Bidirectional Auxetic Structures Using Nitinol Shape Memory Alloys Sheet

Abstract

This paper introduces a simple manufacturing method for auxetic structures using nitinol shape memory alloy (SMA) sheets that deform by under load and recover their original shape by heat. Auxetics are structure or materials that have a negative Poisson's ratio. Auxetic structures are of kirigami-based designs that expand when stretched and contract when compressed, unlike most materials that behave in the opposite way. The proposed auxetic structure using SMA sheets can be easily manufactured by laser cutting machine and is functional immediately after the cutting process. This auxetic design allows for bidirectional motion as compressing or stretching according to the direction of the force. Then, by heating the auxetic structures using SMA sheets over As (Austenite start temperature), they start to recover their original shape by the intrinsic property of SMA. In the experiment, the auxetic structure exhibited an original shape recovery at a deformation rate of approximately 10% when heat was applied. At the maximum deformation rate, a load of approximately 3N was measured in the Martensite, while approximately 5N of load was measured in the Austenite. This concept is implemented in a robotic gripper that can grasp small objects.