certify the composite parts aren’t even really given the right information.

What information are the tests missing?

Well, one example is an ASTM standard test that is used to certify parts with solvent resistivity. When you take a composite panel and submerge it in skydrol at 70 C for a certain time, take it out, rinse it, dry it and test the mechanical properties, you discover you’ve lost X amount or the material has swelled by so much. It’s a specific test, but it’s not realistic test. Airplane builders are discovering that if you take a composite part, put it under tension and then submerge it under skydrol, the whole thing fails in a fraction of the time. Right now that’s not an official test, but it’s a very realistic test because at some point airplane parts are under tension or being loaded.

Is aerospace focused on invention or perfection?

Right now, they’re struggling with the materials that are there. Like everyone else in composites, the aerospace industry is struggling with the fact that on one end you have fiber and on the other there is the resin and these two things just don’t like each other. That’s just a fact. People have become handy over the years, playing around with processing and sizing to make the fiber like the resin or vice versa. But all of this, in my opinion, is poor man’s approach to get something that is semi-workable. As a result, it’s a big push to design a fiber and resin from the same chemistry so they inherently like each other.

Where is there the greatest potential for composite growth?

 

It is an interesting question in the sense that if you ask what is driving the industry, an obvious answer is fuel prices driving to lighter cars and electric cars. To make those plausible, auto manufacturers have to cut weight, so composites a good thing. I’d say I see the most potential within transportation in general, but also think designers have a big impact on this as well. Designers look to see what they can make/design and it’s all about what they feel people want, at the end of the day.

Where is there room for improvement within composites?

 

When a company purchases an all-composite aircraft, they ideally want it to fly for at least 30 years. I’m not confident that we understand composites properties enough to guarantee that it will work that long, especially if you think about fatigue properties. In general, there is a large gap between the fiber and resins. Toughness, fiber adhesion, solvent resistivity and temperature use are all characteristics that need to be improved. There are also gaps that need to be filled in relation to fatigue under use, fatigue under solvent exposure, repairability and property characteristics after impact. Right now, those are the major show stoppers that keep composites from become a real “adult material.”

What do you think it will take for composites to reach adulthood?

Materials that have been introduced over the years have several maturity cycles. For example, in the ‘80s everyone was making things out of carbon fiber and then they realized that it’s not a simple technology so stepped away from it. Some went bankrupt, some were bought out and over the years the technology matured. Now, people know how to process things, they know a bit more about the resin/fiber properties and we’re once again in the upswing. I think what the composites industry will realize is that there is still a lot to learn, that we’re not quite there yet. In my opinion it’s not a mature technology. I think there is a lot to learn, especially on the chemical level.

Are there indicators you look for to say we’ve reached that potential?

That’s hard to say because the aerospace industry is a slow-adapting industry, and rightly so. The rapid adaptors that are going for new applications are sporting goods like composite bikes and sports cars. I think that’s where a lot of innovation will come from. More people will learn how to design, process and fine-tune properties.

The major learning curve that needs to occur is for manufacturers, especially within aerospace and automotive, to step away from thinking composites are metal mimics. They need to realize that a composite part may look different when it’s built because it’s not a metal mimic. If they don’t, that’s when things fail. You can’t just replace one part made out of composites! For example, if you take a racing bike, then look at a single tube and say ‘that’s what it looks like now’ and match it with carbon, that just doesn’t work. It will fail.

People are now starting to understand and appreciate that everything can’t be made out of composites. Well they can, but they’ll be expensive. Wind turbines are at the brink of moving more into composites, as well as automotive. If that happens, they’ll be major drivers because while it’s fun to own a plane, that’s not quantity. Cars are quantity.

What is on the horizon for composites?

Something that doesn’t get a lot of attention but should is compounding, which I think is going to be a problem solver for a lot of composite applications. Making continuous fiber composites is expensive. but it is used because properties are instantly better than those using a heat resin property. Compounding is a field that is in between and very promising. In compounding you take chopped resin, approximately 2 cm in length, that can be injection molded into complex shapes and parts that will be—property wise—an in-between for continuous fiber composites and heat resin and can be made at a much faster pace.

Where will compounding have the greatest impact?

In any application where one has to deal with complex shapes. One of the challenges of continuous fiber composites is making complex shapes. It’s very expensive, whereas in compounding you can make large quantities with properties similar to a continuous fiber composite part.

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Melinda Skea is ACMA’s managing editor.

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