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Cracking the Origin of Metal Cracking

Tuesday, July 14, 2015

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TEMPE, AZ--Small cracks hold the seeds of catastrophically large structural failures, prompting Arizona researchers to find a new way of keeping a nano-sized eye on them.

Research led by Arizona State University materials science and engineering professor Karl Sieradzki is uncovering new knowledge about the causes of stress-corrosion cracking in alloys used in pipelines for transporting water, natural gas and fossil fuels—as well as for components used in nuclear-power generating stations and the framework of aircraft.

Photos: Arizona State University

Corrosion of a silver-gold alloy spontaneously causes the formation of nanoscale porous structures that undergo high-speed cracking under the action of a tensile stress.

Sieradzki's team’s findings are detailed in "Potential-dependent dynamic fracture of nanoporous gold," published June 22 in the journal Nature Materials.

200 Meters Per Second

The work focuses on the interactions of metal alloys at the nanometer and atomic scales, in order to prevent the failure of massive systems critical to public and industrial infrastructure.

Using ultra-high-speed photography and digital image correlation, the team has been able to closely observe events that trigger the origination of stress-corrosion fracture in a model silver-gold alloy—and to track the speed at which cracking occurs.

And that speed can be devastating.

The team measured cracks moving at speeds of 200 meters per second, corresponding to about half of the shear wave sound velocity in the material, ASU said in a research announcement.

Modeling Clay

Sieradzki called that result remarkable, given that only brittle materials such as glass typically fracture in this manner.

Gold alloys are among the most malleable metals, so long as they are not used in a corrosive environment, he said.

Karl Sieradzki

Karl Sieradzki's team studied the stress-corrosion behavior of metals that threatens the mechanical integrity of engineered structures.

In most other environments, the alloys behave like children’s modeling clay, Sieradzki explained. Roll modeling clay into a cylindrical shape, and you can stretch it by about 100 percent before it slowly tears apart.

In a corrosive environment, however, "silver is selectively dissolved from the alloy, causing porosity to form," the university said. "If this occurs while the alloy is stressed, the material fails as if it were made of glass."

The Stress-Corrosion Connection

The research provides "a deeper understanding of the stress-corrosion behavior of such metals as aluminum alloys, brass and stainless steel that threatens the mechanical integrity of important engineered components and structures," the university said.

The discoveries provide a guide for “designing alloys with different microstructures so that the materials are resistant to this type of cracking,” Sieradzki said.

The research has been funded by the Department of Energy’s Basic Energy Science program.


Tagged categories: Accidents; Corrosion; Cracking; Infrastructure; North America; Pipeline; Power Plants; Quality Control; Research

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