Papers by Author: Kyung Su Kim

Abstract: This paper experimentally investigates the hysteretic behaviors of yield stress in electrorheological (ER) and magnetorheological (MR) materials which are known as smart materials. As a first step, the PMA-based ER material is prepared by dispersing the chemically synthesized polymethylaniline (PMA) particles into non-conducting oil. For the MR material, commercially available one (Lord MRF-132LD) is chosen for the test. Using the rheometer, the torque resulting from the shear stress of the ER/MR materials is measured, and then the yield stress is calculated from the measured torque. In order to describe the hysteretic behavior of the fielddependent yield stress, a nonlinear hysteresis model of the ER/MR materials is formulated between input (field) and output (yield stress). Subsequently, the Preisach model is identified using experimental first order descending (FOD) curves of yield stress in discrete manner. The effectiveness of the identified hysteresis model is verified in time domain by comparing the predicted field-dependent yield stress with the measured one.

Abstract: In the present paper, the dynamic characteristics of smart hull structure are investigated for the feasibility of Macro Fiber Composite (MFC) actuator in active vibration control of smart hull structure. MFC is an advanced anisotropic piezoceramic actuator. Finite element technique was used to ensure application to practical geometry and boundary conditions of smart hull structure. Using finite element method, modal analysis and strain distribution were first conducted to investigate dynamic characteristics of smart hull structure. Based on numerical modal analysis, the locations of MFC actuators are determined to achieve maximum control authority. Then, the MFC actuators are attached to the hull structure and natural frequencies and damping coefficients are identified through experimental modal test.

Abstract: In this study, an experimental method has been studied to measure the residual stresses on a specimen with I-groove weldment. The specimens were extracted from I-groove weldment which was joined by SAW (Submerged Arc Welding) with CO2 shield gas. A FEA (Finite Element Analysis) model was developed for the estimation of the residual stresses for the specimen. Measurements were carried out using ESPI(Electronic Speckle Pattern Interferometry) system which can measure the strain distribution on the surface of specimen. The residual stresses were estimated by the value of strain measured by ESPI system. Strain gages were added to evaluate the accuracy of ESPI system. In addition, a three-dimensional FE model was used to estimate the residual stresses generated by the welding procedure. A thermal elasto-plastic analysis was performed by the FEA. The stresses measured by the experiments were compared with the results of FEA. Also, discussed are the difference and agreement between the stresses obtained by experiments and FEA, respectively.

Abstract: The turbine wheels of a turbocharger are operated at high revolution speed in high temperature inlet gas. Alloy 713LC blades of the turbine wheel broke in an hour the during a model test. Two failures and several cracks were found in the turbine blades. Failures in blades are suspected to occur as a result of thermal mechanical stresses or fatigue load and other cause such as creep-rupture and resonant vibration. The present study investigates the possible causes of the failure of these blades. FEM (Finite Element Method) was used to calculate the thermal centrifugal stresses and natural frequency to find the cause of failures. LCF (Low Cycle Fatigue) life of blades was roughly estimated by using the stress and strain level calculated by FEM. The investigation indicates that the failures were associated with resonant forces and HCF (High Cycle Fatigue).

Abstract: Low cycle fatigue cracks are mainly detected at discontinuous welded locations with high stresses under repeated cyclic static loads due to cargo loading and unloading. Theoretical and analytical methods have been used to estimate the local stress and strain, which affect the prediction of fatigue life, but these methods have difficulties considering stress concentration at notched locations and complicated material behavior of welded joints or heat affected zones. Electronic speckle pattern interferometry(ESPI) system is a nondestructive and non-contact measurement system, which can provide relatively accurate full field strain at critical positions such as welded zones and structurally discontinuous locations. In this study, local strain was measured by ESPI system at the welded cruciform joint, and then low cycle fatigue test was performed. Effect of local strain on low cycle fatigue life was examined by using the strain value measured by ESPI. In order to verify the relations between local strains and fatigue lives, after theoretical local strains and stresses were calculated by using Neuber’s rule, the measured local strains corresponding the experimental fatigue lives were compared with the results of Neuber’s rule and established codes of the British standard and DNV curve.

Abstract: It is not easy to solve the troubles around the openings at the detail design stage, because there are a lot of concerning locations. There are not also clear design rules of classification societies. For more detailed strength analysis around holes on the longitudinal girder or transverse web below deck plate, it is also very difficult to decide the boundary condition (B.C) at both ends. Grillage analysis, which considers B.C and more detailed member arrangement, is used in simple analysis. Furthermore, refined mesh method, which use a part of member in the strength analysis, is used to decide S.I.Fs around holes. Component loads are applied to a refined mesh model separately and S.I.Fs according to component loading are calculated. S/F and B/M are decided from the grillage analysis and S.I.F for a given hole location is deduced from database and the results of grillage analysis.

Abstract: The structural design problems are acknowledged to be commonly multicriteria in nature. The various multicriteria optimization methods are reviewed and the most efficient and easy-to-use Pareto optimal solution methods are applied to structural optimization of grillages under lateral uniform load. The result of the study shows that Pareto optimal solution methods can easily be applied to structural optimization with multiple objectives, and the designer can have a choice from those Pareto optimal solutions to meet an appropriate design environment.

Abstract: For many fatigue-critical parts of machines and structures, the load history under operating conditions generally involves variable amplitude loading rather than constant amplitude loading. An accurate prediction of fatigue crack propagation life under variable amplitude loading requires a thorough evaluation of the load interaction effects. In this study, fatigue tests under both constant and variable amplitude loading were carried out to investigate the overload effects on fatigue crack propagation of the notched specimens. Strain distributions around the crack tip before and after a tensile overloading were measured using the ESPI (Electronic Speckle Pattern Interferometry) system. The size of the plastic zone was determined from the measured strain distributions. The study proposes a crack propagation prediction model that incorporates the overload ratio effect. A comparative work for the overload ratio effect demonstrated that the prediction by the proposed model was in good agreement with the experimental results. The prediction of fatigue crack propagation including multiple overloads with the proposed model show also a good agreement with test results.

Abstract: The effective breadth of a stiffened plate under bending was investigated. The design parameter of an effective breadth was used when a stiffened plate subjected to a lateral loading was considered. Due to the lateral loads there was shear transmission through the plate from the stiffener, which leads to a non-uniform longitudinal stress distribution across the plate width the effective breadth that takes into account a shear lag can be calculated by the simple beam theory. For a tanker, the existing effective breadth concept has been used when longitudinal stiffeners are fixed at adjacent web frames. However, the effective breadth of each classification’s formula can’t be applied to flexible support condition such as the one prevailing in Ro-Ro vessel. The effective breadth according to the rules of several classification societies was compared and a way to get an accurate analysis as suggested.

Abstract: This paper presents experimental results of tensile and compressive behaviors of smart electrorheological (ER) materials. Two different ER materials ; chemically treated starch particles and polymethylaniline particles are synthesized followed by devising a sqreeze mode type apparatus associated with motion controller and data acquisition system. The field intensity, electrode velocity and initial gap are chosen as important test parameters that influence on the tensile and compressive characteristics. The maximum tensile stresses are evaluated at each condition and compared between two ER materials.