• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1722-1729
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• 1994

At early stage of creep-fatigue crack growth tests, initial transient behavior which implies high crack growth rate has been generally observed by some researchers. Since the influence of the initial transient crack growth behavior on the remaining life of components is significant, cause of it should be further studied. In this study, characteristics of the initial transient behavior of 1Cr-1Mo-0.25V steel is studied experimentally by performing creep-fatigue crack growth tests at $538^{\circ}C$ in air under trapezoidal waveshapes. It is verified that the cause of the initial transient behavior is not high ${(C_t)}_{avg}$ values due to the small scale creep condition at the early stage of test, but oxidation-dominated crack growth mechanism during the transient period which is different from the creep-dominated crack growth mechanism in steady crack growth period.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.11
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• pp.1339-1347
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• 2013

Creep through nanoindentations has attracted increasing research attention in recent years. Many studies related to indentation creep tests, however, have simply focused on the characteristics of steady-state creep, and there exist wide discrepancies between the uniaxial test and the indentation test. In this study, we performed a computational simulation of spherical indentations, and we proposed a method for evaluating the creep properties considering transient creep. We investigated the material behavior with variation of creep properties and expressed it using regression equations for normalized variables. We finally developed a program to evaluate the creep properties considering transient creep. By using the proposed method, we successfully obtained creep exponents with an average error less than 1.1 and creep coefficients with an average error less than 2.3 from the load-depth curve.

• Journal of Welding and Joining
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• v.19 no.1
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• pp.82-87
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• 2001

The creep-rupture and low cycle fatigue properties of transient liquid phase bonded joints of Ni-base single crystal superalloy, CMSX-2 was investigated using MBF-80 insert metal. The (100) orientation of bonded specimen was aligned perpendicular to the joint interface. CMSX-2 was bonded at 1523K for 1.8ks in vacuum, optimum bonding condition. The creep rupture strength and rupture lives of the joints were the almost identical to ones of the base metal. SEM observation of the fracture surfaces of joints after creep rupture test revealed that the fracture surfaces classified three types of region, ductile fracture surface, cleavage fracture surface and interfacial fracture surface. The low cycle fatigue properties of the joints were also the same level as those of base metal. The elongation and reduction of area values of joints were comparable to those of base metal while fell down on creep rupture condition of high temperature.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.1
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• pp.81-90
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• 2012

The indentation test, which is one of the testing methods for evaluating the mechanical properties of materials, can be applied to the evaluation of creep properties. Many studies related to the indentation creep test, however, have just focused on the characteristics of the steady-state creep, so there are wide discrepancies between the uniaxial test and the indentation test. To obtain accurate creep properties, it is therefore important to consider the effects of transient creep. In the present work, the Ogbonna et al.'s work on the spherical indentation test including the transient creep was expanded and applied to the conical indentation creep test. The characteristics of the transient creep were analyzed via finite element simulations and compared with those obtained through spherical indentation. Other effects, such as elastic strain, indenter shape, contact area, and representative strain, which have not been considered properly in prior studies on the creep test, are also discussed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.9
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• pp.851-857
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• 2015

In this paper, we estimate the time-dependent C(t) integrals under elastic-plastic-creep conditions. Finite-element (FE) transient creep analyses have been performed for single-edge-notched-bend (SEB) specimens. We investigate the effect of the initial plasticity on the transient creep by systematically varying the magnitude of the initial step load. We consider both the same stress exponent and different stress exponents in the power-law creep and plasticity to elastic-plastic-creep behavior. To estimate the C(t) integrals, we compare the FE analysis results with those obtained using formulas. In this paper, we propose a modified equation to predict the C(t) integrals for the case of creep exponents that are different from the plastic exponent.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.10
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• pp.1001-1010
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• 2015

This paper estimates the time-dependent crack-tip stress fields under elastic-plastic-creep conditions. We perform Finite-Element (FE) transient creep analyses for a Single-Edge-notched-Bend (SEB) specimen. We investigate the effect of the initial plasticity on the transient creep by systematically varying the magnitude of the initial step-load. We consider both the same stress exponent and different stress exponent in the power-law creep and plasticity to determine the elastic-plastic-creep behaviour. To estimation of the crack-tip stress fields, we compare FE analysis results with those obtained numerically formulas. In addition, we propose a new equation to predict the crack-tip stress fields when the creep exponent is different from the plastic exponent.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2015.05a
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• pp.59-60
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• 2015

In this study, stress-strain, thermal expansion strain, total strain and high temperature creep strain of ultra-high-strength concrete with compressive strengths of 80, 130, and 180MPa were experimentally evaluated considering elevated temperature and loading condition. Also, transient creep strain has been calculated by using the results of experiment. Experimental coefficient K was proposed with application of non-steady state creep model. It is considered that the experimental results of this study could be baseline data for deformation behavior analysis of ultra-high-strength concrete.

• Journal of the Korea Concrete Institute
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• v.23 no.5
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• pp.627-636
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• 2011

Recently, to prevent explosive spalling of high-strength concrete (HSC) members, the usage of nylon fiber instead of polypropylene fiber has increased. Past experimental studies have been conducted to examine the spalling and mechanical properties of HSC with nylon fibers when exposed to elevated temperature. However, the previous studies on HSC with nylon fibers subjected to high temperatures were performed only on the properties such as spalling, compressive strength, and elastic modulus rather than investigations on to the behaviors such as thermal strain, total strain, steady state creep, and transient creep. Therefore, in this study thermal strain, total strain, steady state creep, and transient creep of HSC mixed with nylon fibers with water to binder ratio of 0.30 to 0.15 were tested. The experimental results showed that nylon fibers did not affect the performance of HSC with nylon fibers at high temperatures. However, HSC with nylon fibers generated a larger transient creep strain than that of HSC without fibers and normal strength concrete.

• 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.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.6
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• pp.609-617
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• 2011

There is a trend towards the progressive use of higher operating temperatures and stresses to achieve improved efficiencies in power-generation equipment. It is important to perform the crack assessment under hightemperature and high-pressure conditions. The C(t)-integral is a key parameter in crack assessment for transient creep states. The estimation of the C(t)-integral is complex when considering the mechanical and thermal loads simultaneously. In this paper, we study estimation of C(t)-integral under combined mechanical and thermal load depending on loading conditions.