Yigil Cho - Academia.edu (original) (raw)

Papers by Yigil Cho

Bulletin of the American Physical Society, Mar 2, 2015

Submitted for the MAR15 Meeting of The American Physical Society Designing 3D Structure by 5-7 Ki... more Submitted for the MAR15 Meeting of The American Physical Society Designing 3D Structure by 5-7 Kirigami 1 XINGTING GONG, YIGIL CHO, TOEN CASTLE, DANIEL SUSSMAN, RANDALL KAMIEN, University of Pennsylvania-The purpose of this talk is to explore how one can create 3D structures from 2D materials through the art of kirigami. Kirigami expands upon origami by allowing not only folds, but also cuts, into materials. If we take an incompressible material such as paper and remove a hole from it, the paper will buckle into the third dimension once that hole is sealed in order to relieve strain. Thus, orienting cuts and folds in certain places throughout a sheet of paper can influence its "pop-up," 3D structure. To narrow down the inverse design problem, we confined ourselves to making only one kind of cut (which we call the "5-7 cut") on a honeycomb grid, and we show how this single cut can give rise to arbitrarily complex three dimensional structures. A simple set of rules exists: (a) one 5-7 cut divides the material into 2 sections which can choose to pop-up or down independently of each other, (b) rows of uniform cuts must pop up or down in unison, giving (nearly) arbitrary 2D structure, and (c) the 5-7 cuts can be arranged in various ways to create 6 basic pop-up "modes," which can then be arranged to give (nearly) arbitrary 3D structure. These simple rules allow a framework for designing targeted 3D structure from an initial 2D sheet of material.

Acta Materialia, Sep 1, 2015

Metal-based cellular materials with periodic structures are currently the preferred choice as bon... more Metal-based cellular materials with periodic structures are currently the preferred choice as bone/ cartilage implants under load-bearing conditions due to their controlled pore interconnectivity and porosities. This report presents a new methodology for the structural analysis of periodic cellular materials using X-ray microtomography and dual-level finite element modeling (FEM). A three-dimensional (3D) structure of periodic titanium foam produced using selective laser melting (SLM) is obtained using an X-ray microtomography. A dual-level FEM based on the 3D structure is used to simulate the deformation behavior of titanium foam with a regular structure under uniaxial compression, and the computed results are compared directly with the interrupted uniaxial compression experiments performed on the deformed 3D structures. The deformation behaviors of simplified structures with cylindrical and hexahedral struts are simulated, and the computed results demonstrate that unavoidable defects in the actual structure affect the mechanical stability significantly. Additionally, buckling-induced deformation behavior is analyzed by introducing an imperfection in the actual and simplified structures. The effects of certain selected process variables, such as internal angle and diameter, are also examined through a series of process simulations.

Modelling and Simulation in Materials Science and Engineering, Aug 16, 2013

Advanced Materials, Oct 21, 2015

Figure 5. Finite element simulation of the contact process between various tapered epoxy nanostru... more Figure 5. Finite element simulation of the contact process between various tapered epoxy nanostructures and nanoindenter tip as a function of indentation depth. a) Pencil-like, b) stepwise, c) tall cone-shaped, and d) short cone-shaped structures. (Contour: von Mises stress.

Advanced Materials, Mar 23, 2015

Figure 4. FE simulation of pre-twisted kagome lattices with different pre-twisting angles under u... more Figure 4. FE simulation of pre-twisted kagome lattices with different pre-twisting angles under uniaxial compression. A-C) Mode I. D-F) Mode II. A,D) The original non-deformed structures for modeling. B,E) Stress-strain relationship. C,F) The corresponding evolution of the Poisson's ratio.

Advanced Functional Materials, Dec 16, 2013

Periodically structured materials, whose physical properties are functions of the structural para... more Periodically structured materials, whose physical properties are functions of the structural parameters, including shape, geometry, size, orientation and arrangement, are of wide interests for applications, such as controlling the light, sound or heat wave propagation, [ 1,2 ] wetting, [ 3,4 ] adhesion, [ 5,6 ] and cell sensing and proliferation. [ 7 ] In many applications, high aspect ratio (AR = height/width) structures are desired. For example, as plasma etching masks, they offer better etching resistance and structure fi delity. [ 8,9 ] As grating structures, high AR could lead to new properties, such as blazed transmission gratings via total external refl ection on the grating sidewalls for X-rays incident at graze angles. [ 10 ] As photonic crystals, high AR structures have higher intensity refl ection peak at the photonic stop band. [ 11 ] However, high AR structures are mechanically unstable. When the fi lm is developed in the lithographic process it tends to collapse due to capillary force [ 12-15 ] or to be buckled due to anisotropic swelling. [ 16,17 ] Specifi cally, it has been shown that one-dimensional (1D) structures can be laterally buckled into irregular two-dimensional (2D) wavy patterns due to compressive residual stress generated in the fi lm confi ned on a rigid substrate, for example, by deposition of a thin layer of metal or semiconductor, [ 18,19 ] or by swelling. [ 16 ] Because the compressive stresses induced by swelling and heating/ cooling are isotropic laterally, most of the buckling structures reported in literature are random. It will be intriguing to harness such instability for pattern transformation, specifi cally, to create highly ordered, high AR 2D wavy patterns by lateral buckling of 1D high AR structures, leading to very different physical properties. Here, we created highly ordered 2D nanowaves from a commonly used negative-tone photoresist SU-8 during two-beam holographic lithography (HL) by buckling of high AR (up to 6) 1D nanowalls (periodicity of 600 nm). During the development stage, the 1D pattern went through a constrained swelling in the good solvent, leading to the global buckling. The degree of lateral undulation could be controlled by tuning the pattern AR and exposure dosage. Different from literature, the nanowalls in our system were buckled in the same direction with long-range ordering. Between the nanowalls, interconnecting nanofi bers (30-50 nm in diameter) were formed between nanowalls when exposed to high dosages. By comparing experimental results with fi nite-element analysis, we confi rmed that nanofi bers formed only in the buckled fi lm when the neighboring walls were close enough; they prevented the recovery of the deformed nanowalls to their original state, thus, minimizing random instability after critical point drying. The nanowave structure showed weaker refl ecting color under an ambient light and lower transmittance compared to the nanowalls. Using double

Metallurgical and Materials Transactions, Sep 28, 2015

A three-dimensional (3D) thermo-mechanical model is developed considering the phase transformatio... more A three-dimensional (3D) thermo-mechanical model is developed considering the phase transformation occurring during the friction stir welding (FSW) of steel, and the simulated result is compared with both the measured temperature distribution during FSW and the microstructural changes after FSW. The austenite grain size (AGS) decreases significantly because of the frictional heat and severe plastic deformation generated during FSW, and the decreased AGS accelerates the diffusional phase transformation during FSW. The ferrite phase, one of the diffusional phases, is developed mainly in mild steel, whereas the bainite phase transformation occurs significantly in high-strength steel with large hardenability. Additionally, transformation-induced heat is observed mainly in the stir zone during FSW. The measured temperature distribution and phase fraction agree fairly well with the predicted data.

Permanent deformation that originates fro m transformation p lasticity has favorable aspects for ... more Permanent deformation that originates fro m transformation p lasticity has favorable aspects for steels with improved strength and ductility. However, it also causes undesirable deformation of products or specimens, lead ing to their degradation. This article rev iews recent investigations of transformat ion plasticity. A co mb ination of newly suggested models, nu merical analyses, and novel experiments has attempted to reveal the mechanism. Since the nature of the transformation p lasticity is still unclear, there are significant challenges still to be solved. Fundamental understanding of transformation plasticity will be essential for the develop ment of advanced steels.

$Research paper thumbnail of Engineering the shape and structure of materials by fractal cut$

Proceedings of the National Academy of Sciences of the United States of America, Nov 24, 2014

iScience

Highlights A defect-free single crystalline Au nanowire shows the Euler buckling deformation The ... more Highlights A defect-free single crystalline Au nanowire shows the Euler buckling deformation The buckling and postbuckling deformation is utilized for the electrical measurement Contact regime changes from the Sharvin to the Holm during buckling deformation This study suggests a nanoscale probe for the electrical and magnetic characterization

Advanced Analysis Center, Korea Institute o South Korea Department of Materials Science and Engin... more Advanced Analysis Center, Korea Institute o South Korea Department of Materials Science and Engin South Korea Materials Science and Engineering Program Texas 78712, USA. E-mail: gshwang@che.u 512 471 4847 High Temperature Energy Materials Resear Technology, Seoul 130-650, South Korea. E 958 5449; Tel: +82 2 958 6622 Institute of Physics and Applied Physics, Yo Department of Chemical Engineering, Unive USA † Electronic supplementary informa 10.1039/c4ra14953j ‡ These authors are contributed equally t Cite this: RSC Adv., 2015, 5, 17438

Materials & Design, 2016

A finite element model was developed to predict the deformation, temperature history, carbon diff... more A finite element model was developed to predict the deformation, temperature history, carbon diffusion, phase fraction, and hardness during the carburizing heat treatment of automotive annulus gear ring, initially made of a medium carbon steel. Carburizing gas with a constant carbon potential for entire surfaces of the gear was assumed. The temperature and pressure driven carbon diffusion was solved by the finite element simulation based on Fick's law. Both the diffusional and displacive phase transformations during the heat treatment were modeled incorporating the carbon concentration inside the gear. The constitutive equation of the transformation plasticity was incorporated into the finite element model. Strains due to the phase transformation, transformation plasticity, and thermal expansion/contraction were calculated by the finite element model. The prediction accuracy for the phase evolution, hardness distribution, and dimensional change of the gear ring was verified with the measurement data.

Metallurgical and Materials Transactions A, 2015

RSC Advances, 2015

Through a combined density functional theory andin situscanning electron microscopy study, we dem... more Through a combined density functional theory andin situscanning electron microscopy study, we demonstrate the ultrafast chemical lithiation of a singlec-Si nanowire brought into direct contact with Li metal in the absence of an electric field.

Soft matter, Jan 7, 2017

Thin films that exhibit spatially heterogeneous swelling often buckle into the third dimension to... more Thin films that exhibit spatially heterogeneous swelling often buckle into the third dimension to minimize stress. These effects, in turn, offer a promising strategy to fabricate complex three-dimensional structures from two-dimensional sheets. Here we employ surface topography as a new means to guide buckling of swollen polymer bilayer films and thereby control the morphology of resulting three-dimensional objects. Topographic patterns are created on poly(dimethylsiloxane) (PDMS) films selectively coated with a thin layer of non-swelling parylene on different sides of the patterned films. After swelling in an organic solvent, various structures are formed, including half-pipes, helical tubules, and ribbons. We demonstrate these effects and introduce a simple geometric model that qualitatively captures the relationship between surface topography and the resulting swollen film morphologies. The model's limitations are also examined.