Facoltà di Ingegneria Civile e Industriale

Usually to deform a structure we need to spend some energy. The energetic cost needed to obtain a unitary level of deformation is called stiffness: more stiffness means more energy needed to produce a given strain.
Professor Stefano Vidoli from DISG-ICI, in collaboration with colleagues from Sorbonne Université (Paris, France) applied suitable plastic deformations to a metallic thin disk and were able to produce a cylindrical shell having very peculiar properties.
In such a shell some deformation modes do have a vanishing stiffness. In particular, the energetic cost to change the curvature direction is vanishing, whilst altering the level of curvature does have a cost.
Once clamped on a shaker (see the Figure) and subjected to oscillations of given amplitude and feequency, this pre-stressed cylindrical shell shows a bizarre dynamical response. For instance, for a large range of frequencies and amplitudes, the response is a precession of the curvature axis having angular speed independent from the actual amplitude and multiple of the imposed frequency. In the Youtube video (linked below) we show this peculiar response. The reader is advised: the shell seems to rotate but it is not; since it is clamped at the center it is actually deforming in a continuous fashion varying its curvature direction.
Not only such a structure is relevant per se as dynamical system, but it could have many engineering applications as the dynamical response is, at least for some aspects, completely independent from the applied forcing.

More info: https://doi.org/10.1016/j.eml.2022.101755