• Journal of Powder Materials
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• v.5 no.2
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• pp.122-128
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• 1998

NiAl based ODS (Oxide Dispersion Strengthened) intermetallic alloys have been produced by mechanical alloying (MA) process and consolidated by hot extrusion. Subsequent thermomechanical treatments have been applied to induce secondary recrystallization in an attempt to improve creep resistance in this material. The creep behavior of secondary recrystallized MA NiAl has been investigated and compared with those of as-extruded condition. Minimum creep rate were shown to be approximately two orders of magnitude lower than that in as-extruded condition. The improvement in creep resistance is believed due to the grain coarsening, restricting of dispersoid coarsening as well as increase in grain aspect ratio. Creep threshold stress behavior, below which no measurable creep rate can be detected, has been discussed on the basis of particle-dislocation interaction theory. The threshold stress becomes negligible after secondary recrystallization in MA NiAl, presumably due to dispersoid coarsening and a decrease in grain boundary area during secondary recrystallization.

• Geomechanics and Engineering
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• v.7 no.3
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• pp.279-295
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• 2014

A series of one-dimensional consolidation tests and triaxial creep tests were performed on Nansha clays, which are interactive marine and terrestrial deposits, to investigate their time-dependent behaviour. Based on experimental observations of oedometer tests, normally consolidated soils exhibit larger secondary compression than overconsolidated soils; the secondary consolidation coefficient ($C_{\alpha}$) generally gets the maximum value as load approaches the preconsolidation pressure. The postsurcharge secondary consolidation coefficient ($C_{\alpha}$') is significantly less than $C_{\alpha}$. The observed secondary compression behaviour is consistent with the $C_{\alpha}/C_c$ concept, regardless of surcharging. The $C_{\alpha}/C_c$ ratio is a constant that is applicable to the recompression and compression ranges. Compared with the stage-loading test, the single-loading oedometer test can evaluate the entire process of secondary compression; $C_{\alpha}$ varies significantly with time and is larger than the $C_{\alpha}$ obtained from the stage-loading test. Based on experimental observations of triaxial creep tests, the creep for the drained state differs from the creep for the undrained state. The behaviour can be predicted by a characteristic relationship among axial strain rate, deviator stress level and time.

• Journal of the Korean Geotechnical Society
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• v.23 no.12
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• pp.43-52
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• 2007

This study analyzes the characteristics of creep deformation behavior of weathered granite soils used in road embankments. The creep strain under the unconfined compressive state demonstrated an excellent agreement with the theoretical analysis of the burgers substance. The elastic deformation showed a termination in its characteristics after a long-term period owing to the increase in applied loads. The primary creep strain was 0.0028 and concluded that the deformation completed within $3{\sim}5$ days after applying the loads. Also, the completing time of creep deformation in the embankment soils increased in proportion to the height of embankment soils. The secondary creep strain is about 50% of the primary creep strain.

• Journal of the Korean Society for Precision Engineering
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• v.26 no.7
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• pp.99-104
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• 2009

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft as polymers or used as mechanical elements at high temperatures. One of the popular thermo-elastic plastics, Polyethylene(PE) which is used broadly for engineering purposes, as it has good properties and merits compared to other plastics, was studied for creep characteristic at various level of stresses and temperatures. From the experimental results, the creep limit of PE at room temperature is 75% of tensile strength. Also the creep limits decreased exponentially as the temperatures increased, up to 50% of the melting point. Also the secondary stage among the three creep stages was nonexistent nor was there any rupture failure which occurred for many metals.

• Journal of Advanced Marine Engineering and Technology
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• v.22 no.4
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• pp.490-497
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• 1998

An elast-biscoplastic finite element analysis is performed to investigate detailed growth behavior of creep cracks and the numerical results are compared with experimental results. The results of mesh translation method are compared with those of node release method. Load line displancement curve obtained from the crack growth analysis by mesh translation shows the improved results than that obtained from the crack growth by node release method when the secondary creep rate is only used as creep material property. The results of accounting for primary creep rate and instantaneous plasticity shows a good agreement with the experimental result.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.7 s.166
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• pp.1120-1128
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• 1999

An elastic-viscoplastic finite element analysis is performed to investigate detailed growth behavior of creep cracks and the numerical results are compared with experimental results. In Cr-Mo steel stress fields obtained from the crack growth method by mesh translation were compared with both cases that the secondary creep rate is only used as creep material property and the primary creep rate is included. Analytical stress fields, Riedel-Rice(RR) field, Hart-Hui-Riedel(HR) field and Prime(named in here) field, and the results obtained by numerical method were evaluated in details. Time vs. stress at crack tip was showed and crack tip stress fields were plotted. These results were compared with analytical stress fields. There is no difference of stress distribution at remote region between the case of 1st creep rate+2nd creep rate and the case of 2nd creep rate only. In case of slow velocity of crack growth, the effect of 1st creep rate is larger than the one of fast crack growth rate. Stress fields at crack tip region we, in order, Prime field, HR field and RR field from crack tip.

• Journal of Power System Engineering
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• v.6 no.1
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• pp.55-60
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• 2002

There has been no report on the life prediction for gas turbine materials at high temperatures based on the creep properties and their relationship with the AE(acoustic emission) properties as a means of real-time non-destructive testing. One of the important issues is thus to develop a reliable method of evaluating creep properties by the AE technique. In this paper, the real-time evaluation of high temperature creep time and AE cumulative counts for nickel-based superalloy Udimet 720 was performed on round-bar type specimens under pure load at the temperatures of 811, 922 and 977K. The total AE cumulative counts until the starting point of secondary creep($N_1$) and that of tertiary creep($N_2$) have quantitative relationship with the tertiary creep time and the rupture time. It is thus possible to construct the life prediction system based on creep and the prevention system of tertiary creep by using AE technique.

• 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.

• Journal of The Korean Society of Agricultural Engineers
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• v.66 no.2
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• pp.1-12
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• 2024

In order to integrate two consolidation theories of Terzaghi's consolidation theory and Mesri's secondary compression theory and to identify a model suitable for analyzing stress-strain behavior over time, numerical analysis on consolidation tests were conducted using a modified cam-clay model and a soft soil creep model and the following conclusions were obtained. The results of numerical analysis applying the theory that a linear proportional relationship is established between the void ratio at logarithmic scale and the permeability coefficient at logarithmic scale is better agreement with the result of oedometer test than the results of applying constant hydraulic conductivity. The modified cam-clay model is a model that does not include secondary compression, but the slope of the normal consolidation line corresponding to the compression index of the standard consolidation test includes secondary compression, so the actual settlement curve over time is lower than the predicted value through numerical analysis. It always gets smaller. Other previous studies that applied Terzaghi's consolidation theory to consolidation test analysis showed the same results and were cross-confirmed. The soft soil creep model, which includes secondary compression in the theory, showed good agreement in all sections including secondary compression in the consolidation test results. It was judged appropriate to use a soft soil creep model when performing numerical analysis of soft clay ground.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.3
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• pp.91-96
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• 2018

Microstructural development of ferritic 11Cr-3.45W heat-resistance steel for ultra-supercritical power plant during creep and thermal aging was investigated using electron microscopy. The test samples were isothermally aged at $700^{\circ}C$ for up to 4000 hours and subjected to creep loading at $700^{\circ}C$ for predetermined periods of lifetime to prepare the damaged materials. In this structural material, a various secondary phases are the primary influence on mechanical properties of ferritic heat-resistance steel. The typical precipitates of $M_{23}C_6$, MX and $M_2X$ secondary phases had been analyzed through qualitative and quantitative manner. Coarsening of precipitates and increase of lath width were observed during creep and thermal aging. This phenomenon was remarkable for creep process compared with isothermal aging process.