Bend, Stretch and Break - Materials Testing at Northwestern
Mark Seniw has watched students bend, stretch and break a number of materials since he began overseeing Northwestern University’s Central Laboratory for Materials Mechanical Properties (CLaMMP) in 1984.
The materials fatigue they study involves the kind of properties linked to the five-foot rip in the fuselage of the Southwest Airline flight out of Phoenix a few weeks ago. The pilot made a successful emergency landing. But Seniw's lab is developing and testing materials that can withstand more fatigue.
In the CLaMMP lab, student researchers perform mechanical testing on glass, wood, and ceramics, and have even experimented with skin in order to see how much stress a particular material can withstand before it gives way.
Seniw talks about how CLaMMP promotes scientific innovation and promises the reliability of products we use every day.
Could you explain what role metal fatigue and inspections play in aircrafts?
In order to go to high altitudes you have to pressurize the cabin, or else you have to be on oxygen because the air is so thin up there. So they pressurize the cabin and that stretches the aluminum that’s on there. While it’s flying, the airplane is being buffeted around, and that’s causing stress on the aluminum skin also. When planes land, they decompress and the metal goes back to its original shape. So each time it takes off and flies it undergoes that fatigue cycle.
In an Aloha Airlines accident, the inspections where done based on the standards made on the mainland, where flights go longer distances. But in Hawaii, they could do much more takes-offs and landings and not do as many miles as a mainland aircraft. They changed the inspections after the accident. They realized they need to include not just the flight time - how much time they’re in the air – but also how many take offs and landings they do, because that’s considered a cycle.
What interesting research projects are students working on?
One student is testing materials in use for replacing tendons. You want the tendon tissue to grow on this material and [have the material] eventually dissolve away. So instead of taking a graft from another area, you would [use this material] and your cells would grow on there, complete the bridge across the severed part, heal and be functional.
What exactly is metal foam, a light, strong material created in the laboratory of David Dunand, who co-directs CLaMMP?
If you think of foam rubber, it’s a plastic or polymer with air pockets in it, and [metal foam is a] similar thing. So how do you get air pockets in a metal? One of the ways is to pour a salt in a mold. They’ll sift it until they get similar size salts, then they pour the molten metal in the mold. It goes in between the salts and, once the metal solidifies, they dissolve out the salt with water or acid. Once the salt is gone, you now have this metal [with a] foam structure inside. In cases when you want a lighter material [that’s] still strong, you might use that.
What machines are used to test materials?
They’re called universal testing machines, and there are five in this lab. They’re different sizes and hold different capacities. Some have ovens to do elevated temperature tests, and we can put chambers on them to do low temperature tests to see how [materials] perform at lower temperatures.
They’re all pretty basic – apply a load, see what the response is, and measure how the material responds. Some of them do tension and compression, some do fatigue. We can test metals, polymers, ceramics, composites, wood, cement, glass, and biological materials.
We’ve tested tissue before. Someone from the medical school was working on battlefield laser sutures. They were trying to come up with a laser technique where they could hold the tissue together and use a laser that would seal the tissue to immediately stop bleeding or [prevent] an infection from occurring. I believe it was pig skin. They would cut a laceration and use the regular suture technique, and we would pull that to see how strong it was before the sutures gave way. They’d do a similar [test] with the laser technique to see how that was compared to the suture, and to see how much force it would take to pull it apart. They’ve tested bone and other materials too.
How would you explain materials science to someone who doesn’t understand what it is and how it affects their lives?
The strength of materials is important to everyone. Everything that you encounter in life has been manufactured and tested for its mechanical properties. Some are tested so that they’re safe. Others are tested so manufacturers can come up with the least amount of material they would need for making a product that’s functional, like an aluminum can. It’s very thin and they make it as small as they can [while still] able to hold the soda or the beer inside, and stack it up with crates and cases on top of each other. You can make an indestructible can, but it would be very expensive and not be very profitable. So some research is driven by safety, some driven by costs – at some point everything gets tested mechanically.
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