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Characterizing cracked crystals

Walter Drugan and Wendy Crone

Walter Drugan and Wendy Crone (22K JPG)

Wendy Crone's copper beryllium sample

Wendy Crone's copper beryllium sample (29K JPG)

A crack in the sidewalk generally is at most a minor obstruction; at least, a cosmetic flaw. Put that same crack in an airplane propeller, however, and it draws more concerned attention. Professors Walter Drugan and Wendy Crone are two of a handful of people who might be happy to see such a flaw — if only to make a material stronger by learning from its weaknesses. In one of Drugan's areas of expertise, fracture mechanics, he uses mathematical tools to study flaws in materials.

Currently, fracture mechanics theory applies mainly to polycrystalline materials, made up of thousands of tiny grains, or crystals, that are oriented in various directions. "Most of the structural components you see are made of polycrystalline materials," says Drugan, a professor of engineering mechanics in the Department of Engineering Physics.

But with colleague Crone, a cutting-edge mechanics experimentalist, and expert numerical analysts from Brown University and the University of Groningen in the Netherlands, Drugan is analyzing the effects of cracks in single-crystalline materials.

Unlike their polycrystalline counterparts, single-crystalline materials do not have grain boundaries, which tend to be sites of impurities and flaws. "If you can get rid of grain boundaries, it gives you the hope of making materials with fewer flaws and impurities, and greater resistance to creep-type deformation (strain caused by stress) at higher temperatures," says Drugan.

Metallic single-crystal materials are used in a variety of industries, including aerospace and microelectronics. "As an example, single-crystal metals are incorporated in the advanced high-pressure turbo pumps that serve in the Space Shuttle engines," says Crone, an assistant professor. Among their uses, single-crystal metals also appear in transformer cores for magnetic devices, collimators for X-ray machines and focusing elements for particle-optical devices.

In addition, they are essential to the multibillion-dollar turbine-engine industry; General Electric announced last year that it has $25 billion in orders for gas-turbine engines that employ single-crystal metallic materials.

Although increased strength, reliability, deformation control and heat tolerance are some of their advantages, single-crystal materials invariably have flaws as well. And those flaws have very different characteristics than flaws in polycrystalline materials do, making efforts to analyze them more complicated, says Crone, who grew and tested single-crystal copper beryllium samples for the project.

Load, or force, applied to a polycrystalline metallic material produces stresses that radiate from a crack tip and activate slip systems (atomic planes that slide over each other) in the material's randomly oriented grains. The stresses that occur are, on average, the same regardless of how the crack-load system is oriented related to the material. In a single-crystalline material, in which atomic planes are aligned the same way, the group's studies show that a flaw's orientation strongly affects how the slip systems respond.

"The slip system that produces the major amount of plastic (permanent) slipping changes when the crack orientation is changed. This will have a dramatic effect on how fracture occurs," says Drugan.

The team developed not only an experimental basis for exploring the phenomenon, but reinforced it with theoretical and analytical evidence. The collaboration is unique, says Drugan, because the researchers' individual conclusions mesh to illuminate the whole picture. "As far as I know, there is no group that has put together all the components like we have," he says.

The group already has published three recent back-to-back papers in the Journal of the Mechanics and Physics of Solids, and is working on another paper that addresses cracks in copper beryllium with four different orientations in relation to the material's atomic planes. In each case, Crone's experiments show the behavior is different — and the analytical and numerical studies are able to show why. "We're going to try to explain all the behavior at once," says Drugan.


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Date last modified: Monday, 09-Jul-2001 13:56:42 CDT