Wichita State researchers describe their design
Delamination is the bane of many an aerospace engineer’s life, because it means the layers in a strong-but-lightweight carbon-fiber composite structure are beginning to separate — risking a critical fracture. One possible solution would be to create composites whose long carbon microfibers are much more tightly anchored to the epoxy resin they are embedded in, but until now, no one has come up with a practical way of doing it.
To the rescue come researchers from Wichita State University in Kansas, who have developed a manufacturing technique that uses coiled carbon nanotubes instead of straight ones to bind the resin and fibers more tightly together.
“Our helical carbon nanotubes, dispersed in the resin, entangle together but also entangle with the microfibers, providing the highest possible interlocking reinforcement,” says Davood Askari, the nanocomposites engineer who led the research. “It’s like you walk into a very dense jungle where all the tree branches have been entangled and mechanically interlocked together. It’s going to be very difficult to pull them all away and walk amongst the trees.”
Wichita announced this composites-enhancing research in mid-April, though the basic science underpinning it was first described in a January paper in the Journal of Composite Materials. The researchers reported boosting the fracture toughness and other parameters of aerospace-grade carbon composites by populating them with helical nanotubes.
What is so special about this shape of nanotube? The common single-walled and multi-walled varieties of these hollow tubes of carbon atoms have been shown to increase the strength and rigidity of composite resins but come with some downsides. Namely, their smooth sides mean that they often clump, disperse poorly in polymers and — most importantly for delamination — do not bind to the microfibers. However, Askari and his colleagues reasoned that a nanotube in the shape of a wound-up, spring-like coil should be able to “entangle mechanically” with the microfibers, stiffening the resin and therefore reinforcing the entire composite structure.
Even with these advantages, however, they decided to take an additional step to boost bonding performance by “activating” the tubes before adding them to the resin. In this technique, the tubes are doused in concentrated acids — sulfuric, nitric and hydrochloric — that deposit additional hydrogen and oxygen-based ions, called hydroxyl groups, to various sites along the lengths of the tubes. The tubes are then washed to remove excess acid.
The result? The hydroxyl ions help “in the formation of crosslinks with the epoxy resin, resulting in improved bonding and dispersion,” the researchers wrote in their paper.
The results have been impressive, says Askari, with the resulting composite showing improvements in metrics, including “fracture toughness, one of the main material properties that accounts for the delamination and inter-laminar properties,” he says.
Specifically, the team achieved:
- 5.7% increase in tensile strength;
- 35.1% higher fracture toughness;
- 49.5% boost in its strain-to-failure performance; and
- 60% hike in hardness.
Since concluding their initial study at the end of 2023, they have continued to refine this coiled nanotube technique. Askari is hopeful that in a future paper, they will be able to report fracture toughness was boosted up to 50%.
“And we have achieved this by only adding 0.1% by weight of the helical nanotubes,” he says.
Askari notes that these helical nanotubes also increase the composite’s overall electrical and thermal conductivities. On an aircraft, he says, it means the composite could absorb a higher load of electric current in a lightning strike. Today, a high-density copper mesh inside the aircraft skin shields against such strikes. If the skin were constructed of the Wichita composite, designers could decrease the density of the copper mesh, saving weight.
He also sees potential space applications. The nanotubes could be incorporated into a ceramic composite, like the reinforced carbon-carbon and silicon carbide composites that often comprise spacecraft heat shields.
For now, Askari says that Wichita State is exploring potential applications with a raft of industry partners in the aerospace sector and other industries. And one of those companies indicates just how strong and impact-resistant this nanomaterial might be: Leading Technology Composites Inc., also of Wichita, is exploring using it for bulletproof vests.
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