The technique, described today in an open-access paper published by Science Advances, takes advantage of the stress-relaxing effect of folding creases in paper and other materials, geekwire.com reported.
“If you were wearing a football helmet made of this material and something hit the helmet, you’d never feel that hit on your head. By the time the energy reaches you, it’s no longer pushing. It’s pulling,” senior author Jinkyu Yang, associate professor of aeronautics and astronautics at the University of Washington (UW), said in a news release.
That’s not to say that future football helmets will be made of paper. But the engineering principles that Yang and his colleagues tested with paper models could well be translated into new types of shock-absorbing structures for rocket landing legs, automotive vehicles and other applications.
The team investigated how metamaterials could be structured to respond in novel ways to an impact force.
“Metamaterials are like Legos. You can make all types of structures by repeating a single type of building block, or unit cell as we call it,” Yang explained.
“Depending on how you design your unit cell, you can create a material with unique mechanical properties that are unprecedented in nature.”
For their experiments, the researchers came up with a variety of patterns that could be laser-cut out of paper and assembled into origami-type unit cells.
“Origami is great for realizing the unit cell,” said coauthor Yasuhiro Miyazawa, a UW aeronautics and astronautics doctoral student.
“By changing where we introduce creases into flat materials, we can design materials that exhibit different degrees of stiffness when they fold and unfold. Here we’ve created a unit cell that softens the force it feels when someone pushes on it, and it accentuates the tension that follows as the cell returns to its normal shape.”
The researchers assembled a chain consisting of 20 of the unit cells, stacked one after the other. Then they pushed on one end of the chain, and tracked the effect with six GoPro cameras.
What the researchers recorded might sound counterintuitive: At first, the unit cells passed along the compression wave. But as the wave progressed, the force of the push was absorbed by the creases in the paper, and translated into an opposing pushback when the creases returned to normal.
The original force of the compression wave never made it down to the other end of the chain. Instead, it was replaced by the tension force associated with the pushback. So instead of registering a ‘push’ on the far end of the chain, the researchers registered a ‘pull’.
“Impact is a problem we encounter on a daily basis, and our system provides a completely new approach to reducing its effects. For example, we’d like to use it to help both people and cars fare better in car accidents,” Yang said.
“Right now it’s made out of paper, but we plan to make it out of a composite material. Ideally, we could optimize the material for each specific application.”