• 한국자동차공학회논문집
• /
• 제11권1호
• /
• pp.143-150
• /
• 2003

Experimental research has been carried out to investigate the characteristics of the fatigue crack initiation and propagation behavior of Tailor Welded Blank(TB) sheet used for vehicle body. We used three types of specimens which were machined of the same base metal: one is 1.4mm thick, another is 1.6mm thick, and the third(TB specimen) is laser-welded of two specimens(1.4mm and 1.6mm thick ones). The results of tensile and hardness test indicate that the yield strength of the TB specimen is the highest, and the hardness around welding bead is higher than that of base metal. Fatigue strength and fatigue limit of the TB specimen are much superior to those of the base metal up to $10^6$ cycles. The fatigue crack propagation of the heat-affected zone of the TB specimen is slower than that of the base metal. Welding bead has the fastest crack Propagation in the low stress intensity factor range$(\DeltaK)$ region, but the slowest in the high $\DeltaK$ region. The fatigue propagation characteristic of the TB specimen is relatively stable in comparison with that of the base metal in the high ${\Delta}K$ region around over $28MPa\sqrt{m}$.

• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2009년도 제38회 동계학술대회 초록집
• /
• pp.53-53
• /
• 2010

Car industry has required light-weight steels, but still with strong mechanical strength. To meet this requirement, a variety of researches on Fe-Al alloys have been performed. As Al is being added in a disordered manner, alloys become more ductile and show higher yield stress. At a certain concentration of Al, however, the Fe-Al alloy system falls in a second phase whose mechanical strength is worsened. To understand the microscopic role of Al, we investigate the stability and the elastic properties of various Fe-Al alloys using ab initio density functional theory. At agiven Al concentration, the equilibrium geometry is obtained among several disordered Fe-Al alloy structures by performing the geometry relaxation. The formation energies and elastic properties such as bulk moduli of the equilibrium structures are also computed as a function of Al concentration. We also investigate the effects of different elements such as Si and Mn. Fe-Si alloy systems exhibit unusual mechanical behaviors requiring further investigation to understand their physical origin. Especially, the microscopic role of Mn is investigated to find its physical origin of preventing the Fe-Al alloy system from forming an unfavorable second phase. The effect of manganese on mechanical properties of Fe-based alloys is also explored.

• Structural Engineering and Mechanics
• /
• 제74권3호
• /
• pp.381-394
• /
• 2020

The structural performance of cold-formed steel (CFS) built-up battened columns were numerically investigated in this paper. The built-up column sections were formed by connecting two-lipped channels back-to-back, with a regular spacing of battens plates, and have been investigated in the current study. Finite element models were validated with the test results reported by the authors in the companion paper. Using the validated models, the parametric study was extended, covering a wider range of overall slenderness to assess the accuracy of the current design rules in predicting the design strengths of the CFS built-up battened columns. The parameters viz., overall slenderness, different geometries, plate slenderness (b/t ratio) and yield stress were considered for this study. In total, a total of 228 finite element models were analyzed and the results obtained were compared with current design strength predicted by Effective Width Method of AISI Specifications (AISI S100:2016) and European specifications (EN1993-1-3:2006). The parametric study results indicated that the current design rules are limited in predicting the accuracy of the design strengths of CFS built-up battened columns. Therefore, a design equation was proposed for the AISI and EC3 specifications to predict the reliable design strength of the CFS Built-up battened columns and was also verified by the reliability analysis.

• 한국기계가공학회지
• /
• 제20권6호
• /
• pp.59-69
• /
• 2021

Directed energy deposition (DED) is an additive manufacturing technology involving a focused high-power laser or electron beam propagating over the substrate, resulting in melt pool formation while simultaneously supplying metal powder to the melt pool area to deposit the material. DED is performed to repair and strengthen parts in various applications, as it can be easily integrate local area cladding and cross-material deposition. In this study, we characterize stainless steel 316 L parts fabricated via DED based on various deposition conditions and geometries to widen the application of DED. The deposition characteristics are investigated by varying the laser power and powder feed rate. Multilayer deposition with a laser power of 362 W and a powder feed rate of 6.61 g/min indicate a height closest to the design value while affording high surface quality. The microhardness of the specimen increases from the top to the bottom of the deposited area. Tensile tests of specimens with two different deposition directions indicate that horizontally long specimens with respect to a substrate demonstrate a higher ultimate tensile strength and yield strength than vertically long specimens with lower elongation.

• Steel and Composite Structures
• /
• 제31권4호
• /
• pp.409-417
• /
• 2019

The choice of individual material for industrial application is primarily based on knowledge of its behavior in similar applications and similar environmental conditions. Contemporary design implies knowledge of material behavior and knowledge in the area of structural analysis supported by large capacity computers. Bearing this in mind, this paper presents and analyzes the experimental results related to the mechanical properties of the material considered (30CrNiMo8/1.6580/AISI 4340) at different temperatures as well as its creep and fatigue behavior. All experimental tests were carried out as uniaxial tests. The test results related to the mechanical properties are presented in the form of engineering stress-strain diagrams. The results related to the creep behavior of the material are shown in the form of creep curves, while the fatigue of the material is shown in the form of stress - life (S - N) diagram. Based on these experimental results, the values of the following properties are determined: ultimate tensile strength (${\sigma}_{m,20}=696MPa$), yield strength (${\sigma}_{0.2,20}=355.5MPa$), modulus of elasticity ($E_{,20}=217GPa$) and fatigue limit (${\sigma}_{f,20,R=-1}=280.4MPa$). Results related to fatigue tests were obtained at room temperature and stress ratio R = -1.

• Steel and Composite Structures
• /
• 제38권6호
• /
• pp.705-716
• /
• 2021

An innovative partially precast partially encased composite beam (PPECB) is put forward based on the existing research. In order to study the flexural performance of the new composite beam which has precast part and cast-in-place part, six prefabricated specimens and one cast-in-place specimen are designed with considering the influence of the production method, the steel flange thickness, the concrete strength grade and the stirrup process on the behavior of the composite beam. Through four points loading and test data collection and analysis, the behavior of partially prefabricated specimen is similar to that of cast-in-place specimen, and the casting method, the thickness of the steel flange, the concrete strength grade and the stirrup process have different influence on the crack, yield and peak load bearing capacity of the component. Finally, the calculation theory of plastic bending of partially precast partially encased concrete composite beams is given. The calculation results are in good agreement with the experimental results, which can be used for practical engineering theory guidance. This paper can provide reference value for further research and engineering application.

• 한국공간구조학회논문집
• /
• 제23권1호
• /
• pp.61-68
• /
• 2023

Buckling Restrained Braces can not only express the strength considered at the time of design, but also reduce the seismic load by energy dissipation according to the plastic behavior after yield deformation of the steel core. The physical characteristics and damping effect may be different according to the buckling prevention method of the steel core by the lateral restraint element. Accordingly, in this study, To compare hysteresis characteristics, Specimen(BRB-C) filled with mortar, specimen(BRB-R) combined with a buckling restraint ring and Specimen(BRB-EP) filled with engineering plastics was fabricated, and a cyclic loading test was performed. As a result of the cyclic loading test, the maximum compressive strength, cumulative energy dissipation and ductility of each test specimen was similar. But in case of the cumulative energy dissipation and ductility, BRB-C filled with the mortar specimen showed the lowest. This is considered to be because the gap between the steel core and the reinforcing material for plastic deformation was not uniformly formed by pouring mortar around the core part.

• 열처리공학회지
• /
• 제29권1호
• /
• pp.8-14
• /
• 2016

This article presents a study on the tensile properties of hypoeutectoid steels with different ferrite-pearlite microstructures. Nine kinds of hypoeutectoid steel specimens were fabricated by varying carbon content and isothermal transformation temperature. The microstructural factors such as ferrite & pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured and then tensile tests were carried out on the specimens in order to investigate the correlation of the microstructural factors with strength and ductility. The pearlite volume fraction usually increased with decreasing transformation temperature, while the pearlite interlamellar spacing and cementite thickness decreased mostly with decreasing transformation temperature, irrespective of carbon content. The tensile test results showed that the yield and tensile strengths of all the steel specimens increased and their ductility was also improved as the transformation temperature decreased. For the steel specimens investigated, the difference in the transformation temperature dependence of strength and ductility could be explained by the fact that the variation in pearlite fraction with transformation temperature noticeably affected various microstructural factors such as pearlite interlamellar spacing and cementite thickness associated with pearlite fracture mechanism such as void initiation, cementite necking, and cracking.

• Steel and Composite Structures
• /
• 제18권4호
• /
• pp.985-1000
• /
• 2015

The method of cross-section analysis for different sections in a structural frame has been widely investigated since the 1960s for determination of sectional capacities of beam-columns. Many hand-calculated equations and design graphs were proposed for the specific shape and type of sections in pre-computer age decades ago. In design of many practical sections, these equations may be uneconomical and inapplicable for sections with irregular shapes, leading to the high construction cost or inadequate safety. This paper not only proposes a versatile numerical procedure for sectional analysis of beam-columns, but also suggests a method to account for residual stress and geometric imperfections separately and the approach is applied to design of high strength steels requiring axial force-moment interaction for advanced analysis or direct analysis. A cross-section analysis technique that provides interaction curves of arbitrary welded sections with consideration of the effects of residual stress by meshing the entire section into small triangular fibers is formulated. In this study, two doubly symmetric sections (box-section and H-section) fabricated by high-strength steel is utilized to validate the accuracy and efficiency of the proposed method against a hand-calculation procedure. The effects of residual stress are mostly not considered explicitly in previous works and they are considered in an explicit manner in this paper which further discusses the basis of the yield surface theory for design of structures made of high strength steels.

• 터널과지하공간
• /
• 제24권5호
• /
• pp.395-403
• /
• 2014

본 연구에서는 특수 재질의 경량 고강도 강관 록볼트를 개발하였다. 그라우트 주입확인시험을 실시하여 완전충전이 이루어지는지 알아보았고 인장시험을 통해 항복내하력을 조사하였으며 이를 기존의 이형봉강 록볼트와 비교하였다. 또한, RMR 분류등급이 서로 다른 국내 3개 현장에서 실시한 현장인발시험을 통해 본 록볼트의 강도 및 신장특성을 확인하였다. 개발된 록볼트는 이형봉강 록볼트에 비해 항복내하력이 클 뿐 아니라 경량이라 취급이 용이하며, 그라우트 완전충전 효과에 따라 부식성 환경에서 내구성이 클 가능성이 있는 것으로 생각된다.