To learn how tough a material is, engineers find its breaking point.

A recent study examined a transparent material, used in high-impact applications such as helicopter windshields, at the molecular level to measure its toughness. Researchers at the University of Illinois Urbana-Champaign and the University of California Irvine followed the crack along a grain boundary in magnesium aluminate spinel, or transparent aluminum, adding new data about this unique crystal-structured material.

“A company in Maryland was working with us on this crystalline material. When they changed their processing a bit to make it more transparent, simpler, or cheaper they got vastly different strengths and they didn't know why.  We thought there must be something happening at the grain boundaries because of changes in their processing,” said John Lambros, an aerospace engineering professor in The Grainger College of Engineering.

The research team cut out very small samples from a larger plate of the material to do bi-crystal experiments -- just two grains, one to the left and one to the right of a single grain boundary -- to study under very high magnification how a crack might grow.

Lambros said this method has been done before, but always for an opening fracture in which the boundary is pulled apart.

“Those results were being used universally. But grain boundary toughness depends on how you load it. Pulling it gives you one value. Shearing or sliding it gives you very different values. We developed both an opening and a shearing configuration, measuring each number independently. What we found makes a lot of physical sense. The shear toughness is much, much higher than the opening toughness.”

Lambros used a wooden chopstick as an example.

“To break it, you apply pressure at both ends and it snaps in the middle. You can’t break it by sliding it. You’d need a huge amount of force. We were able to measure that sort of bending versus. sliding force for the spinel material.”

Earlier, the team conducted an experiment using amorphous material. The molecules in materials like glass aren’t structured so they’re easier to work with. That study was a proof of concept that they could make precise optical measurements at a very small scale using a transmission electron microscope.

“The whole point of this project was to study the properties and grain boundary response in crystalline materials. At the molecular scale, crystalline materials are ordered, say in a cubic structure. The shear toughness number is the key result from this study.”

The magnesium aluminate spinel they studied is a substitute for traditional glass. It’s much stronger and tougher, more impact and thermal shock resistant. It’s often referred to as transparent aluminum and typically used in military applications, such as bullet-proof glass on vehicles.

“You can get the same or better performance in terms of ballistic penetration and thermal shock with much thinner and lighter sections.”

Lambros said in crystalline materials, boundaries make a big difference. Depending on the strength of the boundary, toughness and so on, it may fail at very different loadings. He said the most difficult aspect of this study was getting the grain sample right.

“I've done a lot of work on fracture mechanics and optical methods and my colleague at UC Irvine has done a lot of work on small scale using a transmission electron microscope. Finding the grain boundary so we could make the initial notch to start the crack was difficult. We cut a 2-millimeter by 2-millimeter slice, which had some markers on it that we could visualize in the focused ion bean or scanning electron microscopes to cut the sample. Because we used different instruments for the cut and to visualize it, we went back and forth a lot. There were many times when we would lose the grain boundary.

“Eventually we developed a scheme that worked.  We took one grain boundary and marched along it. Once we knew it could be done, the job is for someone else to repeat it 100 times at 100 different boundaries to get more statistics.”

Lambros said that at this stage, it’s not about processing tougher material. This is basic research.

“We haven’t solved all the problems needed for production. But because we did the experiment first, then modeled it in 3D, people who design new materials can use the experimental data as a starting point for industry-applied technology readiness levels.”

The work was done in the Advanced Materials Testing and Evaluation Laboratory at U. of I., for which Lambros is the director. Funding was provided by the National Science Foundation.

The study, “Grain boundary fracture of a magnesium alumniate spinel bi-crystal: Microscale experiments with varying mode mixity,” written by Yiguang Zhang, Ph.D. ’23, Shen J. Dillon and John Lambros, is published in the Journal of the American Ceramic Society. DOI: 10.1111/jace.70690