Metamaterial undermines 250-year-old construction principles
Researchers from FOM Institute AMOLF, Leiden University and Harvard University made a rubber beam that bends faster when subjected to less pressure. They published their work on 21 July online in Physical Review Letters.
Pattern of holes
The beam is made from a metamaterial, which gives it the name metabeam. Metamaterials have special properties that do not occur in nature. By providing the metabeam with a carefully chosen pattern of small holes, the researchers managed to cause the strange behaviour. The researchers noticed that at a critical pressure, there is a tipping point at which a self-reinforcement effect occurs: the beam continues to bend further even though the pressure decreases. Group leader Martin van Hecke: "It is just like pushing a car. You expect that you have to push harder to make the car go faster, but here we have a situation in which the car goes faster and faster even though you are pushing less."
The researchers investigated the cause of this self-reinforcement effect. They discovered that the metabeam under a slight pressure compresses easily but stretches with difficulty. In normal materials without holes the difference between compression and stretching only plays a role at high pressures. This sensitivity to the difference between compression and stretching causes the strange effect during the bending of the metabeam. The shape and position of the holes accurately determine the pressure at which this effect occurs. The researchers can therefore specifically tailor the properties of the material by changing the pattern of holes.
Old construction principles
The arrival of the metabeam has undermined centuries-old construction principles. The relationship between beam bending and pressure was established 250 years ago by Leonhard Euler in his universal law about elastic instabilities of beams. Since then, his theory has formed the basis for the construction of houses and bridges. Euler assumed that there is no difference between stretching and compression. The metabeam defies this assumption and with that opens the door for new developments. Group leader Martin van Hecke: 'Bridges will not be constructed using the metabeams. However, it is conceivable that we can make a robot arm that has smart bending build in thanks to mechanical switches based on this material.'
Discontinuous Buckling of Wide Beams and Metabeams
Corentin Coulais, Johannes T. B. Overvelde, Luuk A. Lubbers, Katia Bertoldi en Martin van Hecke, Physical Review Letters, 21 July 2015