Analysis of Soft Kirigami Unit Cells for Tunable Stiffness Architectures

The enhancement of stiffness in many soft actuators and architectures is required to produce a large blocking force or to achieve multidimensional deformations. This paper introduces a new method exploiting Kirigami inspired kinematic architecture. Usually, Kirigami structures are made of rigid materials (i.e., thin sheets). Instead, in this study we investigate Kirigami unit cells made of soft material. To validate this approach, different unit cells, having aspect ratio (w/l) ranging from 0.25 to 1 and wall thickness ratio (t/l) ranging from 0.03 to 0.12, are investigated by means of FEM simulations. A fatigue analysis is also carried out to study unit cell reliability for the imposed load. Results show that a wall thickness ratio over 0.08 enhances the axial stiffness rapidly with a good structure stability and a high design life, while with lower wall thickness the maximum strain can be increased. Then, a trade-off of these two parameters can be found in order to tune the mechanical characteristics of the structure (i.e., large deformation ratio with a relevant axial stiffness, high design life), according to the specific application. For instance, with an aspect ratio of 0.5 and wall thickness ratio of 0.09, the axial stiffness is 0.86 N/mm, and the maximum strain is 300%. Noticeably, this approach presents a design strategy for implementing multidimensional motions in complex architectures. Indeed, arranging the unit cells with different aspect ratios would allow the desired behavior (bending, torsional, etc.), while the different wall thicknesses can ensure the desired stiffness.