TAGS: Polymer Reinforcement
MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nano stitching”. This technique involves depositing chemically grown microscopic forests of carbon nanotubes between composite layers.
The tiny, densely packed fibers grip and hold the layers together, like ultra strong Velcro, preventing the layers from peeling or shearing apart.
Improving Material’s Resistance to Cracks by 60%
To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely
lightweight and durable structures.
But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.
In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nano stitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.
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Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “
We’re showing that nano stitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”
Using Chemical Vapor Deposition to React to Catalysts
At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nano stitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes, hollow fibers of carbon, each so small that tens of billions of the nanotubes can stand in an area smaller than a fingernail.
To grow the nanotubes, the team used a process of chemical vapor deposition to react to various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.
The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.
In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nano stitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.
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This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nano stitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.
Source: Massachusetts Institute of Technology