STRESS AND CORROSION CRACKING OF METALS
HOUGHTON,MI--Imagine a world where we could predict when an airplane rivet was about to break or when a pipe was about to burst; the part could be replaced before it caused a serious problem. Although that world is still a ways away, Tamara Olson, an assistant professor in mathematics at Michigan Technological University, and Brent Adams from Carnegie Mellon University are looking for ways to make it happen.
Metal cracking often occurs due to a combination of stress and corrosion. Metals are constantly under stress: bridges support the weight of vehicles, forks withstand pressure from our hand. In addition to stress there is the problem of corrosion: chemicals, air pollution, and even normal water or soil conditions. A crack that starts on the edge spreads deeper into the material as corrosive elements seep in and stress is applied. Unfortunately, these cracks often go unnoticed until they cause failure in the material.
Olson and Adams are interested in materials which experience stress and corrosion cracking, but they are looking at the cracks on a small scale. "Some examples of cracks I have come across include rivets in rotor blades of helicopters and landing gear of airplanes," said Olson. In addition to aircraft, she said metal cracking is a concern in maintaining pipelines for natural gas.
On a very small scale, metals consist of tiny particles called crystallites arranged edge to edge. When stress and corrosion are applied to the metal, cracks form along some of the boundaries between these crystallites, forcing them to come "unglued" from one another.
Each crystallite in the metal has its own orientation. The orientation may be horizontal, vertical, or on a diagonal. When two adjacent crystallites have the same orientation the metal tends to be stronger in that area; when the orientations don't line up the metal is more likely to crack.
Olson is doing statistical analysis of metal samples provided by Adams' group. She is trying to determine the probability of a crack going beyond a certain point, and the direction the crack will go. This probabilistic research is called a Markov Chain Model: the outcome at one stage (where will the crack go?) depends on the outcome of the previous stage (where has the crack gone so far?). This information could help engineers predict which parts or materials were likely to break and when, allowing time to replace them rather than waiting for a break.
So far, Olson and Adams have developed a two-dimensional model of the probability of metal cracking by following the paths of cracks in metals and observing where and how far they went. Because materials are three-dimensional, however, they intend to extend the model with more research.
"Because stress corrosion cracking is a common and catastrophic phenomenon in certain materials, it would be invaluable for engineers to be able to estimate a material's susceptibility to cracking by measuring microstructure and using the model," Olson said.
Olson said that processing has an influence on the orientation of the particles. Two common processes are rolling the metal and melting and casting the metal. When rolling metal, the direction of pressure can influence the orientation of the particles, while in the melting and casting process, particles depend upon how the material cools. According to Olson, the ultimate goal is to identify the relationship between the process and the ensuing structure of the metal, and then, by looking at the structure, to determine the material's susceptibility to cracking.
Research is being funded by the National Science Foundation. For more information, contact Olson at (906) 487-2191 or email [email protected].
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