• Transactions of the Korean Society of Automotive Engineers
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• v.23 no.3
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• pp.279-286
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• 2015

To achieve lightweight design, research & development of various lightweighting technologies such as hydroforming are underway worldwide. In the case of hydroforming, application of ultra high strength steel is essential for weight reduction of the car. However, considering common high-strength carbon steel, it is not suitable to the actual hydroformed parts since the lack of formability. DP steel offers an outstanding combination of strength and formability as a result of their microstructure. DP steel has high strength and good formability but it's difficult to secure stable quality of welding section because of softening of weld section and chemical composition. Therefore, most of companies use LASER welding when making high strength tube. Electric resistance welding is excellent production method for steel tube manufacturing considering the productivity. Optimum electric resistance welding technology is needed to be developed for application of high strength hydroformed parts using DP steel. This study is comprehensive research & development from electric resistance welding to actual formabililty evaluation.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2011.11a
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• pp.209-210
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• 2011

This study is aimed to draw a optimum combined fiber mix condition to improve spalling resistance and flowability of ultra high-strength concrete. As a result, W/B 12.5% concrete specimens were prevented spalling with PE0.05+ PP0.1, PE0.05+NY0.1 and W/B 12.5% concrete specimens were prevented spalling with all of combined organic fiber mix condition. But There is no significant influence of steel fiber under 5% volume ratios to prevent spalling. In the scope of this study, we suggest that condition of optimum volume ratio PE0.05+NY0.1 is to improve spalling resistance, flowability and residual compressive strength.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2013.11a
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• pp.36-37
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• 2013

It is the study on the mechanical properties of the material when mixing hybrid fiber as steel and synthetic fiber to improve fire resistance and toughness of concrete. Finally, The purpose is to identify of mechanical properties of Hybrid Fiber-Reinforced Ultra High Strength Concrete such as flexural strength.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2017.05a
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• pp.204-205
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• 2017

In this study, flowability and strength on ultra high performance concrete(UHPC) pre-mixed binders with fiber was investigated. The flow of UHPC with pre-mixed binders was higher than that of seperate mixing conditions. The UHPC using PVA fiber with high specific surface area showed a low flow compared to steel fiber. An pre-mixing method led to improved strength of UHPC and low deviation of specimens due to dispersion effect of each materials.

• Journal of Welding and Joining
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• v.32 no.5
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• pp.44-49
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• 2014

The effect of weldbond hybrid joining process on the mechanical behavior of single lap and L-tensile joints was investigated for the newly developed 1.2GPa grade ultra high strength TRIP(transformation induced plasticity) steel. In the case of single lap shear behavior, the weldbond joint of 1.2GPa TRIP steel showed lower maximum tensile load and elongation than that of the adhesive bonding only. It was considered to be due to the reduction of real adhesion area, which was caused by the degradation of adhesive near the spot weld, and the brittle fracture behavior of the spot weld joint. In the case of L-tensile behavior, however, the maximum tensile load of the weldbond joint of 1.2GPa TRIP steel was dramatically increased and the fracture mode was change to the base metal fracture which is desirable for the spot weld joint. These synergic effect of the weldbond hybrid joining process in 1.2GPa TRIP steel was considered to be due to the stress dissipation around the spot weld joint by the presence of adhesive which resulted in the change of crack propagation path.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.573-576
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• 2004

High performance SFRC composed of mira-sized ultra fine particles is characterized by high strength, high ductility and excellent durability. therefore many researches about materials based on new composition like this are performed recently. many researchers have reported that adding steel fiber to concrete improved its tensile and flexural strength significantly. the main objective of this research is to examine the effect of adding steel fiber on the tensile strength of high performance SFRC. variables considered in this study are w/c ratio and fiber volume fraction.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4A
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• pp.267-279
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• 2009

A numerical model that can simulate the nonlinear behavior of ultra high strength fiber reinforced concrete (UHSFRC) structures subjected to monotonic loading is introduced. The material properties of UHSFRC, such as compressive and tensile strength or elastic modulus, are different from normal strength reinforced concrete. The uniaxial compressive stress-strain relationship of UHSFRC is designed on the basis of experimental result, and the equivalent uniaxial stress-strain relationship is introduced for proper estimation of UHSFRC structures. The steel is uniformly distributed over the concrete matrix with particular orientation angle. In advance, this paper introduces a numerical model that can simulate the tension-stiffening behavior of tension part of the axial member on the basis of the bond-slip relationship. The reaction of steel fiber is considered for the numerical model after cracks of the concrete matrix with steel fibers are formed. Finally, the introduced numerical model is validated by comparison with test results for idealized UHSFRC beams.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.11a
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• pp.647-650
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• 2005

This paper is to investigate the shrinkage properties of ultra high strength concrete(UHSC) incorporating $5\%$ of expansive additives(EA) along with $1\%$ of shrinkage reducing agent(SRA). UHSC subjected to steam curing and incorporated with steel fiber exhibited higher compressive strength than control UHSC by as much as50MPa at 7days, while at 28days, noticeable change in compressive strength was not observed between UHSC mixtures. Control UHSC subjected to steam curing had a $922{\times}10^6$ of autogenous shrinkage strain value, which was 6.7 times of drying shrinkage strain value at 42 days. The combination of EA and SRA resulted in a decrease in autogenous shrinkage by as much as half of control mixture. Steam curing contributed to the reduction of autogenous shrinkage by as much as $11\%$ compared with that of standard curing.

• Steel and Composite Structures
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• v.35 no.4
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• pp.463-474
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• 2020

In this paper, the influence of adding multi-walled carbon nanotube (MWCNT) on the pull behavior of steel and GFRP bars in ultra-high-performance concrete (UHPC) was examined experimentally and numerically. For numerical analysis, 3D nonlinear finite element modeling (FEM) with the help of ABAQUS software was used. Mechanical properties of the specimens, including Young's modulus, tensile strength and compressive strength, were extracted from the experimental results of the tests performed on standard cube specimens and for different values of weight percent of MWCNTs. In order to consider more realistic assumptions, the bond between concrete and bar was simulated using adhesive surfaces and Cohesive Zone Model (CZM), whose parameters were obtained by calibrating the results of the finite element model with the experimental results of pullout tests. The accuracy of the results of the finite element model was proved with conducting the pullout experimental test which showed high accuracy of the proposed model. Then, the effect of different parameters such as the material of bar, the diameter of the bar, as well as the weight percent of MWCNT on the bond behavior of bar and UHPC were studied. The results suggest that modifying UHPC with MWCNT improves bond strength between concrete and bar. In MWCNT per 0.01 and 0.3 wt% of MWCNT, the maximum pullout strength of steel bar with a diameter of 16 mm increased by 52.5% and 58.7% compared to the control specimen (UHPC without nanoparticle). Also, this increase in GFRP bars with a diameter of 16 mm was 34.3% and 45%.

• Laser Solutions
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• v.13 no.4
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• pp.1-6
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• 2010

Recently the use of ultra high strength steels (UHSS) in structural and safety component is rapidly increasing in the automotive industry. For example, 1.5GPa grade hot stamping with die-quenching of boron steel 22MnB5 could apply crash-resistant parts such as bumpers and pillars. The development of laser welding process of hot stamping steels, fundamental bead-on-plate welding and lap joint welding test were carried out using 3kW Nd:YAG laser. Local hardening & HAZ softening occurred in hot stamping steel as a result of metallurgical change caused by the welding heat input in the Nd:YAG laser welding process. The size of soft zones in the hot stamping steel was related to the welding heat input, being smaller at high speeds which generated a smaller heat input. Also in the case of lap joint design structure, same welded characteristics were shown. The HAZ softening degree was controlled to ensure the joint strength.