• 한국항공우주학회지
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• 제48권5호
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• pp.363-372
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• 2020

고 변형률 속도에 대한 소재의 동적 압축 물성은 고속 충돌 및 고속 성형 등 동적 환경에서의 유한요소 해석의 신뢰성 향상을 위해 필수적이다. 일반적으로 고 변형률 속도에 대한 소재의 동적 압축 물성은 SHPB(Split Hopkinson Pressure Bar) 장비를 통해 획득 가능하다. 본 연구에서는 최근 무인 항공기에 확대 적용되고 있는 Woven type CFRP(Carbon Fiber Reinforced Plastic) 소재에 대한 충돌 해석에 대응하기 위해 SHPB 장비를 활용하여 해당 소재의 동적 압축 물성을 획득하였다. 또한 Pulse shaper를 활용하여, Elastic-brittle 특성을 지니는 소재에 대한 일정한 변형률 속도 확보 및 실험 데이터에 대한 신뢰도를 향상시켰다. CFRP 소재의 경우 방향 별 기계적 물성이 다른 이방성 소재이므로 두께 방향과 면내 방향 시편을 제작하여 각각 실험을 수행하였다. SHPB 실험 결과 면내 방향 시편의 경우 일정한 변형률 속도 영역에 도달하기 전, 시편의 파단이 발생하여 데이터의 재현성 및 신뢰성 확보에 어려움이 있는 반면, 두께 방향의 시편의 경우 시편 전·후면 응력일치도가 우수하여 데이터 신뢰도가 높으며, 일정한 변형률 속도 영역을 획득할 수 있다. LS-dyna를 활용한 유한요소해석을 통해, 압력봉으로부터 측정되는 데이터는 시편과 압력봉의 변형에 의해 변형률이 과도하게 예측되는 것을 확인하였다.

• 한국군사과학기술학회지
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• 제20권2호
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• pp.206-216
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• 2017

This paper studies on the dynamic properties of Ti-6Al-4V alloy. After forming the four different micro structures(equiaxed, lamellar, and 2 bimodals) through heat treatments, static and dynamic properties of each structure were investigated quantitatively. Dynamic behaviors of the alloy are observed by the compressive split Hopkinson pressure bar(SHPB) tests. In additon, parameters of Johnson-Cook equation were determined from the SHPB test results. In order to verify the suitability of the parameters, high velocity impact tests were performed and the results were compared with the numerical analysis results. Although the flow stress and the fracture strain of the bimodal structures were higher than those of the equiaxed structure at the static tests, the superior dynamic properties were observed at the equiaxed structure due to the effects of higher maximum flow stress and fracture strain. From the numerical analysis, J-C parameters which are determined on this study describe well the dynamic behavior of Ti-6Al-4V alloy. Experimental and analysis results are consistent with ${\pm}5%$ of an average error.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1996년도 봄 학술발표회 논문집
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• pp.192-197
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• 1996

An experimental study was performed to examine the feasibility of using silicic wastes as construction materials for civil structures, and investigate its utility as a replacement for the favored nature resource to prevent the economic loss. In order to achieve this objective, mechnical properties of concrete containing silicic wastes is tested by investigating the strength development through parameters of water-binder ratios replacement 10 percent ratio with respect to curting conditions. The effect of stringth development is investigated for curing conditions when silicic wastes of 10 percent of cement-binder ratios is containde. Comparision on compressive strength of normal concrete and concrete containing silicic wastes at 28 day is conducted. The concrete with silicic wastes have larger compressive strength than of normal concrete by about 20 percent, when cured at 80 degree. The wastes concrete using silica sand shows increased strength, fracture toughness, elastic modulus and strain than the normal concrete, although the silicic wastes concrete could be able to satisfy the generally required strength for conventional concrete structures.

• 한국공작기계학회논문집
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• 제17권3호
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• pp.115-121
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• 2008

A knowloedge of the residual stress distribution at circumferential welds can increase the prediction accuracy of a fracture assessment in pipe lines. In this study, in order to predict the residual stress distribution in the circumferential butt-welded pipes were measured, using the hole-drilling strain gauge method. Their practical applications were performed in to two kinds of pipes. As the results, the following characteristics were found. On the inner surface of pipes, the circumferential and axial residual stresses were both tensile near the center line of welding and both of them changed from tensile to compressive as the distance from the center line increased. On the outer surface, however, the circumferential residual stress was shown to be tensile wile the axial residual stress was compressive near the center line of welding, and later they were revered at the region far away from the centerline.

• KCI Concrete Journal
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• 제11권3호
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• pp.65-77
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• 1999

A portland cement was reinforced by incorporating carbon fiber(CF), silica powder, and impregnating the pores with styrene monomers which were polymerized in situ. The effects of type, length, and volume loading of CF, mixing conditions, curing time and, curing conditions on mechanical behavior as well as freeze-thaw resistance and longer term stability of the carbon-fiber reinforced cement composites (CFRC) were investigated. The composite Paste exhibited a decrease in flow values linearly as the CF volume loadings increased. Tensile, compressive, and flexural strengths all generally increased as the CF loadings in the composite increased. Compressive strength decreased at CF loadings above approx. 3% in CFRC having no impregnated polymers due to the increase in porosity caused by the fibers. However, the polymer impregnation of CFRC improved all the strength values as compared with CFRC having no Polymer impregnation. Tensile stress-strain curves showed that polymer impregnation decreased the fracture energy of CFRC. Polymer impregnation clearly showed improvements in freeze-thaw resistance and drying shrinkage when compared with CFRC having no impregnated polymers.

• Geomechanics and Engineering
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• 제29권4호
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• pp.451-461
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• 2022

The roadways surrounded by rock and coal will lose their stability or even collapse under rock burst. Rock burst mainly involves an evolution of dynamic loading which behaves quite differently from static or quasi-static loading. To compare the dynamic response of coal and rocks with different static strengths, three different rocks and bituminous coal were selected for testing at three different dynamic loadings. It's found that the dynamic compression strength of rocks and bituminous coal is much greater than the static compression strength. The dynamic compression strength and dynamic increase factor of the rocks both increase linearly with the increase of the strain rate, while those of the bituminous coal are irregular due to the characteristics of multi-fracture and heterogeneity. Moreover, the absorbed energy of the rocks and bituminous coal both increase linearly with an increase in the strain rate. And the ratio of absorbed energy to the total energy of bituminous coal is greater than that of rocks. With the increase of dynamic loading, the failure degree of the sample increases, with the increase of the static compressive strength, the damage degree also increases. The static compassion strength of the bituminous coal is lower than that of rocks, so the number of small-scale fragments was the largest after bituminous coal rupture.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.74-79
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• 2003

When workability at the a certain bulk deformation process is defined as the maximum plastic deformation capability that the workpiece can sustain without causing any cracks or fracture, the workability is dependent on the microstructure, initial workpiece shape, stress state developed during the deformation process, strain rata and presence of the interfacial friction between workpiece and tool. For a review purpose, the workability definition and test methods are summarized depending on the applied stress state at bulk deformation process in Table 1 at the text. In this study, the cold workabilities of as-cast A16061 bulk material have been measured and comparatively analyzed at the primary tensile stress state by using tensile specimens, the primary compressive stress state by using cylindrical specimens, and the forming limit diagram by ductile fracture.

• Structural Engineering and Mechanics
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• 제9권6호
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• pp.589-600
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• 2000

This paper attempts to provide a theoretical basis for the design of high-strength concrete columns in terms of the spacing of lateral reinforcement. In order to achieve this, important concepts had to be addressed such as the choice of a measure of ductile behaviour and a realistic high-strength concrete stress-strain model, as well as limiting factors such as longitudinal steel buckling and lateral steel fracture. A design method incorporating above factors are suggested in the paper. It is shown that both buckling of longitudinal steel and hoop fracture will not demand a reduction in spacing of lateral ties with increase in compressive strength of concrete.

• Geomechanics and Engineering
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• 제34권6호
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• pp.683-696
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• 2023

The destruction and fracture of rock masses are crucial components in engineering and there is an increasing demand for the study of the influence of rock mass heterogeneity on the safety of engineering projects. The numerical manifold method (NMM) has a unified solution format for continuous and discontinuous problems. In most NMM studies, material homogeneity has been assumed and despite this simplification, fracture mechanics remain complex and simulations are inefficient because of the complicated topology updating operations that are needed after crack propagation. These operations become computationally expensive especially in the cases of heterogeneous materials. In this study, a heterogeneous model algorithm based on stochastic theory was developed and introduced into the NMM. A new fracture algorithm was developed to simulate the rupture zone. The algorithm was validated for the examples of the four-point shear beam and semi-circular bend. Results show that the algorithm can efficiently simulate the rupture zone of heterogeneous rock masses. Heterogeneity has a powerful effect on the macroscopic failure characteristics and uniaxial compressive strength of rock masses. The peak strength of homogeneous material (with heterogeneity or standard deviation of 0) is 2.4 times that of heterogeneous material (with heterogeneity of 11.0). Moreover, the local distribution of parameter values can affect the configuration of rupture zones in rock masses. The local distribution also influences the peak value on the stress-strain curve and the residual strength. The post-peak stress-strain curve envelope from 60 random calculations can be used as an estimate of the strength of engineering rock masses.

• 콘크리트학회지
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• 제4권4호
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• pp.115-122
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• 1992

폐쇄 폐콘크리트를 재활용한 재생콘크리트의 강도특성을 천연골재를 사용한 일반콘크리트와 비교하였다. 동결융해 처리수의 압축강도와 3점휨 재하시험하의 변형율을 측정하였다. 재생콘크리트는 동결융해 처리 후 압축강도 보존율이 더 높았다. 재생콘크리트는 또한 높은 변형율과 처짐에 민감함을 보였으나 파괴와 관련된 다른 성질들은 일반콘크리트와 유사하거나 더 좋은 것으로 나타났다. 그러므로 폐콘크리트를 구조용 콘크리트 제조에 재 사용이 가능할 것으로 보여진다. 그러나 실제 사용을 위하여는 콘크리트에 있어서 중요한 성질인 압축강도가 더 증진되어야 하며 최대 변형율도 보다 자세히 점검되어야 한다.