• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.5 s.236
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• pp.689-697
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• 2005

This paper presents the reliability estimation of door hinge for home appliances, which consists of bushing and shaft. The predominant failure mechanism of bushing made of polyoxymethylene(POM) is brittle fracture due to decrease of strength caused by voids existing, and that of shaft made of acrylonitrile-butadiene-styrene(ABS) is creep due to plastic deformation caused by excessive temperature and lowering of glass transition temperature by absorbed moisture. Since the brittle fracture of bushing is overstress failure mechanism, the load-strength interference model is used to estimate the failure rate of it along with failure analysis. By the way, the creep of shaft is wearout failure mechanism, and an accelerated life test is then planned and implemented to estimate its lifetime. Through the technical review about failure mechanism, temperature and humidity are selected as accelerating variables. Assuming Weibull lifetime distribution and Eyring model, the life-stress relationship and acceleration factor, $B_{10}$ life and its lower bound with $90\%$ confidence at worst case use condition are estimated by analyzing the accelerated life test data.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.303-311
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• 2004

This paper presents the reliability estimation of door hinge for home appliances, which consists of bushing and shaft. The predominant failure mechanism of bushing made of polyoxymethylene(POM) is brittle fracture due to decrease of strength caused by voids existing, and that of shaft made of acrylonitrile-butadiene-styrene(ABS) is creep due to plastic deformation caused by excessive temperature and lowering of glass transition temperature by absorbed moisture. Since the brittle fracture of bushing is overstress failure mechanism, the load-strength interference model is used to estimate the failure rate of it along with failure analysis. By the way, the creep of shaft is wearout failure mechanism, and an accelerated life test is then planned and implemented to estimate its lifetime. Through the technical review about failure mechanism, temperature and humidity are selected as accelerating variables. Assuming Weibull lifetime distribution and Eyring model, the life-stress relationship and acceleration factor, B$_{10}$ life and its lower bound with 90% confidence at worst case use condition are estimated by analyzing the accelerated life test data.a.

• International Journal of Concrete Structures and Materials
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• v.9 no.4
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• pp.453-462
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• 2015

A critical review on existing creep theories in calcium-silicate-hydrate (C-S-H) is presented with an emphasis on several fundamental questions (e.g. the roles of water, relative humidity, temperature, atomic ordering of C-S-H). A consensus on the rearrangement of nanostructures of C-S-H as a main consequence of creep, has almost been achieved. However, main disagreement still exists on two basic aspects regarding creep mechanisms: (1) at which site the creep occurs, like at interlayer, intergranular, or regions where C-S-H has a relatively higher solubility; (2) how the structural rearrangement evolutes, like in a manner of interlayer sliding, intra-transfer of water at various scales, recrystallization of gelled-like particles, or dissolution-diffusion-reprecipitation at inter-particle boundary. The further understanding of creep behavior of C-S-H relies heavily on the appropriate characterization of its nanostructure.

• Journal of Mechanical Science and Technology
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• v.20 no.8
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• pp.1209-1216
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• 2006

An AZ31 magnesium alloy was tested at constant temperatures ranging from 423 to 473 K (0.46 to 0.51 Tm) under constant stresses. All of the creep curves exhibited two types depending on stress levels. At low stress (${\sigma}/ G < 4 {\times}10^{-3}$), the creep curve was typical of class A (Alloy type) behavior. However, at high stresses (${\sigma}/ G > 4 {\times}10^{-3}$), the creep curve was typical of class M (Metal type) behavior. At low stress level, the stress exponent for the steady-state creep rate was of 3.5 and the true activation energy for creep was 101 kJ/mole which is close to that for solute diffusion. It indicates that the dominant deformation mechanism was glide-controlled dislocation creep. At low stress level where n=3.5, the present results are in good agreement with the prediction of Fridel model.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.10
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• pp.1137-1143
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• 2012

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft material as polymers or used as mechanical elements at high temperatures. One of the popular thermo-plastic polymers, Acrylonitrile Butadiene Styrene (ABS) which is used broadly for machine elements material, as it has excellent mechanical properties such as impact resistance, toughness and stiffness compared to other polymers, was studied for creep characteristic at different levels of stress and temperatures. From the experimental results, the creep limit of ABS at room temperature is 80 % of tensile strength which is higher than PE and lower than PC or PMMA. Also the creep limits decreased to linearly as the temperatures increased, up to $80^{\circ}C$ which is the softening temperature of Butadiene ($82^{\circ}C$). Also the secondary stage of creep among the three creep stages for different levels of stress and temperature was non-existent which occurred for many metals by strain hardening effect.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.227-232
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• 2004

The failure of geogrids can be defined as an excessive creep strain which causes the collapse of slopes and embankments. In this study, the accelerated creep tests were applied to two different types of polyester geogrids, at 75, 80, 85$^{\circ}C$ by applying 50% load of ultimate tensile strengths using a newly designed test equipment which is allowed the creep testing at higher temperatures. And then the creep curves were shifted and superposed in the time axis by applying time-temperature supposition principles. In predicting the lifetimes of geogrids, the underlying distribution for failure times were determined based on identification of the failure mechanism. The results indicate that the conventional procedures with the newly designed test equipment are shown to be effective in prediction of the lifetimes of geogrids with shorter test times. In addition, the predicted lifetimes of geogrids having different structures at various creep strains give guidelines for users to select the proper geogrids in the fields.

• Journal of the Korean Society of Safety
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• v.24 no.6
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• pp.20-26
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• 2009

The creep deformation behavior of AZ31 magnesium alloy was examined in the temperature range from 573 to 673K (0.62 to 0.73 Tm) under various constant stresses covering low strain rate range from $4{\times}10^{-9}\;s^{-1}$ to $2{\times}10^{-2}\;s^{-1}$. At low stress level, the stress exponent for the steady-state creep rate was ~3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ~5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute-atmospherebreakaway concept.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1177-1185
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• 1996

The crack growth under creep condition is one of the major damage mechanisms which determines remaining life of the component operating at high temperatures. In this paper, the creep crack growth by grain boundary cavitation is studied, which is frequently observed failure mechanism for creep brittle materials. As a result of diffusive growth of creep cavities, it is shown that the crack-tip stress field is modified from the original stress distribution by the amount of singularity attenuation parameter which is function of crack growth rate and material properties. Also, the stress relaxation at crack-tip results in the extension of cavitating area by the load dump effect to meet the macroscopic force equilibrium conditdion.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.3
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• pp.177-182
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• 2005

The steady state cyclic mechanism, and the behaviour of Nicoseal(Fe-29Ni-17Co) have been examined under the condition of square wave stress cyclic tension creep test at the temperature, stress and frequency range of $430{\sim}470^{\circ}C$($0.41{\sim}0.43T_m$), 353~383 MPa, and 3 cpm, respectively. Also, the relationship between cyclic creep and static creep have been examined. The stress exponents(n) for the static creep deformation of this alloy were 11.6, 10.0, 8.4 and 7.9 at the temperature of 430, 445, 460 and $470^{\circ}C$, respectively. The apparent activation energies (Q) for the static creep deformation were 54.2, 51.8, 49.7 and 46.8 kcal/mole for the stress of 353, 363, 373 and 383 MPa, From the above results, it could be considered that the cyclic creep accelaration phenomena was obtained and that the cyclic deformation for Nicoseal seemed to be controlled by dislocation climb over the range of experimental conditions. Nicoseal alloy under the cyclic creep conditions was obtained as P=(T+460)(logt+17). The failure plane observed by SEM showed up transgranular fracture at all range.

• Transactions of Materials Processing
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• v.17 no.5
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• pp.364-372
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• 2008

In this study, the circumferential creep behaviors ofpilgered advanced Zirconium alloy tubes such as Zr-Nb-O and Zr-Nb-Sn-Fe were investigated in the temperature range of $400\sim500^{\circ}C$ and in the stress range of 80$\sim$150MPa. The test results indicate that the stress exponent for the steady-state creep rate of the Zr-Nb-Sn-Fe alloy decreases with the increase of stress(from 6$\sim$7 to 4), while that of the Zr-Nb-O alloy is nearly independent of stress(5$\sim$6). The activation energy of creep deformation is found to be nearly the same as the activation energy for Zr self diffusion. This indicates that the creep deformation may be controlled by dislocation climb mechanism in Zr-Nb-O. On the other hand, the transition of stress exponent(from 6-7 to 4) in Zr-Nb Sn-Fe strongly suggests the transition of the rate controlling mechanism at high stresses. The lower stress exponent at high stresses in Zr-Nb-Sn-Fe can be explained by the dynamic deformation aging effect caused by interaction of dislocations with Sn substitutional atoms.