The researchers created microstructures to shield a defect shaped like a rabbit. Illustrations by Paulino et al.
We often don’t realize the engineering that underpins both large structures like airplane windows and small yet essential components like engine cables.
These structures often need openings in their manufacturing, but the gaps can reduce structural integrity.
The standard methods of strengthening openings in structures aren’t flawless and can even cause new areas of stress.
This leads to the question: What if we could make these openings seem to disappear to the forces acting on the structure?
Engineers at Princeton and Georgia Institute of Technology have now created a new technique to achieve this.
“Think about a plate with a hole in it. If you put it under stress, if you pull on it, you are going to get a concentration of stress where the plate fails sooner than it would without the hole,” said Emily D. Sanders, an assistant professor of mechanical engineering at Georgia Tech and one of the authors.
“We want to design something around this hole, or defect, so it seems like the hole does not exist,” Sanders added.
The color scale indicates the density of the microstructures in the physical cloak. Red areas are densest, yellow are in the middle, and blue are the least dense. Paulino et al/Princeton University
Opening surrounded by microstructures
Traditional reinforcement is often designed to counteract particular types of stress or pressure that engineers anticipate the structure will face.
For example, a window frame might be built to withstand wind pressure from a certain direction or the general stress of the aircraft fuselage during flight.
This new technique actively changes the way forces flow through the material around the opening. It’s not literally filling the hole, but rather engineering the surrounding material in such a way that the forces acting on it behave as if the hole weren’t even there.
The research details how surrounding openings with carefully designed microstructures can offer protection against a multitude of loads – those external forces that cause stress, movement, or deformation.
The secret lies in the precise shape and orientation of these microstructures, calibrated to counteract the most challenging loads the structure might face. This allows engineers to address multiple stresses simultaneously.
“Any structure can potentially have an infinite number of loads. Every time you drive your car, the loads are different, the wind may blow in different directions, or the temperature may fluctuate,” said Glaucio Paulino, a principal author.
Knots in trees
The inspiration for this ingenious technique comes from an unlikely source: nature itself. The researchers were fascinated by knots in trees.
It appears that the microstructures within these knots naturally direct force around intrusions like branches or roots, allowing the tree to maintain its strength.
The team identified the specific loads that would pose the greatest threat to the structure.
The researchers discovered that by focusing on calculating just six to ten of these “worst-case” load scenarios, they could achieve effective results.
The team then ascertained the most efficient way to design and arrange the microstructures surrounding the opening to achieve the desired cloaking effect.
“The optimization technique introduced by the authors represents a breakthrough methodology for achieving the invisibility of a defect, irrespective of the direction of any externally applied force,” said Davide Bigoni, a professor at the Universita’ di Trento in Italy.
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“This results in omnidirectional cloaking, a property with broad applications. These include ensuring mechanical stress neutrality in organ tissue replacement, modifying structural elements to facilitate the passage of installations in machinery or civil infrastructure, and enhancing the restoration of artwork.”
The findings were published in the Proceedings of the National Academy of Sciences journal.
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ABOUT THE AUTHOR
Mrigakshi Dixit Mrigakshi is a science journalist who enjoys writing about space exploration, biology, and technological innovations. Her work has been featured in well-known publications including Nature India, Supercluster, The Weather Channel and Astronomy magazine. If you have pitches in mind, please do not hesitate to email her.
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