A continuous pleated birod for converting contractions into twisting through instability

The mechanical behavior of a structure made by joining two flexible flanges with a deformable pleated web, simply called continuous birod, is investigated. When axially loaded, the continuous birod shows an unconventional buckling phenomenon that couples axial contractions and helical twist, with a softening-hardening postcritical force–displacement response. In this work we propose an analytical model capable to predict the critical load and displacement of a continuous birod under axial compression. Our model does not contain any heuristic term: all the parameters have a clear physical-geometrical interpretation and can be easily identified. The obtained closed-form expressions for the critical load and displacement are in good agreement with experimental evidences and finite element simulations. The model can be exploited for the design of compliance and multistable devices for advanced applications.