Seung Tae Choi

On the unified approach to anisotropic and isotropic elasticity for singularity, interface and crack in dissimilar media

Abstract Proposed in this paper is the equivalence between anisotropic and isotropic elasticity for two-dimensional deformation under certain conditions. That is, the isotropic elasticity can be reconstructed in the same framework of the anisotropic elasticity, when the interface between dissimilar media lies along a straight line. Therefore, many known solutions for an anisotropic bimaterial are valid for a bimaterial, of which one or both of the constituent materials are isotropic. The usefulness of the equivalence is that the solutions for singularities and cracks in an anisotropic/isotropic bimaterial can easily be obtained without solving the boundary value problems directly. The interaction solutions of singularities, interfaces, and cracks in infinite anisotropic bimaterial are summarized, to be used for the cases of isotropic/isotropic and anisotropic/isotropic bimaterials. Conservation integrals also have the similar analogy between anisotropic and isotropic elasticity so that J integral and J-based mutual integral M are expressed in the same complex forms for anisotropic and isotropic materials, when both end points of the integration paths are on the straight interface. The use of J and M integrals together with the present equivalence are exemplified to obtain energy release rate, stress intensity factors, and T-stresses of interfacial cracks lying in the interface of anisotropic/anisotropic, isotropic/isotropic, or anisotropic/isotropic solids.

Opto-mechanical analysis of nonlinear elastomer membrane deformation under hydraulic pressure for variable-focus liquid-filled microlenses.

Nonlinear large deformation of a transparent elastomer membrane under hydraulic pressure was analyzed to investigate its optical performance for a variable-focus liquid-filled membrane microlens. In most membrane microlenses, actuators control the hydraulic pressure of optical fluid so that the elastomer membrane together with the internal optical fluid changes its shape, which alters the light path of the microlens to adapt its optical power. A fluid-structure interaction simulation was performed to estimate the transient behavior of the microlens under the operation of electroactive polymer actuators, demonstrating that the viscosity of the optical fluid successfully stabilizes the fluctuations within a fairly short period of time during dynamic operations. Axisymmetric nonlinear plate theory was used to calculate the deformation profile of the membrane under hydrostatic pressure, with which optical characteristics of the membrane microlens were estimated. The effects of gravitation and viscoelastic behavior of the elastomer membrane on the optical performance of the membrane microlens were also evaluated with finite element analysis.

Study on residual stress in viscoelastic thin film using curvature measurement method

Using LSM (laser scanning method), the radius of curvature due to thermal deformation in polyimide film coated on Si substrate is measured. Since the polyimide film shows viscoelastic behavior, i.e., the modulus and deformation of the film vary with time and temperature, we estimate the relaxation modulus and the residual stresses of the polyimide film by measuring the radius of curvature and subsequently by performing viscoelastic analysis. The residual stresses relax by an amount of 10% at 100°C and 20% at 150°C for two hours.

Measurement of time-dependent adhesion between a polymer film and a flat indenter tip

We revisited an elasticity problem of flat indentation on an elastic film bonded to a rigid substrate and obtained the force–depth relation in a simple form. With the obtained force–depth relation, Kendalls elastic equilibrium theory of adhesion was extended to the adhesion between a flat tip and a compressible elastic film. Thus, the thermodynamic work of adhesion at the moment of debonding of a flat tip from an elastic film was expressed in terms of pull-off force, elastic constants and geometric parameters. It is worth noting that the obtained relation for elastic films is still valid for viscoelastic films if viscoelastic losses are limited to the process zone of debonding. This makes it possible to study the time-dependent adhesion of viscoelastic polymer films. Indentation experiments with a flat diamond tip were performed on SU-8 films, and the results verified that the extended form of Kendalls theory correctly compensates the effect of the finite thickness of the films on the work of adhesion. The indentation results also showed that the work of adhesion is strongly dependent on the unloading velocity of the tip, while indentation depth and dwell time have only minor effects on the work of adhesion.

Laser-Induced Particle Adsorption on Atomically Thin MoS2.

Atomically thin molybdenum disulfide (MoS2) shows great potential for use in nanodevices because of its remarkable electronic, optoelectronic, and mechanical properties. These material properties are often dependent on the thickness or the number of layers, and hence Raman spectroscopy is widely used to characterize the thickness of atomically thin MoS2 due to the sensitivity of the vibrational spectrum to thickness. However, the lasers used in Raman spectroscopy can increase the local surface temperature and eventually damage the upper layers of the MoS2, thereby changing the aforementioned material properties. In this work, the effects of lasers on the topography and material properties of atomically thin MoS2 were systematically investigated using Raman spectroscopy and atomic force microscopy. In detail, friction force microscopy was used to study the friction characteristics of atomically thin MoS2 as a function of laser powers from 0.5 to 20 mW and number of layers from 1 to 3. It was found that particles formed on the top surface of the atomically thin MoS2 due to laser-induced thermal effects. The degree of particle formation increased as the laser power increased, prior to the thinning of the atomically thin MoS2. In addition, the degree of particle formation increased as the number of MoS2 layers increased, which suggests that the thermal behavior of the supported MoS2 may differ depending on the number of layers. The particles likely originated from the atmosphere due to laser-induced heating, but could be eliminated via appropriate laser powers and exposure times, which were determined experimentally. The outcomes of this work indicate that thermal management is crucial in the design of reliable nanoscale devices based on atomically thin MoS2.

Stress intensity factors and kink angle of a crack interacting with a circular inclusion under remote mechanical and thermal loadings

A problem of a circular elastic inhomogeneity interacting with a crack under uniform loadings (mechanical tension and heat flux at infinity) is solved. The singular integral equations for edge and temperature dislocation distribution functions are constructed and solved numerically, to obtain the stress intensity factors. The effects of the material property ratio on the stress intensity factor (SIF) are investigated. The computed SIFs are used to predict the kink angle of the crack when the crack grows.

Pressure-induced relaxor-to-ferroelectric crossover in vinylidene fluoride relaxor terpolymer: a possible explanation to the high performance of the terpolymer nanocomposites

Understanding ferroelectricity in polymers presents a formidable challenge, as specific requirements in the crystalline ordering of the polymer chains must be met that run counter to the propensity of polymers to form largely disordered structures. Consequently, poly(vinylidene fluoride) (PVDF:[-CH2-CF2-]n and its copolymers formed with poly(trifluorethylene) (PFTE:[-CHF-CF2-]n have been investigated very extensively because of their scientifically challenging and technologically important piezoelectric and ferroelectric (FE) properties. Early work has shown that irradiation (as well as the addition of 1, 1-chlorofluoroethylene (CFE) forming the terpolymers) irreversibly converts these FE copolymers to relaxors. In this presentation, we will recall some properties of the relaxor, and present experimental data obtained with the terpolymer under controlled hydrostatic pressure. We will show how the passage of the α conformation in the relaxor to the β conformation can be achieved by the application of stress. We will also show, that for terpolymer/nanocomposites (with graphene), some induced stress could be partially generated by the change of bipolar state ((terpolymer/ nanoparticles) subjected to an electric field) to a unipolar state due a possible ionization of the medium (high electronegativity of the Fluor). This stress leads to an induced relaxor to ferroelectric phase crossover at lower field.

Strength Design and Minimization of Residual Stresses in Reversible GaAs Wafer Bonding Process

The GaAs wafer bonding process is investigated to reduce the mechanical failures of GaAs wafer based on strength design concept. Three-point bending experiment is performed to measure the fracture strength of GaAs wafer, of which cleavage takes place on (110) plane. We propose a simple method for minimizing the thermal residual stress in a three-layer structure, of which the basic idea is to use an appropriate steady-state temperature gradient to the wafer bonding process. The optimum bonding condition of GaAs/wax/sapphire structure is determined based on the proposed method. The effect of material anisotropy on the thermal residual stress is also analyzed by finite element method.

Measurement of Mechanical Properties and Residual Stresses of Bridged Gold Films and Circular Gold Membranes

In order to measure the mechanical properties of gold films on silicon substrate, two types of specimens, i.e., bridged films and circular membranes, are manufactured. Using a wedge tip, the bridged gold films are indented so that the films are pushed off, which is called as V-peel test. The load-deflection curves obtained by the V-peel test are analyzed with the concept of geometrically nonlinear beam by using the minimum potential energy theory together with Ritz method. Thus, Young’s modulus and residual stress of the bridged gold films are obtained. Blister test is also conducted to measure the Young’s modulus and residual stress of a circular gold membrane, of which deformation is measured by Twyman-Green interferometer. By gradually increasing the external pressure applied on the membrane, the interfacial fracture toughness between the gold membrane and silicon substrate is measured based on the concepts of interfacial fracture mechanics.

Elastic Singularity Interacting With Various Types of Interfaces

The elastic solution for a singularity in an anisotropic trimaterial with perfectly bonded interfaces was obtained in the previous work by Choi and Earmme. The term trimaterial denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. It is shown in this paper that when the interfaces of an anisotropic trimaterial are one of the following types: (i) perfectly bonded, (ii) rigid, (iii) separated, (iv) separated without slip, and (v) slipping interfaces, the elastic solution for a singularity in the trimaterial has the same form as that for a singularity in a trimaterial with perfectly bonded interfaces, but with the bimaterial matrices properly altered.