Dynamic Hardness Testing: It's Not So Hard Anymore

July 11, 1997

If you've ever fooled around with Silly Putty, then you already know a little about statics and dynamics. Pull the goo slowly and it stretches into a long rope; jerk it and it breaks.

Sand behaves the same way, only different. Push a knife slowly into a sandbag, and you can slide it right through. But fire a gun into the same sandbag and the bullet stops within inches.

It's pretty easy to examine how materials behave in the slow-knife scenario. The static hardness test that determines how materials respond to that kind of force has been around for decades. But dynamic hardness testing--which measures how materials respond when acted upon very quickly, as by a bullet--has been another matter. It's a time-consuming, complex task, even for highly trained individuals. Which means that measuring the dynamic indentation hardness of materials--which has applications ranging from the MIR space station hull to bullet-proof vests--is both expensive and difficult.

But research by Michigan Technological University scientists could render these elaborate and costly tests obsolete. Assistant Professor Ghatu Subhash and his doctoral candidate Brian Koeppel of MTU's Deparetment of Mechanical Engineering- Engineering Mechanics have developed a method of measuring dynamic indentation hardness in less time than it takes a fast ball to reach the catcher's mitt. "Our test happens," Koeppel said, "in 200 microseconds"--one five-thousandth of a second. That's the fast part. Now for the cheap and ingenious part. With a small company grant, Subhash has managed to put together a model dynamic hardness tester for about $5,000. True, it is somewhat unwieldy. "At ten feet long, you wouldn't want to keep it on your desk," Subhash notes. However, researchers swear they could make an eighteen-inch working device with the R&D equivalent of chicken scratch.

"If a company gives us $50,000, we can do it," Subhash asserts. They also have the expertise to add a read-out function, which would make the entire process automatic. Right now, they have to make a few manual measurements.

Koeppel's work on the project has already attracted attention in professional circles, earning one award after another as the research progressed. He took top honors in the Society for Experimental Mechanics Student Paper Competition at the VII International Congress on Experimental and Applied Mechanics, in 1996; first place at Michigan Tech's 1997 Sigma Xi Research Colloquium; and, most recently, won $1,000 for presenting the best student paper at the 1997 Joint American Society of Mechanical Engineers/American Society of Civil Engineers/Society of Engineering Science summer meeting at Northwestern University.

Business interests are also beginning to take notice. Static hardness testers are standard equipment in machine shops and research labs worldwide. If mass produced, the MTU dynamic hardness tester could have a similar market.

"It has tremendous potential," Subhash said. "We have had communications with several companies." Just in the area of crash worthiness, the device could be used to evaluate how a car bumper or windshield reacts to pebbles kicked up by trucks, or how space vehicles would react following a meteor collision. Canadian and U.S. patents are pending.

And, with the help of a National Science Foundation grant, Professor Abhijit Chandra, and Associate Professor Young Huang , Subhash is using the new device to develop an experimental model of the grinding process, particularly as it applies to ceramics.

Grinding is VERY dynamic, with super-hard materials such as diamond chips ripping away at the ceramic matrix. Reseachers hope to determine the best ways to minimize damage (such as cracking) to the ceramic workpiece during the process, with the goal of making the smoothest possible surface.

Subhash credits Koeppel for bringing the research to near-fruition. "I started thinking about it a long time ago, when Professor Chandra wanted to collaborate on modeling ceramic grinding processes, but Brian made it work," he said. "He brought it to the highest level. He also conveys the information well, presents the papers elegantly."

Koeppel's award-winning paper presentations have not come without a certain amount of personal sacrifice. "I'd rather go to the dentist than do public speaking," he says.

For further information, contact Dr. Ghatu Subhash at 906-487-3161 or e- mail: subhash@mtu.edu.

Michigan Technological University
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