Chin-Hyung Lee

Numerical Investigation on the Behavior of Circumferentially Butt-Welded Steel Circular Hollow Section Flexural Members

This paper presents a finite element (FE) analysis to quantify the effects of weld-induced residual stresses on the flexural behavior of circumferentially butt-welded steel circular hollow section (CHS) members. FE modeling of the weld-induced residual stresses is first described. Nonlinear FE analysis in which the behavior of the CHS members subjected to bending is explored incorporating the residual stresses is discussed next. Two FE CHS tube models with different failure modes are developed in order to clarify the effects that the residual stresses have on the flexural behavior. Simulated results show that welding residual stresses can have significant effects on the initial stiffness and the ultimate strength of the CHS flexural members depending on the loading position; hence, they should be taken into account in assessing the behavior of steel CHS members with circumferential weld under bending.

Analysis of residual stress in stainless steel pipe weld subject to mechanical axial tension loading

This paper presents the characteristics of welding residual stresses in circumferentially butt-welded stainless steel pipe by utilizing three-dimensional (3-D) uncoupled thermo-mechanical finite element (FE) analysis method. Moreover, stress variations in welded joints of the pipe under superimposed mechanical axial tension loading are further investigated employing the welding residual stresses and plastic strains obtained from the thermo-mechanical FE analysis as an initial condition. Results show that spatial variations of the welding residual stresses are present along the circumference and a rapid change of the residual stresses exists at the welding start/stop position, hence 3-D FE analysis is essential to accurately simulate circumferential welding of a pipe component. When mechanical axial tension loading is applied to the circumferentially butt-welded stainless steel pipe, bending moment is generated at the welded joints caused by the circumferential shrinkage of the weld region during welding; thus affecting the axial and hoop stress evolutions in the course of the superimposed mechanical loading.

J-INTEGRAL FOR A 3-D INTERFACE CRACK CONFIGURATION IN WELDS OF DISSIMILAR STEELS

In this study, path-independent values of the J-integral in the finite element context for an arbitrary three-dimensional interface crack configuration in welds of dissimilar steels are presented. For the fracture mechanics analysis of an interface crack in welds of dissimilar steels, residual stress analysis and fracture analysis must be performed sequentially. In the analysis of cracked bodies containing residual stress, the usual domain integral formation results in path-dependent values of the J-integral. And unlike cracks in homogeneous materials, an interface crack in welds of dissimilar steels always induces both opening and shearing modes of stress in the vicinity of the crack tip. Therefore, this paper discusses modifications of the conventional J-integral that yield path independence in the presence of residual stress and the total J values which can characterize the severity of an interface crack tip in welds of dissimilar steels. A finite element method which can evaluate the J-integral for an interface crack in three-dimensional residual stress bearing bodies is developed using the modified J-integral definition and total J values. The situation when residual stresses only are present is studied as is the case when mechanical stresses are applied in conjunction with a residual stress field.

Influence of weld-induced residual stresses on the hysteretic behavior of a girth-welded circular stainless steel tube

The present study attempts to characterize the relevance of welding residual stresses to the hysteretic behaviour of a girth-welded circular stainless steel tube under cyclic mechanical loadings. Finite element (FE) thermal simulation of the girth butt welding process is first performed to identify the weld-induced residual stresses by using the one-way coupled three-dimensional (3-D) thermo-mechanical FE analysis method. 3-D elastic-plastic FE analysis equipped with the cyclic plasticity constitutive model capable of describing the cyclic response is next carried out to scrutinize the effects that the residual stresses have on the hysteretic performance of the girth-welded steel tube exposed to cyclic axial loading, which takes the residual stresses and plastic strains calculated from the preceding thermo-mechanical analysis as the initial condition. The analytical results demonstrate that the residual stresses bring about premature yielding and deterioration of the load carrying capacity in the elastic and the transition load ranges, whilst the residual stress effect is wiped out quickly in the plastic load domain since the residual stresses are nearly wholly relaxed after application of the cyclic plastic loading.

Damage detection of a prototype building structure under shaking table testing using outlier analysis

The civil engineering community is becoming increasingly interested in monitoring structural behavior of civil infrastructure and in evaluation of the structural performance. The demand has largely been driven by deficiencies in structural performance due to the aging of the infrastructure, excessive loading, and natural disasters such as an earthquake, a landslide, a typhoon and a tsunami. In this study, a structural health monitoring methodology using acceleration responses is proposed for damage detection of a three-story prototype building structure during shaking table testing. A damage index is developed using the acceleration data and applied to outlier analysis, one of unsupervised learning based pattern recognition methods. A threshold value for the outlier analysis is determined based on confidence level of the probabilistic distribution of the acceleration data. The probabilistic distribution is selected according to the feature of the collected data.

Test-bed for the remote health monitoring system for bridge structures using FBG sensors

This paper reports on test-bed for the long-term health monitoring system for bridge structures employing fiber Bragg grating (FBG) sensors, which is remotely accessible via the web, to provide real-time quantitative information on a bridges response to live loading and environmental changes, and fast prediction of the structures integrity. The sensors are attached on several locations of the structure and connected to a data acquisition system permanently installed onsite. The system can be accessed through remote communication using an optical cable network, through which the evaluation of the bridge behavior under live loading can be allowed at place far away from the field. Live structural data are transmitted continuously to the server computer at the central office. The server computer is connected securely to the internet, where data can be retrieved, processed and stored for the remote web-based health monitoring. Test-bed revealed that the remote health monitoring technology will enable practical, cost-effective, and reliable condition assessment and maintenance of bridge structures.

PREDICTION OF GROUND VIBRATIONS AROUND THE TURNOUT SYSTEM INDUCED BY THE TILTING TRAIN

In Korea, it has been planned to apply high speed tilting train, which operates at the maximum speed of 200km/h, to conventional lines by the year of 2010. However, for the application of the tilting train to conventional lines, it is prerequisite to establish a stable turnout system allowing the tilting train to pass through it without reducing speed. Therefore, the improved turnout system for the speed-up of conventional lines has been developed and the prototype of the turnout system has been constructed. However, levels of impact-induced ground vibrations from the discontinuous part of the turnout system have not been clarified. In this study, analysis of ground vibrations around the improved turnout system is performed in order to predict the generation level of ground vibrations due to the passage of the tilting train through the improved turnout system.