Papers by Author: Seung Ho Han

Abstract: Mechanical post treatments for welded structures have been applied in various industrial fields and, in most cases, have been found to cause substantial increase in their fatigue strength. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In hammer peening, the weld profile is modified due to removal or reduction of minute crack-like flaws; compressive residual stresses are also induced by repeated hammering of the weld toe region with blunt-nosed chisel. In this study, a hammer peening procedure, using commercial pneumatic chipping hammer, was developed; a quantitative measure of fatigue strength improvement was performed. The fatigue life of hammer-peened specimen was prolonged by approximately 10 times in
S=240MPa, and was doubled for the as-welded specimen.

Abstract: In an early stage of design process for the lower arm in automobile suspension module, an easy and fast FE modeling, static and durability analysis supported by parametric study considering its geometric changes are required. The FE modeling support system has been developed, which is implemented on the platform of MSC.Patran. The CAD file produced by taking into account of automatic 3D parametric model in CATIA V5 can be transferred to this system. For the process automation of various design activities including the parametric study, human interactions are excluded practically, in which all processes are dealt with the XML-wrapper, and in- and output data are linked each other and treated in the engineering data management. The developed FE modeling support system reduces time and cost to design and analyze engineering problems.

Abstract: The hot spot stress or the notch strain alleviates the welding detail dependency of S-N curve to some extent. This paper suggests a new stress model which alleviates the dependency further, thus the fatigue strengths of several welding details of same material can be evaluated with the S-N curve of the base material. A stress at the hot spot of a weld joint is decomposed into two components; linear rising one, and rapid rising one which is inversely proportional to the distance from the hot spot. For the stress decomposition, a formula is proposed with which the configuration of stress distribution near a hot spot is fitted exactly. The new stress model makes use of a geometric characteristic of the stress distribution curve by the formula. The stress model is applied to five different weld joints. As the result, the experimental fatigue data are plotted very closely to the S-N curve of the base material

Abstract: The technical development of construction equipment such as wheel loader leads to an increase of not only amount of workload per hour, but also magnitude of allowable stress range. Since welded structures of the wheel loader occupy approximately 40-60% of its total weight and are subjected to incessant fatigue loads, fatal fatigue failures occur occasionally around the welded zone during its practical operation in the field. To reasonably evaluate the fatigue life, the effect of these geometries and welding residual stress on the fatigue life should be taken into account. In this paper, the modified notch strain approach developed previously by the authors is applied practically to assess the fatigue life of welded joints in boom and front frame structures of the wheel loader.

Abstract: For the fatigue design of a large scaled welded structure, the readily accessibility of various engineering tools is required for the static, fatigue, dynamic analysis and parametric study. In this paper, a multi-agent system is suggested for the development of distributed, open and intelligent design system. The developed design system is applied to assess cumulative fatigue damage on the welded bogie frame of railway vehicles. The engineering tools for fatigue durability analysis are chosen as I-DEAS, ANSYS and BFAP, in which iterative analysis is performed automatically caused by geometrical changes of transom support bracket attached in the bogie frame. The prototype of design system is implemented successfully for the assessment of cumulative fatigue damages of the welded bogie frame.

Abstract: The automation of fatigue durability analysis for welded bogie frames according to the UIC-code is proposed by using the ModelCenter, which enables several tools used in fatigue durability analysis to be integrated, i.e. I-DEAS, ANSYS and BFAP, and an iterative analysis of geometrical changes in transom support bracket to be performed. In additions wrapping programs to control I/O-data and interfaces of these tools were developed. The developed automation technique brings not only a significant decrease in man-hour required for durability analysis, but also provides a platform for multidisciplinary engineering activities.

Abstract: Multiple collinear surface cracks distributed that are randomly along a weld toe have a strong influence on the fatigue crack propagation life of welded joints. This issue is investigated using statistical approaches based on series of systematic experiments, in which initial crack numbers, their locations and crack sizes, i.e. depth and length, are taken into account. The number of initial cracks follow a normal distribution, and the probability of initial crack depths and lengths can be accurately described by the Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of the mutual interaction and the coalescence of multiple cracks are considered as well as the Mk-factors. The automatic calculation of fatigue crack propagation life is achieved by the application of NESUSS, where parameters such as the number, location and size of cracks are all treated as random variables. The random variables are dealt through a Monte-Carlo simulation with sampling random numbers of 2,000. The results of the simulation provide the statistical characteristics of the fatigue crack propagation life for welded joints as a function of the number of initial cracks. The sum of the simulation results and the fatigue crack initiation life referred from a previous paper is in good agreement with the experimental results.

Abstract: For a rational design of a welded joint, it is necessary to repeatedly assess the fatigue life of the joint with various dimensions and welding conditions. In this paper, an automated, repeatable/repetitive fatigue life assessment process for a welded cruciform joint was studied. The process consists of a structural analysis to obtain the stress distribution in the vicinity of the weldtoe, a thermal elasto-plastic analysis to determine the welding residual stress, and a fatigue life assessment based on the analyzed stress distribution and welding residual stress. With changes in design conditions including dimensions and/or welding heat input, the aforementioned tasks have to be performed. Using a commercial tool for system integration, automation of a repeated process for a welded cruciform joint based on 2D modeling was achieved. In this automated system, data exchanges between programs, regardless of whether they are commercial or in-house, work well, and parametric studies for optimal design can be performed.

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.