Designing instabilities in inhomogeneous soft auxetic structures

This monograph-styled thesis includes material published in two peer-reviewed articles, along with one article uploaded on arXiv, which is currently in preparation for submission. Each of these works constitutes a separate chapter. The primary objective of this thesis is to explore mechanical instabilities in inhomogeneous soft auxetic structures, with a specific emphasis on controlling the onset of wrinkles and their directions at desired locations. Auxetics, also known as negative Poisson’s ratio materials, exhibit expansion (or contraction) in all directions when stretched (or compressed), unlike conventional materials. This unique characteristic finds applications in various fields such as bioprosthetics, energy harvesting, flexible electronics, and sports helmets. Our study investigates the finite strain deformation behavior of auxetic structures under different loads, revealing complex surface patterns due to tailored distribution of material properties, essential for their practical applications. As a proof of concept, we investigate the wrinkling behavior of functionally-graded or inhomogeneous auxetic structures under various loading conditions. This includes (a) studying the wrinkling of thin and soft functionally-graded auxetic membranes under in-plane tensile loads, (b) designing instabilities in inflated circular and square auxetic membranes under uniform pressure loading, and (c) investigating the wrinkling instability of 3D isotropic auxetic bilayer systems under tension. Additionally, we present initial findings on the influence of loading direction in cylindrical bilayer systems, the effect of microstructural geometry on wrinkling instability in thin rectangular sheets, and experimental results on 3D printed thermoplastic polyurethane auxetic samples subjected to uniaxial tension. These findings are compared with our Finite Element simulations. In summary, we demonstrate that by tailoring the spatial inhomogeneities of the Young modulus and Poisson ratio along with the direction of loading, we can produce non-trivial wrinkling patterns at targeted locations in auxetic structures.

Funder

European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 956401

Publisher

University of Galway

Rights

Attribution-NonCommercial-NoDerivatives 4.0 International

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Collections

University of Galway Theses (PhD Theses)