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Multiscale Modeling of Defects in Solids

Computational Materials Institute at Cornell Web site

What makes defects form in the wing of an airplane, and how do these grow and change until they eventually make the wing break? Until a few years ago, scientists and engineers had to wait until a defect, or crack, was big enough to be visible before they could predict how much longer the wing could be safely used. But a multi-disciplinary team from Cornell University, with support from the National Science Foundation, has devised a way to study the evolution of these cracks from the time that they involved only a few atoms. Their project was called "Multiscale Modeling of Defects in Solids."

The Cornell team consisted of Principal Investigator James Sethna, from the Laboratory of Atomic and Solid State Physics; and co-PIs Christopher Myers, from the Cornell Theory Center; Paul Dawson, from the Mechanical and Aerospace Engineering Department; and Anthony Ingraffea, the Dwight C. Baum Professor of Engineering. The group explored novel software engineering techniques to support making computer models at multiple length and time scales.

They did this by developing a virtual laboratory, a software infrastructure called the Digital Material, which they used to explore and develop new theories and models of defect dynamics, and to test and validate those models over a wide range of length scales, from the atomic to the microscopic. They used two computer languages, one to write the code that did all the work, and a second, high-level, language called Python, to steer the applications of the code. They also used Design Patterns, a programming technique that supports flexible program composition.

Until the early 1990s, experts focused on what happened from the time a crack was large enough to be seen. However, according to Dr. Ingraffea, an expert in aerospace structural engineering, "What we weren't able to do was predict how long it would take from the time the airplane was first put into service until the time the cracks were a millimeter long, and that's most of the life. So we were doing a really good job of predicting 10 percent of the life and a miserable job of predicting 90 percent of it."

Today that has changed. According to Dr. Sethna, "We try to understand the defects—starting from the atoms—well enough to tell you what they will do under various circumstances." Doing this involves more than measuring one defect or crack. Researchers also need to know how the defect is affected by its environment as well as how it affects its environment. "The crack by being there changes the behavior of the material near it," said Dr. Sethna. "Other defects might fly towards the crack or fly away from it due to the crack's existence." This is why it was important for the team to systematically measure a crack and extract all the information that engineers might want to know.

This involved a change in approach for both the physicists and the engineers. As Dr. Sethna explains, "Engineers by and large like to try and understand the materials ignoring the atoms. And the physicists spend a lot of time thinking about the atoms. Our approach is to try to extract the information you need from the atomistic scale to communicate to the longer wavelength scale."

The outcome has important applications not only in the airline industry, but also in reusable launch vehicles used by NASA, and in gears used in transportation from helicopters to submarines. It's also of use in the fields of heavy industry and microelectronics. And ultimately, research could yield results that allow metals to be designed from the atomic scale up to be more resistant to the development of defects.

In addition to the principle investigators, the project also involved faculty, senior researchers and research associates, postdocs, and graduate students from Cornell's Laboratory of Atomic and Solid State Physics, the Cornell Theory Center, and the Departments of Mechanical and Aerospace Engineering, Materials Science and Engineering, Civil and Environmental Engineering, and Theoretical and Applied Mechanics.

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