Papers by Author: Hyeon Gyu Beom

Abstract: Single or multiple of delaminations have been found frequently on the fracture surface of X70 pipeline steel. In this study, the delamination cracks and their influence on the fracture of pipeline are investigated by both experiment and three-dimensional fracture analyses. It is shown that the three-dimensional stress state is prerequisite for delamination crack and the strength distribution of material influences the form and direction of delamination crack. The delamination cracks are produced on the weak interfaces among the material by the tensile stress perpendicular to them before the fracture passes. The direction of delamination crack depends on the three-dimensional stress fields and strength distribution of material near the crack tip or notch root. The delamination cracks of the fracture through thickness of pipe wall make the effective thickness decrease and the delamination cracks of surface crack are perpendicular to the direction of fracture propagation direction. The delamination cracks reduce the stress triaxiality near crack tip and in turn, improve the fracture toughness of X70 pipeline steel.

Abstract: A thin electrode layer at the interface between two dissimilar linear piezoelectric materials under electromechanical loading is investigated. The complex function theory is employed to obtain the exact solution to a finite thin conductive layer. Special consideration is devoted to the structure of singular stress and electric fields near the tip of the thin electrode between two dissimilar piezoelectric materials. The stress and electric field are found to have an inverse square root singularity. The electric field intensity factor characterizes uniquely the singular fields close to the edge of the conductive line sheet.

Abstract: In this paper, we present an efficient method for conducting a finite element analysis of a structure with cyclic symmetry and apply the method to analyze the natural vibration and linear and non-linear static characteristics of a blower impeller. A blower impeller is composed of circumferentially repeated substructures. The whole structure is partitioned into substructures, and the finite element analysis can thus be performed with one representative substructure by using the transformed equations for each number of nodal diameters, which are derived from a discrete Fourier transform. We calculated the natural vibration and linear and non-linear static characteristics of a blower impeller without a stiffening ring, and with small as well as large stiffening rings, respectively. The accuracy and efficiency of the presented method are verified by comparison of the results obtained from the analysis using a substructure to those obtained using the whole structure.

Abstract: The bolted end-plate composite beam-CCSHRC column connection was validated to be ductile and offered an alternative to pre-Northridge connection. This study aims at the beam lower flange fracture in the connection test, and applies the J-integral criteria to examine the connection’s nonlinear fracture behavior. Advanced 3-D connection models containing initial crack in the high stress zone at lower flange are created, and the J values at the crack tip are calculated with considering the influences of certain parameters. The results demonstrate that the J values are strongly affected by the initial crack length and interstory drift. For 0.94, 1.35, 1.86 and 2.50 mm long crack, the J values sharply increase during loading history. The crack with a length of 2.50 mm propagates at a 66 mm drift, while the 1.35 mm long crack grows at a 120 mm drift. For 0.94 or 0.61 mm long crack, it keeps stable without growing upon loading. Besides, the J values exhibit a weak sensitivity to the beam concrete strength and tensile reinforcement ratio for beam. Under the same drift, the J-integral increases by about 3.5% when concrete strength changes from 15 to 24 MPa, and the J values at 0.6% tensile reinforcement ratio for beam are 1.5% larger than those at 0.3% or 1% reinforcement ratio.

Abstract: A crack on the conductive interface between two dissimilar piezoelectric ceramics under electromechanical loading is investigated. The closed form of the singular crack tip fields for the interface crack is derived here using an analysis based on analytic functions. It is shown that the interfacial crack-tip field consists of a pair of oscillatory singularities. A closed form of the solution for a finite crack on the conductive interface between dissimilar piezoelectric media is also derived.

Abstract: Based on detailed three-dimensional finite element analyses, elastic stress and strain field of ellipse major axis end in plates with different thickness and ellipse configurations subjected to uniaxial tension have been investigated. The plate thickness and ellipse configuration have obvious effects on the stress concentration factor, which is higher in finite thickness plates than in plane stress and plane strain cases. The out-of-plane stress constraint factor tends the maximum on the mid-plane and approaches to zero on the free plane. Stress concentration factors distribute ununiformly through the plate thickness, the value and location of maximum stress concentration factor depend on the plate thickness and the ellipse configurations. Both stress concentration factor in the middle plane and the maximum stress concentration factor are greater than that under plane stress or plane strain states, so it is unsafe to suppose a tensioned plate with finite thickness as one undergone plane stress or plane strain. For the sharper notch, the influence of three-dimensional stress state on the SCF must be considered.

Abstract: The Charpy V notch specimens of X70 pipeline steel with different notch orientation are tested under the static and dynamic conditions at different temperatures. By analyzing the load versus displacement curves and fracture appearances of specimens the relation of fracture behavior and loading rate is investigated. The maximum load increases and the displacement corresponding to maximum load reduces with test temperature decreasing. Both under the static conditions are larger than that under the dynamic conditions. The fracture sections of all test specimens are reduced in the thickness direction and quantity reduced depends on the load rate, notch orientation and test temperature. At the higher temperature, delamination cracks are found on the fracture surface both under the static and dynamic conditions, which are perpendicular to the thickness direction in T-L specimen and perpendicular to the main crack propagation direction in T-S specimen. Influence of loading rate on the delamination crack size and amount is obvious. The fracture initiation energy and total fracture energy increase with test temperature increasing. Influence of loading rate on the total fracture energy is unobvious at the higher temperature, but is obvious at the lower temperature. So the loading rate effects on total fracture energy are relevant to test temperature and notch orientation.

Abstract: The quasi-static tests on the endplate bolted connections of the new structure system consisting of SCC beam and CCSHRC column are briefly introduced in this paper. Meanwhile, the 3-D solid finite element models of the connections with pre-existing cracks in the lower flange’s high stress fields are set up by ANSYS. The material nonlinearities of concrete, steel and bars, together with the contact between the endplate and column surface are all considered in the model. With the transverse load applied on top of columns, the fracture parameters are calculated by APDL. The results indicate that the opening mode crack will happen mainly. When the pre-existing crack length is 2.50mm and the inter-storey drift is less than 6mm, the stress intensity factor values agree well with the converting values derived from J-integral and the crack tip fields are in elastic state. The J values are highly influenced by the pre-existing crack length, while seldom influenced by the concrete compression strength. Moreover, the J-integral have the trend to sharply increase when the pre-existing crack length is larger than 0.61mm, and the pre-existing crack will grow during loading when its length is larger than 1.35mm.

Abstract: This paper presents electro-mechanical characteristics of a micro-machined vibrating silicon ring gyroscope which can measure angular velocity components about three orthogonal axes. The ring gyroscope has a ring connected to the gyroscope main body by support ligaments that are arranged with cyclic symmetry. The natural modes of its vibration can be distinguished into the in-plane motion and the out-of-motion that are coupled by the gyro-effect due to the rotation of the gyroscope main body. The motions of the ring are electro-statically driven, sensed and balanced by electrodes. The equations of motion are formulated with considering the electrostatic effects of electrodes. The measuring method of angular velocities of the gyroscope main body by force-torebalance is proposed. The dynamic characteristics of the ring gyroscope are discussed.

Abstract: In this paper, a finite element code especially for micro-magnetostrictive actuators was developed. Two significant characteristics of the presented finite element code are: (1) the magnetostrictive hysteresis phenomenon is effectively taken into account; (2) intrinsic geometric feature of typical thin film structures of large length to thickness ratio, which makes it very difficult to construct finite element mesh in the region of the thin film, is considered reasonably in modeling micro-magneostrictive actuators. For verification purpose, magnetostrictive thin films were fabricated and tested in the form of a cantilevered actuator. The Tb-Fe film and Sm-Fe film are sputtered on the Si and Polyimide substrates individually. The magnetic and magnetostrictive properties of the sputtered magnetostrictive films are measured. The measured magnetostrictive coefficients are compared with the numerically calculated ones.