Carbon it time to hybrid-it?

Posted by DSM on Dec 23rd 2015

Carbon fiber: great for everything but impact resistance

Modern airplane designs make extensive use of carbon fiber composites in the fuselage and wings. Carbon fiber is strong yet light. But it’s not so good at withstanding impacts. Instead of yielding when hit, carbon fiber bends and then shatters. This means it is not so good for applications in which impact loads are a risk. This makes carbon-reinforced polymers a good choice for a plane’s fuselage, and the flaps and flat surfaces of a wing, but not for the wing’s leading edge, which is prone to high impact bird strikes. Similarly, you can make a vehicle’s doors, roof and other similar panels with carbon composites, but not its bumpers. But suppose you could.

The scientific explanation is that while carbon fiber shows the best strength-to-weight ratios for tension, compression and bending loadings, it shows poorer performance on impact loadings. “It’s not bad, but it is not as excellent as the other properties,” notes DSM Dyneema scientist and part-time professor at Delft University of Technology Roel Marissen. This is because carbon fiber is only linearly elastic and cannot dissipate the energy released when it is hit by a small, pointed object. “The amount of deformation energy you can put into the material is not that big.”

The secret lies in hybridizing carbon fiber with Dyneema®

For the curious and imaginative, this raises the question of whether it is possible to create a carbon fiber composite that offers dramatically better impact resistance. A carbon fiber composite that outperforms other materials on tension, compression, bending and impact loadings could open the way to new and exciting applications.
And the answer is, we can. Like a perfect marriage, combining – hybridizing – carbon fiber with Dyneema® UHMwPE fiber (ultra-high molecular weight polyethylene) results in carbon/Dyneema® hybrid composites that handle impact forces significantly better. From 50% up to 100% better, depending on the amount and performance level of the Dyneema® used.

Take the best and mix: carbon fiber plus Dyneema®

A DSM Dyneema research paper – Carbon/Dyneema® Intralaminar Hybrids: A New Strategy to Increase Impact Resistance or Decrease the Mass of Carbon Fiber Composites – explains the science. The paper was written with the aerospace industry in mind, but the conclusions are widely applicable. “Dyneema® fibers are extremely strong and light in one loading direction, tension,” says Marissen, “and the impact performance of Dyneema® is really excellent.” This is why, for example, Dyneema® Force Multiplier Technology has quickly become the go-to material for bullet-resistant vests and anti-ballistic vehicle armor. “So if,” Marissen continues, “you mix carbon fibers that are about the best in the world in all kinds of loadings – except impact – with Dyneema® fiber that is about the best in the world in impact, you get a material that has increased impact resistance, a slight loss on flexural strength properties, but still a very good overall performance.”

A thicker composite can replace lost bending strength

This ability to replace up to half the carbon with Dyneema® without significantly reducing the final composite’s typical ‘carbon fiber’ performance, reflects another useful property of Dyneema®. Dyneema® fibers have a lower density than carbon fibers. So, on a weight-for-weight basis, you can make the composite that contains Dyneema® thicker than one that doesn’t. “Bending strength is determined by both the strength of the material and its thickness,” explains Marissen. “If you make a piece of wood twice as thick, the flexural strength is four times as great. It’s not just the material property that provides bending strength, but also the increased thickness, related to decreased density at the same weight.”

In other words, the lower density of the Dyneema® fibers compensates for the nominal loss of carbon stiffness and strength. The upshot is a composite that, because it’s a bit thicker, shows similar tension, compression and bending load performance to a ‘normal’ carbon fiber composite. Plus, the much increased impact loading performance as well.

Dyneema® SK99 makes the potential gains even greater

In fact, the results today could be even better than indicated in the paper. “The best high performance polyethylene fibers will give the best composite performance,” says Marissen. Today we have Dyneema® SK99, an even stronger fiber than the SK75 used in the research. With Dyneema® SK99 we have something for carbon fiber applications that is unsurpassed. Dyneema® SK99 would give even better results than we found with SK75. Everything I know points in that direction.”

It all looks so promising. So why aren’t we seeing more carbon/Dyneema® hybrids? Marissen points to the nature of the composites market: fragmented, specialized and broad, ranging from automotive and aerospace applications to sports and sporting goods, renewables and beyond. Some high performance applications also require extensive testing and certification. This means creating prepregs (pre-impregnated but not yet cured mixtures of fibers and resins), something that lies beyond the focus of the DSM Dyneema business. But with carbon/Dyneema® hybrid laminates already being used in glider nose-cones and other small applications, Marissen remains hopeful that with the right partners, the idea will move forward to spawn a new generation of breakthrough carbon and Dyneema® fiber applications.

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