Sung-Kie Youn

Asymptotic homogenization of viscoelastic composites with periodic microstructures

A systematic way of obtaining the effective viscoelastic moduli in time and frequency domain is presented for viscoelastic composites with periodic microstructures. The problem of estimating the effective moduli is formulated using the asymptotic homogenization method. For theoretical aspects, the memory effects due to the homogenization are shown in general form and a sufficient condition for the effects to disappear is fully discussed. The computational procedure is divided into two steps. The effective relaxation moduli are computed in Laplace transformed domain and are numerically inverse-transformed into time domain. The effective complex moduli are then readily obtained by using simple formulae of the Fourier transform. Several numerical examples are presented to illustrate and verify present approach and to discuss the memory effects.

A study on the shape extraction process in the structural topology optimization using homogenized material

Abstract The shape and topology optimization of structures as an optimal density distribution problem is considered. An artificial material model, whose elastic moduli are contrived to stay close to the lower side of the Hashin-Shtrikman bounds, is introduced to obtain the efficient relationship between the effective elastic moduli and the density of the given material. The introduction of the new model results in optimal density distribution which is readily applicable to real structural patterns with the least possible porous regions. Also, a density redistribution algorithm is suggested in order to suppress checker-board patterns without the use of the higher order elements. This algorithm combined with the new artificial material introduced is shown to effectively work in extracting the optimal structure shape from the computed density distribution.

Design of microstructures of viscoelastic composites for optimal damping characteristics

Abstract An inverse homogenization problem for two-phase viscoelastic composites is formulated as a topology optimization problem. The effective complex moduli are estimated by the numerical homogenization using the finite element method. Sensitivity analysis shows that the sensitivity calculations do not require the solution of any adjoint problem. The objective function is defined so that the topology optimization problem finds microstructures of viscoelastic composites which exhibit improved stiffness/damping characteristics within the specified operating frequency range. Design constraints include volume fraction, effective complex moduli, geometric symmetry and material symmetry. Several numerical design examples are presented with discussions on the nature of the designed microstructures. From the designed microstructures, it is found that mechanism-like structures and wavy structures are formed to maximize damping while retaining stiffness at the desired level.

Analysis of temperature distribution in a rolling tire due to strain energy dissipation

Abstract This paper addresses a systematic procedure using a sequential approach for the analysis of the coupled thermomechanical behavior of a steady state rolling tire. Not only knowledge of mech...

On the collapse of circular confined rings under external pressure

Abstract The paper presents a study of the large deflection collapse of circular rings confined in a rigid cavity under external pressure. The ring is assumed to be inextensional and to have an initial localized imperfection which causes a small section of its circumference to be detached from the confining wall. The cavity formed is pressurized and its growth examined. The formulation is general enough to allow for large deflections of the ring as well as material nonlinearities. The pressure v s change in volume response of the confined ring is found to be characterized by a limit load for both elastic and inelastic material behavior. The limit load is shown to be dependent on both the geometry of the initial imperfection as well as the yield and post yield characteristics of the ring material. The response beyond the limit load is unstable until the crown of the ring touches the opposite side after which it becomes stable again.

A nonlinear viscoelastic constitutive model of solidpropellant

Abstract A nonlinear viscoelastic constitutive model for solid propellant is proposed. Theviscoelastic dewetting criteria is developed and the softening of the solid propellant due todamage is treated by the modulus decrease. In the calculation of the modulus decrease, themodulus of void, which is created by dewetting, is modeled with non zero constant values. Thevalues of adhesion energy between the binder and AP particle are obtained by a modified 180°peel test. The nonlinearities during cyclic loading are accounted for by the functions of theoctahedral shear strain measure. The model is evaluated with different loading conditions and thepredicted values well matched the measured ones. The model provides a simple and convenientmeans to predict solid propellant behavior without requiring a complex micromechanicaldescription.

A three-dimensional nonlinear viscoelastic constitutive model of solid propellant

Abstract A nonlinear viscoelastic constitutive model for the solid propellant is proposed. In their earlier work, the authors have developed an isotropic constitutive model and verified it for one-dimensional case. In the present work, the validity of the model is extended to three-dimensional cases. Also, implementation of the model into a commercial code for the use of the constitutive model in the finite element analysis of solid propellant is discussed. Large deformation, dewetting and cyclic loading effects are treated as main sources of nonlinear behavior of the solid propellant. The softening of the solid propellant due to dewetting is treated by the modulus decrease. The nonlinearities during cyclic loading are accounted for by the functions of the octahedral shear strain measure. The constitutive equation is implemented into a finite element code for the analysis of propellant grains. A commercial finite element package ‘ABAQUS’ is used for the analysis and the model is introduced into the code through a user subroutine. The model is evaluated with different loading conditions and the predicted values are in good agreement with the biaxial test results.

Thermoviscoplastic analysis of hypersonic structures subjected to severe aerodynamic heating

llie aeroclyiiarnic skiii consists of con- duction combined with surface radiation. lleat transrcr between the aerotlynarnic skin, the lieat exchanger sur- laces and the primary structiire is by conduction at tlic solid-fluid interface. The finite clcnient representation of concliiction lieat transfer with radiation boundary coii- ditions follows

Lightweight mirror design method using topology optimization

Lightweight mirrors experience optical image degradation due to mechanical loadings such as self-weight, polishing pressure, and vibration. Optical surface deformation of a lightweight primary mirror is an important factor that affects optical performance. We use topology optimization to design a lightweight primary mirror under self-weight and polishing pressure. For the optimization, we used a 3-D model of the mirror and based our calculations on the rms surface error of the mirror as an objective function constrained by the maximum weight of the mirror. In the first example of topology optimization, we consider the mirrors self-weight loading. In the second example, we include the polishing pressure. We present the results of the optimized design topology for the mirror with various mass constraints. To examine the optimal design results, we manufacture a prototype of the mirror.

Three Dimensional Analysis of High Frequency Induction Welding of Steel Pipes With Impeder

High frequency induction welding is widely employed for longitudinal seam welding of small scale tubes and pipes due to its relatively high processing speed and efficiency. This research is aimed at understanding the variables that affect the quality of the high frequency induction welding. The welding variables include the welding frequency, weld speed, vee angle, and tube thickness. Temperature distribution of the tube is calculated through three dimensional coupled electromagnetic and thermal finite element analysis. The skin and proximity effects are considered in the electromagnetic analysis. The influence of the impeder is also analyzed. The effects of the operating welding variables on the temperature distribution are investigated quantitatively by exhibiting the heat affected zone. The results explain the mechanism of significant enhancement of welding efficiency when the impeder is used. Not only good weld state can be obtained but also overheated edge can be avoided by understating the effect of welding variables. Suggestions are made for the better induction welding conditions.