Scientists 3D-print materials that stop vibrations cold

Now, scientists may be reaching a turning point in that gradual journey. Researchers from the University of Michigan and the Air Force Research Laboratory (AFRL) have demonstrated a way to 3D print intricate tubular structures whose unique internal geometry allows them to suppress vibrations in ways never seen in natural materials. These creations belong to a class known as mechanical metamaterials -- engineered substances with properties that come entirely from their design rather than their composition.

The ability to block or reduce vibrations could be valuable across many industries, from transportation to construction and beyond. The team's findings, published in Physical Review Applied, build on decades of theory and computer modeling to produce real-world structures that can passively disrupt vibrations traveling through them.

"That's where the real novelty is. We have the realization: We can actually make these things," said James McInerney, a research associate at the AFRL. McInerney was previously a postdoctoral fellow at U-M working with Xiaoming Mao, a professor of physics, who is also an author of the new study.

"We're optimistic these can be applied for good purposes. In this case, it's vibration isolation," McInerney said.

The project received partial funding from the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research, and also involved support from the U.S. National Research Council Research Associateship Program, administered by the National Academies of Sciences, Engineering and Medicine.

Contributors included Serife Tol, an associate professor of mechanical engineering at U-M; Othman Oudghiri-Idrissi of the University of Texas; and Carson Willey and Abigail Juhl of AFRL.

"For centuries, humans have improved materials by altering their chemistry. Our work builds on the field of metamaterials, where it is geometry -- rather than chemistry -- that gives rise to unusual and useful properties," Mao said. "These geometric principles can apply from the nanoscale to the macroscale, giving us extraordinary robustness."

Structural foundations

According to McInerney, the study brings together classical structural engineering, modern physics, and cutting-edge manufacturing tools such as 3D printing.

"There's a real probability that we're going to be able to manufacture materials from the ground up with crazy precision," he said. "The vision is that we're going to be able to create very specifically architectured materials and the question we're asking is, 'What can we do with that? How can we create new materials that are different from what we're used to using?'"

As Mao noted, the team is not altering a material's chemistry or molecular makeup. Instead, they are exploring how controlling shape and structure at a fine scale can produce new and advantageous mechanical properties.

In nature, this approach already exists. Human bones and plankton shells, for instance, use intricate geometries to gain remarkable strength and resilience from simple materials. With technologies like 3D printing, scientists can now replicate and enhance that natural design principle in metals, polymers, and other substances to achieve effects that were previously out of reach.

"The idea isn't that we're going to replace steel and plastics, but use them more effectively," McInerney said.

New-school meets old-school

While this work does rely on modern innovations, it has important historical underpinnings. For one, there's the work of the famous 19th century physicist, James Clerk Maxwell. Although he's best known for his work in electromagnetism and thermodynamics, he also dabbled in mechanics and developed useful design considerations for creating stable structures with repeating subunits called Maxwell lattices, McInerney said.

Another key concept behind the new study emerged in the latter half of the 20th century, as physicists found that interesting and perplexing behaviors emerged near the edges and boundaries of materials. This led to a new field of study, known as topology, that's still very active and working to explain these behaviors and to help capitalize on them in the real world.

"About a decade ago, there was a seminal publication that found out that Maxwell lattices can exhibit a topological phase," McInerney said.

Over the last several years, McInerney and colleagues have explored the implications of that study as they pertain to vibration isolation. The team has built up a model explaining that behavior and how to design a real object that would exhibit it. The team has now proved that its model is at its most advanced stage yet by actually making such objects with 3D printed nylon.

A cursory look at the structures reveals why making them previously was such a challenge. They resemble a chain-link fence that's been folded over and rolled up into a tube with a connected inner and outer layer. Physicists call these kagome tubes, a reference to traditional Japanese basket weaving that used similar patterns.

This is, however, just the first step in realizing the potential of such structures, McInerney said. For instance, the study also showed that the better a structure is at suppressing vibrations, the less weight it can support. That is a costly, potentially even unacceptable, tradeoff in terms of applications, but it highlights interesting opportunities and questions that remain at a fundamental level, he said.

As such novel structures are made, scientists and engineers are going to need to build new standards and approaches to test, characterize and assess them, which is a challenge that excites McInerney.

"Because we have such new behaviors, we're still uncovering not just the models, but the way that we would test them, the conclusions we would draw from the tests and how we would implement those conclusions into a design process," he said. "I think those are the questions that honestly need to be answered before we start answering questions about applications."