A new research discovery at Rensselaer Polytechnic Institute could lead to tougher, more durable composite frames for aircraft, watercraft, and automobiles.
Epoxy composites are increasingly being incorporated into the design of new jets, planes, and other vehicles. Composite material frames are extremely lightweight, which lowers the overall weight of the vehicle and boosts fuel efficiency. The downside is that epoxy composites can be brittle, which is detrimental to its structural integrity.
Professor Nikhil Koratkar, of Rensselaer's Department of Mechanical, Aerospace, and Nuclear Engineering, has demonstrated that incorporating chemically treated carbon nanotubes into an epoxy composite can significantly improve the overall toughness, fatigue resistance, and durability of a composite frame.
When subjected to repetitive stress, a composite frame infused with treated nanotubes exhibited a five-fold reduction in crack growth rate as compared to a frame infused with untreated nanotubes, and a 20-fold reduction when compared to a composite frame made without nanotubes.
This newfound toughness and crack resistance is due to the treated nanotubes, which enhance the molecular mobility of the epoxy at the interface where the two materials touch. When stressed, this enhanced mobility enables the epoxy to craze - or result in the formation of a network of pillar-like fibres that bridge together both sides of the crack and slow its growth.
'This crazing behaviour, and the bridging fibres it produces, dramatically slows the growth rate of a crack,' Koratkar said. 'In order for the crack to grow, those fibres have to first stretch, deform plastically, and then break. It takes a lot of energy to stretch and break those fibres, energy that would have otherwise gone toward enlarging the crack.'
Results of the study were published this week in the journal Small.
Epoxy composites infused with carbon nanotubes are known to be more resistant to cracks than pure epoxy composites, as the nanotubes stitch, or bridge, the two sides of the crack together. Infusing an epoxy with carbon nanotubes that have been functionalised, or treated, with the chemical group amidoamine, however, results in a completely different bridging phenomenon.
At the interface of the functionalised nanotubes and the epoxy, the epoxy starts to craze, which is a highly unusual behaviour for this particular type of composite, Koratkar said. The epoxy deforms, becomes more fluid, and creates connective fibres up to 10 microns in length and with a diameter between 100 nanometres and 1,000 nanometres.
'We didn't expect this at all. Crazing is common in certain types of thermoplastic polymers, but very unusual in the type of epoxy composite we used,' Koratkar said. 'In addition to improved fatigue resistance and toughness, the treated nanotubes also enhanced the stiffness, hardness, and strength of the epoxy composite, which is very important for structural applications.'
Koratkar said the aircraft, boat, and automobile industries are increasingly looking to composites as a building material to make vehicle frames and components lighter. His research group plans to further investigate crazing behaviour in epoxy composites, in order to better understand why the chemical treatment of nanotubes initiates crazing.