Kyong-Ho Chang

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.

Characteristics of the residual stress distribution in welded tubular T-joints

Tubular T-joints are structural discontinuities that can be easily involved with stress concentrations. It is therefore necessary to estimate an internal stress distribution of T-joints. However, the complicated residual stresses are unavoidably produced adjacent to the joints by welding. In this paper, the residual stress distributions in welded tubular T-joints were analyzed by using a three-dimensional non-steady heat conduction analysis and a three-dimensional thermal elastic-plastic analysis. Characteristics of the residual stress distribution in welded tubular T-joints are investigated by the thermal-mechanical analysis results.

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.

A STUDY ON HYSTERETIC MODEL FOR WELD METAL AND ITS EFFECT ON BEHAVIOR OF STEEL PIPES WITH A WELDED JOINT

The weld metals used in welding process influences on behavior of steel structures under monotonic and cyclic loading. Because weld metals have different characteristics than structional steels for stress-strain relationship and mechanical properties. Therefore, to predict behavior of steel structure manufactured by welding process, a hysteretic model for weld metal is necessary. In this paper, to formulate the hysteretic model for E71T-1 weld metal, tensile tests and low cycle fatigue tests were carried out. A formulated hysteretic model applied to 3-dimensional elastic-plastic finite element analysis was proposed by the authors. To investigate the effect of weld metal on behavior of steel pipe members, numerical analyses of steel pipe with a welded joint were carried out under monotonic and cyclic loading. The effect of weld metal was clarified by comparing analytical results both steel pipe with consideration of weld metal and without consideration of weld metal.

The occurrence time estimation of oil contaminated source considering hydro-geological features

As industrial activities increase, the risk of geological contamination in Korea is becoming higher. The possibility of contamination is especially rising due to the increase of oil leakage and the corrosion of oil storage facilities. In this study, which was based on oil leakage cases in Korea, the occurrence time of oil contamination trace and source was hydro-geologically, such as groundwater flow considered soil composition, physical properties of soil and level of soil layers etc, estimated by examining the cause and ranges of an oil contamination event. It has been verified that the contamination history of potential contamination sources as well as the ranges and features of contamination sources were analyzed based on the test results of well, soil, and groundwater contamination status. The influential range of contaminated grounds and the occurrence time of oil leakage, considering hydro-geological features in the sites, can also be estimated. The occurrence time of oil leakage was estimated as 430 days when the mean void velocity was 0.52 m/day. The distance of pollutant migration was also estimated using the same method at 225 m.

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.

Verification of Validity of Governing Factors in High Accurate Prediction of Welding Distortion

Abstract The legitimacy of dominating factor in the high accuracy prediction of welding distortion was investigatedfor butt welding and fillet welding. When out-of-plane distortion was measured by the experiment objecting to butt welding, if tack welding was easily performed, the position of a neutral axis was variously changed by the irregularity. Then, there have been a case that out-of-plane distortion was generated in the unexpecteddirection. This case should be especially noted. New model for the experiment was proposed so as to solve this problem. As it was elucidated by the case of fillet welding, it was verified that the analysis should be carried out with satisfying the yield condition (especially at high temperature above 700 degree Celsius) and with closely simulating the penetration shape (heat input in weld metal) in order to solve the proposition that is the high accuracy prediction of welding distortion. It was confirmed that residual stress is highly predicted because welding distortion is highly predicted, too.Key Words : Butt welding, Fillet welding, Welding distortion, Residual stress, High accurate prediction, Governingfactor in prediction

Prediction of Welding Imperfection with Idealization of Welding and Their Accuracy

Abstract In order to reduce a grand compute time in prediction of welding distortion and residual stress by 3D thermal elastic plastic analysis, idealization of welding that is methods to heat input simultaneously in all weld metal on the same welding direction is carried out on two weld joints(butt welding and fillet welding). Then, the accuracy of acquired results is investigated through the comparison of the high accuracy prediction results. The thermal conduction analysis results by idealization of welding, the temperature is raised accompany with beginning of heat input because all of weld metal is heated input at the same time. On the other side,the temperature witch predicted with high accuracy is raised at the moment heating source passes the measuring points. So, there is difference of time between idealization of welding and considering of movingheat source faithfully. However, temperature history by idealization of welding is well simulated a high accuracy prediction results. Key Words : Welding imperfection, Welding distortion, Residual stress, Idealization of welding, CPU time, 3D thermal elastic- plastic analysis

Specification of Governing Factors for High Accurate Prediction of Welding Distortion

Abstract In carrying out the elastic-plastic analysis, four conditions (equilibrium equation, constitutive equation, condition of compatibility and yield condition) should be satisfied. In welding, the temperature largely changed from a melting temperature to a room temperature. So, yield stress of materials largely changed, too. In particular, yield stress becomes about zero over 700℃. The analysis should be carried out under thecondition that equivalent stress generated in temperature increment ΔT did not exceed yield stress of materials at high temperature over 700℃. It should be sufficiently recognized that the obtained results were not reliable if this condition was not satisfied.Key Words : Bead on plate, Welding distortion, Angular distortion, Accuracy, Shape of deposit metal, Thermal Elastic-Plastic analysis, Governing factor 특집논문: 고정도 용접변형 예측기술 Fig. 1 Gas heating correction (box girder) 1. 서 론 교량 및 선박 등 판과 보강재로 구성된 박판 구조물의 제작에 다수 적용되고 있는 용접공정에는 변형과 응력이 필연적으로 발생 1) 하고 있다. 그 중 변형은 좌굴, 피로 등 역학적요인과 구조물의 제작 정밀도 및 미관 저하에 따른 상품 신뢰성 저하 등 제작 전반에 걸쳐 효율을 저감시키는 요인이 되고 있다. 허용치 이상의 변형에 대하여 현장에서는, 여러 가지 후처리 방법으로 교정되고 있으나, 교정에는 시간/비용 증가와 함께 경험적인 숙련 기술이 요구되고 있으나, 숙련기술자의 인력은 최근 감소경향에 있어, 근본적인 해결 방법이 요구되고 있다. 한편, 현장 교정방법으로 가열교정(Fig. 1)에는, 많은 화석에너지와 부생물인 온실가스가 발생, 이들은 지구 온난화에 영향을 미치고 있으며, 일본의 경우, 온실