A car that dashes against an obstacle suffers damages, first to its fenders. There is a keen interest to develop materials that can be sandwiched in the fender system which will absorb the shock and prevent the interiors from being damaged. Origami metamaterials that crumple rather than tear, and take the impact, can play an important role in such situations. Researchers from Indian Institute of Technology Madras have developed such a material, which could have many such uses.
Poisson ratio
When you crush or stretch a material along a particular direction, it undergoes a modification in the perpendicular, or lateral, direction. For example, take a clay cube and compress it along one face, it will then bulge out in the sides. The ratio between the deformation along the force and the deformation in a direction lateral to the force is called the Poisson ratio. The Poisson ratio can be positive or negative. While, as in the example of the clay cube, we can easily visualise a material with a positive Poisson ratio, it is somewhat counter-intuitive to consider a material with a negative Poisson ratio. In fact, there is a lot of interest in such materials – they are called auxetics. One uses of auxetic materials is in lining the soles of sports shoes, where it offers better support when running or jumping. “If we try to crush or impact an auxetic material, it offers resistance to the crushing load as the material below will try to contract inwards, making it ‘denser’ and therefore, preventing the crushing load from moving further into the material,” says Phanisri Pradeep Pratapa from the Structural Engineering Laboratory of the Department of Civil Engineering at the Indian Institute of Technology, Madras.
In order to be useful, materials need to maintain a constant Poisson ratio when they crumble under pressure. However, they are prone not to do so, and the Poisson ratio varies as they deform. In the last decade, scientists have developed materials with constant Poisson ratio under pressure. But these are soft materials, which again limits their usability in preventing damage during an accident or impact, for instance.
Using origami
Into this scenario enter a special class of materials called origami metamaterials. These combine the Japanese art of paper folding (origami) and the existing material of choice and fold it to obtain desired properties.
Dr. Pratapa, with his PhD student, has developed a special class of origami metamaterials which show a constant value of Poisson Ratio when subjected to stress. “The origami metamaterials we have developed are mechanism-based systems. These are manufactured by joining panels along their edges to form ‘creases’ about which the structure locally ‘folds’ or rotates about,” says Dr. Pratapa. The benefit is that the observed property does not depend on whether it is made from a sheet of paper, polymer or metal. What matters is that under impact the sheet folds up along the creases.
Morph cell
According to a paper published in Journal of Engineering Mechanics the material the researchers have developed has a “nearly constant Poisson function in the range –0.5 to 1.2 over a finite stretch of up to 3.0 with a minimum of 1.1.” An idea that the researchers had, played a crucial role in developing this origami metamaterial.
The crux of the idea is a unit cell called Morph that Dr. Pratapa and collaborators developed earlier. “This cell can transform into two contrasting geometries. One which exhibits positive Poisson ratio and the other which exhibits negative Poisson ratio,” explains Dr. Pratapa. It is possible to combine these two geometries to join and deform together as a single system, by joining them along their edges. This is what made it possible for the researchers to develop a material which showed a constant Poisson ratio when stress was applied. When the Morph cell undergoes folding, it attains two distinct configurations that look different, but have the boundaries in such a way that they can be combined without restricting its natural folding behaviour, he explains.
