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Why Carbon Fiber Is Not as Strong as It Could Be

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At Utah-based Rock West Composites, supplying bicycle manufacturers with carbon fiber tubing is a normal part of the job. A few states away, line workers at Boeing’s assembly plant use carbon fiber panels to construct airplane wings and fuselage sections. All over the country, carbon fiber is being used as a lightweight replacement for aluminum and steel because of its extreme tensile strength. So what do you say if someone told you that carbon fiber is not as strong as it could be?

As strong as carbon fiber is, it has never lived up to its theoretical strength in real world applications. And until a couple of years ago, no one knew why. But research out of Rice University unlocked the secret and presented a solution at the same time. Should composite manufacturers ever figure out a way to put the research to good use, the strength of carbon fire materials could be dramatically increased.

  • A Hidden Weakness

Long chains of carbon molecules in tight alignment are what give carbon fiber its strength. This is one of the reasons carbon fiber recycling is so challenging. The only process we currently have for recycling results in shorter carbon chains that are not as tightly aligned, resulting in a weaker material. But that is a discussion for another time. Right now, we want to address the fact that even the best carbon fiber material is not as strong as it could be.

Researchers at Rice University were curious as to why real-world carbon fiber applications did not live up to their theoretical counterparts in terms of strength. They began looking at manufacturing processes to see if they could find a culprit. And find one they did.

Researchers discovered that the long chains of carbon atoms that make up carbon fiber tend to become misaligned during manufacturing. In an October 2016 article discussing the research, they compared this misalignment to what happens when a zipper gets misaligned. When the teeth of a faulty zipper do not line up, it throws the entire garment off.

The Rice University team set up computer simulations to measure the frequency of misalignment in a typical manufacturing process that involves heating polyacrylonitrile (PAN) to burn off all but the desired carbon molecules. They discovered that heating causes the chains to buckle, similar to how a ribbon might buckle if you stretched it across a table and then began pushing the two ends together.

Buckling leads to the creation of what the researchers called ‘D-loops’. The loops are not able to correctly ‘zip’ into place during manufacturing, so the finished product is only about 25% as strong as theory otherwise dictates.

  • A Possible Solution

The question now on the minds of scientists is whether the misalignment problem can be fixed or not. It looks like it can be, by introducing graphene to the equation. Researchers at the UK’s Manchester University say that graphene has the potential to improve the performance of most composites when added in correct volumes.

The key to making this work is using a single layer of graphene as the composite additive. Unlike other materials, graphene actually gets stronger as it gets thinner. By introducing the right amount of single layer graphene during the manufacturing process, carbon fiber can be made stronger without sacrificing its lightweight properties.

It turns out carbon fiber is not as strong as it could be. But that could change. All it will take is someone to figure out a process for combining carbon fiber with graphene in an efficient and cost-effective manner. Easy, right?

Sources:

  1. Phys.org – https://phys.org/news/2016-10-strength-gap-graphene-carbon-fiber.html
  2. Cyclist – http://www.cyclist.co.uk/in-depth/302/is-graphene-the-next-carbon-fibre