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Adaptive phononic crystals produce vibration-damping sheet metal that never rattles

EMPA Swiss Federal Laboratories For Materials Science And Technology) : 10 March, 2014  (Technical Article)
Researchers from Empa and ETH Zurich have succeeded in producing a prototype of a vibration-damping material that is not only able to damp vibrations completely, but it can also specifically conduct certain frequencies further. Although the "programmable material’ still only works in a one-dimensional model construction, it has already demonstrated its unusual capabilities.
Metamaterials are artificial media structured on a size scale smaller than the wavelength of external stimuli. Materials of interest exhibit properties not found in nature, such as negative index of refraction. They are cellular assemblies of multiple elements fashioned from materials including metals and plastics, arranged in periodic patterns. Metamaterials gain their properties not from their constituents, but from their exactingly-designed structures. Their precise shape, geometry, size, orientation and arrangement can affect light or sound in a manner that is unachievable with conventional materials.
Such a “metamaterial” could fundamentally revolutionise mechanical engineering and plant construction. Until now, the vibration properties are determined by the selection of material and the geometry of the part. In future, the material could react to current vibration readings and adapt its vibration properties at lightning speed.
The working model used by the researchers consists of a 1m x 1cm aluminium plate that is 1mm thick. This sheet-metal strip can vibrate at different frequencies. In order to control the wave propagation, ten small aluminium cylinders (7mm thick, 1 cm high) are attached to the metal. Between the sheet and the cylinders sit piezo discs, which can be stimulated electronically and change their thickness in a flash. This ultimately enables the team headed by project supervisor Andrea Bergamini to control exactly whether and how waves are allowed to propagate in the sheet-metal strip. The aluminium strip thus turns into a so-called adaptive phononic crystal – a material with adaptable properties.  
Adaptation in fractions of a second
The piezo controls can now be set in such a way that waves are able to propagate through the sheet-metal strip “perfectly normally”, as though no aluminium cylinders were attached to it. Another configuration enables a certain frequency spectrum of the waves to be absorbed. And this muffling is variable, as the piezo elements can alter their elastic properties electronically in fractions of a second – from low to high stiffness.
Bergamini explains what could develop from the research results: “Imagine you produce a sheet of metal, imprinted with an electronic circuit and small piezo elements at regular intervals. This sheet metal could be programmed electronically to block a certain vibration frequency. The interesting thing is that even if you cut off part of the sheet, the waves in the cropped section would largely spread in the same way as in the initial piece. This method could be used on three-dimensional components."
Further research
During the Phononic Crystal with Adaptive Connectivity research project, Empa-researcher Bergamini collaborated with Paolo Ermanni’s group at ETH Zurich and Massimo Ruzzene from the Georgia Institute of Technology. In a follow-up project, the programmability of the prototype is to be expanded: “Until now, every piezo element has reacted to vibrations alone, independent of its neighbour,” explains Bergamini. “As the next step, we want to interconnect the elements with each other to be able to control them jointly or in a coordinated fashion.”
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