• Journal of Food Hygiene and Safety
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• v.5 no.4
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• pp.187-196
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• 1990

This study was carried out to develop new model for gastric ulcer and to investigate some factor which effect it. Rats were immersed for 8 hours. Ulcer index was measured from total length of occured ulcer. Ulcer index (UI) of restraint and immersion group was higher significantly than that of only restraint group (p<0.001) and ulcer was occurred within only glandular stomach. UI of low water temperature group was higher significantly than that of high water temperature group (p

• Magazine of the Korea Concrete Institute
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• v.7 no.2
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• pp.136-145
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• 1995

A method for lmprovlng strengths of recycled concrete was studied to make use of it in nolmal concrete structures. Recycled conc~ete was prepared by replacing 50% by weight of coarse aggregate with recycled aggregate. Mix design rnet hod for crushed aggregates was used and specirriens were cured by normal moisture curing method. A plastlciser and a fly ash were added to the mix to improve performance of recycled concrete. Compressive strength, flexural strength, tensile strength, elastic modulus, stress-strain relationship, long-term compressive strength and fracture toughness were evaluated and compared with those of rlormal concretes. Recycled concrete showed, in general, lower compressive strength than normal concreks. It also showed lower elastic modulus, lower tensile and flexural strengths, and higher strain under the same stress level. However, by reducing w /c ratio down to 35% using the plasticiser. average compressive strength(${\alpha}_{28}$) of recycled concrete was reached. with slump of $16{\pm}2$cm, to $225kg/cm^2$ or hlgher, which is an acceptable strmgth level for normal structural concrete. I-Iowevei., elastic modulus and strain should be improved further for practical use of recycled concrctc: in normal structure. Fly ash addition in both concretes showed an effect of irnprovilig long term compressive strength and reducing strengths.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.4 no.4
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• pp.1-11
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• 1984

Developed is a nonlinear fracture theory which can model the complex fracture behavior of concrete. This theory is based on the nonlinear behavior due to progressive microcracking and strain-softening in the fracture process zone of concrete cracks. The simplified realistic fracture model which preserves the same fracture energy for the different fracture process zone widths is also derived. By modeling fracture through stress-strain behavior, the effect of compressive stresses parallel to the crack plane can be easily taken into account. The comparisons of the present theory with valuable fracture test data available in the literature show good agreements, and the existing linear theory exhibits in many cases large deviations from the actual test results. A simple approximate formula for the fracture energy of concrete which should, be necessary for the fracture analysis of concrete structures is derived. Finally, the application of fracture theory to reinforced concrete and the necessity of further research are discussed.

• Journal of the Korea Concrete Institute
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• v.21 no.4
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• pp.419-429
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• 2009

This study focuses on the effect of the cutting order of prestressing strands on the strain change in the strands and on the state of stress of concrete, experimentally and numerically. In the experiment, strain of strands and of transversal reinforcement were measured for three different cutting orders during detensioning process by using flame-cutting procedure. The experimental results were compared with those obtained from the FE analysis. As a results of the experiment, it is confirmed that the cutting order of prestressing strands affected on the strain of strands as well as of transversal reinforcement. The FE analysis gave similar results to those obtained from the experiment. Therefore, the cutting order should be chosen appropriately to when the strands get detensioned.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.5
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• pp.337-343
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• 2017

As a shop practice, a strength estimation method for die cast parts is suggested, in which various defects such as pores can be allowed. The equivalent porosity is evaluated by combining the stiffness data from a simple elastic test at the part level during the shop practice and the theoretical stiffness data, which are defect free. A porosity equation is derived from Eshelby's inclusion theory. Then, using the Mori-Tanaka method, the porosity value is used to draw a stress-strain curve for the porous material. In this paper, the Hollomon equation is used to capture the strain hardening effect. This stress-strain curve can be used to estimate the strength of a die cast part with porous defects. An elastoplastic theoretical solution is derived for the three-point bending of a die cast beam by using the plastic hinge method as a reference solution for a part with porous defects.

• Tunnel and Underground Space
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• v.19 no.2
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• pp.158-166
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• 2009

As a basic study for investigating the development of the stress-induced crack in Hoek-Brown rock, a homogenization technique of elastic cracks is proposed. The onset of crack is monitored by Hoek-Brown empirical criterion, while the orientation of the crack is determined by the critical plane approach. The concept of volume averaging in stress and strain component was invoked to homogenize the representative rock volume which consists of intact rock and cracks. The formulation results in the constitutive relations for the homogenized equivalent anisotropic material. The homogenization model was implemented in the standard FEM code COSMOSM. The numerical uniaxial tests were performed under plane strain condition to check the validity of the propose numerical model. The effect of friction between the loading plate and the rock sample on the mode of deformation and fracturing was examined by assuming two different contact conditions. The numerical simulation revealed that the homogenized model is able to capture the salient features of deformation and fracturing which are observed commonly in the uniaxial compression test.

• Tunnel and Underground Space
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• v.32 no.3
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• pp.219-230
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• 2022

The Geo-Reservoir Experimental Analogue Technology (GREAT) cell was designed to recreate the thermal-hydro-mechanical conditions of deep subsurface in the laboratory. This apparatus can generate a polyaxial stress field using lateral loading elements, which rotate around the longitudinal axis of a sample and is capable of performing a fluid flow test for samples containing fractures. In the present study, numerical simulations were carried out for triaxial compression tests using the GREAT cell and the mechanical behavior of samples under different conditions of lateral loading was investigated. We simulated an actual case, in which triaxial compression tests were conducted for a polymer sample without fractures, and compared the results between the numerical analysis and experiment. The surface strain (circumferential strain) of the sample was analyzed for equal and non-equal horizontal confining pressures. The results of the comparison showed a good consistency. Additionally, for synthetic cases with a fracture, we investigated the effect of the friction and type of fracture surface on the deformation behavior.

• Geomechanics and Engineering
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• v.37 no.1
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• pp.49-64
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• 2024

Rocks undergoing repeated loading and unloading over an extended period, such as due to earthquakes, human excavation, and blasting, may result in the gradual accumulation of stress and deformation within the rock mass, eventually reaching an unstable state. In this study, a CNN-CCM is proposed to address the mechanical behavior. The structure and hyperparameters of CNN-CCM include Conv2D layers × 5; Max pooling2D layers × 4; Dense layers × 4; learning rate=0.001; Epoch=50; Batch size=64; Dropout=0.5. Training and validation data for deep learning include 71 rock samples and 122,152 data points. The AI Rock Constitutive Model learned by CNN-CCM can predict strain values(ε₁) using Mass (M), Axial stress (σ₁), Density (ρ), Cyclic number (N), Confining pressure (σ₃), and Young's modulus (E). Five evaluation indicators R², MAPE, RMSE, MSE, and MAE yield respective values of 0.929, 16.44%, 0.954, 0.913, and 0.542, illustrating good predictive performance and generalization ability of model. Finally, interpreting the AI Rock Constitutive Model using the SHAP explaining method reveals that feature importance follows the order N > M > σ₁ > E > ρ > σ₃.Positive SHAP values indicate positive effects on predicting strain ε₁ for N, M, σ₁, and σ₃, while negative SHAP values have negative effects. For E, a positive value has a negative effect on predicting strain ε₁, consistent with the influence patterns of conventional physical rock constitutive equations. The present study offers a novel approach to the investigation of the mechanical constitutive model of rocks under cyclic loading and unloading conditions.

• Computers and Concrete
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• v.33 no.5
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• pp.605-619
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• 2024

The increase of reclaimed asphalt pavement (RAP) content in recycled asphalt concrete (RAC) is accompanied by the degradation of low-temperature cracking resistance, which has become an obstacle to the development of RAC. This paper aims to reveal the meso-scale mechanisms of the low-temperature fracture behavior of RAC and provide a theoretical basis for the economical recycling of RAP. For this purpose, micromechanical heterogeneous peridynamic model of RAC was established and validated by comparing three-point bending (TPB) test results against corresponding numerical simulation results of RAC with 50% RAP content. Furthermore, the models with different aggregate shapes (i.e., average aggregates circularity (${\bar{C_r}}=1.00$, 0.75, and 0.50) and RAP content (i.e., 0%, 15%, 30%, 50%, 75%, and 100%) were constructed to investigate the effect of aggregate shape and RAP content on the low-temperature cracking resistance. The results show that peridynamic models can accurately simulate the low-temperature fracture behavior of RAC, with only 2.9% and 13.9% differences from the TPB test in flexural strength and failure strain, respectively. On the meso-scale, the damage in the RAC is mainly controlled by horizontal tensile stress and the stress concentration appears in the interface transition zone (ITZ). Aggregate shape has a significant effect on the low-temperature fracture resistance, i.e., higher aggregate circularity leads to better low-temperature performance. The large number of microcracks generated during the damage evolution process for the peridynamic model with circular aggregates contributes to slowing down the fracture, whereas the severe stress concentration at the corners leads to the fracture of the aggregates with low circularity under lower stress levels. The effect of RAP content below 30% or above 50% is not significant, but a substantial reduction (16.9% in flexural strength and 16.4% in failure strain) is observed between the RAP content of 30% and 50%. This reduction is mainly attributed to the fact that the damage in the ITZ region transfers significantly to the aggregates, especially the RAP aggregates, when the RAP content ranges from 30% to 50%.

• Korean Journal of Materials Research
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• v.28 no.8
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• pp.435-439
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• 2018

MA Al alloys are examined to determine the effects of alloying of Mg and Cu and rolling on tensile deformation behavior at 748 K over a wide strain rate range($10^{-4}-10^3/s$). A powder metallurgy aluminum alloy produced from mechanically alloyed pure Al powder exhibits only a small elongation-to-failure(${\varepsilon}_f$ < ~50%) in high temperature(748 K) tensile deformation at high strain rates(${\acute{\varepsilon}}=1-10^2/s$). ${\varepsilon}_f$ in MA Al-0.5~4.0Mg alloys increases slightly with Mg content(${\varepsilon}_f={\sim}140%$ at 4 mass%). Combined addition of Mg and Cu(MA Al-1.5%Mg-4.0%Cu) is very effective for the occurrence of superplasticity(${\varepsilon}_f$ > 500%). Warm-rolling(at 393-492 K) tends to raise ${\varepsilon}_f$. Lowering the rolling-temperature is effective for increasing the ductility. The effect is rather weak in MA pure Al and MA Al-Mg alloys, but much larger in the MA Al-1.5%Mg-4.0%Cu alloy. Additions of Mg and Cu and warm-rolling of the alloy cause a remarkable reduction in the logarithm of the peak flow stress at low strain rates (${\acute{\varepsilon}}$< ~1/s) and sharpening of microstructure and smoothening of grain boundaries. Additions of Mg and Cu make the strain rate sensitivity(the m value) larger at high strain rates, and the warm-rolling may make the grain boundary sliding easier with less cavitation. Grain boundary facets are observed on the fracture surface when ${\varepsilon}_f$ is large, indicating the operation of grain boundary sliding to a large extent during superplastic deformation.