Jeong Woo Han

Bio: Jeong Woo Han is an academic researcher. The author has contributed to research in topics: Fatigue limit & Welding. The author has an hindex of 2, co-authored 14 publications receiving 23 citations.

Fatigue Crack Initiation and Propagation Life of Welded Joints

Abstract: The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of occurrence in 2.3, 50 and 97.7%, in which the cyclic response of the local stress/strain in the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated in consideration of the Mk-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life as a sum of crack initiation and propagation life was found to be in good agreement with the experimental results.

Research for Fatigue Life Extension Techniques in Weldments via Pneumatic Hammer Peening

Abstract: Fatigue failures are often occurred at welded joints where stress concentrations are relatively high due to the joint geometry. Although employing good detail design practices by upgrading the welded detail class enables to improve the fatigue performance, in many cases, the modification of the detail may not be practicable. As an alternative, the fatigue life extension techniques that reduce the severity of the stress concentration at the weld toe region, remove imperfections and introduce local compressive welding residual stress, have been applied. These techniques are also used as definite measures to extend the fatigue life of critical welds that have failed prematurely and have been repaired. In this study, a hammer peening procedure for using commercial pneumatic chipping hammer was developed, and the effectiveness is quantitatively evaluated. The pneumatic hammer peening makes it possible to give the weld not only a favorable shape reducing the local stress concentration, but also a beneficial compressive residual stress into material surface. In the fatigue life calculation of non-load carrying cruciform specimen treated by the pneumatic hammer peening, the life was lengthened about ten times at a stress range of 240MPa, and fatigue limit increased over 65% for the as-welded specimen.

TOPSIS-Based Multi-Objective Shape Optimization for a CRT Funnel

Abstract: Key Words : TOPSIS(Technique for Order Preference by Similarity to Ideal Solution), Multi-objective Shape Optimization(다중목적 형상최적설계), CRT Funnel(CRT 후면유리), Design of Experiments(실험계획법), Rational Bezier Curve(베지어 곡선), MADM(Multi-Attribute Decision Making: 다속성 의사결정) 초록: TOPSIS(Technique for Order Preference by Similarity to Ideal Solution)는 상충되는 다수의 속성이 존재하는 상황에서 의사결정이 요구되는 다속성 의사결정법(Multi Attribute Decision Making) 중 하나이다. 이는 선택된 대체안이 최선의 이상적 대체안으로부터 가장 가까운 거리에 위치해야 하고, 동시에 부정적으로 이상적인 대체안으로부터는 가장 멀리 위치해야 한다는 논리에 입각한 의사결정 기법이다. TOPSIS 는 최소화와 최대화가 공존하는 다목적함수 형상 최적설계에 적용이 가능하다. 본 연구에서는 TOPSIS 와 베지어 곡선(Rational Bezier Curve)을 적용하여 CRT(Cathode Ray Tubes) 후면유리의 다중목적 형상최적설계를 수행하였다. 무게와 1 차 주응력의 두 가지 다중목적 함수를 최적화하기 위하여, 다중목적 함수의 성능지표를 TOPSIS의 상대적 근접도로 정의하고 이를 반응표면모델로 구성하여 다중목적 형상최적설계가 가능한 방법론을 제안하였다. 이를 통해 하나의 최적해가 아닌 최적해의 군이 선정되어, 무게와 주응력 최적해의 모순관계를 확인하면서 다양한 설계요구 스펙을 만족시켜줄 수 있는 방안을 설계자가 스스로 선택하도록 하였다. Abstract: The technique for order preference by similarity to ideal solution (TOPSIS) is regarded as a classical method of multiple attribute decision making (MADM), often used to solve various decision-making or selection problems. It is based on the concept that the chosen alternative should have the shortest distance from the positive ideal solution and the farthest distance from the negative ideal solution. The TOPSIS can be applied to a design process for carrying out multi-objective shape optimization wherein the best and worst alternatives are to be decided. In this paper, multi-objective shape optimization using the TOPSIS and Rational Bezier curve was applied to the funnel of a cathode-ray tube (CRT). In order to minimize the weight and first principal stress, a new multi-objective shape optimization methodology is proposed, wherein the relative-closeness coefficients of the TOPSIS are defined as the performance indices of a multi-objective function and evaluated by response surface models. This methodology enables the designer to decide on the best solution from a number of design specification groups by examining the various conflicts between the weight and the first principal stress.

Fatigue Life Improvement for Cruciform Welded Joint by Mechanical Surface Treatment using Hammer Peening and UNSM

Abstract: For the improvement of fatigue strength of welded structures, mechanical post treatments have been applied in various industrial fields and have in most cases been founded to give substantial increases in their fatigue lives. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In mechanical surface treatments, e.g. PHP (pneumatic hammer peening) and UNSM (ultrasonic nano-crystal surface modification), the weld profile is modified due to remove or reduce minute crack-like flaws, and compressive residual stresses are also induced. In this study, a pneumatic hammer peening procedure and a UNSM device were introduced, and a quantitative measure of fatigue strength improvement was performed. The fatigue strength at 2 × 106 cycles of hammer-peened and UNSM treated on a non-load carrying cruciform welded joint shows 220 and 260MPa, respectively, which are more than two times higher than that of as-welded specimen. Especially, the surface layer in the vicinity weld toe treated by the UNSM provides nano-crystal structure created by an ultrasonic cold forging and introduces very high welding residual stress in compression.

Three-dimensional weld toe magnification factors for various welded joints

Abstract: To effectively calculate the stress intensity factors in welded components, a weld toe magnification factor is introduced here that will allow for the influence of geometrical effects. This factor was derived from the data obtained in a parametric study performed by FEM. Several sets of weld toe magnification factor solutions have been presented, but these are applicable only to non-load-carrying cruciform or T-butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints such as cruciform, cover plate and longitudinal stiffener joints, the currently used weld toe magnification factors do not adequately describe the behavior of weld toe cracks. In the present study, the weld toe magnification factor solutions with exponents and fractions that have polynomial functions in terms of a/c and a/t — i.e., crack depths normalized by corresponding crack lengths and specimen thickness — were developed through a parametric study using 3-dimensional finite elements for the above mentioned three types of welded joints. These weld toe magnification factor solutions showed a tendency to increase at a/t < 0.2. Meanwhile, for 0.2 < a/t < 0.7, the effect of the attachment on the weld toe magnification factor decreased asymptotically. When the a/t ratio exceeded this range, the weld toe magnification factor became almost one of unity. The fatigue crack propagation life was evaluated by using the proposed weld toe magnification factor and by considering the propagation mechanisms of multiple-surface cracks, and it showed good agreement-to within a deviation factor of 2-between the experimental and calculated results for the fatigue crack propagation life for all welded joint.

Influence of weld geometry and residual stresses on the fatigue strength of longitudinal stiffeners

Abstract: Typically, for a fatigue strength assessment of a welded structure, the influence of welding residual stresses has to be considered. An easy applicable approach is given within the framework of the IIW recommendations; the design S-N curves can be upgraded in case of medium or low residual stresses. A relaxation or redistribution of tensile residual stresses, which is often accompanied by an increase in fatigue life, cannot be considered. To obtain a better understanding of the residual stresses and their influence on the fatigue strength, fatigue tests have been performed on longitudinal stiffeners in as-welded and stress relieved state with both stress ratios R = −1 and R = 0. Local strains and stresses were measured using strain gauges and X-ray diffraction technique to get insight into the local material’s response due to global loading close to the weld toe. The crack initiation and propagation phase was detected by a camera. FE models were set up to reproduce the observed material behavior in the notch for the crack initiation phase. By numerical analysis it could be shown that, due to sharp notches at the weld toe with an average toe radius of r = 0.05 mm, a stress redistribution occurs during the first load cycle. This leads to a fatigue strength, which is, particularly at higher load amplitudes, independent from mean as well as from initial residual stresses. A comparison of fatigue data derived on longitudinal stiffeners from literature confirms this effect. For lower load amplitudes instead, mean and residual stresses gain more influence. In this investigation a reliable assessment of the crack initiation period could be performed with strain-based concepts using damage parameter PSWT for a mean stress assessment. All fatigue tests could be transformed into a single damage parameter PSWT-N curve with a low scatter if residual stresses as well as the distortion of the specimens are regarded.

Fatigue life improvement for cruciform welded joint by mechanical surface treatment using hammer peening and ultrasonic nanocrystal surface modification

Abstract: For the improvement of the fatigue strength for welded structures, mechanical posttreat- ments have been applied in various industrial fields and in most cases have been found to give substantial increases in their fatigue lives. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In mechanical surface treatments, for example, PHP (pneumatic hammer peening) and UNSM (ultrasonic nanocrystal surface modification), the weld profile is modified due to removed or reduced minute crack-like flaws, and compressive residual stresses are also induced. In this study, a PHP procedure and a UNSM device were introduced, and a quantitative measure of fatigue strength improvement was performed. The fatigue strength at 2 × 10 6 cycles of hammer-peened and UNSM treated on a non-load-carrying cruciform welded joint shows 220 and 260 MPa, respectively, which are more than two times higher than that of as-welded specimen. Especially, the surface layer in the vicinity weld toe treated by the UNSM provides nanocrystal structure created by an ultrasonic cold forging and introduces very high welding residual stress in compression.