When good metals go bad

10/19/04

Nanoscientists probe the mysteries of stress corrosion to make planes and ships safer— and even cozier

Air travel may become safer as a result of research USC will carry on as its $2 million part of a $3.8 million investigation into corrosion-induced failure in high-performance metals used in aerospace and other demanding applications.

Supercomputing specialist Priya Vashishta and his colleagues will model the behavior of hundreds, thousands and millions of individual atoms to gain greater understanding of how and why alloys of titanium and other metals suffer "stress corrosion cracking" -- potentially catastrophic damage resulting from mechanical strain in chemically unfriendly environments.

In addition to the obvious safety implications, passengers may reap a coziness bonus.

"Anyone who has traveled on an airplane knows the air is extremely dry," said Vashishta, who has joint appointments in the USC Viterbi School of Engineering departments of materials science, biomedical engineering and computer science, and in the USC College of Letters, Arts, and Sciences department of physics and astronomy. "And this is deliberate."

"The purpose is to minimize corrosion of the airplane -- which is accelerated by moisture -- and extend its life," the scientist explained. "But if we can we understand more precisely how corrosion takes place, we may be able to find ways that will deal with the problem with less discomfort for travelers, while still keeping planes airworthy."

Vashishta and longtime collaborators Aichiro Nakano and Rajiv K. Kalia will be carrying out their investigations as part of an Information Technology Research project funded by the National Science Foundation. Like Vashishta, Nakano and Kalia hold joint appointments in the Viterbi School departemnts of computer science, materials science and (for Kalia) biomedical engineering; and in the USC College department of physics and astronomy.

The trio will partner with Caltech and Purdue on the effort, one of 120 ITR projects funded by the NSF.

Vashishta and co-investigators will use new techniques of nanoscience to supplement the traditional structural engineering approach, known as "continuum mechanics." This technique involves extensive testing of pieces of material to establish parameters of performance, which are then be expressed as predictive equations engineers use to design structures.

This works well, Vashista said, in providing reliable forecasts of how the material will behave when new. But it offers little guidance into how and when materials may fail because of stress corrosion cracking (SCC) -- damage from corrosion that starts when ordinary strain on the metal produces tiny cracks that allow the entrance of moisture and oxygen.

Nanoscientific analysis can supply such guidance, Vashishsta said. The idea is to go down to the basic atomic structure of the material and simulate the behavior of individual atoms at the point where cracks appear in the surface.

"We start by accurately modeling the behavior of collections of a few hundred atoms at one point; proceed from there to modeling thousands of atoms along the surface, going to millions of atoms over a larger area," he explained.

The results of the nanoanalysis have to produce the same predictions for behavior as the traditional continuum approach, Vashishta said. "But by understanding exactly what is going on, in detail, at the point where the material is failing, we can find better ways to prevent damage, and create more corrosion resistant materials."

"Corrosion is an enormously complex technological and economic problem with an annual cost of about 3% of the US gross domestic product," according to the proposal for the study. "Most critical here is premature and catastrophic failure of materials resulting from chemically influenced corrosion. …Safety and reliability [of structures] … is endangered by the uncertainties in ...SCC. To prevent SCC … requires that we understand the atomistic mechanisms underlying SCC.

Such understanding demands huge computational resources. Vashishta's group has its own 480-CPU supercomputer, and also uses the 2048-CPU supercomputer at the USC center for High Performance Computing and Communications (HPCC).

Vashishta's team is supplementing their expertise in materials science, computing and physics with contributions from other schools. Caltech is contributing expertise in the force fields created by chemical corrosion reactions, (William Goddard, Ferkel Professor of Chemistry, Materials Science and Applied Physics) and in traditional continuum mechanics (Michael Ortiz, Hayman Professor of Aerospace and Mechanical Engineering). Purdue computer scientist Ananth Y. Grama is also part of the project.

Source: Eurekalert & others

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