Jeong-Ung Park

Effect of welding sequence to minimize fillet welding distortion in a ship’s small component fabrication using joint rigidity method:

This study was performed to investigate the effect of welding sequence on fillet welding distortion. It proposes a new model of joint rigidity method, which can determine the welding sequence for minimum welding distortion, through a reliability validation done via experiments and welding distortion analysis using the equivalent load method. A new joint rigidity method considers the gap between base plate and stiffener and the fillet welding sequence at left/right bead. Fillet welding sequence at left/right bead could be considered by setting welding beads at left/right as a separate solid element after welding sequence was drawn by new joint rigidity method. Analysis and experiment for welding distortion for the test specimen with different stiffener distances were conducted. It was confirmed that the experimental and analysis results were well-matched. The validity of welding sequence using joint rigidity method was followed after the comparison of the welding distortion for the small components for ship building. Note that the welding sequence by which welding distortion became maximum and minimum was drawn using joint rigidity method for the small assembly components. Welding distortion was also analyzed using equivalent load method. Analysis results revealed that the maximum welding distortion less than 10 mm was generated. Welding distortion less than around 5 mm occurred in welding sequence which minimized welding distortion compared with welding sequence, generating maximum welding distortion.

Comparison of Measured Residual Stress Distributions in Extra-Thick Butt Welds Joined by One-Pass EGW and Multipass FCAW

This study is to measure the welding residual stress distributions in a 70 mm-thick butt weld by one-pass electron gas welding using both the inherent strain method and neutron diffraction method, respectively. Based on the measurement results, the characteristics of residual stress distribution through thickness were compared between one-pass electron gas welding and multipass flux-cored arc welding. Residual stresses in the specimens of electron gas welding measured by the inherent strain method and neutron diffraction method were well matched. The longitudinal residual stress in the multi-pass flux-cored arc welding is tensile through all thicknesses in the welding fusion zone. Meanwhile, longitudinal residual stress in electron gas welding is tensile on both surfaces and compressive at the inside of the plate. The magnitude of residual stresses due to electron gas welding is lower than that due to flux-cored arc welding.

Development of Fatigue Life Improvement Technology of Butt Joints Using Friction Stir Processing

Burr grinding, tungsten inert gas (TIG) dressing, ultrasonic impact treatment, and peening are used to improve fatigue life in steel structures. These methods improve the fatigue life of weld joints by hardening the weld toe, improving the bead shape, or causing compressive residual stress. This study proposes a new postweld treatment method improving the weld bead shape and metal structure at the welding zone using friction stir processing (FSP) to enhance fatigue life. For that, a pin-shaped tool and processing condition employing FSP has been established through experiment. Experimental results revealed that fatigue life improves by around 42% compared to as-welded fatigue specimens by reducing the stress concentration at the weld toe and generating a metal structure finer than that of flux-cored arc welding (FCAW). Hot-spot stress, structural stress, and simplified calculation methods cannot predict the accurate stress at the weld toe in case the weld toe has a smooth curvature as in the case of the FSP specimen. On the contrary, a finite element calculation could reasonably predict the stress concentration factor for the FSP specimen because it considers not only the bead profile but also the weld toe profile.

Measurement of Welding Residual Stress in a 25-mm Thick Butt Joint using Inherent Strain Method

Abstract Overlay welding is carried out to improve the corrosion resistance, wear resistance and heat resistance on the surface of the chemical plant and steelmaking plant structures. In overlay welding, control of the bead size and the temperature distribution of weldment are particularly important because that is directly connected to the improvement of quality and productivity. The aim of this study is to model the welding heat source that is very useful to analyze the bead size and temperature distribution of weldment. To find the welding heat source model, numerical analyses are performed by using FE software MSC-marc.Key Words : Overlay welding, Finite element analysis, Bead size, Temperature distribution 1. 서 론 용접에 의해 발생하는 잔류응력은 강구조물의 피로성능, 파괴 등에 영향 1) 을 준다. 따라서 구조물의 용접이음부의 안전한 설계를 하기 위해서는 용접 잔류응력 2-4) 의 분포를 정확히 예측 및 측정하여 그 영향을 평가하는 것이 중요하다. 용접잔류응력을 측정 및 예측하는 방법에는 실험적 방법과 해석적 방법이 있다. 해석적 방법은 컴퓨터 수치계산능력과 범용적인 수치해석 코드의 발달로 열탄소성해석 5-7) 에 의한 잔류응력을 용이하게 계산할 수 있다. 그러나 이러한 해석결과는 비교할 수 있는 측정된 결과가 없어 해석결과에 대해 신뢰성을 확인하기가 어렵다. 실험적 방법에 의한 잔류응력은 일반적으로 표면의 잔류응력을 측정할 수 있는 홀드링법

A Study on Serviceability of Oversized Bolt Hole in High-Tension Bolt Joint Subjected to Bending

If a design load exceeding the frictional force of the contact surface is applied to the connection of steel members using a high-tension bolt friction joint, sliding occurs and the connection of the steel members bears the design load through the shear strength and bearing strength of the bolt and the base plate. The sliding distance can be determined by the tensile force of the bolt, the friction coefficient of the contact surface, and the position of the bolt in the base plate hole. This study measured and analyzed sliding according to standard bolt hole and oversize bolt hole when pure bending moment and tensile force were applied to high-tension bolt joints with different sizes of bolt holes made in the base plate and the cover plate. In a high-tension bolt joint receiving pure bending moment and tensile force, the load causing sliding in an oversize bolt hole was of that in a standard bolt hole. In a member receiving tensile force, the sliding load ratio was lower when the size of oversize bolt holes in the base plate and the cover plate was large. In addition, the size of the oversize bolt hole in the base plate was more closely correlated with the change of sliding loadthan the size of the oversize bolt hole in the base plate.

Fracture Toughness of Thick Steel Plate for Ship Building

조선업을 포함한 다양한 산업 분야에서 후판 강재의 수요량의 증가와 사용범위의 폭이 넓게 되고 있다. 후판 의 사용에 따라서 강재의 파괴인성에 문제가 여러 측면 에서 제기 되고 있는 실정이다. 특히, 최근 선박 산업에 서는 컨테이너선으 대형화에 따라서 H/C을 비롯한 여 러 부분에서 극후판이 사용되고 있으며, 조선산업에서 용접이 차지하는 비중은 약 40%정도에 달하고 있다. 주 로 파괴인성은 이러한 용접에 의해서 더욱 취약하게 나 타나고 있으며, 선박의 모든 연결부는 용접프로세스를 사용하고 있으며, 선체구조물의 용접부 및 모재의 안전 성평가는, 선체구조물의 안전성과 직결된다. 선체에 용 접프로세스를 적용한 이래 제2차 세계대전 중 미국에서 건조된 전용접 전시표준함의 용접부 취성파괴에 의한 손상으로 많은 연구가 수행되어 모재 및 용접재료의 개 발과 연구가 진행되었고, 이를 통해 여러 가지 기준을 제시할 수 있는 단계가 되었다. 최근, 이러한 기준은 고강도 강재의 개발, 선박건조의 생산성 향상을 위한 용접프로세스 개발(대입열용접), 컨 테이너선의 초대형화에 따른 극후판 강재의 사용에 따 라서, 일부 선급협회(현재는 NK:일본선급협회)에서는 극후판 대입열용 TMCP강에 대하여 취성균열 전파 정 지 특성에 대하여 문제를 제기 하고 있으며, 용접부 건 전성평가방법의 재검토가 요구되고 있는 실정이다. 따라 서 여기서는 선체구조물의 용접부 취성파괴 발생의 원 인과, 파괴인성평가법 및 특성에 대해 기술 하고자 한 다.

Ductile Crack Initiation Criterion with Mismatched Weld Joints Under Dynamic Loading Conditions

Brittle failure of high toughness steel structures tends to occur after ductile crack initiation/propagation. Damages to steel structures were reported in the Hanshin Great Earthquake. Several brittle failures were observed in beam-to-column connection zones with geometrical discontinuity. It is widely known that triaxial stresses accelerate the ductile fracture of steels. The study examined the effects of geometrical heterogeneity and strength mismatches (both of which elevate plastic constraints due to heterogeneous plastic straining) and loading rate on critical conditions initiating ductile fracture. This involved applying the two-parameter criterion (involving equivalent plastic strain and stress triaxiality) to estimate ductile cracking for strength mismatched specimens under static and dynamic tensile loading conditions. Ductile crack initiation testing was conducted under static and dynamic loading conditions using circumferentially notched specimens (Charpy type) with/without strength mismatches. The results indicated that the condition for ductile crack initiation using the two parameter criterion was a transferable criterion to evaluate ductile crack initiation independent of the existence of strength mismatches and loading rates.

Bead Shape and Conditions of Friction Stir Processing to Improve Fatigue Strength

Abstract Burr grinding, Tungsten Inert Gas (TIG) dressing, ultrasonic impact treatment, and peening are used to improve fatigue life in steel structures. These methods improve the fatigue life of weld joints by hardening the weld toe, by improving the bead shape, and by creating the compressive residual stress. In this study, a new post-weld treatment method improving the weld bead shape and metal structure at the welding zone using Friction Stir Processing (FSP), a welding process, is proposed to enhance fatigue life. For that, a pin-shaped tool and processing condition employing Friction Stir Processing (FSP) is established through experiments. Experimental results revealed that fatigue life is improved by around 50% compared to as-welded fatigue specimens by reducing the stress concentration at the weld toe and by generating a metal structure finer than that of flux-cored arc welding (FCAW). Key Words : Friction stir processing, Fatigue strength, Bead shape, Butt welding, Stress concentration, Microstructure

A Study on the Change of Load Carrying Capacity of High-tension Bolt Joints by Critical Sections

This study conducted a static tensile test in order to prevent the lowering of load carrying capacity caused by critical sections made by over bolt holes in the base plate and the cover plate of steel member joints using high-tension bolts. The change of the load carrying capacity of joints was examined by comparison of the maximum load on joint fracture obtained from the tensile test with critical section rate and design strength. According to the results, the rate of decrease in strength was higher when the critical section rate was high, and in particular, decrease in strength was affected much more by critical sections in the base plate than by those in the cover plate. In high-tension bolt joints using over bolt holes for the base plate and the cover plate, load carrying performance was somewhat lower than that in joints using standard bolt holes, but the maximum tensile strength on facture was over 15% higher than design fracture strength. According to the results of this study, the use of over bolt holes in high-tension bolt joints had an insignificant effect on the lowering of load carrying capacity, so the allowance of over bolt holes in the joints of steel members is expected to enhance to the economy and efficiency of the works.

The Effect of Welding-Pass Grouping on the Prediction Accuracy of Residual Stress in Multipass Butt Welding

The residual stress analysis of a thick welded structure requires a lot of time and computer memory, which are different from those in thin welded structure analysis. This study investigated the effect of residual stress due to welding-pass grouping as a way to reduce the analysis time in multipass thick butt welding joint. For this purpose, the parametric analysis which changes the number of grouping passes was conducted in the multipass butt weld of a structure with a thickness of 25 mm and 70 mm. In addition, the residual stress by thermal elastoplastic FE analysis is compared with the results by the neutron diffraction method for verifying the reliability of the FE analysis. The welding sequence is considered in order to predict the residual stress more accurately when using welding-pass grouping method. The results of the welding-pass grouping model and half model occurred between the results of the left/right of the full model. If the total number of welding-pass grouping is less than half of that of welding pass, a large difference with real residual stress is found. Therefore, the total number of the welding-pass grouping should not be reduced to more than half.