Purdue University engineers are working with General Motors to develop an energy-absorbing material that could offer greater protection on the football field or in an earthquake zone.

PXCMs use a honeycomb architecture containing unit cells with multiple stable configurations. Image credit: Purdue University/Pablo Zavattieri.The phase transforming cellular materials (PXCMs) use a honeycomb architecture that contains unit cells with multiple stable configurations. The structures can flex and remain in position almost indefinitely, not unlike a flexing playing card (see video here).

"It has two stable positions," says Pablo Zavattieri, an associate professor at Purdue's Lyles School of Civil Engineering. "I push it and it goes to the other position. If you remove the force and the card returns to the original position, the mechanism is said to be meta-stable. Then you could combine many of these building blocks and have bi-stable or meta-stable materials, which gives us the flexibility to design these materials for specific needs."

He says that materials with meta-stable unit cells might be used for head protection in helmets. Materials with bi-stable unit cells could be incorporated into buildings to protect against earthquake damage.

The energy dissipation due to the mechanical behavior of the unit cells adds to the intrinsic energy dissipation of the base material, says staff researcher Nilesh Mankame, of the Smart Materials and Structures Group at the General Motors Global Research and Development Center.

"Many emerging materials like aluminum, magnesium and fiber-reinforced composites that play an increasingly important role in the transportation, defense and construction industries suffer from low intrinsic energy dissipation," he says. "The energy absorption capability of structures that are made of such base materials can be increased by incorporating PXCMs into the structures."

The work may be extended to a variety of geometric configurations such as cubes, tetrahedral or pentagonal for use in helmets. Like other phase-transforming materials such as shape-memory alloys, PXCMs could be controlled using heat or other external stimuli. This concept also could be used to make soft robots that are flexible and able to conform to various shapes.