• Steel and Composite Structures
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• 제53권2호
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• pp.243-252
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• 2024

Conventional carbon mild steel is a type of steel known for its low carbon content and generally used in the construction industry. Its easily formable and weldable properties make this steel a widely preferred material for buildings, bridges and various construction projects. Other advantages of these steels are their low cost and good mechanical properties. However, high temperatures have an impact on the microstructure and mechanical characteristics of these materials. When high temperatures are present during a fire, steels show significant microstructural changes. Elevated temperatures often decrease the mechanical characteristics of steels. For this purpose, evaluating the post-fire behavior of conventional structural mild steel is an important issue in terms of safety. A combined experimental and parametric study was conducted to estimate fire damage to steel buildings, which is an important issue in the construction field. Tensile test coupons were cut from conventional structural S235JR mild steel sheets with thicknesses ranging from 6 mm to 12 mm. These samples were exposed to temperatures as high as 1200 ℃. After heat treatment, the specimens were allowed to naturally cool to ambient temperature using air cooling before being tested. A tensile test was performed on these coupons to evaluate their mechanical properties after fire, such as their elastic modulus, yield strength, and ultimate tensile strength. The mechanical behavior of conventional S235JR structural steel changed significantly when the heating temperature reached 600℃. The thickness of the steel had a negligible effect on yield strength loss, with the highest measured loss being 50% for 8 mm thickness at 1200℃. For thinner sections (6 mm), yield strength decreased by up to 40%, while thicker samples (12 mm) showed similar reductions. Ultimate tensile strength also showed minimal changes up to 600℃, but beyond this point, a notable decline occurred, with approximately 30% strength loss at 1200℃. The modulus of elasticity remained almost constant up to 800℃, but at 1200℃, the loss reached around 20% for thicker sections (10 mm and 12 mm) and up to 35% for thinner sections (6 mm and 8 mm). Overall, high temperatures led to significant deterioration in both yield and ultimate strength, with a general loss of load-bearing capacity above 600℃. A new equation was formulated from experimental results to predict changes in the mechanical properties of S235JR steels. This equation offers a precise evaluation of buildings made from conventional structural S235JR mild steel after fire exposure. Furthermore, the empirical equation is applicable to low-strength steels with yield strengths ranging from 235 MPa to 420 MPa.

• Steel and Composite Structures
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• 제27권2호
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• pp.229-242
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• 2018

High strength steel is widely used in industrial applications to improve the load-bearing capacity and reduce the overall weight and cost. To take advantage of the benefits of this type of steel in construction, an innovative hybrid fabricated member consisting of high strength steel tubes welded to mild steel plates has recently been developed. Component-scale uniaxial and multiaxial cyclic experiments have been conducted with simultaneous constant or varying axial compression loads using a multi-axial substructure testing facility. The structural interaction of high strength steel tubes with mild steel plates is investigated in terms of member capacity, strength and stiffness deterioration and the development of plastic hinges. The deterioration parameters of hybrid specimens are calibrated and compared against those of conventional steel specimens. Effect of varying axial force and loading direction on the hysteretic deterioration model, failure modes and axial shortening is also studied. Plate and tube elements in hybrid members interact such that the high strength steel is kept within its ultimate strain range to prevent sudden fracture due to its low ultimate to yield strain ratio while the ductile performance of plate governs the global failure mechanism. High strength material also significantly reduces the axial shortening in columns which prevents undesirable frame deformations.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1994년도 봄 학술발표회 논문집
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• pp.181-188
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• 1994

Generally, the structural material shows rate dependent behaviors, which require to constitute different strain-stress relations according to strain rates. Conventional rate- independent constitutive equations used in general purpose finite analysis programs are inadequate for dynamic finite strain problems. In this paper, a rate dependent constitutive equation for elastic-plastic material was developed. The plastic stretch rate was modeled based on slip model with dislocation velocity and density so that there is no yielding condition, and no loading conditions. Non-linear hardening rule was also introduced for finite strain. Material constants of present constitutive equation were determined by experimental data of mild steel. The constitutive equation was applied to uniaxile tension. It was appeared that the present constitutive equation well simulates rate dependent behaviors of mild steel.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1810-1815
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• 2003

Tube hydroforming provides a number of advantages over conventional stamping process, including fewer secondary operation, weight reduction, assembly simplification, adaptability to forming of complex structural components and improved structural strength and stiffness. In this study, the effect of the heat treatment on the hydro-formability has been investigated. By using the mild steel tube bulging test is performed at various heat treatment conditions to evaluate the hydro-formability.

• Steel and Composite Structures
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• 제34권2호
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• pp.189-213
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• 2020

Stainless-clad (SC) bimetallic steels that are manufactured by metallurgically bonding stainless steels as cladding metal and conventional mild steels as substrate metal, are kind of advanced steel plate products. Such advanced composite steels are gaining increasingly widespread usage in a range of engineering structures and have great potential to be used extensively for large civil and building infrastructures. Unfortunately, research work on the SC bimetallic steels from material level to structural design level for the applications in structural engineering field is very limited. Therefore, the aim of this paper is to investigate the material behaviour of the SC bimetallic steels under the cyclic loading which structural steels usually could encounter in seismic scenario. A number of SC bimetallic steel coupon specimens are tested under monotonic and cyclic loadings. The experimental monotonic and cyclic stress-strain curves of the SC bimetallic steels are obtained and analysed. The effects of the clad ratio that is defined as the ratio of the thickness of cladding layer to the total thickness of SC bimetallic steel plate on the monotonic and cyclic behaviour of the SC bimetallic steels are studied. Based on the experimental observations, a cyclic constitutive model with combined hardening criterion is recommended for numerical simulation of the cyclic behaviour of the SC bimetallic steels. The parameters of the constitutive model for the SC bimetallic steels with various clad ratios are calibrated. The research outcome presented in this paper may provide essential reference for further seismic analysis of structures fabricated from the SC bimetallic steels.

• Structural Engineering and Mechanics
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• 제13권6호
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• pp.591-614
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• 2002

This paper presents experimental and numerical analyses of the thermomechanical behaviour that takes place in SAE1020 mild steel cylindrical specimens during the conventional tensile test. A set of experiments has been carried out in order to obtain the stress-strain curve and the diameter evolution at the neck which allow, in turn, to derive the elastic and hardening parameters characterizing the material response. Temperature evolutions have also been measured for a high strain rate situation. Moreover, a finite element large strain thermoelastoplasticity-based formulation is proposed and used to simulate the deformation process during the whole test. Some important aspects of this formulation are discussed. Finally, the results provided by the simulation are experimentally validated.

• 대한조선학회논문집
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• 제34권1호
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• pp.77-86
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• 1997

최근들어 안전하고 합리적인 구조를 설계하기 위하여 구조물의 내충돌 또는 내충격에 대한 요구와 관심이 높아지고 있는데, 이러한 문제들은 아주 짧은 시간동안에 대변형이 일어나는 비선형문제라는 특징이 있다. 구조재료는 변형속도가 빨라짐에 따라 정적인 범주에서 보이는 거동과는 달리 변형률 의존적인 거동을 보인다. 따라서 대변형 소성문제인 충돌해석 등에는 종래 사용하여 온 변형률 비의존 재료구성방정식으로는 한계가 있다. 이 논문에서는 이러한 점을 개선하기 위하여 연강의 소성거동을 잘 나타낼 수 있는 소성슬립모델을 채용하고, 비선형경화를 도입하여 변형도 적용범위를 확장한 대변형 탄소성 변형률의존 재료구성방정식을 제시하였다. 본 구성방정식의 특징으로 항복조건과 하중조건이 필요없기 때문에 계산이 간편하며, 전위밀도와 속도로써 소성을 표현하기 때문에 보다 물리적인 의미를 가지고 금속재료의 소성현상을 나타낼 수 있다.