Thursday, 25 June 2020

Speaker: Prof. Paolo Celli

Department of Civil Engineering, Stony Brook University, Stony Brook, USA

From patterned sheets to functional morphing structures

Abstract

Shape-changing mechanical systems are designed to predictably achieve large shape changes in response to applied loads. Their applications range from everyday objects like foldable chairs, to drug delivery capsules and to large-scale civil and space structures. Some of the recent efforts in this realm have been directed towards: (i) expanding the range of achievable shape changes; (ii) reducing the part count of these often-complex systems; (iii) exploring non-mechanical actuation strategies. Our first objective is to answer the curiosity-driven question of finding a simple strategy to design flat sheets that can transform into 3D surfaces with non-zero Gaussian curvature. After illustrating a method to achieve this goal via frustrated mechanisms, I will present the subsequent steps we took to try and bridge the gap between a strategy that is only bound to work at the tabletop scale using rubbery materials, and larger-scale structural applications. Along the way, I will make mechanistic considerations on how to construct metallic shape-changing structures by resorting to assemblies of twisted ribbons, and on how to leverage displacement-amplification mechanisms to create structures that morph in response to temperature changes.

Biography

Paolo Celli is an Assistant Professor in the Department of Civil Engineering at Stony Brook University. Prior to joining SBU in January 2020, he was a postdoc at Caltech. There, he worked in the lab of Chiara Daraio and collaborated extensively with NASA JPL’s Materials Development and Manufacturing Technology Group. Trained as a mechanical engineer in Italy, he obtained his PhD in Civil Engineering from the University of Minnesota in 2017. Paolo’s research interests are in solid mechanics, dynamics and vibrations, and smart structures. Using a combination of experiments and numerical models, his main goal is to develop structural systems with innovative properties and reconfigurable attributes, and to understand their mechanics.

Notes

by Marco Moscatelli.

With the final objective of producing three-dimensional structures with nonzero Gaussian curvature from initially flat configurations, elastic sheets made up of rubber and featuring different non-periodic cut patterns are produced by means of a laser cutting technique, experimentally tested and numerically analyzed.

The inhomogeneous distribution of strains, caused by an intentionally leverage geometric frustration, induces global buckling modes that make the sheets bend out of plane when subjected to tensile loads at some specific points of the boundaries.

The initial local behavior of these systems is well understood by considering pin-jointed truss analogs that undergo zero-energy mechanisms.

The idea of shape changing structures is extended to materials relevant to engineering applications by using metallic glass ribbons. These components are pre-twisted into structural elements with regions that behave as compliant hinges with different preferred bending axes. Their final configuration is selected by means of numerical and analytical models. The chosen shape is then employed as a building block for three-dimensional structures, that are finally tested.

The thermo-mechanical behavior of metallic components is exploited to create displacement amplifying units that can experience large deformations in response to thermal stimuli.