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Failure Model May Avert the Real Thing

Friday, May 1, 2015

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LINCOLN, NE—A new model that can simulate sub-surface corrosion damage could help head off structural failures and design stronger materials, its developers say.

The unique model looks at the rate and profile of pitting corrosion to assess sub-surface damage and predict its spread.

Such technology could be used to get a fix on the potential for future failures and to design materials that are less susceptible to degradation, say engineers Florin Bobaru and Ziguang Chen, who developed the model at the University of Nebraska-Lincoln.

Understanding the Interface

The trick is to be able to assess the interface between the substrate and the corrosion—a challenge that Bobaru and Chen are the first to unlock, the team says.

People have been trying to model this for many years, but they've struggled because it's difficult to track how the interface between a corroding solution and the solid material evolves over time," Bobaru said.

University of Nebraska-Lincoln corrosion research
Scott Schrage / University Communications, UNL

Florin Bobaru and Ziguang Chen have developed a model that simulates sub-surface corrosion damage.

According to Bobaru, these struggles were partly due to misconceptions of that interface. Previous research often devised the interface as a membrane separating the metal surface from the corrosive chemical solution.

Bobaru and Chen, however, treated the interface as an initial layer of corrosion that indicated less perceptible, but advancing, degradation below the surface.

"Once you have that understanding, you can say, 'I know this piece is corroded, but it's going to last two more years,' or, 'I better replace this next month because there's a high protential that a crack will run through and damage the whole structure,'" Bobaru said.

Replicating Corrosion

In their research, Bobaru and Chen examined current experimental data on corrosion in one-dimensional components that included wire. Then they calibrated their model's parameters accordingly and found that the simulations of corrosion-related damage were very close to the experimental outcomes.

After a few modifications, the research duo was also able to replicate corrosion of two- and three-dimensional structures. The model can be applied to metallic and non-metallic materials.

Their model simulates two separate, yet related, processes: It captures corrosion from escaping ions, which migrate from the metal to the interface of the corroded pit, and it calculates how the loss of ions contributes to mechanical bond deterioration.

©iStock.com / lammerst333

"People have been trying to model this for many years, but they've struggled because it's difficult to track how the interface between a corroding solution and the solid material evolves over time."

"It tells us what concentration of ions we have to lose in order for a mechanical bond to break," Bobaru explained. "So the ion diffusion problem is coupled with the mechanical damage to the material."

Exploring Fractures

Bobaru and Chen are now working on getting the model to simulate the fractures that result from broken mechanical bonds.

"Our contribution was the modeling of this sub-surface layer as one that eventually weakens a material and could influence how potential cracks grow from there," Bobaru explained.

But this is just the first step, he adds.

"Once you have corrosion pits forming, a crack can emerge. This crack growth is influenced by corrosion, and the corrosion is influenced by the crack growth. The solution can get in there, speeding up the material degradation and potential catastrophic failure of your system. So we're now exploring that."

A study on the model, "Peridynamic modeling of pitting corrosion damage," appeared earlier this year in the Journal of the Mechanics and Physics of Solids.

   

Tagged categories: Colleges and Universities; Computer generated modeling; Corrosion; North America; Quality Control; Research

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