• 소성∙가공
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• 제28권6호
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• pp.368-374
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• 2019

The past recent years have seen an increasing use of high-strength steel sheets in the automotive industry. However, the formability and damage prediction of these materials requires accurate acquisition of necking and fracture strains. Digital image correlation (DIC) is used to accurately capture the necking and fracture strains during testing. The fact that single time points of capturing vary with frame rate makes the need for an investigation necessary. For the high-strength steel DP980, the frame-rate dependences of the final necking and fracture strains values are analyzed here. To eliminate the influence of gauge length, the strains were measured locally by DIC. Results for three specimen shapes obtained with frame rates of 1 and 900 fps (frames per second) were considered and based on them, triaxiality failure diagrams (TFD) are established. It was observed that after diffuse necking, the deformation path departed from the initially linear one, and the stress triaxiality grew with ongoing deformation. It was further revealed that the frame rate-dependence of the necking strain was rather low (< 2%), whereas the fracture strain could be underestimated by up to 8% when the lower frame rate of 1 fps was used (compared with 900 fps). In this study, this issue is investigated while taking into consideration the three different triaxialities. These results demonstrate the importance of choosing an appropriate frame rate for the determination of necking and fracture strains in particular.

• 소성∙가공
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• 제12권3호
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• pp.208-213
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• 2003

The methodology to design initial blanks considering the reduction of both springback and flange trimming amounts is studied. Three dimensional forming analysis of a trial blank Is first carried out using FEM and the tentative Initial blank shape is then determined by cutting the outer edge of the trial blank whose shape is nearly matched with the trimming line. During the shaping the blank edge, tile movement of blank outer line is described with random variables to reduce the sensitivity of initial blank geometry. After performing 2-D FEM forming and springback simulations for selected sections and optimizing the trimming and springback amounts in terms of section length of the blank, the initial blank is finally determined. In order to see tile springback reduction in the initial blank determined by the proposed method, a stepped s-rail is stamped and the sppingback is measured. The springback of newly designed initial blank of stopped s-tail is tremendously reduced.

• 한국산업융합학회 논문집
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• 제7권1호
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• pp.69-82
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• 2004

Recently, in the food, paper, steel and plastic industries, plate or sheet type products have been produced by the rolling drum. Steam heating drums are introduced into plastic products facilities in order to keep the density, microstructure, and strength of material uniformly. The drum journal can not help being concentrated by stresses due to the bending and torsion. Especially the drum, heated by high pressure steam, might be exposed in the steam leakage accident. First of all, the stresses on the steam drum journal are to be analyzed, and a case study proper to the subject was performed with a scraped journal, in order to investigate the failure characteristics as well as the initiation and propagation of fatigue cracks, and most probable circumstances of crack initiation. As the result of this study, it is suggested that newly installed drum journal be thoroughly inspected at the next periodic maintenance intervals for evidence of cracking, the microstructure examination and hardness measurements to prevent steam drum from the failure accident.

• 한국표면공학회지
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• 제48권1호
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• pp.27-32
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• 2015

Advanced high strength steels undergo recrystallization annealing in reducing gas atmosphere before galvanizing to improve mechanical properties. The selective oxidations of elements such as Mn, Si, Cr and Al during annealing decrease wettability of liquid zinc, resulting in bare spots and other defects. In this work, Fe-3wt%Mn steel sheet was annealed at $780^{\circ}C$ for 1200 sec. in 5% $H_2-N_2$ atmosphere and then dipped into zinc bath held at $460^{\circ}C$, which contained 0.2wt% dissolved Al. MnO crystallines in the average size of 200 nm were formed on the surface after annealing. It is estimated that MnO has been detached into bath with the formation and growth of inhibition layer with longer immersion time during galvanizing. No evidence of aluminothermic reduction of MnO has been found in this study.

• 소성∙가공
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• 제32권2호
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• pp.87-91
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• 2023

In the present work, the ductile fracture behaviors of ultra-high strength steel sheets along the different loading directions are investigated under various loading paths. Three loading paths, i.e., in-plane shear, uniaxial tension, plane strain tension deformations, are considered, and the corresponding specimens are described. The experiments are conducted using the digital image correlation (DIC) system to analyze the strain at the onset of the fracture. The experimental results show that the loading path for each specimen sample is linear, and different values of the fracture strains for the loading direction from the plane strain tension are observed. The ductile fracture model of the modified Mohr-Coulomb (MMC) is constructed based on the experimental data and evaluated along the rolling direction and transverse direction under various loading paths.

• 소성∙가공
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• 제31권1호
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• pp.11-16
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• 2022

In this paper, wear characteristics of PVD coatings were compared on the die surface for cold stamping of MART1470 steel sheet with the finite element analysis and the pin-on-disc wear test. Three types of PVD coatings (CrN, TiAlCrN, and MoS2TiCr(W)N) were considered for the tool surface made of STD11 material. The stamping process of an auto-body part was analyzed with the finite element method. Ranges of process variables for the wear test such as contact pressure, relative speed, and sliding distance were predicted from analysis results. In order to quantitatively analyze wear characteristics of each coating, the amount of wear was measured and compared according to process variables with the pin-on-disc wear test. The influence of each process variable was investigated and the wear characteristics of the three coating layers were quantitatively compared. It was confirmed that the wear characteristics of MoS₂TiCr(W)N coating were better than those of CrN and TiAlCrN. It was noted that the proposed prediction approach could predict and respond to the wear phenomenon occurring in the stamping process.

• 한국강구조학회 논문집
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• 제20권3호
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• pp.365-375
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• 2008

yLRC 합성기둥은 기존 합성기둥의 장점과 거푸집 공사의 절감을 가져올 수 있으며, CFT와 비교하였을 때 강재량 감소의 효과와 함께 콘크리트의 충전상태를 육안으로 쉽게 확인할 수 있는 등의 장점을 가지고 있다. 실험결과와 연구를 통하여 yLRC 합성기둥의 구조체로서의 내력은 검증이 되었으나 내화성능에 관한 연구는 수행되지 않았다. 이에 본 논문에서는 yLRC 합성기둥의 내화성능 평가를 위해 콘크리트 면적비, 보강판, 피복유무 등을 변수로 두어 실험과 해석을 수행하였으며, 실험과 해석결과를 비교 고찰하여 내화 성능 평가를 하였다.

• 소성∙가공
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• 제31권6호
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• pp.351-364
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• 2022

The yield criterion, or called yield function, plays an important role in the study of plastic working of a sheet because it governs the plastic deformation properties of the sheet during plastic forming process. In this paper, we propose a modified version of previous anisotropic yield function (Trans. Mater. Process., 31(4) 2022, pp. 214-228) based on J2 and J3 stress invariants. The proposed anisotropic yield model has the 6th-order of stress components. The modified version of the anisotropic yield function in this study is as follows. f(J₂⁰,J₃⁰) ≡ (J₂⁰)³ + α(J₃⁰)² + β(J₂⁰)^3/2 × (J₃⁰) = k⁶ The proposed anisotropic yield function well explains the anisotropic plastic behavior of various sheets such as aluminum, high strength steel, magnesium alloy sheets etc. by introducing the parameters α and β, and also exhibits both symmetrical and asymmetrical yield surfaces. The parameters included in the proposed model are determined through an optimization algorithm from uniaxial and biaxial experimental data under proportional loading path. In this study, the validity of the proposed anisotropic yield function was verified by comparing the yield surface shape, normalized uniaxial yield stress value, and Lankford's anisotropic coefficient R-value derived with the experimental results. Application for the proposed anisotropic yield function to AA6016-T4 aluminum and DP980 sheets shows symmetrical yielding behavior and to AZ31B magnesium shows asymmetric yielding behavior, it was shown that the yield locus and yielding behavior of various types of sheet materials can be predicted reasonably by using the proposed anisotropic yield function.

• 한국강구조학회 논문집
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• 제18권5호
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• pp.543-551
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• 2006

최근까지 벽식 스틸하우스에 구조재로 주로 적용되어오고 있는 박판냉간성형형강 스터드의 경우 열교현상에 의한 단열상의 문제를가지고 있기 때문에 추가적인 단열재를 사용하여야 한다. 이러한 문제를 해결하기 위한 새로운 개념의 아연도금강판(t =1.0mm-1.2mm)과 난연 강화플라스틱(GFRP) 패널 (t=4.0mm-6.0mm)로 구성된 복합스터드가 개발되었다. 복합스터드 패널을 주택 에 적용하기 위하여, 복합스터드의 구조적인 거동 및 내하력 평가를 수행하였다. 본 논문에서는 ATTM(Axial/Torsional Test machine)을 이용하여 수행된 압축력과 비틂을 동시에 받는 복합스터드의 실험적인 연구결과를 기술하였다. 압축-비틂 실험의 주요 변수는 복합스터드의 길이, 초기 압축력, 가력방법 등이며, 초기 압축력을 일정하게 유지한 상태에서 스터드 시험체가 종국적인 파괴에 이르도록 비틂 하중을 점차 증가시키는 방법으로 실험을 수행하였다. 또한 고강도 아연도금강판과 GFRP의 기계적인 특성을 고려한 비선형 유한요소해석을 수행하여 실험결과와 비교, 검증하였다.

• Journal of the Korean Wood Science and Technology
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• 제42권5호
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• pp.571-578
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• 2014

As a way of developing wooden joint development, a glass fiber reinforced wood plate was manufactured to replace a steel plate. Also, the fracture toughness was evaluated. Through application to a cantilever-type specimen made of a column and a beam, the moment resistance performance was evaluated. For the fracture toughness specimen of the wood plate, 12 types were manufactured by varying the combination of a main member (veneer and plywood) and reinforcement (glass fiber sheet and glass fiber cloth). The results of the fracture toughness test indicated that the 5% yield load of the specimen using plywood was 18% higher than that of the specimen using veneer, and that the specimen reinforced by inserting glass fiber sheets between testing materials (Type-3-PS) had the highest average 5% yield load 4841 N. Thus, a moment resistance strength test was performed by applying Type-3-PS to a column-beam joint. The results of the test indicated that compared to the specimen using a steel plate and a drift pin (Type-A), the maximum moment ratio of the specimen using a glass fiber reinforced wood plate (Type-3-PS) and a drift pin (Type-B) was 0.79; and that a rupture occurred in the wood plate due to high stiffness of the drift pin. The maximum moment ratio of the specimen using a glass fiber reinforced wood plate (Type-3-PS) and a glass fiber reinforced wooden laminated pin (Type-C) was 0.67, which showed low performance. However, unlike Type-A, a ductile fracture occurred on Type-C, and the load gradually decreased even after the maximum moment.