A team of engineers from Princeton University and the Georgia Institute of Technology has developed a technique that can maintain the structural integrity around openings by essentially hiding them from the surrounding forces.
Whether designing a window in an airliner or a cable conduit for an engine, manufacturers devote a lot of effort to reinforcing openings for structural integrity. But the reinforcement is rarely perfect and often creates structural weaknesses elsewhere.
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Rather than reinforcing the opening to protect against a few select forces, the new approach reorganises nearly any set of forces that could affect the surrounding material to avoid the opening.
The researchers surrounded openings with microstructures designed to protect against many different loads. The microstructures’ shape and orientation are calibrated to work with the most challenging loads that the structure faces, allowing designers to counter multiple stresses at once.
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‘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. ‘We want to design something around this hole, or defect, so it seems like the hole does not exist.’
Glaucio Paulino, the Margareta Engman Augustine professor of engineering at Princeton, said that designers typically reinforce the structure at openings such as windows or tunnels. But, he said, by increasing structural strength in one direction, reinforcement can introduce other problems by creating new stress in a different direction. The goal of the cloaking technique is to protect the structure by redirecting the force without creating new or undesirable stress levels.
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The researchers were inspired by knots in trees, in which it seems as though microstructures direct force around the site of intrusions such as branches or roots and maintain structural strength. The researchers wanted to know if they could engineer structures to do the same thing in manufactured materials.
Paulino said that the technique relies on two optimisation problems that are designed to select the best solutions from a range of choices. The first problem uncovers the loads that will produce the greatest challenge to the object’s structure. This is more challenging than it sounds because loads on a structure or a machine can change with circumstances.
‘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,’ Paulino said.
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The researchers found that calculating six to ten of the worst-case loads for a structure yields the most effective results. With that information, they run a second optimisation problem to find the most effective way to create and deploy microstructures surrounding the window or conduit.
‘The optimisation 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 of solid and structural mechanics at the Universita’ di Trento in Italy. ‘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.’
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The idea is similar to cloaking techniques that have been developed to hide objects on the electromagnetic spectrum, such as stealth aircraft. Paulino explained that the equations for solid material can be more challenging than those for electromagnetism, but the goal is the same. ‘Any elastic disturbance is hidden by the cloak,’ he said. ‘It is like it does not exist.’
The research has been published in the Proceedings of the National Academy of Sciences.