• Journal of the Korean Society for Precision Engineering
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• v.21 no.2
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• pp.168-176
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• 2004

Interrupted creep tests were carried out on the Mod.9Cr-1Mo steel in order to investigate the structural degradation during creep. The ranges of creep stress and temperature were from 71 to 167MPa and 873 to 923k, respectively. The change of hardness and tempered martensitic lath width were measured in the grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, but was not stable during creep, and the structural evolution was enhanced by creep strain. The relation between the change in lath width and strain was described in the from, $\delta$W= a ($W_s-W_o$)$cdot;varepsilon$, where $\varepsilon$ is the strain, $W_o$is the initial lath width, $W_s$ is the final lath width depending solely on stress, and a is the constant of the magnitude of 0.67 $\mu$m /strain. The change in Victors hardness was expressed by a one-valued function of creep life consumption ratio. Based on the empirical relation between strain and lath width, a model was proposed to explain the relation between change in hardness and creep life consumption ratio. The model revealed that about 65$%$ of dislocations in lath structures were eliminated by the migration of subboundaries.

• Korean Journal of Materials Research
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• v.14 no.7
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• pp.451-456
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• 2004

Thermal creep properties of the zirconium tube which was developed for high burnup application were evaluated. The creep test of cladding tubes after various final heat treatment was carried out by the internal pressurization method in the temperature range from $350^{\circ}C to 400^{\circ}C$ and from 100 to 150 MPa in the hoop stress. Creep tests were lasted up to 900days, which showed the steady-state secondary creep rate. The creep resistance of zirconium claddings was higher than that of Zircaloy-4. Factors that affect creep resistance, such as final annealing temperature, applied stress and alloying element were discussed. Tin as an alloying element was more effective than niobium due to solute hardening effect of tin. In case of advanced claddings, the optimization of final heat treatment temperature as well as alloying element causes a great influence on the improvement of creep resistance.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.1
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• pp.171-178
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• 2002

An effect is made in this study to deepen understanding of small punch(SP) creep testing which has been a round for about 10 years as a substitute for the conventional uniaxial creep testing. Even though considerable numbers of SP creep test program have been performed, most of the tests were aimed at measuring creep rupture lives only. Very flew studies showed interest on the meaning of what we were really measuring during the SP creep tests. In this paper meanings of the parameters measured during the SP creep testing, such as punch load and punch displacement rate are investigated using finite element analysis. It was shown that the measured parameters must represent the stress and strain rates of the material at the annular region located at about 0.65 mm from the center of the SP specimen. The material in this location would go through constant maximum stress and strain rate during the testing. Experimental verification is also discussed.

• Korean Journal of Metals and Materials
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• v.49 no.7
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• pp.515-521
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• 2011

Small punch creep testing has received attention due to the convenience of using smaller specimens than those of conventional uniaxial creep tests, which enables creep testing on developing or currently operational components. However, precedent studies have shown that it is necessary to consider friction between the punch and specimen when computing uniaxial equivalent stress from a finite element model. In this study, small punch creep behaviors of AISI 316L stainless steel, which is widely used in high temperature-high pressure machineries, have been compared for the two different ceramic balls such as $Si_3N_4$ and $Al_2O_3$. The optimal range of the friction coefficient is 0.4~0.5 at $650^{\circ}C$ for the best fit between experimental and simulation data of AISI 316 L stainless steel. The higher the friction coefficient, the longer the creep rupture time is. Therefore, the type of ceramic ball used must be specified for standardization of small punch creep testing.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.3
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• pp.183-189
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• 2005

The effects of duplex-treatment of plasma carburization and CrN coating onto Ti-6Al-4V alloy on its creep properties were investigated by means of a constant stress creep tester. Applying duplex-treatment, specimens having an inner carburized layer of about $150{\mu}m$ in depth and outer CrN layer of about $7.5{\mu}m$ in thickness were prepared. The hardness of duplex-treatment surface was about 1,960 VHN. It also appeared that the duplex-treatment improved the roughness of the surface significantly; $Ra=0.045{\mu}m$ for treated alloy while $Ra=0.321{\mu}m$ for untreated alloy. The steady-state creep behaviors were investigated in a temperature range of $510{\sim}550^{\circ}C$ ($0.42{\sim}0.44T_m$) under an applied stress range of 200~275 MPa. The stress exponent, n, was derived assuming the power law creep behavior. The surface treatment showed a decrease in a value from 9.32 (untreated) to 8.79 (treated). Also the activation energy obtained from an Arrhenius plot increased from 238 to 257 kJ/mol.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.3
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• pp.198-208
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• 1997

The-steady-state creep mechanism and behavior of Zircaloy-4 used as cladding materials in PWR have been investigated in air environment over the temp, ranges from 600 to $645^{\circ}C$ and stress ranges from 4 to $7kg/mm^2$. The stress exponents for the creep deformation of this alloy, n were decreased 4.81, 4.71, 4.64, and 4.56 at 600, 615, 630 and $645^{\circ}C$, respectively; the stress exponents decreased with increasing the temperature and got closer to about 5. The apparent activation energies, Q, were 62.1, 60.0, 57.9 and 55.4 kcal/mole at stresses of 4, 5, 6, $7kg/mm^2$, respectively; the activation energies decreased with increasing the stress and were close to those of volume self diffusion of Zr in Zr-Sn-Fe-Cr system. In results, it can be considered that the creep deformation for Zircaloy-4 was controlled by the dislocation climb over the ranges of this experimental conditions. Larson-Miller parameter, P, for the crept specimens was obtained as P=(T+460)(logt,+23). The failure plane observed by SEM slightly showed up intergranular fracture at this experiment ranges. However, it was essentially dominated by the dimple phenomenon, which was a characteristics of the transgranular fracture.

• Steel and Composite Structures
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• v.31 no.4
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• pp.409-417
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• 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.

• Journal of the Korea Concrete Institute
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• v.16 no.2 s.80
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• pp.185-194
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• 2004

It is difficult to analyze and predict the long-term behavior of concrete structures and members under multiaxial stresses because most of existing researches on creep of concrete were mainly concerned about uniaxial stress state. Therefore, the main objective of this paper is the investigation of creep properties of concrete under multiaxial stresses. This paper presents experimental study on creep of concrete under multiaxial compression. Twenty seven cubic specimens($20{\times}20{\times}20 cm$) for three concrete mixes were tested under uniaxial, biaxial, and triaxial stress states. Creep strains were measured in three directions of principal stresses. Poisson's ratio at the initial loading was obtained, as was Poisson's ratio due to creep stain and Poisson's ratio due to the combined creep strain and elastic strain. These Poisson's ratios were approximately equal for each concrete mix. The Poisson's ratio at the initial loading and the Poisson's ratio for the combined strain Increased slightly as the strength of the concrete increased. In addition, the volumetric creep strain and deviatoric creep strain were linearly proportional to volumetric stress and deviatoric stress, respectively.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2002.05a
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• pp.35-40
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• 2002

In the present study, we examined the influence of prestrain on creep strength of Class M alloy(STS310S) and Class A(STS310J1TB) alloys containing precipitates. Prestrain was given by prior creep at a higher stress than the following creep stresses. Creep behaviour before and after stress change and creep rate of pre-strianed specimens were compared with that of virgin specimens. Pre-straining produced the strain region where the strain rate was lower than that of a virgin specimen both for STS310J1TB and STS310S steels. The reason for this phenomenon was ascribable to the viscous motion of dislocations, the interaction between dislocations and precipitates in a STS310J1TB steel, and the interaction of dislocations with sub-boundaries in a STS310S steel which has the higher dislocation density and smaller subgrain size resulted from pre-straining at higher stress.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.9
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• pp.949-956
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• 2009

The C(t)-integral describes amplitude of stress and strain rate field near a tip of stationary crack under transient creep condition. Thus the C(t)-integral is a key parameter for the high-temperature crack assessment. Estimation formulae for C(t)-integral of the cracked component operating under mechanical load alone have been provided for decades. However, high temperature structures usually work under combined mechanical and thermal load. And no investigation has provided quantitative estimates for the C(t)-integral under combined mechanical and thermal load. In this study, 3-dimensional finite element analyses were conducted to calculate the C(t)-integral of elastic-creep material under combined mechanical and thermal load. As a result, redistribution time for the crack under combined mechanical and thermal load is re-defined through FE analyses to quantify the C(t)-integral. Estimates of C(t)-integral using this proposed redistribution time agree well with FE analyses results.