By Daniel Reasa
LEGO cylinders on a plate form a simplified model of a forest, and Prof. Stefano Gonella has experimentally shown that physical waves (analogous to earthquakes) can be guided along complex paths created with these LEGO structures. Gonella, Associate Professor in the Department of Civil, Environmental, and Geo-Engineering at the University of Minnesota, covered this in his talk in the Mechanics Seminar Series at UW-Madison, titled “Mechanics of Inter-Modal Tunneling in Nonlinear Metamaterials.”
Gonella’s research focuses on metamaterials, a class of materials that are designed to exhibit unusual properties as compared to something like a solid block of metal. These materials are often made of repeating unit cells, which in this case is a forest of LEGO cylinders on a plate. The cylinders don’t make the plate any stiffer, however, under dynamic loading the extra mass of the cylinders effects how waves propagate through the plate.
By adjusting the size of the cylinders, Gonella showed that he can create metamaterials that attenuate different wave frequencies. The wave attenuation can be spatially design by changing the cylinder arrangement on the base plate. Experimentally, Gonella created these structures out of cylindrical LEGO pieces on a flat LEGO baseplate. Stacking multiple cylindrical bricks allowed for the spectral (frequency) tuning of the setup, and spatial tuning was achieved by leaving paths without cylinders, creating undamped channels for waves energy to propagate.
Gonella excited this setup with a shaker, and measured the resultant vibrations using laser vibrometers that measured the microscopic motion of the plate. Real world structures won’t be built out of LEGO pieces, but the results confirmed both the spectral and spatial damping properties of the metamaterial, as waves only existed in the undamped areas at the predicted frequencies. In the end, this was a strong proof of concept for a versatile experimental setup showing that wave energy at desired frequencies can be concentrated in desired locations through the design of the metamaterial.