• Proceedings of the KWS Conference
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• 2002.10a
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• pp.378-383
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• 2002

In previous study, the decrease and recovery of total stiffness in welded structure was discussed on the basis of experimental examination through tensile loading and unloading test of welded specimen. The recovery of structure stiffness was caused by the release of welding residual stress through mechanical loading. In this study, analysis model that is indispensable for the effective application of MSR method was established on the basis of test and measurement result. Thermal elasto-plastic analysis for welding process was performed by non-coupled analysis. Analysis results of welding process were transfer to elasto-plastic model for tensile loading & unloading by restart technique. In elasto-plastic analysis model for mechanical loading & unloading, hardening appearance of weld metal was considered by rezoning technique and tying technique was used for JIG condition of test machine.

• International Journal of Korean Welding Society
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• v.2 no.1
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• pp.40-44
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• 2002

Residual stress by welding should be reduced because that decreases the reliability on strength of welded structure. The reason is that the total stiffness of structure decreases by non-linear behavior of weldment under external load. The release of residual stress by mechanical loading and unloading is often performed in the fabrication of box structure for steel bridge. The proper degree of loading and unloading is significant at release method of residual stress by mechanical loading because that degree is changed by material and geometric shape of welded structure. Therefore, the simulation model that could exactly analyze the release of residual stress by mechanical loading is to be necessary. This simulation model should be established on the based of variable and accurate measurement data. In this study, the non-linear behavior of weldments under external loading and unloading, such as the decrease and increase of structure stiffness, was investigated by monitoring of nominal stress and strain. Tensile loading and unloading test under variable load was performed and the proper degree of stress relaxation was measured by sectioning technique using strain gauge.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.6
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• pp.59-66
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• 2001

Fatigue strength of the welding structure is governed by the residual stress at the weldment toe and static tensile overloads were known as relieving the residual stresses. In this study, static tensile overloads were applied to the welding structures which caused the relief of residual stresses. The amount of residual stress relief was found as proportional to the change of fatigue limit at the given conditions. Based on the fact of the proportionality between the change of fatigue limit and that of residual stress, simple measurement technique is proposed. Modified stress-life curves base on proposed technique gave good agreement with test results.

• Journal of Welding and Joining
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• v.7 no.4
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• pp.46-55
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• 1989

Zirconia coatings are performed by the plasma spraying on the substrate of Al-Si alloy. In case of plasma sprayed ceramic coatings, it is important to control properly residual stress occurred during cooling process. Residual stress in coating layer varies with sprayed conditions and is influenced greatly by the coating layer thickness. Surface residual stress due to coating layer thickness is measured by X-ray diffraction method and the residual stress in coating layer is estimated by the deflection of coating layer when the restraint force in substrate was removed. When zirconia was coated on the substrate, tensile residual stress remains on zirconia coated surface layer. The tensile stress is increased to 0.35mm thickness and after 0.45mm thickness it is decreased abrouptly. A thick bond and composite coating reduce the zirconia surface stress and composite coating controls effectively the thick zirconia surface stress.

• Nuclear Engineering and Technology
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• v.48 no.1
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• pp.259-267
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• 2016

Most manufacturing processes, including welding, create residual stresses. Residual stresses can reduce material strength and cause fractures. For estimating the reliability and aging of a welded structure, residual stresses should be evaluated as precisely as possible. Optical techniques such as holographic interferometry, electronic speckle pattern interferometry (ESPI), Moire interferometry, and shearography are noncontact means of measuring residual stresses. Among optical techniques, ESPI is typically used as a nondestructive measurement technique of in-plane displacement, such as stress and strain, and out-of-plane displacement, such as vibration and bending. In this study, ESPI was used to measure the residual stress on the welded part of butt-welded American Society for Testing and Materials (ASTM) A36 specimens with $CO_2$ welding. Four types of specimens, base metal specimen (BSP), tensile specimen including welded part (TSP), compression specimen including welded part (CSP), and annealed tensile specimen including welded part (ATSP), were tested. BSP was used to obtain the elastic modulus of a base metal. TSP and CSP were used to compare residual stresses under tensile and compressive loading conditions. ATSP was used to confirm the effect of heat treatment. Residual stresses on the welded parts of specimens were obtained from the phase map images obtained by ESPI. The results confirmed that residual stresses of welded parts can be measured by ESPI.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.43-48
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• 2003

The problem of welding stresses and fatigue behavior is the main concerns of welding research fields. The residual stresses and distortion of structures by welding is exert negative effect on the safety of mechanical structures. That is, expansion of material by high temperature and distortion by cooling during welding process is caused of tensile and compressive residual stresses on welding material, and this residual stresses reduce fracture and fatigue strength of welding structures. The accurate prediction of residual stress and relaxation due to loading and post weld heat treatment of weld zone is very important to improve the quality of weldment. In this study, a finite element modeling technique is developed to simulate the relaxation of residual stresses due to loading and post weld heat treatment of weld zone. The accuracy of finite element models is evaluated based on experimental results and the results of the analytical solution.

• Proceedings of the KSR Conference
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• 2002.10a
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• pp.225-231
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• 2002

The problem of welding stresses and fatigue behavior is the main concerns of welding research fields. The residual stresses and distortion of structures by welding is exert negative effect on the safety of mechanical structures. That is, expansion of material by high temperature and distortion by cooling during welding process is caused of tensile and compressive residual stresses on welding material, and this residual stresses reduce fracture and fatigue strength of welding structures. The accurate prediction of residual stress and redistribution due to fatigue crack propagation of weld zone is very important to improve the quality of weldment. In this study, a finite element modeling technique is developed to simulate the redistribution of residual stresses due to fatigue crack propagation of weld zone.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.668-674
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• 2002

Structural integrity assessment is indispensable for preventing catastrophic failure of industrial structures/components/facilities. This diagnosis of operating components should be done periodically for safe maintenance and economical repair. However, conventional standard methods for mechanical properties have the problems of bulky specimen, destructive and complex procedure of specimen sampling. Especially, the mechanical properties at welded zone including weldment and heat affected zone could not be evaluated individually due to their size requirement problem. So, an advanced indentation technique has been developed as a potential method for non-destructive testing of in-field structures. This technique measures indentation load-depth curve during indentation and analyzes the mechanical properties related to deformation such as yield strength, tensile strength and work-hardening index. Also indentation technique can evaluate a residual stress based on the concept that indentation load-depth curves were shifted with the direction and the magnitude of residual stress applied to materials. In this study, we characterized the tensile properties and welding residual stress of various industrial facilities through the new techniques, and the results are introduced and discussed.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2007.04a
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• pp.265-270
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• 2007

According to previous research, welding-induced residual stresses in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Thee welding-induced residual stresses need be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation. In this study, a fatigue Analysis technique for steel structures with welding induced residual stress is presented. First, We calculate the history of temperature according with welding process. Secondly, residual stress with a welding thermal history was evaluated by non-linear thermal stress analysis and lastly, fatigue strength is estimated with modified Goodman equation which can consider the effect of mean stress level.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.58-58
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• 2010

The purpose of this study is to evaluate the structural safety at the topside weldment of hull structure, which was welded after launching. For it, the variations of residual stress and distortion at the topside weldment with loading conditions such as hull girder hogging bending moment after launching and free initial loading state was evaluated by using FEA. And the maximum stress range at the weldment under design loads specified by classification society was evaluated by FEA. In this case, the residual stress and welding distortion at the topside weldment was assumed to be initial imperfection. In accordance with FEA results, regardless of initial loading condition, tensile residual stress was found. However, the residual stress and welding distortion at the topside weldment produced under hogging condition was less than those of topside weldment under free loading state. That is, the amount of residual stress at the topside weldment decreased with an increase in the amount of tension load caused by hogging condition. It was because the compressive thermal strain at the topside weldment produced during welding was reduced by tensile load. However, the maximum stress range at the topside weldment under maximum hull girder bending moment was almost similar regardless of initial loading condition. So, if the problem related to the soundness of weldment is not introduced by initial load, the effect of initial loading condition during welding on fatigue strength of topside weldment could be negligible.