• Journal of Biosystems Engineering
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• v.16 no.1
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• pp.37-48
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• 1991

Rice plants are subjected to various forces such as natural force of wind and mechanical force of cultivating machines. Rheological behavior of the rice stem can be expressed in terms of three variables : stress, relaxation and time. The objectives of this study are to examine stress relaxation, creep and recovery characteristics on the rice stem in case of axial and radial loading. Stress relaxation with time was studied on three levels of loading rate and on four levels of applied stress. The results were summarized as follows : 1. The hysterisis losses of the rice stem distinctly observed at the radial compression in comparison with axial compression. The hysterisis loss implied that the stem to absorbed energy without being deformed beyond the yield point. 2. Ageneralized Maxwell model consisting of three elements gave a good description of the relaxation behavior of the rice stem. Rate of loading was more significant on the observed relaxation behavior within the short relaxation time, but there were little influences of rate of loading on the relaxation time. 3. The stress relaxation intensity and the residual stress increased in magnitude as the applied stress increased, but the relaxation time was little affected by the applied stress. 4. The coefficients of the stress relaxation model showed much differences in the radial compression and the axial compression, especially the higher relaxation stress of the third element was observed in the radial compression. 5. The behaviors of rice stem in creep and recovery test also might be represented by a four element Burger's model. But the coefficients of the creep model were different from those of the recovery model. 6. The steady-state phenomena of creep appeared at the stress larger than 20 MPa in Samkang and 1.8 MPa in Whajin. 7. The elastic modulus of the stem showed the range from 40 to 60 MPa. It could be considered, as a result, the rice stems had viscoelastic properties.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.3
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• pp.355-361
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• 1987

Creep and fatigue tests were carried out on crack growth properties of small surface cracks in 304 stainless steel at 538.deg.C, 593.deg. C and 650.deg. C in air, by using small plate specimens with a small artificial pit. All the data of the crack growth rate per hour obtained in the present tests were correlated with the maximum stress intensity factor, so that the applicability of linear fracture mechanics to the crack growth of surface cracks at elevated temperature was investigated. In the creep test, relatiion of .sigma.$\^$n/.t$\_$f/=C is obtained between failure time and nominal stress at each temperature level, where n has the value of 11-14 depending on the temperature level. In the creep and fatigue crack growth properties of surface cracks at the elevated temperatures, the maximum stress intensity factor, $_{4}$$\_$max/, is some extent applicable parameter to describe the surface crack growth rate under the present experimental conditions. The crack growth rate per hour increases when the holding time decreases, and creep crack growth rate per hour becomes the lowest limit of crack growth rate per hour in this tests.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.1
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• pp.125-132
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• 2010

In the recent construction industry, an external strengthening method using fiber reinforced polymers has been widely used. Since reinforced concrete structures strengthened with fiber reinforced polymers are always under sustained loads, influence of creep and shrinkage on the structures is inevitable. Due to the creep and shrinkage, behaviors of the structures, such as deflection, deformation, recovery capability, strength and so on are also under the influence of creep and shrinkage. Thus, in order to estimate efficacy, creep recovery and residual strength of FRP strengthened RC beams, long-term flexural experiments and static flexural experiments were carried out. As the result of the experiments, FRP strengthened RC beams were very effective in terms of deflection control. Furthermore, the strengthened beams had higher immediate deformation recovery than immediate deformation. Through the static flexural experiments, it was shown that the CFRP strengthened beam had high residual strength. It seems that the sustained loads did not affect bond and residual strength of the beams.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.119-124
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• 2001

A general procedure to implement the sensitivity analysis of PSC flexural members is proposed based on the analytical calculation of the gradients of stresses and strains with respect to the 21 design variables in a closed format. The formulation covers the long term losses including concrete creep, shrinkage, and PS steel relaxation as well as load effects. The derived formulation is applied to the rectangular section PSC beam with prestressing and nonprestressing steels for the sensitivity analysis. The analytically calculated sensitivity results are compared with those numerically calculated.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.11a
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• pp.405-408
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• 2003

In a slender column under sustained eccentric compression, the deformations caused by creep and shrinkage can lead to an increase in the loads on the structure and a reduction in strength. This study presents a methodlogy and improved computer program for the analysis of time-dependent long column in considering slender effects and nonlinear behaviors. In this result, when slenderness ratio is greater than 80, we know that magnified moment methods may be not applied in long columns.

• Nuclear Engineering and Technology
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• v.20 no.4
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• pp.241-245
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• 1988

It is very important to investigate the behaviors of the material parameters governing PCI fuel failure during power ramp because PCI fuel failure is considered to be related to the operations limits of power reactors. In this study, the behavior characteristics of the material parameters such as hoop stress, hoop strain, ridge height, creep strain rate and strain energy in cladding were studied as a function of the operating parameters such as power shock and ramp rate. The FEMAXI-IV fuel rod performance analysis code was used for this study.

• Journal of Korean Society of Steel Construction
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• v.12 no.2 s.45
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• pp.133-142
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• 2000

This study deals with behavior of steel box girder bridges according to the concrete slab casting sequences and sectional types. The time dependent behavior of bridges caused by the differential setting of slab concrete resulting from time gap for each part of slab deck in a sequential placing method produces is analyzed. In correlation studies between girder section types and placing sequences, time dependent effects of concrete creep and shrinkage are implemented in the analytical model proposed in the previous study. Finally, field recommendations in terms of concrete slump and relative humidity are suggested to prevent early transverse cracking of concrete slabs.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.2
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• pp.113-119
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• 2002

In the prestressed concrete structures, stresses are gradually redistributed with time due to the creep and shrinkage of concrete and the stress relaxation of prestressed steel. In this study a numerical procedure and computer program is developed to analyze the behavior of prestressed concrete structures considering the time-dependent properties of material. It accounts for the aging, creep and shrinkage of concrete and the stress relaxation of prestressed steel. The structural model uses two dimensional plane frame elements with three nodal degree of freedom and is analyzed based on the finite element method. Member cross section can consist of concrete, reinforcement and prestressing steel. Two different set of equations for the prediction of time-dependent material properties of concrete are presented, which are ACI, CEB-FIP. Analytical studies for different examples of prestressed concrete structures have been performed to demonstrated the capabilities and practical applicabilities of the developed program.

• Journal of Korean Association for Spatial Structures
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• v.11 no.3
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• pp.115-123
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• 2011

It is needed to investigate the material properties of ETFE foil under various temperatures because ETFE membrane foils have very thin depth and relatively high flexibility. In this paper, the material properties of ETFE membrane foil obtained from 3 testes under various temperatures are presented. First, the uniaxial test under four temperatures as -20$^{\circ}C$, 0$^{\circ}C$, +20$^{\circ}C$ and +40$^{\circ}C$ was performed. Each 5 specimen was tested and the yield stress, tensile strength and the Young's modulus of the foils are obtained. Second, the creep testes under three temperatures as 25$^{\circ}C$, 40$^{\circ}C$ and 60$^{\circ}C$, 3MP, 6MP and 9MP tension load was subjected to the specimen and the creep characteristics was investigated. Finally, the tear test under $5^{\circ}C$, $^0{\circ}C$ and $20^{\circ}C$ was performed. It is concluded that the shape of stress-strain curve or general behaviors are similar with that of normal temperatures but the mechanical characteristics of ETFE membrane foils were affected by the temperatures, obviously.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.105-108
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• 2008

For the early seismic isolation design in Korea, foreign products of isolation bearings were used. But these days, the application of domestic products of isolation bearings is increasing. However various experimental studies can be found very seldom on the extreme and lonr term behaviors of isolation bearings. In this study, we considered the laminated rubber type isolation bearings that have many application cases in Korea and we evaluated their shear strength, long term characteristics such as aging and creep affecting shear strength of bearings in long term period. For the reality of experiments, fabricated isolation bearing specimens are designed for a real structure and shear loading was applied under design compressive loads. To evaluated aging effect, the specimens were exposed to high temperature environment for certain period and their shear properties were measured to compare with their original values. Also we measured creep amount of isolation bearings under constant compressive load for 1,000 hours and estimated creep amount after 60 years compatible with general life cycle of bridges.