Dr Anna Lee

Postdoctoral Associate
Institute of Mechanical Engineering

École Polytechnique Fédérale de Lausanne

Anna Lee is a Postdoctoral Associate at the École Polytechnique Fédérale de Lausanne (EPFL), where she moved to on January 1st, 2018. She received her BS and MS both in Mechanical and Aerospace Engineering from the Seoul National University (SNU) in South Korea (2011 & 2013, respectively). She worked as a Research Associate at the Institute of Advanced Machinery and Design in SNU (2013-2014) and then earned her PhD in Mechanical Engineering from the Massachusetts Institute of Technology (2018). She is interested in the fluid-structure interaction of bird wings, focusing on the effects of their mechanical structure and topography on the aerodynamic performance. She is also interested in the buckling of thin shells, which was the topic of her PhD study. She is a member of the American Physical Society (APS) and the Korean Society of Mechanical Engineers (KSME).

Fabrication and Buckling of Thin Spherical Shells Containing Precise Geometric Imperfections

We revisit the classic problem of buckling of thin spherical shells under uniform pressure and explore the effect that geometric imperfections can have on their buckling behavior. Since the 1960s, numerous theoretical and computational studies have addressed the imperfection sensitivity of buckling of thin elastic shells. However, there is a lack of precise experiments to corroborate these predictions, especially for spherical shells, which is the central topic of this thesis.

 

First, we develop a novel fabrication technique to produce thin hemispherical elastic shells by the coating of spherical molds with a polymer solution. Upon curing the thin liquid film yields the elastic structure of nearly constant thickness. We experimentally investigate the drainage dynamics, the final thickness, and its uniformity. Our results are directly compared with theoretical and numerical analyses. Secondly, we study the buckling of spherical shells that contain a precisely engineered geometric imperfection. Our shell fabrication technique allows us to introduce a single dimple-like defect with controllable geometric properties. We systematically vary the amplitude and width of the defect, and then we present a quantitative relationship between the critical buckling pressure and the defect geometry. Our results can be predicted by both the finite element method and numerical simulations of a reduced shell theory model. Finally, we fabricate hemispherical bilayer shells containing a defect. To do so, we coat two different polymer solutions, layer by layer, onto the hemispherical molds containing a defect. We find that the bilayer shell can self-repair or self-aggravate the geometric imperfections due to residual swelling. Hence, the critical buckling pressure can be increased or decreased over time depending on the order of coating of each polymer layer. The fabrication technique and experimental results presented in this thesis open exciting new avenues in the study of the buckling of spherical shells, and we hope that it will instigate a resurgence of interest in this classic but important field of mechanics.