• Journal of the Korea Concrete Institute
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• v.13 no.5
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• pp.415-422
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• 2001

The present paper focuses on the development of a realistic analysis model for the deformation calculation of reinforced concrete beams subjected to fatigue loadings. The proposed model considers the effect of cyclic creep, which arises from the repeated loading, to calculate the deformation of reinforced concrete beams. A comprehensive experimental program has been set up to identify the deformation accumulation of reinforced concrete beams under repeated loadings. The major test variables were the concrete compressive strength and the magnitude of fatigue loads. The model was calibrated from the present test results. The proposed model allows more realistic analysis of reinforced concrete beams under fatigue loads, especially deformation accumulation of such beams.

• Journal of the Korea institute for structural maintenance and inspection
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• v.8 no.2
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• pp.195-204
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• 2004

A series of constant creep and repeated creep tests are performed to investigate the behavior of visco-elasto-plastic materials of chemical grouted sands. In the result of constant creep test, the material exhibits three types of shear strain : elastic, plastic, viscoelastic. The elastic, plastic and viscoelastic strains are linear, i.e., the strains are proportional to the stresses for loading. Good agreement is found between the predicted viscoelastic and test results by the power law and the generalized model. In the repeated creep test, the instantaneous recoverable strain is time-independent and the magnitude of accumulated plastic strain increases with number of cycles. Also it is seen that the accumulated plastic strains are approximately proportional to stress. There are no significant differences between test results predicted values for first cycle, and the differences increase relatively insignificantly with number of cycles.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.4
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• pp.321-331
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• 2011

The present work is aimed to analyze the creep characteristics of a turbojet engine turbine blade using the theta projection method. The theta projection method has been widely used due to its advantages and flexibility. For the creep characteristic analysis of the turbine blade, tests are performed considering the operating conditions and the non-linear material properties. Results from the creep test are fitted using the four theta model. The predicted proprieties using the four theta model are compared with the prediction model and creep test results. To obtain an optimum value of the four theta parameters in non-linear square method, a number of computing processes in the non-linear least square method were carried out to obtain full creep curves. Results using the theta model has more than 0.95 value of $R^2$. The results between the experimental values and predicted four theta model has about 90.0% accuracy. The theta projection method can be utilized for a design purpose to predict the creep behavior.

• Polymer(Korea)
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• v.36 no.1
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• pp.88-92
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• 2012

Long-term performance of polymer under constant sustained load has been the main research focus, which created a need for the accelerated test method providing proper lifetime assessment. Cycling fatigue loading is one of the accelerated test method and has been of great interest. Microstructure change of high density polyethylene under cyclic fatigue loading and creep was examined utilizing a tensile device specially designed for creep and fatigue test and also can be attachable to the X-ray diffractometer. In this way, the crystal morphology change of polyethylene under creep and cyclic fatigue load was successfully monitored and compared. Despite the marked differences in macroscopic deformation between the creep and cyclic fatigue tests, crystal morphology such as crystallinity, crystal size, and $d$-spacing was as nearly identical between the two test cases. Specimens pre-deformed to different strains, i.e., before yield point (BYP), at yield point (YP) and after yield point (AYP), however, showed markedly different changes in crystal morphology, especially between AYP and the other two specimens.

• Magazine of the Korea Concrete Institute
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• v.11 no.1
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• pp.279-288
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• 1999

The creep of concrete structures caused by variable stresses is generally calculated by step-by-step method based on the superposition of creep function. Although most practical application is carried out by this linear assumption. significant deviations between predictions and experiments have been observed when unloading takes place, that is. stress is reduced. This shows that the superposition of creep function does not describe accurately the effect of sustained compressive preload. The main purpose of this study is to propose a creep analysis model which is expressed with both creep function and creep recovery function where increase or decrease of stress is repeated. In these two function method, the creep behavior is modelled by using linear creep law for loading and creep recovery law for unloading. To apply two function method to time analysis of concrete structures, the calculation method of creep strain increment under varying stress is proposed. The calculation results based on the present method correlates very well with test data, but the conventional superposition method exhibits large deviation from test results. This paper provides a more accurate method for the time dependent analysis of concrete structures subjected to varying stress, i.e. increasing or decreasing stress. The present method may be efficiently employed in the revision of future concrete codes.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.555-560
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• 2003

Fatigue strength of concrete is ususlly presented by the Wohler Curve. But, new dimension T(time) from the view point of cyclic creep concept should be considerd. This paper presented four variable F-N-T-R relationship, this four variable relationship simultaneously accounts for the time effect and the effect of load rate. And analytical models are presented to predict fatigue strength of R/C beam strengthened with steel plate and carbon fiber sheet. Also, the correlation between the ratio of stress and the fatigue life was investigated.

• Journal of the Korean Geosynthetics Society
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• v.4 no.1
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• pp.3-6
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• 2005

The failure of geogrids can be defined as an excessive creep strain which causes the collapse of slopes and embankments. In this study, the lifetime of knitted polyester geogrids was predicted by using SIM(Stepped Isothermal Methods using TTS principal) and statistical data analysis techniques. The results indicate that the creep strain was 8.74, 8.79, 8.80% with 2.16~2.20% of CV% at 75, 100, 114 years, respectively and the creep strain reaches 9.3% after 100 years of usage at $27^{\circ}C$ which meets the required lifetime(creep strain less than 10% after 100 years of usage) in the fields. The SIM method is shown to be effective in reduction of uncertainty associated with inherent variability of multi-specimen tests and shorter test times than conventional TTS(Time-Temperature Superposition).

• Journal of the Korean Geotechnical Society
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• v.18 no.4
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• pp.105-117
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• 2002

In the design of foundations for the super-structures such as transmission towers and oil-platforms, the foundations must be considered as a medium to resist cyclic tensile forces. In this study, the uplift capacity of the drilled shaft used as the medium resisting to this pattern of forces is investigated by performing cyclic uplift test of a small model-drilled shaft constructed in compacted granite soil in a steel chamber. In this test, the behavioral difference between a pile loaded on the top of the pile and a pile loaded at the bottom of the pile was investigated intensively. The load transfer curves obtained from the test were investigated by changing the confining pressure in the chamber. The load tests also included creep test and cyclic test. It is found from the tests that uplift capacity of the shaft loaded at the bottom is greater than that of the shaft loaded on the top of the pile. It is found also from the creep test that the pile loaded at the bottom was more stable than the shaft loaded on the top. If a pile loaded at the bottom is pre-tensioned, the pile will be most effective to the creep displacement. It is found also from the cyclic tests that apparent secant modulus obtained in a cycle of the load increases with the number of cycles.

• Proceedings of the KIEE Conference
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• 2003.07d
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• pp.2726-2728
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• 2003

본 논문에서는 디지털 로드 셀의 국산화 결과를 보고한다. 개발된 로드 셀의 특성은 다음과 같다. A/D분해도: 20 bit, 변환속도: 100 Hz, 정도: < 0.02% R.O., 반복특성: < 0.01% R.O. 사용온도범위: $-20{\sim}80^{\circ}C$, 온도특성: < 0.03% R.O./$10^{\circ}C$, 진단기능: 자체내장, 자체보정기능: 온도, 편심오차, 크리프 보정기능 내장, 보호회로: surge lightening.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04a
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• pp.365-370
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• 1998

The creep strain of the compression zone of concrete beams subjected to cyclic loading should be a significant factor in increasing strain and deflections. An analytical model which is similar to a previous one is presented to predict the increase in cyclic creep strain and the damage using the properties of the constituent materials: concrete and steel. The analytical expressions are compared with our experimental data. The effect of concrete-creep is accounted by the term En, Icr,n, and Mcr,n. In this study, it is proved that cyclic creep exponents 'n' in Cyclic Creep Model, according to the parameters -strength, range of stress- have the various values. It is hoped that with the availability of more experimental data and better understanding of some of the complex behavior, the model could be further improved.