Researchers butter up the old ‘scratch test’ to make it tough

May 26, 2011

By Denise Brehm
Civil & Environmental Engineering

It might not seem like scraping the top of a cold stick of butter with a knife could be a scientific test, but engineers at MIT say the process is very similar to the “scratch test,” which is perhaps the oldest known way to assess a material’s hardness and strength — or, in scientific language, its resistance to deformation.

Using the scraping of butter as a starting point, the engineers launched a study to see if the age-old scratch test could be used to determine a material’s toughness, or how well it resists fracturing after a small crack has already formed. The answer: The scratch test is indeed measuring crack resistance rather than strength and is valid on material samples of any size. This means that engineers now have a simple “new” test for assessing a material’s fracture properties.

“Fracture mechanics has not reached the same level of pervasiveness in most engineering practice as strength theories, and this is due to the fact that it is difficult to determine fracture properties of materials, from soft clay to hard concrete,” says Franz-Josef Ulm, the George Macomber Professor of Civil and Environmental Engineering (CEE) at MIT. “The test which we propose here is just this: a straightforward test for the engineering practice.”

In a paper in Physical Review Letters that appeared online May 20, co-authors Ulm; Pedro Reis, the Esther and Harold E. Edgerton Assistant Professor of Civil and Environmental Engineering and Mechanical Engineering; and CEE graduate student Ange-Therese Akono — who is first author on the paper — describe their research and findings.

They performed laboratory scratch tests on paraffin wax, which is similar to butter but more stable at room temperature, Reis says, and used theory and mathematics to pare the process down to its essential components. They then created a mathematical model of the entire physical “scratch” process, which shows that the area of contact