• International Journal of Highway Engineering
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• v.2 no.3
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• pp.129-144
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• 2000

This paper presents the characteristics of viscoelastic properties and fatigue performance of SBS modified asphalt mixtures depending on the aggregate gradation. Dynamic shear rheometer (DSR) and uniaxial tensile creep tests are performed to analyze the thermomechanical behavior of asphalt binders and mixtures, respectively. Uniaxial tensile fatigue tests for seven different asphalt mixtures are conducted to evaluate the effects of aggregate gradations and SBS modifier on the fatigue performance of the mixtures. DSR and uniaxial tensile creep tests results show that the SBS modified asphalt mixtures have better rutting resistance than the unmodified mixtures at high temperatures regardless of the aggregate gradations used. Fatigue factor $G^*sin\delta$ in Superpave binder specification may not be adequate for evaluating the fatigue Performance of asphalt mixtures. It is observed from uniaxial tensile fatigue tests that SBS modified asphalt mixtures compared to unmodified mixtures have ten times longer fatigue lives regardless of the aggregate gradations(dense, SMA, and Superpave gradations) used in the mixtures. The better fatigue performance of the SBS modified mixtures is observed even after long-term aging process. The effect of aggregate gradations on the fatigue performance is not as significant as the SBS modifier. The cellulose fiber added in the SMA mixture has negligible effects on the viscoelastic Properties and fatigue performance of the mixture, but is effective in reducing draindown. Although the SBS modified asphalt binder is used, it may be necessary to add the cellulose fiber into the SMA mixture to prevent the draindown.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.8
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• pp.1398-1408
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• 2003

Tensile and LCF(low cycle fatigue) tests were carried out in air at wide temperature range 20$^{\circ}C$-750$^{\circ}C$ and strain rates of 1${\times}$10$\^$-4//s-1${\times}$10$\^$-2/ to ascertain the influence of strain rate on tensile and LCF properties of prior cold worked 316L stainless steel, especially focused on the DSA(dynamic strain aging) regime. Dynamic strain aging induced the change of tensile properties such as strength and ductility in the temperature region 250$^{\circ}C$-600$^{\circ}C$ and this temperature region well coincided with the negative strain rate sensitivity regime. Cyclic stress response at all test conditions was characterized by the initial hardening during a few cycles, followed by gradual softening until final failure. Temperature and strain rate dependence on cyclic softening behavior appears to result from the change of the cyclic plastic deformation mechanism and DSA effect. The DSA regimes between tensile and LCF loading conditions in terms of the negative strain rate sensitivity were well consistent with each other. The drastic reduction in fatigue resistance at elevated temperature was observed, and it was attributed to the effects of oxidation, creep and dynamic strain aging or interactions among them. Especially, in the DSA regime, dynamic strain aging accelerated the reduction of fatigue resistance by enhancing crack initiation and propagation.

• Nuclear Engineering and Technology
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• v.50 no.1
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• pp.116-125
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• 2018

Modified 9Cr-1Mo ferritic steel is a preferred material for steam generators in nuclear power plants for their creep strength and good corrosion resistance. Austenitic stainless steels, such as type 316LN, are used in the high temperature segments such as reactor pressure vessels and primary piping systems. So, the dissimilar joints between these materials are inevitable. In this investigation, dissimilar joints were fabricated by the Shielded Metal Arc Welding (SMAW) process with Inconel 82/182 filler metals. The notch tensile properties and Charpy V-notch impact toughness properties of various regions of dissimilar metal weld joints (DMWJs) were evaluated as per the standards. The microhardness distribution across the DMWJs was recorded. Microstructural features of different regions were characterized by optical and scanning electron microscopy. Inhomogeneous notch tensile properties were observed across the DMWJs. Impact toughness values of various regions of the DMWJs were slightly higher than the prescribed value. Formation of a carbon-enriched hard zone at the interface between the ferritic steel and the buttering material enhanced the notch tensile properties of the heat-affected-zone (HAZ) of P91. The complex microstructure developed at the interfaces of the DMWJs was the reason for inhomogeneous mechanical properties.

• Journal of the Korea Institute of Building Construction
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• v.17 no.1
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• pp.31-37
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• 2017

The use of half PC slab has been increasing to shorten construction period. Because the drying shrinkage of topping concrete is restrained by PC slab, the tensile stress is generated at the topping concrete and the cracks can be occurred at the topping concrete due to drying shrinkage. Therefore, it is important to predict the tensile strain of half PC slab due to drying shrinkage to improve the quality of half PC slab. However, there is no studies on prediction of shrinkage behavior of half PC slab yet. Therefore, in this study, half PC slab was made, and the predictability of tensile strain generated at half PC slab due to drying shrinkage was investigated. The step by step method considering creep was used to estimate the tensile strain of half PC slab. In result, good agreement was obtained between the analytical and experimental values.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.1
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• pp.131-138
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• 2002

This paper provides the fully plastic J solutions for circumferential cracked pipes with inner, semi- elliptical surface cracks, subject to internal pressure and global bending. Solutions are given in the form of two different approaches, the GEF/EPRl approach and the reference stress approach. For the GE/EPRl approach, the plastic influence functions for fully plastic J are tabulated based on extensive 3-D FE calculations using the Ramberg-Osgood (R-O) materials, covering a wide range of pipe and crack geometries. The developed GEf/EPRl-type fully plastic J estimation equations are then re-formulated using the concept of the reference stress approach for wider applications. Based on the FE results, optimized reference load solutions for the definition of the reference stress are found for internal pressure and for global bending. Advantages of the reference stress based approach over the GE/EPRl-type approach are fully discussed. Validation of the proposed reference stress based J estimation equations will be given in Part II, based on 3-D elastic-plastic or elastic creep FE results using typical tensile properties of stainless steels and generalized creep- deformation behaviours.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.11 no.1
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• pp.12-19
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• 2015

As a measure of improving the mechanical properties of a diffusion bonded joint of a ferritic/martensitic steel (FMS), the post-bonding heat treatment (PBHT) is applied. In the temperature range of normalizing condition ($950-1,050^{\circ}C$), diffusion bonding is employed with compressive stress (6 MPa). Due to the martensite structure distributed in the matrix, Vicker's hardness values of the as-bonded are much higher than those of the as-received. Through the PBHT for 1 h at $720^{\circ}C$, hardness values are recovered to as low as those of the as-received condition. Also, tensile properties of PBHT are similar to those of the as-received at up to the test temperature of $550^{\circ}C$, when the diffusion bonding is carried out over $1,000^{\circ}C$. Based on the creep-rupture testing performed at $650^{\circ}C$ in air environment, the joint efficiency of the PBHTed specimens is about 80% in, which is higher than that of the as-bonded specimens.

• Nuclear Engineering and Technology
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• v.46 no.6
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• pp.857-862
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• 2014

Mechanical alloying under various gas atmospheres such as Ar, an Ar-$H_2$ mixture, and He gases were carried out, and its effects on the powder properties, microstructure and mechanical properties of ODS ferritic steels were investigated. Hot isostatic pressing and hot rolling processes were employed to consolidate the ODS steel plates. While the mechanical alloyed powder in He had a high oxygen concentration, a milling in Ar showed fine particle diameters with comparably low oxygen concentration. The microstructural observation revealed that low oxygen concentration contributed to the formation of fine grains and homogeneous oxide particle distribution by the Y-Ti-O complex oxides. A milling in Ar was sufficient to lower the oxygen concentration, and this led a high tensile strength and fracture elongation at a high temperature. It is concluded that the mechanical alloying atmosphere affects oxygen concentration as well as powder particle properties. This leads to a homogeneous grain and oxide particle distribution with excellent creep strength at high temperature.

• Journal of Ocean Engineering and Technology
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• v.3 no.2
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• pp.125-137
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• 1989

To check technical improvement in the soundness and strength of 12% Cr steel rotor, a 25 tons of rotor with 65 tons of ingot was made in real size and was cut to pieces to take test samples, and the various mechanical tests such as impact, tensile, creep, and fatigue were carried out. The strengths are compared with those of 1% Cr-Mo-V rotor of same size. Microstructures of the samples are examined and reviewed. The results can be summarized as follows. 1) Fracture appearance transition temperatures are 80.deg. C at the center part and 60.deg. C near surface of 12% Cr rotor, and 8.deg. C near surface of 1% Cr-Mo-V rotor. 2) Comparative rapid softening occurs at higher temperatures above 600.deg. C for 12% Cr steel and 550.deg. C for 1% Cr-Mo-V steel in tension tests. 3) Fatigue crack propagation rate of 12% Cr steel is almost same as that of 1% Cr-Mo-V steel at the same corresponding surface part of the rotors. The crack growth rate of center part of 12% Cr rotor is faster than near surface part of the rotor, and the crack growth rate at the load condition of R=0.04 is slower than that of the load condition of R=0.5 for both 12% Cr steel and 1% Cr-Mo-V steel. 4) Crack growth rate of radial direction near surface of 12% Cr rotor is faster than that of transverse direction at the same part because of the difference in residual stresses. 5) Both creep and fatigue strengths of 12% Cr steel are superior to those of 1% Cr-Mo-V steel and the difference is thought the effect of climb and glide controlled creep by solid solution of alloying elements and dispersion of carbides.

• Journal of Ocean Engineering and Technology
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• v.3 no.2
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• pp.625-625
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• 1989

To check technical improvement in the soundness and strength of 12% Cr steel rotor, a 25 tons of rotor with 65 tons of ingot was made in real size and was cut to pieces to take test samples, and the various mechanical tests such as impact, tensile, creep, and fatigue were carried out. The strengths are compared with those of 1% Cr-Mo-V rotor of same size. Microstructures of the samples are examined and reviewed. The results can be summarized as follows. 1) Fracture appearance transition temperatures are 80.deg. C at the center part and 60.deg. C near surface of 12% Cr rotor, and 8.deg. C near surface of 1% Cr-Mo-V rotor. 2) Comparative rapid softening occurs at higher temperatures above 600.deg. C for 12% Cr steel and 550.deg. C for 1% Cr-Mo-V steel in tension tests. 3) Fatigue crack propagation rate of 12% Cr steel is almost same as that of 1% Cr-Mo-V steel at the same corresponding surface part of the rotors. The crack growth rate of center part of 12% Cr rotor is faster than near surface part of the rotor, and the crack growth rate at the load condition of R=0.04 is slower than that of the load condition of R=0.5 for both 12% Cr steel and 1% Cr-Mo-V steel. 4) Crack growth rate of radial direction near surface of 12% Cr rotor is faster than that of transverse direction at the same part because of the difference in residual stresses. 5) Both creep and fatigue strengths of 12% Cr steel are superior to those of 1% Cr-Mo-V steel and the difference is thought the effect of climb and glide controlled creep by solid solution of alloying elements and dispersion of carbides.

• Journal of the Korean Recycled Construction Resources Institute
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• v.5 no.1
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• pp.117-124
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• 2010

The aggregate, which does not satisfy the standard of KS F 2573, was selected for this investigation. The 28day compressive strength of recycled aggregate concrete without pozzolan material was 21.7MPa, which was less than the strength of concrete made with crushed stone. However, the compressive strength at 28 days was improved by mixing early rapid hardening cement to the cement at the weight ratio of 2.5%. Furthermore, the compressive strength at 91 days and 180 days increased significantly by adding fly ash, slag powder, and diatom powder. The tensile strength of recycled aggregate concrete with pozzolan material also increased about 40% compared to the general concrete. Futhermore, the shrinkage and creep of recycled aggregate concrete with fly ash and slag powder was a little decreased that of recycled aggregate concrete with fly ash and diatom powder. Relationship between compressive strength and creep coefficient was shown to the linear relation like as ${\sigma}_c=-30CF+404$.