• 한국기계가공학회지
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• 제11권4호
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• pp.138-146
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• 2012

This research is to analyze the wave propagation characteristics of anisotropic materials subjected to the low-velocity impact. For this purpose, a higher-order finite element program is used to simulate the dynamic behaviors according to the changes of material property, stacking sequence and dimension etc.. Materials for simulation are composed of $[0^{\circ}]_{10s}$, $[45^{\circ}/-45^{\circ}]_{5s}$ and $[90^{\circ}]_{10s}$ stacking sequences. Finally, the results of this simulation are compared with those of wave propagation theory and then the impact responses and wave propagation phenomena are investigated.

• Wind and Structures
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• 제3권3호
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• pp.143-158
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• 2000

This report presents the findings of a one-year monitoring effort to empirically characterize and evaluate the nature of near-ground winds for structural engineering purposes. The current wind engineering practice in the United States does not explicitly consider certain important near-ground wind characteristics in typical rough terrain conditions and the possible effect on efficient design of low-rise structures, such as homes and other light-frame buildings that comprise most of the building population. Therefore, near ground wind data was collected for the purpose of comparing actual near-ground wind characteristics to the current U.S. wind engineering practice. The study provides data depicting variability of wind speeds, wind velocity profiles for a major thunderstorm event and a northeaster, and the influence of thunderstorms on annual extreme wind speeds at various heights above ground in a typical rough environment. Data showing the decrease in the power law exponent with increasing wind speed is also presented. It is demonstrated that near-ground wind speeds (i.e., less than 10 m above ground) are likely to be over-estimated in the current design practice by as much as 20 percent which may result in wind load over-estimate of about 50% for low-rise buildings in typical rough terrain. The importance of thunderstorm wind profiles on determination of design wind speeds and building loads (particularly for buildings substantially taller than 10 m) is also discussed. Recommendations are given for possible improvements to the current design practice in the United States with respect to low-rise buildings in rough terrain and for the need to study the impact of thunderstorm gust profile shapes on extreme value wind speed estimates and building loads.

• Journal of Welding and Joining
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• 제7권3호
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• pp.19-27
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• 1989

Both the SS41 steel and the M.E.F(martensite encapsulated islands of frrite) dual phase steel made of SS41 steel by heat treatment were welded by friction welding, and then manufactured machinemade Vnotch standard Charpy impact specimens and precracked with a fatigue system at BM(base metal), HAZ(heat affected zone) and WZ(weld interface Zone). The impact test of them was performed with an instrumented impact test machine at a number of temperatures in constant loading velocity and the dynamic fracture characteristics were studied on bases of the absorbed energy, dynamic fracture toughness and fractography from the test. The results obtained are as follows; At the room temperature, the absorbed energy is HAZ.geq.WZ.geq.BM in case of the M.E.F. dual phase steel: BM.geq.HAZ.geq.WZ in case of the SS41 steel, HAZ.geq.BM.geq.WZ at the low temperature. The absorbed energy is decreased markedly with the temperature lowering; it is highly dependent on the temperature. The dynamic fracture toughness of the M.E.F. dual phase steel is HAZ.geq.WZ.geq.BM at the room temperature; BM.geq.WZ.geq.HAZ below-60.deg. C. Therefore the reliability of friction welding is uncertain at the low temperature(below-60.deg. C). The dynamic fracture toughness of the SS41 steel; HZA.geq.WZ.geq.BM at overall temperature region. The flaw formed by rotational upsetting pressure was shown y SEM; in this region. The absorbed energy per unit area and dynamic fracture toughness were low relative to other region.

• Structural Engineering and Mechanics
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• 제42권6호
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• pp.831-847
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• 2012

Impact from water-borne debris during tsunami and flood events pose a potential threat to structures. Debris impact forces specified by current codes and standards are based on rigid body dynamics, leading to forces that are dependent on total debris mass. However, shipping containers and other debris are unlikely to be rigid compared to the walls, columns and other structures that they impact. The application of a simple one-dimensional model to obtain impact force magnitude and duration, based on acoustic wave propagation in a flexible projectile, is explored. The focus herein is on in-air impact. Based on small-scale experiments, the applicability of the model to predict actual impact forces is investigated. The tests show that the force and duration are reasonably well represented by the simple model, but they also show how actual impact differs from the ideal model. A more detailed three-dimensional finite element model is also developed to understand more clearly the physical phenomena involved in the experimental tests. The tests and the FE results reveal important characteristics of actual impact, knowledge of which can be used to guide larger scale experiments and detailed modeling. The one-dimensional model is extended to consider water-driven debris as well. When fluid is used to propel the 1-D model, an estimate of the 'added mass' effect is possible. In this extended model the debris impact force depends on the wave propagation in the two media, and the conditions under which the fluid increases the impact force are discussed.

• 한국습지학회지
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• 제19권4호
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• pp.484-490
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• 2017

복단면 형태를 이루는 직선하도 내 사행하는 저수로의 형태에 따른 흐름 특성을 파악하기 위해, 국내 대표적인 하도 형태를 상정해 실내 수리모형을 실시해서 3차원 수치모의의 유효성을 확인하고, 이를 바탕으로 다른 유형의 하도 형태에 대해서도 수치모의로 검토를 실시하였다. 본 연구결과, 수리모형 실험에서 관측한 수심별 유속값을 이용하여 수치모형의 검정을 수행한 결과, 수치모의 결과와 충분히 일치하는 것으로 확인하였다. 이를 토대로, 추가적인 저수로 형태 변화에 따른 유동장에 대해 분석한 바에 따르면, 선행 연구들에서 검토된 이차류 현상이 발생하였음을 확인한 한편, 고수부지 내 유수단면적 확대에 따라 최고유속분포 지점이 이동하는 현상을 확인할 수 있었다. 궁극적으로 저수로 폭 변화가 흐름에 영향을 끼쳐 궁극적으로 하천설계에 중요한 요소인 수충부의 위치와 그 영향 정도를 파악하는 것이 필요하다고 판단된다.

• Advanced Composite Materials
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• 제17권4호
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• pp.373-407
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• 2008

This paper will examine the possibilities of the virtual tests of composite structures by simulating mechanical behaviors by using supercomputing technologies, which have now become easily available and powerful but relatively inexpensive. We will describe mainly the applications of large-scale finite element analysis using the direct numerical simulation (DNS), which describes composite material properties considering individual constituent properties. DNS approach is based on the full microscopic concepts, which can provide detailed information about the local interaction between the constituents and micro-failure mechanisms by separate modeling of each constituent. Various composite materials such as metal matrix composites (MMCs), active fiber composites (AFCs), boron/epoxy cross-ply laminates and 3-D orthogonal woven composites are selected as verification examples of DNS. The effective elastic moduli and impact structural characteristics of the composites are determined using the DNS models. These DNS models can also give the global and local information about deformations and influences of high local in-plane and interlaminar stresses induced by transverse impact loading at a microscopic level inside the materials. Furthermore, the multi-scale models based on DNS concepts considering microscopic and macroscopic structures simultaneously are also developed and a numerical low-velocity impact simulation is performed using these multi-scale DNS models. Through these various applications of DNS models, it can be shown that the DNS approach can provide insights of various structural behaviors of composite structures.

• 상하수도학회지
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• 제25권2호
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• pp.231-240
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• 2011

The objectives of this research are to investigate coagulation efficiencies of two coagulants l.e., alum and polyaluminum chloride and to understand effects of the coagulants on membrane fouling in microfiltration. The turbidity of supernatant from alum coagulation was increased with increasing doses whereas the turbidity from PACl coagulation was maintained at the low values. The observed injection volume of PACl for the same removal was approximately 30 percent less than alum, which produced a low sludge volume. The settling velocity of PACl flocs was greater than alum flocs. The results corresponded well with floc size measurements. Flux decline from alum coagulation was significant due in part to small sizes of flocs. At the low dose, alum floc had less specific cake resistance than PACl floc. However, as the dosage was increased, the increases in specific cake resistances of alum was substantial. Alum coagulation pretreatment needs careful operation to reduce membrane fouling by flocs. In general, PACl coagulants were more effective than alum coagulants for pretreatment of membrane processes because PACl showed the better performance in coagulation and membrane fouling.

• Elastomers and Composites
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• 제35권2호
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• pp.138-148
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• 2000

EPDM 발포체의 정적/동적 특성을 측정하여 재료 특성과 진동 특성과의 연관성을 알아보고 이로부터 충격음 저감재로서의 적용 가능성을 검토하였다. 정적/동적 특성은 material test system (MTS)를 사용하여 시편의 형상, 두께, 초기하중, 발포도 등에 따라 정적 탄성계수, 동적 탄성계수, 전달율의 변화를 조사하였다. 정적 탄성계수와 동적 탄성계수의 경우, 형상이 단순하고 두께가 얇을수록 큰 값을 나타냈고, 전달율과 동적 탄성계수는 상호 비례 관계에 있음을 확인할 수 있었다. 특징적으로 동적 탄성계수가 증가하면, 전달율의 특성 피크치는 같은 주파수 영역에서의 증가 혹은 고주파수 영역으로의 전이의 형태로 일어나게 되는 것을 알 수 있었다. 실험실적 모사 시험장치를 통해 충격에 따른 주파수별 진동 속도측정과 유한 요소 해석 모델을 사용하여 mode shape에 의한 충격 해석 결과를 알아보았는데, EPDM 발포체를 사용함으로써 2.5-3.5배의 진동 속도 저감이 이루어짐을 알 수 있었고, mode shape의 경우 몰타르와 콘크리트만으로 구성된 구조물에 비해 진동 충격에 대한 변위가 급감함으로써 충격음 저감재로서의 EPDM 발포체의 적용 가능성은 상당히 높은 것을 확인할 수 있었다.

• 한국안전학회지
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• 제7권1호
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• pp.39-45
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• 1992

강구에 의한 횡방향 저속 충격을 받게 되는 복합재의 충격 응력 및 파동 전파에 관한 이론적 및 실험적 연구가 행하여 진다. 이론적 해석을 위해서는 실험적 접촉 법칙과 연계된 Whitney와 Pagano의 이론에 근거한 판 유한 요소 모델이 사용되며, 실험적 해석을 위해서는 직접적인 충격실험이 수행된다. 이러한 해석을 위한 시험편은 ［0/45/0/­45/0］$_{2s}$와 (90/45/90/­45/90)$_{2s}$ 적층 순서를 가지는 유리/에폭시 적층 복합재이며 경계 조건은 clamped-simply support이다. 결과적으로 이러한 두 해석 결과들이 비교 검토되며, 적층 복합재의 충격 응력 및 파동 전파 특성이 규명된다.명된다.

• Elastomers and Composites
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• 제57권4호
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• pp.197-204
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• 2022

Recently, the demand has increased for protective clothing materials capable of shielding the wearer from bullets, fragment bullets, knives, and swords. It is therefore necessary to develop light and soft protective clothing materials with excellent wearability and mobility. To this end, research is being conducted on hybrid design methods for various highly functional materials, such as carbon nanotube (CNT) sheets, which are well known for their low weight and excellent strength. In this study, a hybrid protective material using CNT sheets was developed and its performance was evaluated. The material design incorporated a bonding method that used a binder for interlayer combination between the CNT sheets. Four types of binders were selected according to their characteristics and impregnated within CNT sheets, followed by further combination with aramid fabric to produce the hybrid protective material. After applying the binder, the tensile strength increased significantly, especially with the phenoxy binder, which has rigid characteristics. However, as the molecular weight of the phenoxy binder increased, the adhesive force and strength decreased. On the other hand, when a 25% lightweight-design and high-molecular-weight phenoxy binder were applied, the backface signature (BFS) decreased by 6.2 mm. When the CNT sheet was placed in the middle of the aramid fabric, the BFS was the lowest. In a stab resistance test, the penetration depth was the largest when the CNT sheet was in the middle layer. As the binder was applied, the stab resistance improvement against the P1 blade was most effective.