Does that Make Me Crazing?

by Michael Mullaney on March 26, 2009

We released a new story today about professor Nikhil Koratkar’s fascinating nanomaterials research.  For many readers – myself included – it requires learning a new word: crazing.

Crazing occurs when certain materials, usually a heat-sensitive plastic, are exposed to stress. The last time you crushed a plastic soda bottle, or accidentally stepped on your son’s action figure, you probably caused some crazing.

The picture above, taken by Nikhil’s research group, is a clear example of crazing. When the material cracked under strain of applied stress (e.g. the stress you caused by stepping on the toy), it wasn’t a clean break. Instead, the material formed a network of pillar-like fibers that bridge together both sides of the crack, and help to slow the progression of the crack.

Think back to the last time you pulled a marshmallow or a gummy bear in half: No easy task! The force you exert to bifurcate the innocent gummy bear is considerably more than the force you’d need to pull apart a pretzel or potato chip. The gummy bear stretches out, requiring a continuous expenditure of force – force that, if the gummy bear was less gummy, would have been more than enough to break it in two.

That’s a gross oversimplification, of course, of what Nikhil demonstrated in this research project. By infusing an epoxy composite with treated carbon nanotubes, he found that the nanotubes act upon the composite and prompt it to craze. The crazing, by virtue of its pillar-like fibers, helps to slow the progress of the crack. He sums it perfectly: “In order for the crack to grow, those fibers have to first stretch, deform plastically, and then break. It takes a lot of energy to stretch and break those fibers, energy that would have otherwise gone toward enlarging the crack.”

The big plot twist is that this crazing behavior is unheard of in epoxy composites. Generaly crazing activity is observed in thermoplastics, like PET, PVC, and other “everyday” plastics. The epoxy composite Nikhil used in his experiments was not a thermoplastic, but he was able to make it behave like one. As he said: “Completely unexpected.” He attributes the odd behavior to the interaction of the epoxy with chemical he used to line the carbon nanotubes.  The technical types can read all about it in the journal paper.

Needless to say, Nikhil’s curiosity is piqued and he plans to continue investigating epoxy composite’s newfound identity crisis.

On a macro level, Nikhil’s research could have big implications for the composite frames and components that are becoming more and more commonplace in aircraft, cars, and boats. (Fifty percent of the components in Boeing’s forthcoming 787 Dreamliner are composites, as opposed to 11 percent in the Boeing 777.)

The constant struggle is to make lightweight composites stronger. He has demonstrated that epoxy infused with treated carbon nanotubes exhibited a five-fold reduction in crack growth rate as compared to an epoxy infused with untreated nanotubes, and a 20-fold reduction when compared to a composite frame made without nanotubes. Those are significant toughness gains.

This isn’t Nikhil’s first go at trying to make composites, and in turn aircraft, tougher and safer. He also developed an interesting method to detect and repair microcracks – in real time – in composite frames. The study received some solid media attention.

{ 1 comment }

Darryl 06.07.10 at 5:24 pm

What are the downsides of of the composite frames and components that are becoming more and more commonplace in aircraft, cars, and boats?

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