• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1997.10a
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• pp.140-145
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• 1997

In case of spot-welded joints, the fatigue cracks generally originate from the weld interfaces of the neighborhood nugget tips, and propagate toward the outer surfaces of the sheets. Generally, because fatigue crack was observed in nugget around, strain gage was attached at nugget zone. Accordingly, it was very difficult to detect the generation time of fatigue crack in spot-welded joints and to measure the propagation speed of fatigue crack. We developed the non-destructive method, according to which th fatigue crack propagation rate can be quantitatively estimated by utilizing information obtained from strain gages bonded on the electrode indentations of spot welds. The results measured by real crack were compared with the data which was measured by strain gauge method in fatigue testing. And so fatigue strength was evaluated by stress intensity factor. In this study behavior of fatigue crack propagation under repeated load were considered.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.12 no.1
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• pp.32-37
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• 2003

Evaluation of fatigue strength on the spot welded part is very important for strength design of spot welded steel structures. In this paper, we could get the life cycle of the spot welded part using the lethargy coefficient obtained through the quasi-static tensile shear test for the specimen welded by current 10kA. The reliability evaluation of the life cycle is completed by comparing the life cycle calculated under the constant loading rate with the life cycle obtained by dynamic fatigue test. And then the result calculated by the lethargy coefficient is verified through the lift cycle calculated using the dynamic final tensile stress formula under the increased loading speed. This way can make save the time and cost in processing of predicting the life cycle of a structure.

• Proceedings of the KWS Conference
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• 2008.05a
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• pp.8-8
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• 2008

• Journal of the Society of Naval Architects of Korea
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• v.35 no.4
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• pp.65-75
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• 1998

The $CO_2$ welding with 100% $CO_2$ gas is commonly used because of its cost and efficiency. Arc phenomena and spattering ratio of the $CO_2$ welding are influenced by various factors such as chemical compositions of welding wire, shielding gas, welding condition and welding power source etc.. Spattering ratio is predominantly influenced by the welding condition which determines a droplet transfer rode. In this study, arc phenomena and spattering ratio are investigated by using two type of FCW(titania type, semi-metal type). Then, the welding quality and optimum welding condition can be selected. From this study, the following results ware obtained; 1) In low current range(140A), FCW up to welding voltage(22V) resulted in a typical short circuit transfer, increase of spattering ratio and growth of spatter diameter. 2) In high current range(320A), the arc stability in titania FCW of a typical globular transfer is better than that of semi-metal FCW.

• Journal of Korean Society of Steel Construction
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• v.29 no.1
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• pp.25-36
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• 2017

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent. The presence of a concrete slab and resulting composite action was speculated as one of the critical causes of the prevalent bottom flange fracture. In this study, four seismic retrofit schemes are proposed in order to salvage welded steel moment connections with composite floor slabs in existing steel moment frames. Because top flange modification of existing beams is not feasible due to the presence of a concrete floor slab, three schemes of bottom flange modification by using welded triangular or straight haunches or RBS(reduced beam section), and beam web strengthening by attaching heavy shear tab were cyclically tested and analyzed. Test results of this study show that haunch and web-strengthened specimens can eliminate the detrimental effect caused by composite action and ensure excellent connection plastic rotation exceeding 5% rad. Design recommendations for each retrofit scheme together with supplemental numerical studies are also presented.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.1
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• pp.48-53
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• 2006

A methodology is described for predicting the fatigue life of the resistance spot weldment including strain rate effect. Because it is difficult to perform a physical failure test with high strain rate, an analytical method is necessary to get the mechanical properties of various strain rate, To this end, quasi-static tensile-shear tests at several strain rate were performed on spot weldments of SPC. These test provided the empirical data with the strain rate. With these results, we formulated the function of fatigue life prediction using the lethargy coefficient which is the global material property from tensile test. And, we predicted the fatigue life of spot weldment at dynamic strain rate. To confirm this method for fatigue life prediction, analytical results were compared with the experimental fatigue data.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.10
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• pp.1179-1185
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• 2011

Cold and hot-rolled carbon steel sheets are commonly used in railroad cars or commercial vehicles such as the automobile. The sheets used in these applications are mainly fabricated by spot welding, which is a type of electric resistance welding. However, the fatigue strength of a spot-welded joint is lower than that of the base metal because of high stress concentration at the nugget edge of the spot-welded part. In particular, the fatigue strength of the joint is influenced by not only geometrical and mechanical factors but also the welding conditions for the spot-welded joint. Therefore, there is a need for establishing a reasonable criterion for a long-life design for spot-welded structures. In this thesis, ${\Delta}P-N_f$ relation curves have been used to determine a long-life fatigue-design criterion for thin-sheet structures. However, as these curves vary under the influence of welding conditions, mechanical conditions, geometrical factors, etc. It is very difficult to systematically determine a fatigue-design criterion on the basis of these curves. Therefore, in order to eliminate such problems, the welding residual stresses generated during welding and the stress distributions around the weld generated by external forces were numerically and experimentally analyzed on the basis of the results, reassessed fatigue strength of gas welded joints.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.13-13
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• 2009

현재 열간단조 금형을 제작함에 있어 육성용접을 실시하는 방법이 금형강 STD61, STD11 등으로 제작하는 방법에 비해 보수나 비용적인 측면에서 이점을 가지고 있기 때문에 점차적으로 증가하고 있는 추세이다. 열간단조 공정에서 금형은 $1000^{\circ}C$이상의 고온재료와 반복접촉하게 된다. 이때 이형제의 사용은 급속냉각 및 급속가열의 열피로를 가속시킨다. 또한, 금형은 반복충격에 의한 기계적 피로를 받게 된다. 이러한 금형의 사용환경을 고려한 FCW는 종래 고가의 $2.8{\sim}3.2{\Phi}$인 외국산 FCW를 사용하였으나 이를 대체한 $3.2{\Phi}$ 태경 FCW가 국내에서 개발되었다. 하지만 개발된 FCW를 사용하여 제작된 금형의 수명이 부족한 현상이 발생하였다. 이에 금형의 수명을 연장시킬 수 있는 내균열성 및 내열충격성을 확보한 태경 FCW의 개발과 개발된 FCW의 성능평가가 요구되었다. 특히 열간단조 금형에 있어서 중요한 내열충격성의 경우 가열과 냉각의 반복 Cycle에 의한 Thermal shock의 평가가 대부분이며 높은 Cycle로 인해 많은 시간이 걸리며, 또한 가열과 냉각을 오갈 수 있는 고가의 시험장치가 요구된다. 그러므로 개발된 FCW 육성용접부의 내균열성 및 내열충격성을 평가할 수 있는 방법에 대한 연구와 특히 내열충격성을 시간이 적게 걸리면서도 경제적으로 평가할 수 있는 방법에 대한 연구가 필요하다. 본 연구의 목적은 열간단조 금형 육성용접부의 내균열성 및 열충격특성을 평가할 수 있는 방법에 대한 검토와 특히 내열충격성에 대해 J.W.Kim등의 시험방법을 참고하여 시간이 적게 걸리면서 저 비용으로 열 충격특성을 평가할 수 있는 시험법을 고안하는 것이다. 이를 위한 방법으로 육성용접부의 내균열성을 평가하기 위한 상온 Bending을 실시하였고, 내열충격성을 평가하기 위한 염욕로를 이용하는 고온 Bending을 고안하여 실시하였다. 상온 Bending, 고온 Bending 모두 3점 굽힘시험을 적용하였다. 고온 Bending의 가열방법으로는 염욕로를 사용하여 시편이 대기중에서 약 $850^{\circ}C$의 온도가 될 수 있도록 하였다. 시편은 각각 열처리를 하여 요구 경도를 확보하였고, 이를 염욕로에서 5분간 가열 및 유지하여 취출 후 굽힘하중을 가하여 변위의 정도로 열충격을 평가하는 방법을 사용하였다. 상온 Bending은 극한변형량과 파단부 극한응력으로, 고온 Bending은 고온 극한변형량으로 평가를 하였고, 외국산 FCW를 사용한 육성용접부를 비교대상으로 하였다. 평가 결과 개발된 국산 $3.2{\Phi}$ 태경 FCW의 성능은 외국산 FCW와 유사하거나 우수한 것으로 평가되었고, 실제 금형을 제작하여 현장에 적용한 결과 금형의 수명이 연장된 것이 나타났다.

• Journal of Ocean Engineering and Technology
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• v.13 no.3 s.33
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• pp.49-56
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• 1999

Welding is a reliable process and is mainly adopted for fabricating heavy structures. Recently, transverse cracks in the weld metal is serious problem, and they affect cost, efficiency, safety and joint reliability for various welded structures. In this view of point, this study investigated the potential factors for weld metal transverse crack. The main results obtained are as follows; 1) The content of diffusible hydrogen in the commercial flux cored are welding wire was remarkable change by manufacturer. 2) The diffusible hydrogen content was thd main factor for weld metal transverse cracks. 3) Weld metal was immune to transverse cracking under the condition of low diffusible hydrogen content of high restraint condition. 4) The factors for weld metal transverse crack would be the content of diffusible hydrogen and restraint of weld joint.

• Journal of Welding and Joining
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• v.13 no.2
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• pp.106-114
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• 1995

The characteristics of adhesive bonded joint of steels for automobile were investigated. Shear and tear strength were tested and analyzed for the joints of cold rolled steel sheets bonded with three kinds of epoxy and urethane based adhesive. The results showed that the tensile shear strength and the tear strength of adhesive joint were affected by the shape of adhesive joint such as the length and width of adhesive joint. The thickness of adhesive layer was very important factor affecting the bonding strength. The shear strength increased with decrease of the thickness of adhesive layer, while the tear strength decreased as the thickness of adhesive layer decreased. In comparison with the strength of spot welded joint, the shear strength of adhesive Joint was higher than that of spot welded joint, but the tear strength of adhesive Joint was lower than that of spot welded joint.