Composites stronger with CNT 'stitches'

RESEARCHERS have found a way to bond composite material layers together using carbon nanotube (CNT) ‘stitches’ to make stronger and more lightweight frames for aircraft.

New passenger jets are primarily made from advanced composite materials such as carbon fibre reinforced plastic – durable and light materials that reduce the overall weight of the plane by up to 20% compared to aluminium-bodied planes, which translate into fuel savings. However, composites layers are vulnerable to cracking from relatively small impacts, while aluminium frames can withstand relatively large impacts before cracking.

A team of engineers from MIT has devised a method of fastening the composite layers together using strong CNTs. The team embedded tiny 'forests' of CNTs within a glue-like polymer matrix and then pressed the matrix between layers of uncured carbon fibre composites. The nanotubes resemble tiny, vertically-aligned stitches which work themselves into the crevices of each composite layer, serving as a scaffold to hold the layers together.

The team’s L-shape bend experiments to test the material's overall strength before breaking revealed that compared with existing composite materials, the stitched composites were 31% stronger, withstanding greater forces before breaking apart.

Roberto Guzman, former MIT postdoc and researcher at the IMDEA Materials Institute in Spain, said the improvement provided by the method could lead to lighter and more damage-resistant parts for planes, but this is not the full solution.

“More work needs to be done, but we are really positive that this will lead to stronger, lighter planes. That means a lot of fuel saved, which is great for the environment and for our pockets,” added Guzman.

The initial tests also showed that the polymer glue region between the layers is still the weakest part of the material, which the team described as “problematic”. Methods to strengthen it include Z-pinning and 3D weaving – which involve pinning or weaving bundles of carbon fibres through composite layers, similar to pushing nails through plywood, or thread through fabric.

Brian Wardle, a researcher with Swedish aerospace and defence company Saab AB, clarified that mechanical solutions would be damaging to the material and that chemical solutions would only be effective at the nanoscale.

“A stitch or nail is thousands of times bigger than carbon fibres. So when you drive them through the composite, you break thousands of carbon fibres and damage the composite.

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

The team came up with a technique to integrate a scaffold of CNTs within the polymer glue. They first grew a forest of vertically-aligned nanotubes, then transferred it onto a sticky, uncured composite layer and repeated the process to generate a stack of 16 composite plies – typical for a composite laminate – with the nanotubes glued between each layer.

The team performed a tension-bearing test, to measure the material’s strength, where the researchers put a bolt through a hole in the composite, then ripped it out. While existing composites typically break under such tension, the team found the stitched composites were stronger, able to withstand 30% more force before cracking.

They also performed an open-hole compression test, applying force to squeeze the bolt hole shut. The stitched composite withstood 14% more force before breaking, compared to existing composites.

Wardle added: “The strength enhancements suggest this material will be more resistant to any type of damaging events or features. And since the majority of the newest planes are more than 50% composite by weight, improving these state-of-the art composites has very positive implications for aircraft structural performance.”

The next step for the team is to refine its method to achieve the same strength and damage resistance performance as aluminium frames before composites can be used completely on aircraft. The work is being supported by commercial airline manufacturers Airbus and Boeing, and the US Army.

Composites Science and Technology, DOI: 10.1016/j.compscitech.2016.07.006