• 한국세라믹학회지
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• 제33권5호
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• pp.602-615
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• 1996

The purpose of the present study is to investigate the microstructural effect on the R-curve behavior in three aluminas with different grain size distributions by analyzing the bridging stress distribution. The crack opening displacement (COD) according to the distance behind the stationary crack tip was measured using an in situ SEM fracture method. The measured COD values in the fine-grained alumina agreed well with Wiederhorn's sollution while they deviated from Wiederhorn's solution in the two coarse-grained aluminas because of the increase of the crack closure due to the grain interface bridging in the crack wake. A numerical fitting procedure was conducted by the introduction of the power-law relation and the current theoretical model together with the measured COD's in order to obtain the bridging stress distribution. The results indicated that the bridging stress function and the R-curve computed by the current model were consistent with those computed by the power-law relation providing a reliable evidence for the bridging stress analysis of the current model. The strain-softening exponent in the power-law relation n, was calculated to be in the range from 2 to 3 and was closely related to the grain size distribution. Thus it was concluded from the current theoretical model that the grain size distribution affected greatly the bridging stress distribution thereby resulting in the quantitative analysis of microfracture of polycrystalline aluminas through correlating the local-fracture-cont-rolling microstructure.

• Interaction and multiscale mechanics
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• 제1권3호
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• pp.317-328
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• 2008

When carbon-filled rubber specimens are subjected to cyclic loading, they do not return to their initial state after loading and subsequent unloading, but exhibit a residual strain or permanent deformation. We propose a specific form of the pseudo-elastic energy function to represent cyclic loading for incompressible, isotropic materials with stress softening and residual strain. The essence of the pseudo-elasticity theory is that material behaviour in the primary loading path is described by a common elastic strain energy function, and in unloading, reloading or secondary unloading paths by a different strain energy function. The switch between strain energy functions is controlled by the incorporation of a damage variable into the strain energy function. An extra term is added to describe the permanent deformation. The finite element implementation of the proposed model is presented in this paper. All parameters in the proposed model and elastic law can be easily estimated based on experimental data. The numerical analyses show that the results are in good agreement with experimental data.

• 한국지반공학회논문집
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• 제24권4호
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• pp.67-78
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• 2008

본 연구에서는 시멘트의 고결효과를 정량적으로 파악하기 위하여 낙동강하상모래와 소량의 포틀랜드시멘트를 혼합하여 고결된 모래에 대하여 일축압축시험 및 배수조건과 비배수 조건의 삼축압축시험을 수행하였다. 시멘트혼합율의 증가에 따라 첨두강도 및 탄성계수는 증가하였고 시멘트의 결합력에 의하여 다일레이션 및 과잉간극수압이 억제되었으나 시멘트 결합력의 파괴 후 증가된 모래입자크기에 의하여 증가하였다. 그리고 배수조건의 응력-변형률 곡선은 연화거동이 나타났지만 비배수조건에서는 증가된 부(-)의 과잉간극수압에 의한 유효응력의 증가로 경화거동을 나타냈다. 각 조건에 대한 강도증가량의 예측을 위하여 선형을 가정한 다중회귀분석을 실시한 결과 제시된 경험식의 결정계수는 $0.81{\sim}0.91$로 나타나 신뢰성이 높은 것으로 평가되었으며 건조밀도의 경우 고결된 모래의 입도조건을 동시에 고려할 수 있어 고결된 모래의 강도를 결정하는데 중요한 변수로 분석되었다.

• Steel and Composite Structures
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• 제36권2호
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• pp.179-186
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• 2020

In this paper, the forced resonance vibration of porous functionally graded (FG) curved nanobeam is examined. In order to capture the hardening and softening mechanisms of nanostructure, the nonlocal strain gradient theory is employed to build the size-dependent model. Using the Timoshenko beam theory together with the Hamilton principle, the equations of motion for the curved nanobeam are derived. Then, Navier series are used in order to obtain the dynamical deflections of the porous FG curved nanobeam with simply-supported ends. It is found that the resonance position of the nanobeam is very sensitive to the nonlocal and strain gradient parameters, material variation, porosity coefficient, as well as geometrical conditions. The results indicate that the resonance position is postponed by increasing the strain gradient parameter, while the nonlocal parameter has the opposite effect on the results. Furthermore, increasing the opening angle or length-to-thickness ratio will result in resonance position moves to lower-load frequency.

• 대한기계학회논문집A
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• 제27권6호
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• pp.914-922
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• 2003

In this paper, the strain localization of voided ductile material has been analyzed by nonlocal plasticity formulation in which the yield strength not only depends on an equivalent plastic strain measure (hardening parameter), but also on the Laplacian thereof. The gradient terms in yield criterion show an important role on modeling strain-softening phenomena of material. The influence of the mesh size on the elastic -plastic deformation behavior and the effect of the characteristic length parameter for localization prediction are also investigated. The proposed nonlocal plasticity shows that the load -strain curves converge to one curve. Results using nonlocal plasticity also exhibit the dependence of mesh size is much less sensitivity than that for a corresponding local plasticity formulation.

• Advances in nano research
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• 제13권5호
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• pp.465-474
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• 2022

Based on the nonlocal strain gradient (NSG) theory and considering the influence of moment of inertia, the governing equations of motion of porous functionally graded (FG) nanoplates with four edges clamped are established; The Galerkin method is applied to eliminate the spatial variables of the partial differential equation, and the partial differential governing equation is transformed into an ordinary differential equation with time variables. By satisfying the boundary conditions and solving the characteristic equation, the dispersion relations of the porous FG strain gradient nanoplates with four edges fixed are obtained. It is found that when the wave number is very small, the influences of nonlocal parameters and strain gradient parameters on the dispersion relation is very small. However, when the wave number is large, it has a great influence on the group velocity and phase velocity. The nonlocal parameter represents the effect of stiffness softening, and the strain gradient parameter represents the effect of stiffness strengthening. In addition, we also study the influence of power law index parameter and porosity on guided wave propagation.

• 한국유기농업학회지
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• 제23권2호
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• pp.291-299
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• 2015

본 연구에서는 인산가용화 균주인 Kluyvera sp. CL2 처리시기가 과실품질에 미치는 영향을 구명하고자 본 연구를 실시하였다. 토양의 화학성 변화는 경핵기 처리구에서 인산이 가장 많이 증가하였고, K, Ca, Mg도 증가됨을 알 수 있었으며, pH의 변화는 토양 완충능에 의하여 적었다. 인산가용화균 처리에 따른 수용성 인산의 변화는 처리 시부터 20일 까지 효과적이었으며, 과립연화기 처리구부터 인산가용화율이 좋았다. 또한 과립연화기 처리구에서 과방중과 당도가 증가됨을 알 수 있었다. 인산가용화 균주 처리구에서 안토시아닌, 명도, 적색도, 황색도가 경핵기와 과립연화기 처리구에서 유의성을 보였으며, 특히 적색도가 향상되어 착색증진 효과가 있었다. 결론적으로 과실특성과 과실품질을 모두 비교하였을 때 과립연화기에 인산가용화 균주를 투입하는 것이 수량성과 품질면에서 가장 효과적이라고 생각된다.

• Geomechanics and Engineering
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• 제8권6호
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• pp.829-844
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• 2015

In the current paper the results of a numerical simulation that were verified by a well instrumented experimental procedure for studying the arching effect over a trapdoor in sand is presented. To simulate this phenomenon with continuum mechanics, the experimental procedure is modeled in ABAQUS code using stress dependent hardening in elastic state and plastic strain dependent frictional hardening-softening with Mohr Coulomb failure criterion applying user sub-routine. The apparatus comprises rectangular trapdoors with different width that can yield downward while stresses and deformations are recorded simultaneously. As the trapdoor starts to yield, the whole soil mass deforms elastically. However, after an immediate specified displacement, depending on the width of the trapdoor, the soil mass behaves plastically. This behavior of sand occurs due to the flow phenomenon and continues until the stress on trapdoor is minimized. Then the failure process develops in sand and the measured stress on the trapdoor shows an ascending trend. This indicates gradual separation of the yielding mass from the whole soil body. Finally, the flow process leads to establish a stable vault of sand called arching mechanism or progressive collapse of the soil body.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.295-298
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• 2003

High temperature deformation behavior of a Ti-Al intermetallic compound has been investigated. Specimens with a near gamma and a lamella structures were obtained by performing heat treatment at 1200 and 1330$^{\circ}C$, respectively, for 24 hr and stabilized at 900$^{\circ}C$ for 4 hr followed by air cooling. A series of load relaxation tests has been conducted on these samples at temperatures ranging from 850 to 950$^{\circ}C$ to construct flow curves in the strain rate range from 10$\^$-6//s to 10$\^$-3//s. Strain hardening was observed even at the temperature of 950$^{\circ}C$ in both the near gamma and the lamella structures. Further aging treatment for 12 hr at test temperatures has found to cause no softening in both microstructures.

• Computers and Concrete
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• 제15권4호
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• pp.515-545
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• 2015

The paper presents results of FE simulations of the strain-rate sensitive concrete behaviour under dynamic loading at the macroscopic level. To take the loading velocity effect into account, viscosity, stress modifications and inertial effects were included into a rate-independent elasto-plastic formulation. In addition, a decrease of the material stiffness was considered for a very high loading velocity to simulate fragmentation. In order to ensure the mesh-independence and to properly reproduce strain localization in the entire range of loading velocities, a constitutive formulation was enhanced by a characteristic length of micro-structure using a non-local theory. Numerical results were compared with corresponding laboratory tests and available analytical formulae.