Mechanical engineers at UCLA and China’s Tsinghua University have opened a new path toward reusable energy-absorbing materials – ones designed to take an impact, then bounce back to their original shape and strength. The study was published in Advanced Functional Materials.

Currently energy-absorbing items – for example bicycle helmets, car bumpers and packaging – are designed only to protect from just one impact.

The researchers’ new reusable designs use a composition of solid individual units that are connected by stretchable bands or contained in a stretchable membrane. Such materials – ones with complex functions that are enabled by their physical design – are called metamaterials.

Lihua Jin“Conceptually, it’s like a matrix of balls held together by rubber bands, something you might even make at your desk with some office supplies,” said co-corresponding author Lihua Jin, assistant professor of mechanical and aerospace engineering, of one of the designs. “In this case, however, we’re using a combination of specially selected components and precision tuning the overall structure to withstand impacts, and then bounce right back.”

“During an impact, what’s happening is that the elastic parts hold the whole structure together and trap some of the energy from the impact” said Jin, who leads the Mechanics of Soft Materials Lab. “Meanwhile the solid components help dissipate the rest of the energy in the form of friction heat.”

The structure then can bounce back to its original shape and protection level.

The researchers developed three designs to demonstrate the concept.

In one design, they used a matrix of cylinders connected by elastic bands. In a second design, they connected a matrix of spheres with stretchable strings.  And in a third, they filled a stretchable membrane with spheres.

Each design could be “tuned” for impacts from certain directions. However, the third design performed exceptionally well in taking impacts from different directions.

Protective Material

While the designs used in the study were made by hand, the researchers suggest that new manufacturing techniques could make such structures with microscopic components. The materials could be made using traditional 3D printing, while the elastic parts from an emerging technique called digital light processing printing.

The other authors on the study were Kangjia Fu, a doctoral student at Tsinghua University in Beijing and visiting student at UCLA; and Zhihua Zhao, a professor at Tsinghua University. Zhao was the co-corresponding author.



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