• Computers and Concrete
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• 제21권5호
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• pp.513-523
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• 2018

Time dependent strain due to creep is a significant factor in structural design. Multi-gene genetic programming (MGGP) and artificial neural network (ANN) are used to develop two models for prediction of creep compliance in concrete. The first model was developed by MGGP technique and the second model by hybridized MGGP-ANN. In the MGGP-ANN, the ANN is working in parallel with MGGP to predict errors in MGGP model. A total of 187 experimental data sets that contain 4242 data points are filtered from the NU-ITI database. These data are used in developing the MGGP and MGGP-ANN models. These models contain six input variables which are: average compressive strength at 28 days, relative humidity, volume to surface ratio, cement type, age at start of loading and age at the creep measurement. Practical equation based on MGGP was developed. A parametric study carried out with a group of hypothetical data generated among the range of data used to check the generalization ability of MGGP and MGGP-ANN models. To confirm validity of MGGP and MGGP-ANN models; two creep prediction code models (ACI209 and CEB), two empirical models (B3 and GL 2000) are used to compare their results with NU-ITI database.

• Journal of Biosystems Engineering
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• 제14권3호
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• pp.181-187
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• 1989

The mechanical and rheological properties of agricultural materials which influence the machine design or handling are not completely understood. Agricultural materials do not react in a purely elastic manner, and their responses when subjected to stress and strain appear a combination of elastic and viscous behavior. Many researchers have studied the mechanical and rheological properties of the various agricultural materials, but those properties are available mostly for foreign varieties of agricultural products. Rheological properties of rice seedlings become important to formulate the principles governing their mechanical behavior. The objectives of this study were to experimentally determine the creep and recovery behavior of rice seedlings of one japonica-type and one Indica x japonica hybrid in the transplanting age. The results of this study are summarized as follows; 1. The compression creep and recovery behavior of mat-type seedlings could be described by 4-element Burger's model. 2. The steady-state creep appeared at the stress larger than 0.8 MPa and the logarithmic creep appeared at the stress smaller than 0.8 MPa. 3. In the compression creep test of the rice seedlings, the instantaneous elastic modulus of Burger's model showed the range from 20 to 40 MPa. The higher value of absolute viscosity for the rice seedling explained that the rice seedlings were viscoelastic materials. 4. In the recovery test of the rice seedlings, there was a tendency that the higher permanent strain of all samples was observed under the smaller stress being appeared, and the larger permanent strain in Dongjin was observed than in Samkang.

• Smart Structures and Systems
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• 제24권6호
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• pp.759-768
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• 2019

In nuclear fusion reactors, the key structural component (i.e., the plasma-facing component) undergoes high heat flux cyclic loading. To ensure the safety of fusion reactors, an experimental study on the temperature-induced creep of stainless steel under heat flux cyclic loading was performed in the present work. The strains were measured using a stereo digital image correlation technique (3D-DIC). The influence of the heat haze was eliminated, owing to the use of a vacuum environment. The specimen underwent heat flux cycles ($500^{\circ}C-1000^{\circ}C$) with different mechanical preloads (0 kN, 10 kN, 30 kN, and 50 kN). The results revealed that, for a relatively large preload (for example, 50 kN), a single temperature cycle can induce a residual strain of up to $15000{\mu}{\varepsilon}$.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1999년도 봄 학술발표회 논문집
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• pp.105-112
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• 1999

In recent the superior economic benefits and the convenience of installation increased the use of geosynthetics, especially geogrids with the effects of high tensile strength. In this study, various tests were conducted to determine the physical and chemical properties of geogrids which contains durability under various critical conditions, creep behavior and the stability for installation damage in fields. With analysis of test results, the partial and total safety factors were determined and presented the long term design strength of flexible geogrids.

• Nuclear Engineering and Technology
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• 제53권8호
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• pp.2616-2628
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• 2021

The results of effective irradiation swelling in a wide range of burnup levels are numerically obtained for an inert matrix fuel, which are verified with DART model. The fission gas swelling of fuel particles is calculated with a mechanistic model, which depends on the external hydrostatic pressure. Additionally, irradiation and thermal creep effects are included in the inert matrix. The effects of matrix creep strains, external hydrostatic pressure and temperature on the effective irradiation swelling are investigated. The research results indicate that (1) the above effects are coupled with each other; (2) the matrix creep effects at high temperatures should be involved; and (3) ranged from 0 to 300 MPa, a remarkable dependence of external hydrostatic pressure can be found. Furthermore, an explicit multi-variable mathematic model is established for the effective irradiation swelling, as a function of particle volume fraction, temperature, external hydrostatic pressure and fuel particle fission density, which can well reproduce the finite element results. The mathematic model for the current volume fraction of fuel particles can help establish other effective performance models.

• 콘크리트학회논문집
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• 제13권1호
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• pp.38-45
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• 2001

바닥하중과 면내 압축력을 동시에 재하받는 지하층 슬래브에 대해 시간의존적 거동이 고려된 모멘트 증대법을 개발하기 위해서 수치해석연구를 실시하였다. 수치해석을 위해 비선형 유한요소해석 연구가 수행되었다. 수치해석 연구를 통해 균등 바닥하중과 불균등 바닥하중을 각각 재하받는 플랫 플레이트의 장기거동이 연구되었고, 크리프 효과에 의한 강도 및 강성 저하가 규명되었다. 슬래브에 대한 크리프 효과를 나타내기 위하여 크리프에 의한 유효휨강성비가 도입되었으며, 플랫 플레이트의 장기거동을 고려하기 위하여 이 크리프에 의한 유효휨강성비를 사용하는 모멘트증대법이 개발되었다. 또한 제안된 설계식의 검증을 위하여 설계예제가 제시되어 있다.

• 대한금속재료학회지
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• 제47권5호
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• pp.283-289
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• 2009

This paper dealt with a statistical analysis for evaluating the creep crack growth rate (CCGR) for Modified 9Cr-1Mo (ASTM Grade 91) steel. The CCGR data was obtained by the creep crack growth (CCG) tests conducted under various applied loads at $600^{\circ}C$. To obtain logically the B and q values used in the CCGR equation, three methods such as the least square fitting method (LSFM), the mean value method (MVM) and the probabilistic distribution method (PDM) were adopted and their CCGR lines were compared, respectively. In addition, a number of random variables were generated by using the Monte Carlo simulation (MCS), and the CCGR lines were predicted probabilistically. It was found that both the B and q coefficients followed a 2-parameter Weibull distribution well. In the case of the ranges of 10-90% for the probability variables, P(B, q), the CCGR lines were predicted. Fractographic study was conducted from the specimen after the CCG tests.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1998년도 특별강연 및 추계학술발표 개요집
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• pp.11-24
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• 1998

The invention of fusion wilding technology has brought on a revolutionary change in manufacturing industry which enables the construction of large scale high temperature plants in chemical, petrochemical and power generation industries. However, among the failure cases of high temperature components, premature failures of weldments have taken a large percentage that indicates the detrimental effect of welding on structural integrity. The accurate prediction of the high temperature behaviour of welded components is thus becoming increasingly important in order to realise an optimised design and maintenance of a plant life. In the present paper, recent research activities on high temperature behaviour of welded structures are briefly summarised. A local deformation measuring technique is proposed to determine the creep properties of weldment constituents. A damage mechanics approach is introduced to study the life reduction and ductility reduction due to the presence of a weld in high temperature structures. Finally, the high temperature creep crack growth in weldments is discussed.

• Structural Engineering and Mechanics
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• 제20권1호
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• pp.69-83
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• 2005

In many cases, underground structures are built using conventional above-grade structural systems to carry gravity load. This paper proposes the use of underground arches, termed "buried arches", to support gravity loads, wherein the horizontal thrust of the arch is equilibrated by soil pressure. Because the horizontal soil pressure increases with depth, the depth of the arch may be reduced as the depth below grade increases. Critical to the success of such an approach is a proper accounting of creep and shrinkage for arches made of reinforced concrete. This paper addresses the influence of equilibrium, creep, and shrinkage as they affect the design of the arch from a theoretical perspective. Several examples illustrate the use of buried arches for the design of underground parking structures.

• 한국지반환경공학회 논문집
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• 제1권1호
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• pp.35-42
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• 2000

매립지의 침하특성들은 위생매립장을 설계할 때 뿐만 아니라 쓰레기매립토를 복원하는 과정에서도 반드시 고려해야 할 사항들이다. 매립지의 침하를 예측하는 여러 방법들이 연구되었는데 이들 중 특히 Gibson and Lo 모델과 Power Creep Law 방법들은 침하량이 크고 이차압축이 현저하게 나타나는 매립장에 잘 부합된다. 따라서 Gibson and Lo 모델과 Power Creep Law 방법에 사용되는 매개변수들이 유기물함량에 따라 어떠한 변화를 보이는지 대형압밀시험기를 이용하여 관찰하였다. 유기물함량이 각각 20%, 40%, 60%이고 압밀하중을 $0.1{\sim}1.6kg/cm^2$로 가했을 때 Power Creep Law의 m(기준압축성)은 압밀하중의 증가에 따라 감소하였으며, 그 감소율이 유기물함량 증가량에 따라 증가하는 것으로 나타났다. 그리고 현장계측치에 의한 침하량분석결과 Gibson and Lo model보다는 Power Creep Law에 의한 침하예측이 본 현장과 부합되는 것으로 나타났다.