Papers by Author: Tae Hyun Baek

Abstract: This research focused on solving volumetric locking problem of shell structure of incompressible material. Degenerated solid-shell elements are widely applied on curved structure. But, volumetric locking will take place when the structure is made of incompressible material, such as rubber. Due to Poisson’s locking free property of P1-nonconforming element, it is employed to solve volumetric locking problem of shell structure. Furthermore, the study on shell structure is extended to topology optimization design. To verify the volumetric locking free of P1-nonconforming element on shell structure of incompressible material, some structures are studied by different elements. Comparing with the utilization of high order elements to solve volumetric locking problems, P1-nonconforming elements can save calculation time and reduce the numerical cost.

Abstract: Pre-produced triplate transition joint assemblies are widely used in shipbuilding industry to make welds between aluminum and steel for a number of years now. The straight-shaped transition joint assemblies are bent during shipbuilding. So it is necessary to study the residual stresses created by punch forming, which would have heavy effects on the quality of structural parts. ABAQUS is a suite of powerful engineering simulation programs, based on the finite element method. In this paper, ABAQUS was used as the main tool to simulate the residual stresses in a triplate transition joint after unloading. Punch-pressing was carried to simulate bending moment in ABAQUS. The triplate is consisted of baselayer (steel: Lloyd’s Shipplate Gr. A), interlayer (pure aluminum: Al99.5) and superlayer (Al-Mg alloy: AlMg4.5Mn). Results from the ABAQUS analysis showed that increasing the radius of punch significantly reduced the von Mises residual stresses in steel. Changes of von Mises residual stresses in interlayer (Al99.5) and superlayer (AlMg4.5Mn) were negligible.

Abstract: A hybrid stress determination around circular and elliptical holes utilizing photoelastic phase-shifting and nonlinear least-squares methods is presented. The method was demonstrated by calculating fringe orders of distant points along straight lines using 8-step phase-shifting method. The data was used to evaluate the coefficients in the complex stress functions for hybrid analysis. Tangential stresses around the boundary of the holes were obtained using conformal mapping technique. Different number of terms in a power-series representation of the complex type stress function was tested to qualitatively observe the effects of varying stress field. Actual fringes were related with the reconstructed and sharpened fringes along with the change in the number of terms, m. Good agreement was obtained when m in stress functions was equal to nine. At high stress concentration, the result obtained from the hybrid method agrees with FEM by two and five percent for circular and elliptical hole, respectively. The results show that the established numericalexperimental method for stress analysis is considerably reliable.

Abstract: hybrid experimental-numerical method is presented for determining the stresses around a circular hole in a finite-width, tensile loaded plate. Calculated fringes obtained by FEA provided the information about the external boundary of the hybrid element, and those fringes on straight lines were used for hybrid analysis. In order to see the effects of varying stress field, different numbers of terms in a power-series representation of the complex type stress function were tested. Actual and reconstructed fringes were compared. The hybrid results were highly comparable with those predicted by FEA. The result showed that this approach is effective and promising because isochromatic data along the straight lines in photoelasticity can be conveniently measured by use of phase shifting photoelasticity.

Abstract: In order to evaluate the structural integrity of the breech system used for a thick-walled cylinder subjected to pulsating high internal pressure, fatigue life simulation of a breech system was performed. A stress analysis of the breech was performed to locate the critical region vulnerable to crack initiation. Low-cycle fatigue behavior of the breech material was investigated to obtain the fatigue crack initiation properties. Elastic-plastic finite element stress analysis resulted in a stress concentration at the breech ring groove root. Strains at the breech ring and block were experimentally measured using strain gages and resulted in similar values compared to the calculated strains. Local strain approach was employed to estimate the fatigue life of the breech system for crack initiation at the groove root of the breech ring. Fatigue tests using simulation specimens were performed and an averaged fatigue life was obtained, showing a very good agreement with the calculated fatigue life within a factor of two.

Abstract: A multi-purpose polariscope was developed by applying an electro-mechanical control system to a diffused transmission-type circular polariscope. A conventional polariscope is only good for manual control of optical elements. The new polariscope system was devised to be controlled through two stepping motors and two magnetic clutches. The new polariscope can be used not only for the point-wise measurement using Tardy compensation technique but also for the full-field fringe analysis using conventional and/or phase measuring techniques, if applicable. The distributions of digitally determined unwrapped isoclinics and isochromatics using the digitized images obtained from the developed polariscope were in close agreement to manual measurements.

Abstract: Speckle interferometry with phase shifting method is used to measure in-plane displacements of a steel plate with a partly through-thickness circular hole and a steel plate with a through-thickness circular hole. The circular hole of steel plate with a partly through-thickness circular hole is cut on the rear side of the plate, so that it is not visible during experiment. The speckle noises of fringe patterns acquired by optical experiment are processed with image processing algorithm of Gaussian blur and the in-plane displacements of the two specimens are obtained by use of the processed fringe patterns. Also the in-plane displacements of the two specimens are calculated by use of ANSYS. The results of optical experiments are quite comparable to those of calculation with ANSYS.

Abstract: Predicting the behavior of steel during deformation process under service conditions is one of the main challenges in cold drawing. In this paper, finite element method was used to analyze the distribution of residual stress components in a rod with respect to different area reductions in cold drawing process. Cold drawing brought sunken-in deformation at the bottom land of the rod that was in accord with the result obtained from the analysis using ABAQUS. The results obtained from finite element analysis were in close agreement with the determined high-accuracy measurements.