“Brittility Factor” Explains How Soft Materials Fail Under Stress

Understanding what makes a soft material fail when stressed is a key part of addressing major engineering challenges, such as landslide prevention or creating better industrial materials.

Now, a new study from researchers at the University of Illinois at Urbana-Champaign is transforming how scientists speak about these soft materials. By linking a spectrum of other soft material behaviors that were previously thought to be unrelated, the team has identified a new parameter – the “brittility” factor – which they say can help simplify soft material failure models.

Redefining material failure

The study of soft materials is an important part of engineering due to the knock-on effects for industrial, environmental and biomedical applications. Despite its importance, researchers Simon Rogers, an associate professor of chemical and biomolecular engineering at the University of Illinois Urbana-Champaign, and Krutarth Kamani, a graduate student in the university’s Department of Chemical and Biomolecular Engineering, noticed that there was a significant communication breakdown occurring between scientists working in this field.

“At a recent conference, we realized that all of us who study soft materials from all over Europe and North America couldn’t agree what the connection is between brittle and ductile behavior nor how to define it,” the researchers recalled.

When soft materials – synthetic or natural – begin to deform under pressure, they eventually reach a point where they either spring back to their original form like a rubber band, or undergo permanent deformation, like that rubber band snapping. This is known as the material’s “yielding” behavior. If a material has a slow, gradual yielding transition then it is called a “ductile” material, whereas a material with a very abrupt yielding transition is deemed “brittle”.

In a break from this traditional dichotomy, Rogers and Kamani argue that soft materials should not be viewed as being either strictly brittle or ductile. Rather, scientists should consider a wider spectrum of yielding behaviors.

Introducing the brittility factor

In their new study, the researchers show that it is possible to account for the different spectrum of yielding behaviors exhibited by real-world materials in a continuum model by introducing a new parameter – the brittility factor. The higher the brittility factor, the less a soft material will deform permanently before it yields.

“We didn’t expect this study to explain as much as it does,” said Rogers. “What we ended up with was a way to bring a whole bunch of soft material behaviors together under the same physics umbrella. Previously, they’d been studied independently or maybe all been applied simultaneously, but never thought of as being physically or mathematically connected.”

“This single parameter amazingly connects so many puzzling observations researchers have come across over the years,” Kamani added.

The team’s model was also tested by comparing it to the results of real-world rheometry tests conducted on an array of different soft materials, indicating the general applicability of the brittility factor. Kamani and Rogers believe that the brittility factor could be a simple tool for understanding the yielding transition of different materials under various stresses and strains, and could even explain why some materials are more resistant to rapid yielding than others – a question that has perplexed materials scientists for decades.

“This work marks the point at which we are approaching the crest of the hill in understanding soft materials behavior,” Rogers concluded. “We’ve always felt like each step takes us higher, but with no end in sight. Now we can see the top of the hill, and we are closer to the top and free to move forward in whatever direction we would like.”

Reference: Kamani KM, Rogers SA. Brittle and ductile yielding in soft materials. Proc Natl Acad Sci USA. 2024;121(22):e2401409121. doi: 10.1073/pnas.2401409121

This article is a rework of a press release issued by the University of Illinois Urbana-Champaign. Material has been edited for length and content.