Aligned carbon nanotubes boost in-plane strength of composite laminates

New paper published by MIT also shows open hole compression strength upped by 14% and bolt pull out bolstered by 30%.

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The ability of aligned carbon nanotubes (ACNTs) to defeat delamination — traditionally the Achilles’ heel of composite laminates — has been well reported by CW (see “The end of delamination?” and “CNT-enhanced prepregs: commercial & production-capable”). Now, however, researchers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, US) have found that ACNTs ability to be used as a stitching layer between laminate plies provides benefits beyond increased interlaminar strength.

"In-plane strength enhancement of laminated composites via aligned carbon nanotube interlaminar reinforcement," published this week in the journal Composites Science and Technology, show that this technology — already commercialized by N12 Technologies (Cambridge, MA, US) — significantly increases load-carrying capability: tension-bearing (bolt pull out) critical strength by 30%, open-hole compression ultimate strength by 14%, and L-section bending energy and deflection by more than 25%.

As reported by MIT News, Roberto Guzman, who led the work as an MIT postdoc in the Department of Aeronautics and Astronautics (AeroAstro), says the improvement may lead to stronger, lighter airplane parts — particularly those that require nails or bolts, which can crack conventional composites. The study’s co-authors include AeroAstro professor Brian Wardle and researchers from the Swedish aerospace and defense company Saab AB. (CW originally discussed the development from these two groups in “Aerocomposites: Move to multifunctionality” under the heading Aligned CNTs and nanostitching.)

The researchers grew tiny “forests” of carbon nanotubes onto films, which they then layered between plies of carbon fiber composites. The nanotubes, resembling tiny, vertically-aligned stitches, extend into the plies above and below the ACNT layers, acting to hold the laminate together. The result is that traditional delamination damage modes associated with pre-ultimate failure are suppressed in the in-plane loaded laminates. Compared with existing composite materials, test results show that the stitched composites were 30% stronger, withstanding greater forces before breaking apart.

“Size matters, because we’re able to put these nanotubes in without disturbing the larger carbon fibers, and that’s what maintains the composite’s strength,” says Professor Brian Wardle. “What helps us enhance strength is that carbon nanotubes have 1,000 times more surface area than carbon fibers, which lets them bond better with the polymer matrix.”

“More work needs to be done, but we are really positive that this will lead to stronger, lighter planes,” says Guzman, who is now a researcher at the IMDEA Materials Institute, in Spain. “That means a lot of fuel saved, which is great for the environment and for our pockets.”

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