• Transactions of the Korean Society of Automotive Engineers
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• v.7 no.6
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• pp.279-286
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• 1999

Failure of spot-weld joints in car body may cause noise, vibration and safety problems. Systematic procedures for estimating fatigue strength of general spot-weld joints are developed in this paper. Fatigue test results for various spot-welded specimens are analysed using the finite element method. It is found that the maximum principal strain at the nugget boundary on the inner surface governs the fatigue failure of spot-welded joints.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.5
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• pp.278-284
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• 2017

In this study, the heat input conditions suitable for the AZ61 magnesium alloy were derived by controlling the welding speed at a constant rotational speed. In addition, from an economic point of view, industry demands higher welding speeds. Therefore,the effects of the welding speed were studied. The rotational speed applied was 800rpm, and the welding speed was varied from 100 to 500mm/min to evaluate the behavior of the welded regions. Tensile and hardness tests were conducted to examine the mechanical properties. Optical microscopy was used to observe the microstructure and soundness of the welded regions. Defects were observed at the welded region when the welding speed was more than400mm/min. As the welding speed increased, the grain size of the stir zone decreased and the hardness tended to increase proportionally. When the rotational speed was 800 rpm and the welding speed was 200mm/min and 300mm/min, there wereno defects in the welded region and excellent mechanical properties were recorded. In addition, the joint efficiencies were 100.5% and 101.2%, respectively, and the ultimate tensile strength was similar to that of the base metal. Fracture of the tensile specimen occurred between the advancing side and stir zone, and the fracture location coincided with the region where the hardness decreased temporarily.

• Journal of the Korean Institute of Gas
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• v.12 no.2
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• pp.53-56
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• 2008

We have developed a system for nondestructive measurement of the magnetic flux leakage at welding flaws, existing in a gas pipeline by Hall sensor. For measuring the magnetic flux leakage, we designed a reference specimen having four kinds of welding flaws. Magnetic flux leakage is measured around the welding flaws of the specimen. The possibility for classification of different kinds of welding flaws is carried out by means of the peak-peak value and the interval between peak-peak of the magnetic flux leakage.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05b
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• pp.312-317
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• 1998

사용 후 핵연료 수송용기는 자유낙하 충돌사고에 대비해 양 끝단에 충격완충체를 부착하는데 이 충격완충체의 층격흡수특성은 수송용기의 구조적 안전성애 크게 영향을 미친다. 충격완충체의 층격흡수재를 감싸주는 철제 케이스와, 내부 격막판의 용접 접합부는 제작공정상 일부 부분이 부분접합 형태를 갖기 때문에 완전 접합된 부분이나 모재인 판재 부분에 비해 강도가 약하다. 따라서, 본 연구에서는 충격완충체의 부분접합부와 같은 조건의 접합상태를 고려한 용접시편의 시험을 통해서 충격완충체 케이스의 용접 접합부에 대한 접합강도와 기계적 특성을 분석하고, 접합부의 강도특성이 수송용기의 자유낙하 충돌거동에 미치는 영향을 예측하였다.

• Proceedings of the KIPE Conference
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• 2002.11a
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• pp.85-88
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• 2002

저항 스폿용접은 두 개의 금속판을 서로 겹쳐서 양쪽에 전극으로 압력을 가하면서 대전류를 흘려보낼 때 재료의 접촉면의 저항에 의해 발생하는 줄울열을 이용하여 재료를 용융시켜 두 모재를 접합시키는 방법이다 산업현장의 자동 용접라인에서 로봇암의 끝단에 부착하여 사용하던 기존의 공압건은 시편에 주어지는 압력이 일정하여 실시간 가압력 변화가 어렵고, 전극벌림의 임의 조정이 불가능하였다. 본 연구에서는 서보 모터를 채용한 서보건 시스템의 실시간 가압력제어와 용접도중 일정한 파우어를 공급하는 정전력 제어방식을 이용하여 용접 품질을 향상시키고자 한다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.10
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• pp.1213-1219
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• 2012

A remote $CO_2$ laser system can rapidly change both the distance and the direction of the laser beam by moving a lens and rotating mirrors. It is then easy to weld complex patterns of weld lines. A conventional spot weld joint specimen and a remote $CO_2$ laser weld joint specimen with complex weld line patterns were prepared and tested both statically and dynamically. The relationships between the fatigue strength, i. e. the maximum cyclic force, and the fatigue life were obtained. The fatigue strength of the tested welded joints at two million cycles was found to be approximately 10% of the static strength. Furthermore, it was observed that the fatigue fracture mode changed with the level of the applied cyclic force. The fatigue crack origins were confirmed as the highest stress points found in the structural analysis. The maximum cyclic stress for different weld patterns converges as the fatigue life approaches two million cycles.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.23-23
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• 2009

우수한 용접부 품질 때문에 TIG를 이용한 오비탈 용접은 파이프 용접에 널리 사용되고 있다. 하지만 루트갭이 없고 루트페이스가 큰 맞대기 오비탈 용접의 위보기 및 경사상진자세에서는 오목한 이면비드가 형성되기 쉽지만, 이러한 문제를 극복하기 위한 연구는 희박한 실정이다. 본 연구에서는 위보기 및 경사상진자세에서 볼록한 이면비드의 형성을 연구하기위해서 용융지의 제어 방법을 적극적으로 검토하였다. 4mm 두께의 SS400 시편을 위보기 및 경사상진자세에서 각각 Bead-on-plate 용접하고, 이 때 비드성형기의 사용에 따른 비드 형상 변화를 관찰하였다. 텅스텐 전극과 비드 성형기간의 거리(Tip To Former Distance, 이하 TTFD)를 4.5mm에서 7.5mm로 1mm단위로 변경시켜 실험하였으며, TTFD가 증가할수록 위보기 및 경사상진자세에서 이면비드 높이가 감소하였으며 표면비드의 처짐이 증가하였다.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.60-60
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• 2009

본 연구는 보론이 첨가된 저합금강 용접 열영향부에서의 보론 편석 거동 연구를 위해 보론이 10ppm 첨가된 저합금강을 이용하여 다양한 용접 입열량 및 외부 응력에 따른 용접부 CGHAZ의 보론 편석거동을 분석하였다. 이를 위해 Gleeble 시스템을 이용하여 다양한 입열량에 따른 CGHAZ를 열 및 열-응력 사이클을 통하여 재현하였다. 재현된 시편의 미세조직은 OM을 통하여 분석하고, 보론의 편석거동을 SIMS와 PTA 분석법을 통하여 분석하였다. 그 결과 입열량에 따른 보론의 편석 거동은 최초 입열량이 증가함에 따라 보론의 편석이 증가하다가 다시 감소하였는데 이는 비평형 편석 후 고온에서 유지시간이 길어짐에 따라 back diffusion 발생에 따른 영향으로 판단된다. 또한 외부 응력에 의한 보론 편석 거동 분석 결과, 용접 열 사이클 중 작용하는 외부 응력에 의해 결정립계 편석 감소하였는데 이는 외부 응력에 의한 오스테나이트 결정립 크기 감소에 따른 결정립계 증가의 영향으로 판단된다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.3
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• pp.319-325
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• 2015

High-strength steel has replaced mild steel as the material of choice for truck decks or frames, owing to the growing demand for lightweight vehicles. Although studies on the weld fatigue characteristics of mild steel are available, studies on high-strength steels have been seldom conducted. In this study, firstly, we surveyed a chosen number of approaches and selected the Radaj method, which uses the notch factor approach, as the one suitable for evaluating the fatigue life of commercial vehicles. Secondly, we obtained the S-N curves of HARDOX and ATOS60 steel welds, and the F-N curves of the T-weld and overlapped-weld structures. Thirdly, we acquired a general S-N curve of welded structures made of high-strength steel from the F-N curve, using the notch factor approach. Fourthly, we extracted the weld fatigue characteristics of high-strength steel and incorporated the results in the database of a commercial fatigue program. Finally, we compared the results of the fatigue test and the CAE prediction of the example case, which demonstrated sufficiently good agreement.

• Journal of the Korean Society for Railway
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• v.12 no.6
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• pp.895-901
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• 2009

Fatigue fracture of the welded joints of bogie frames is one of unsolved problems. To improve the fatigue strength of the welded joints, various techniques have been applied. In this study, we have investigated the effect of shot peening on the fatigue characteristics of SS400 welded specimens. The fatigue lives of four kinds of welded specimens were examined; welded, PWHT, shot peened, and shot peened after PWHT. The results of the fatigue tests show that the fatigue limit of the shot-peened specimens is higher than that of the welded specimens, and the PWHT specimens have a lower fatigue limit than the welded specimens. It was found that shot peening is a very effective method to improve the fatigue strength of SS400 welded joints.