The ancient Japanese art of origami that uses strategic folding has found many interesting technological applications such as the Miura-Ori method for folding/unfolding antennas in satellites. Perhaps less known is kirigami where, in addition to folding, cutting is also allowed. Kirigami has also been a subject of intense scientific investigation, not least because several natural systems such as bird wings have periodic polyhedral designs that could easily be reproduced using kirigami techniques.
Robin Neville, Fabrizio Scarpa, and Alberto Pirrera, from the University of Bristol, UK, report how a class of kirigami cellular materials show large shape and volume deformations that could find potential applications in many shape-morphing materials. The researchers start with a flat sheet of poly ether ether ketone—a thermoplastic polymer. A pattern of slits is cut into the sheet, which is then corrugated and folded repeatedly to give a honeycomb architecture. The ease of this method allows the process to be applied to many starting materials and to be automated. By varying a small number of initial parameters such as the folding angle and width of the slit, for example, different kinds of honeycombs can be designed.
A kirigami honeycomb material in different stages of evolution. Credit: Robin M. Neville, Fabrizio Scarpa, and Alberto Pirrera.
Using complex numerical analysis and finite element methods, the group was able to extract mechanical information such as the Poisson’s ratio of the cellular material. It was found that if the stiffness of the connection between two cells—modeled as a hinge connector and hence called “hinge stiffness”—is much greater than that of the sheet, the cell walls bend in response to stress. If the material of the sheet is stiffer than the connection, the honeycomb material bends at the folds.
Interestingly, the researchers found that the kirigami honeycomb material shows a “Poisson’s switch.” This refers to the observation that on either side of a critical folding angle, the Poisson’s ratio of the material switches and shows the opposite sign. The researchers reported in a recent issue of Scientific Reports (doi:10.1038/srep31067) that by using smart materials, the fold angle could be manipulated to expand or contract the honeycomb. “Experimental verification of the predicted switch between negative and positive Poisson’s ratio over relatively small changes in fold angle argues well for practical use of this approach in ingenious aerospace applications,” says Anselm Griffin, a professor at Georgia Institute of Technology. Others in the field have commented positively on this work. Daniel Inman from the University of Michigan says, “Morphing has game-changing possibilities from automotive to aircraft and even civil structures. The work is significant as it brings new possibilities to the mechanism side of shape-changing structures allowing many new designs to be considered. As advanced manufacturing moves from polymers to metals, the impact of this work is even greater.”
