Mechanically Tunable Negative Stiffness Metamaterial

by | Apr 24, 2019

The computational design, fabrication, and experimental evaluation reported is the first dynamic demonstration of a mechanically tunable negative stiffness metamaterial.

The invention of the wheel has been touted as one of the greatest human achievements because it revolutionized transportation and the movement of goods and resources. Taking a ride on the first carts with wooden wheels would be an unpleasant experience, though, because every bump on the road would be transmitted directly to the passenger. This is why current metal wheels have rubber tires which function in part to reduce unwanted vibrations.

The wheel of a car highlights that many engineering systems require two specialized components; 1) a stiff, structural component like the metal rim and; 2) a damping component to reduce vibrations like the rubber.

Ideally, to reduce weight and eliminate parts, a single component should be developed, but there is a natural trade-off between stiffness and damping for standard materials.

Metamaterials, which derive their properties from their structure rather than their material composition, have been proposed and developed to overcome this longstanding design trade-off.

Previous metamaterials aimed at achieving preferable combinations of stiffness and damping, however, operate only within a small range of frequencies, only display their behavior when subjected to large amplitude disturbances, or require a precise high-temperature environment.

Recently, researchers at the University of Texas at Austin and Lawrence Livermore National Laboratory have developed a new mechanical metamaterial that overcomes this material trade-off for a broad range of frequencies, small disturbances, and room temperatures.

They used microstereolithography, an emerging 3D printing technology, to manufacture structures with micron-scale features that exhibit negative stiffness or snap-through effects. By carefully designing and embedding a small volume fraction of these structures within a polyurethane elastomer, they drastically increased the loss of the metamaterial without reducing the stiffness.

Furthermore, they can adjust the material properties by carefully pre-straining or compressing the bulk material.

This work is the first demonstration of a purely mechanical metamaterial that increases the damping of its base material while maintaining its mechanical stiffness.

“Ultimately, this novel demonstration could motivate metallic metamaterials that realize high levels of stiffness and loss simultaneously, thereby eliminating the need for dampers in many structural applications,” explains Dr. Clinton Morris, a former graduate student at the University of Texas at Austin.

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