• 한국재료학회지
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• 제24권4호
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• pp.180-185
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• 2014

The current concern about these materials ($MoSi_2$ and $NbSi_2$) focuses on their low fracture toughness below the ductile-brittle transition temperature. To improve the mechanical properties of these materials, the fabrication of nanostructured and composite materials has been found to be effective. Nanomaterials frequently possess high strength, high hardness, excellent ductility and toughness, and more attention is being paid to their potential application. In this study, nanopowders of Mo, Nb, and Si were fabricated by high-energy ball milling. A dense nanostructured $MoSi_2-NbSi_2$ composite was simultaneously synthesized and sintered within two minutes by high-frequency induction heating method using mechanically activated powders of Mo, Nb, and Si. The high-density $MoSi_2-NbSi_2$ composite was produced under simultaneous application of 80MPa pressure and an induced current. The sintering behavior, mechanical properties, and microstructure of the composite were investigated. The average hardness and fracture toughness values obtained were $1180kg/mm^2$ and $3MPa{\cdot}m^{1/2}$, respectively. These fracture toughness and hardness values of the nanostructured $MoSi_2-NbSi_2$ composite are higher than those of monolithic $MoSi_2$ or $NbSi_2$.

• 한국안전학회지
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• 제34권6호
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• pp.1-6
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• 2019

Carbon fiber has excellent specific strength, corrosion resistance and heat resistance. And p-Aramid fiber has high toughness and heat resistance and high elasticity, and is used in various fields such as industrial protective materials, bulletproof helmets and vests, as well as industrial fields. However, carbon fiber is relatively expensive, and is susceptible to brittle fracture behavior due to its low fracture strain. On the other hand, the aramid fiber tends to decrease in elastic modulus and strength when applied to the epoxy matrix, but it is inexpensive and has higher elongation and fracture toughness than carbon fiber. Thus the twill hybrid carbonaramid fiber reinforced composite laminate composite was investigated for a delamination fracture toughness under Mode I loading by 2 kinds of MBT and MCC deduction. The specimen was fabricated with 20 hybrid fabric plies. The initial crack was made by inserting the teflon tape in the center plane with a₀/W=0.5 length. The results show that SERR(Strain Energy Release Rate) as the critical and stable delamination fracture toughness were 0.09 kJ/㎡, 0.386 kJ/㎡ by MBT deduction, and 0.192 kJ/㎡, 0.67 kJ/㎡ by MCC deduction, respectively.

• 대한금속재료학회지
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• 제56권11호
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• pp.796-804
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• 2018

The Charpy impact toughness of the heat affected zone (HAZ) of electro gas welded 390 MPa yield strength grade steel, manufactured by a thermo mechanically controlled process, was investigated. The effects of added Nb on the toughness of the steel and the factors influencing scatter in toughness are discussed in the present work. It was observed that adding Nb to the steel led to the deterioration of HAZ toughness. The presence of soluble Nb in the HAZ increased its hardenability and resulted in a larger amount of low toughness bainitic microstructure. Microstructural observations in the notch root area revealed the significant role of different microstructures in the area. In the presence of a larger amount of bainitic microstructures, the HAZ exhibited a lower Charpy toughness with a larger scatter in toughness. A softened zone with a lower hardness than the base metal was formed in the HAZ. However, theoretical analysis revealed that the presence of the zone might not be a problem in a real welded joint because of the plastic restraint effect enforced by surrounding materials.

• Journal of Welding and Joining
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• 제7권3호
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• pp.44-54
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• 1989

This study was undertaken to evaluate the allowable welding heat input range for high strength steels manufactured by various processes and to compare the weldability of TMCP steel for high heat input welding with that of conventional Ti-added normalized steel. The allowable welding heat input ranges for conventional 50kg/$mm^2$ steel to guarantee D or E grade of ship structural steel were below 150 and 80kJ/cm respectively. Such a limit in welding heat input was closely related with the formation of undesirable microstructures, such as grain boundary ferrite and ferrite side plate in the coarse grain HAZ. In case of 60 and 80kg/$mm^2$ quenched and tempered steels, for securing toughness in weldments over toughness requirements for base metal, each welding heat input had to be restricted below 60 and 40kJ/cm, that was mainly due to coarsened polygonal ferrite in weld metal and lower temperature transformation products in coarse grain HAZ. The TMCP steel could be appropriate as a grade E ship hull steel up to 200kJ/cm, but the Ti-added normalized steel could be applied only below 130kJ/cm under the same rule. This difference was partly owing to whether uniform and fine intragranular ferrite microstructure was well developed in HAZ or not.

• Journal of Welding and Joining
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• 제4권2호
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• pp.40-46
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• 1986

This investigation is concerned with the toughness and microstructure of manneristically simulated HAZ in 12% Cr steel. Unnotched specimens were subjected to weld thermal cycles a weld simulator. The parameters-peak temperatures, cooling rate, influence of PWHT and plastic deformation were considered. After weld simulation, the specimens were heat-treated, V-notched and impact tested. An optical metallographic examination was performed to correlate the HAZ toughness with microstructure. Also a fractographic examination was done to obtain information on the fracture mode. The toughness of the coarse grained zone and the part of HAZ subjected to a peak temperature range 700-800.deg. C are lower than the other parts. However, they are still high enough. The double PWHT cycle could not improve the HAZ toughness in present study. However, if the first PWHT is conducted before the work piece is cooled below $M_f$, it is expected that the double PWHA may be beneficial to the toughness of the HAZ. It is also expected that martensitic welding can be used on production welds.

• Journal of Welding and Joining
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• 제33권2호
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• pp.32-39
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• 2015

This study examined the welding residual stress and fracture toughness of 78mm thick steel electro gas welding (EGW) and flux cored arc welding (FCAW) welded joints by numerical analyses of the thermal elasto-plastic behavior and fracture toughness(KIC). The residual stress, fracture toughness characteristics and production mechanism on the welded joints were clarified. Moreover, the effects of the welding process (EGW and FCAW) on the welding residual stresses and fracture toughness of welded joints were evaluated. The results showed that the new welding process (EGW) appears to be an effective substitute for the existing welding process (FCAW) in a thick steel plate with high strength.

• 한국철도학회논문집
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• 제8권5호
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• pp.444-453
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• 2005

The safety evaluations of railway wheelsets make use of the static fracture toughness obtained in ingot materials. The static fracture toughness of wheelset materials has been extensively studied by experiments, but the dynamic fracture toughness with respect to wheelset materials has not been studied enough yet. It is necessary to evaluate the characteristics of the fracture mechanics depending on each location for a full-scale wheelset for high-speed trains, because the load state for each location of the wheelset while running is different the contact load between the wheel and rail, cyclic stress in the wheel plate, etc. This paper deals with the fracture toughness depend on load rates. The fracture toughness depending on load rate data shows that once the downward curve from quasi-static values was reached, subsequent values showed a slow increase with respect to the impact velocity. This means that dynamic fracture toughness should be considered in the design code of the wheelset material.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2004년도 추계 학술발표회 제16권2호
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• pp.615-618
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• 2004

The primary role of fibers in High performance fiber reinforced cement composites(HPFRCCs) is to improve the toughness, or energy absorption capacity, of the composite material, However, there is still no general agreement as to how this toughness should be characterized, or how it might be used in the design of structures containing HPFRCCs. In this paper, therefore, we focus on test techniques for measuring flexural toughness. For mechanical properties, HPFRCCs can be tested in the same way as fiber reinforced concrete(FRC). Both the significance and the limitations of somewhat different national and industrial standards of FRC are discussed. For flexural toughness, with depend on the presence of fibers, new test methods was developed and verified. We also suggest evaluation method of tensile toughness indices using the moment curvature relationship in flexural tests.

• Advances in concrete construction
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• 제13권 2호
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• pp.123-132
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• 2022

This paper aims to adapt Multilinear regression (MLR) to predict the strength and toughness of SIFCON containing various pozzolanic materials. Slurry Infiltrated Fibrous Concrete (SIFCON) is one of the most common terms used in concrete manufacturing, known for its benefits such as high ductility, toughness and high ultimate strength. Assessment of compressive strength (CS.), flexural strength (F.S.), splitting tensile strength (STS), dynamic elasticity modulus (DME) and impact energy (I.E.) using the experimental approach is too costly. It is time-consuming, and a slight error can lead to a repeat of the test and, to solve this, alternative methods are used to predict the strength and toughness properties of SIFCON. In the present study, the experimentally investigated SIFCON data about various mix proportions are used to predict the strength and toughness properties using regression analysis-multilinear regression (MLR) models. The input parameters used in regression models are cement, fibre, fly ash, Metakaolin, fine aggregate, blast furnace slag, bottom ash, water-cement ratio, and the strength and toughness properties of SIFCON at 28 days is the output parameter. The models are developed and validated using data obtained from the experimental investigation. The investigations were done on 36 SIFCON mixes, and specimens were cast and tested after 28 days of curing. The MLR model yields correlation between predicted and actual values of the compressive strength (C.S.), flexural strength, splitting tensile strength, dynamic modulus of elasticity and impact energy. R-squared values for the relationship between observed and predicted compressive strength are 0.9548, flexural strength 0.9058, split tensile strength 0.9047, dynamic modulus of elasticity 0.8611 for impact energy 0.8366. This examination shows that the MLR model can predict the strength and toughness properties of SIFCON.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.25-28
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• 2002

In the coarse grain HAZ adjacent to the fusion line, most of the TiN particles in conventional Ti added steel are dissolved and austenite grain growth is easily occurred during welding process. To avoid this difficulty, thermal stability of TiN particle is improved by increasing the nitrogen content in steel. In this study, the effect of hlgh nitrogen TiN particle on preventing austenite grain growth in HAZ was investigated. Increased thermal stability of TiN particle is helpful for preventing the austenite grain growth by pinning effect. High nitrogen TiN particle in simulated HAZ were not dissolved even at high temperature such as 1400'E and prevented the austenite grain growth in simulated HAZ. Owing to small austenite grain size in HAZ the width of coarse grain HAZ in high nitrogen TiN steel was decreased to 1/10 of conventional TiN steel. Even high heat input welding, the microstructure of coarse grain HAZ consisted of fine polygonal ferrite and pearlite and toughness of coarse grain HAZ was significantly improved.