• 대한화학회지
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• 제21권5호
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• pp.330-338
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• 1977

고체의 소성변형을 설명하기 위하여 다음과 같은 가정을 하였다. (1) 고체의 소성변형은 크게 두 가지 기구 즉 dislocation 운동과 grain boundary 운동에 의하여 일어난다. (2) Dislocation 운동에 있어서 유동 단위들은 역학적 모형으로 나타내면 다종의 Maxwell 단위들의 평행연결형으로 되고 grain boundary 유동단위들도 다종의 Maxwell 단위들의 평행연결로 표현된다. 이를 물리적으로 설명하면 같은 부류의 유동단위들은 모두 같은 shear plane에서 같은 shear rate로 흐름을 의미한다. (3) Grain boundary 유동단위들과 dislocation 유동단위들 같은 서로 직렬 연결되어 있다. 이는 물리적으로 고체내에서 stress는 균일하게 작용하나 shear rate는 shear plane 의 종류(dislocation 운동면과 grain boundary 운동면)에 따라 달리 나타남을 의미한다. (4) Dislocation 유동단위들과 grain boundary 운동단위들의 운동은 그들의 흐름을 방해하는 장애물 근방의 원자 또는 분자들이 확산해 나가므로써 가능하게 된다. 이러한 가정하에 반응속도론을 적용하여 shear rate와 shear stress를 구하는 일반식을 도출하였다. 본 연구에서는 실제로 중요한 네가지 경우에 대하여 상기 도출한 일반식을 고찰하였다.

• Journal of the Korean Wood Science and Technology
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• 제37권5호
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• pp.465-473
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• 2009

Three failure theories were studied to evaluate the wood strength variation with grain slope. Maximum stress theory, Tsai-Hill theory and Hankinson formula were presented to hypothesize the failure of wood according to grain slope to loading direction. Red pine and Japanese larch were used as materials to simulate failure strength prediction with grain slope. Calculation of strength results was that the strength of wood drops rapidly between parallel to grain orientation (0 degree) and 15 degree grain orientation. The strength of wood with grain orientation were somewhat different at small grain angles among failure theories, and this tendency was due to tension and compression distinction, and shear accounting in each theories. For the above 45 degree grain orientation, the predicted failure strength of wood with grain variation were very close in each failure theories and were useful in assessing failure strength of wood. The applicable these theories should be considered that the wood has different behavior in tension and compression, and this lead to different strength at small grain angles in each theories. Furthermore, reconsideration is needed to assess the failure strength of wood at small grain angles in Hankinson formula and further studies are necessary to accounting for shear behavior at small grain angles.

• Geomechanics and Engineering
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• 제20권3호
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• pp.243-254
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• 2020

Due to the permanent damage to structures during earthquakes, soil liquefaction is an important issue in geotechnical earthquake engineering that needs to be investigated. Typical examples of soil liquefaction have been observed in many earthquakes, particularly in Alaska, Niigata (1964), San Fernando (1971), Loma Prieta (1989), Kobe (1995) and Izmit (1999) earthquakes. In this study, liquefaction behavior of uniform sands of different grain sizes was investigated by using the energy-based method. For this purpose, a total of 36 deformation-controlled tests were conducted on water-saturated samples in undrained conditions by using the cyclic simple shear test method and considering the relative density, effective stress and mean grain size parameters that affect the cumulative liquefaction energy. The results showed that as the mean grain size decreases, the liquefaction potential of the sand increases. In addition, with increasing effective stress and relative density, the resistance of sand against liquefaction decreases. Multiple regression analysis was performed on the test results and separate correlations were proposed for the samples with mean grain size of 0.11-0.26 mm and for the ones with 0.45-0.85 mm. The recommended relationships were compared to the ones existing in the literature and compatible results were obtained.

• Geomechanics and Engineering
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• 제35권2호
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• pp.135-147
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• 2023

Sand columns in clayey soil are considered one of the most economical and environmentally-friendly soil-improving techniques. It improves the shear strength parameters, reduces the settlement, and increases the bearing capacity of clayey soils. The aim of this paper is to study the effect of grain shape in sand columns on their performance in improving the mechanical properties of clayey soils. An intensive series of consolidated-drained triaxial tests were performed on clay specimens only and clay specimens with sand columns. The parameters examined during the experimental work were grain shape in sand columns (angular, rounded, sub-rounded) and effective confining pressure (50 kPa, 100 kPa, 200 kPa). The results indicated that there is a significant improvement in the deviatoric stress and stiffness values of specimens with sand columns. Improving deviatoric stress values in the use of angular sand grains was found to be higher than those in the use of sub-rounded and rounded sand grains. A 187%, 159%, and 153% increment in deviatoric stress values were observed for the sand columns with angular, sub-rounded, and rounded grain shapes, respectively. The specimens were observed to be more contractive as the sand column was installed, and as the angularity of grains in the sand column was increased. Sand column installation improves significantly the angle of internal friction, and the effective angle of internal friction increases as the angularity of the sand grains increases.

• 한국지구과학회지
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• 제42권2호
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• pp.164-174
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• 2021

본 연구는 육상 기원 조립질 하상 퇴적물의 높은 이동성에 영향을 미치는 전단과 파쇄특성을 조사하기 위하여 링전단실험을 수행하였다. 평균 입경 6 mm 자갈에 대하여 링전단시험장치를 사용하여 전단시간(shear time)과 전단속도(shear velocity)에 따른 전단-변형률 역학특성과 입자파쇄 특성을 조사하였다. 특히 배수(장시간 전단)와 비배수(단시간 전단)조건을 고려하기 위하여 초기 전단속도(0.01→0.1→1 mm/sec와 0.1→0.01→1 mm/sec)에 따른 링전단실험을 수행하였다. 실험결과에 따르면, (i) 배수와 비배수조건 모두에서 입자파쇄 특성이 확인되었지만, 비배수조건에서 상대적으로 큰 전단저항을 받는 것으로 나타났다. (ii) 배수조건에 관계없이 수중 자갈의 초기 전단속도는 전단응력-전단변형률 관계곡선을 결정하는 중요한 요인으로 나타났다. (iii) 입자파쇄는 평균 입경에 영향을 받으며 사용된 수중 자갈은 상대적으로 큰 입자파쇄 특성을 보였다. 그리고 (iv) 전단응력 결정에서 가장 크게 영향을 미치는 영향인자는 전단시간과 초기 전단속도임을 확인할 수 있었다. 결론적으로 모래와 자갈을 다량 함유한 조립질 하상 퇴적물은 입자-입자 간 상호접촉, 마모, 맞물림, 마찰 등의 물리적 이동과정을 통해 입자파쇄와 세립토 함량이 증가되고 이러한 현상은 하상 퇴적물의 고유동성을 초래하는 원인이 되는 것으로 판단된다.

• 한국농공학회지
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• 제23권3호
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• pp.96-104
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• 1981

In order to investigate the characteristics of the shear strength of the unsaturated cohesive soils which has mean characters of sand and clay widely used for banking, I selected soil samples from An-sung district and, against it, performed direct shear test and unconfined compression test changing grain size, compaction energy and moisture content and also performed triaxial compression test under optimum moisture content. The results are as follows; 1.As the passing percent of the No. 200 sieve increased from 23.6% to 56.1%, cohesion increased from 0. 202kg/cm2 to 0. 398kg/cm2 under the direct shear test and from 0.38 kg/cm2 to 1. 05kg/cm2 under the tria4al compression test, internal friction angle decreased from 44. 78$^{\circ}$ to 34. 34$^{\circ}$ under the direct shear test and from 31. 88$^{\circ}$ to 13. 31$^{\circ}$ under the triaxial compression test. 2.Cohesion showed it's maximum value around OMC and internal friction angle showed a tendency to increase according to the decrease of water content but it's increasing ratio was relatively slow. 3.Decreasing ratio of cohesion and internal friction angle was relatively sensitive according to the decrease of compaction energy. 4.The smaller of the vertical stress and the coarser of the grain size of samples, changing of the volume showed a tendency to increase and as the increase of water content, the shear displacement (dh) at failure shear stress ($\tau$f) showed maximum and the $\tau$f-dh curve was gentle. 5.To synthesize the results of the direct shear test and the triaxial compression test, cohesion showed higher under the triaxial compression test and internal friction angle showed a tendeney to appear higher under the direct shear test. It seems that we can get correspondent results by removing the side friction of mold with soils and adjusting the vertical stress and shearing speed under the direct shear test.

• 한국지반공학회논문집
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• 제27권6호
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• pp.49-61
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• 2011

본 연구는 토석류의 유통성과 관련하여 세립토의 흐름특성, 유변학적 모델들의 적용가능성 및 액성상태 의존성 유변학적 특성들을 비교 분석하였다. 입자크기에 따른 유변학적 특성을 살펴보고자 점토질이 풍부한 지중해 해저점토와 실트질이 풍부한 캐나다 동부 뉴펀들랜드 와부시 호수에서 채취한 광미에 대한 물성특성을 분석하였다. 점토질이 풍부한 세립토의 경우 전형적인 전단담화(shear thinning) 거동을 보이는 반면, 실트질 광미의 경우는 전단담화와 Bingham 유체 거동을 함께 보인다. 후자의 경우, 전단변형률속도를 높임에 따라 Bingham 유체처럼 거동하였다. 이러한 현상학적 차이는 입자크기에 따른 유동특성곡선의 차이에서 기인한 것이다. 항복응력과 소성점도의 결정은 전단변형에 의한 유동 입자들의 구조적 변화와 응력상태와 관련되기 때문이다. 세립토(< 0.075mm)를 다량 함유한 토석류의 유동성을 역해석하고자 할 때, 퇴적형상(흐름 양상, 퇴적층의 모양, 두께 및 길이 등)은 항복응력과 소성점도에 의해 결정된다. 항복응력과 소성점도는 액성지수의 함수로 나타낼 수 있으므로, 토석류의 발생가능지역에서 액성상태에 따른 토석류의 유동성을 평가할 지표로 활용할 수 있다.

• 대한기계학회논문집A
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• 제22권3호
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• pp.595-602
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• 1998

Specimens of 304 stainless steel were tested to failure at elevated temperatures under multiaxial stress states, uniaxial tension using smooth bar specimens, biaxial shearing using double shear bar specimens, and triaxial tension using notched bar specimens. Rupture times are compared for uniaxial, biaxial, and triaxial stress states with respect to the maximum principal stress, the von Mises effective stress, and the principal facet stress. The results indicate that the principal facet stress gives the best correlation for the material investigated, and this parameter can predict creep life data under multiaxial stress states with rupture data obtained with specimens under uniaxial stresses. The results also suggest that grain boundary cavitation, coupled with localized deformation processes such as grain boudary sliding, controls the lifetimes of the specimens.

• 터널과지하공간
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• 제33권3호
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• pp.189-207
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• 2023

In the present study, the thermoshearing experiment on a rough rock fracture were modeled using a three-dimensional grain-based distinct element model (GBDEM). The experiment was conducted by the Korea Institute of Construction Technology to investigate the progressive shear failure of fracture under the influence of thermal stress in a critical stress state. The numerical model employs an assembly of multiple polyhedral grains and their interfaces to represent the rock sample, and calculates the coupled thermo-mechanical behavior of the grains (blocks) and the interfaces (contacts) using 3DEC, a DEM code. The primary focus was on simulating the temperature evolution, generation of thermal stress, and shear and normal displacements of the fracture. Two fracture models, namely the mated fracture model and the unmated fracture model, were constructed based on the degree of surface matedness, and their respective behaviors were compared and analyzed. By leveraging the advantage of the DEM, the contact area between the fracture surfaces was continuously monitored during the simulation, enabling an examination of its influence on shear behavior. The numerical results demonstrated distinct differences depending on the degree of the surface matedness at the initial stage. In the mated fracture model, where the surfaces were in almost full contact, the characteristic stages of peak stress and residual stress commonly observed in shear behavior of natural rock joints were reasonably replicated, despite exhibiting discrepancies with the experimental results. The analysis of contact area variation over time confirmed that our numerical model effectively simulated the abrupt normal dilation and shear slip, stress softening phenomenon, and transition to the residual state that occur during the peak stress stage. The unmated fracture model, which closely resembled the experimental specimen, showed qualitative agreement with the experimental observations, including heat transfer characteristics, the progressive shear failure process induced by heating, and the increase in thermal stress. However, there were some mismatches between the numerical and experimental results regarding the onset of fracture slip and the magnitudes of fracture stress and displacement. This research was conducted as part of DECOVALEX-2023 Task G, and we expect the numerical model to be enhanced through continued collaboration with other research teams and validated in further studies.

• Structural Engineering and Mechanics
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• 제65권4호
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• pp.465-476
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• 2018

This article investigates buckling behavior of a multi-phase nanocrystalline nanobeam resting on Winkler-Pasternak foundation in the framework of nonlocal couple stress elasticity and a higher order refined beam model. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, and couple stress effects are omitted. Hamilton's principle is employed to derive the governing equations and the related boundary conditions which are solved applying an analytical approach. The buckling loads are compared with those of nonlocal couple stress-based beams. It is showed that buckling loads of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, shear deformation, surface effect, nonlocality and boundary conditions.