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There's new insight into what triggers rock and ice failure
The cracking, splitting and crushing events occurring constantly just beneath the earth's surface can now be linked to similar activity taking place in floating sheets of ice in the polar regions. Two Dartmouth researchers offer a theory about how these brittle substances break under compression.
Erland Schulson, the George Austin Colligan Distinguished Professor at Dartmouth's Thayer School of Engineering, and Carl Renshaw, Associate Professor of Earth Sciences at Dartmouth, believe that most brittle materials, like rocks and ice, crumble or fail in the same manner. In an article in the August 30, 2001, issue of Nature, titled "Universal Behavior in Compressive Failure of Brittle Materials," the duo names a specific type of cracking mechanism that leads to collapse.
"The subtitle of the article could be 'Lessons Learned from Ice.' By looking at ice cracking, it revealed a mechanism that we haven't applied before to rocks," said Schulson. "We showed years ago that as ice is compressed, cracks within it slide together and link up, eventually causing the ice to fail. And we know what triggers this in ice. We set out to determine if the physics of compressive failure are specific to any given material or not."
In their paper, Schulson and Renshaw argue that "comb" cracks, resembling the teeth on a comb, are the key to crushing in both ice and rock. "If comb cracks develop as a brittle material is compressed, we can predict how that material will fail," said Renshaw.
"This paper helps piece together the puzzle to determine what's the critical factor for why things fail when they fail. How much pressure needs to build up in the case of concrete or how much pressure can you support when ice pushes against a structure like an oil or gas rig in the ocean," said Schulson.
Microcracks in ice have a certain character, and as they experience more pressure, they develop little extensions called "wing" cracks. About two years ago, Schulson noticed that in addition to the wing cracks, there were little secondary cracks forming along one side of the small microcracks, creating mini columns. He focused on these "comb" cracks as the trigger for failure in ice. Since this cracking behavior is also found in granite and other kinds of rock, Schulson tapped into Renshaw's expertise to see if the physics would apply outside of ice.
Touting the benefits of interdisciplinary research, Renshaw said, "Lots of people are interested in how fractures fit together, not only from a failure point of view, but also from a fluid flow point of view. I'm working on how fluids and contaminants travel through this connected network underground. Hopefully, this discovery will lead to understanding the nature of the fracture network in the ground, and how that influences the flow of toxic metals, like arsenic, through groundwater systems."
Schulson and Renshaw will now work to see if their theory can be translated to incorporate the giant structures found in nature. "It's one thing to show that the physics look the same, but another to prove it," said Schulson. "It looks like a duck, and walks like a duck, and quacks like a duck. Now we need to prove it is a duck."
Schulson's research is funded by the Office of Naval Research and the Army Research Office. The National Science Foundation supports Renshaw's work.
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