• Steel and Composite Structures
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• 제34권5호
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• pp.743-767
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• 2020

Due to the impressive flexural performance, enhanced compressive strength and more constrained crack propagation, Fibre-reinforced concrete (FRC) have been widely employed in the construction application. Majority of experimental studies have focused on the seismic behavior of FRC columns. Based on the valid experimental data obtained from the previous studies, the current study has evaluated the seismic response and compressive strength of FRC rectangular columns while following hybrid metaheuristic techniques. Due to the non-linearity of seismic data, Adaptive neuro-fuzzy inference system (ANFIS) has been incorporated with metaheuristic algorithms. 317 different datasets from FRC column tests has been applied as one database in order to determine the most influential factor on the ultimate strengths of FRC rectangular columns subjected to the simulated seismic loading. ANFIS has been used with the incorporation of Particle Swarm Optimization (PSO) and Genetic algorithm (GA). For the analysis of the attained results, Extreme learning machine (ELM) as an authentic prediction method has been concurrently used. The variable selection procedure is to choose the most dominant parameters affecting the ultimate strengths of FRC rectangular columns subjected to simulated seismic loading. Accordingly, the results have shown that ANFIS-PSO has successfully predicted the seismic lateral load with R² = 0.857 and 0.902 for the test and train phase, respectively, nominated as the lateral load prediction estimator. On the other hand, in case of compressive strength prediction, ELM is to predict the compressive strength with R² = 0.657 and 0.862 for test and train phase, respectively. The results have shown that the seismic lateral force trend is more predictable than the compressive strength of FRC rectangular columns, in which the best results belong to the lateral force prediction. Compressive strength prediction has illustrated a significant deviation above 40 Mpa which could be related to the considerable non-linearity and possible empirical shortcomings. Finally, employing ANFIS-GA and ANFIS-PSO techniques to evaluate the seismic response of FRC are a promising reliable approach to be replaced for high cost and time-consuming experimental tests.

• Steel and Composite Structures
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• 제35권6호
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• pp.799-813
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• 2020

Fatigue cracks of rib-to-deck (RD) joints have been frequently observed in the orthotropic steel decks (OSD) using conventional U-ribs (CU). Thickened edge U-rib (TEU) is proposed to enhance the fatigue strength of RD joints, and its effectiveness has been proved through fatigue tests. In-depth full-scale tests are further carried out to investigate both the fatigue strength and fractography of RD joints. Based on the test result, the mean fatigue strength of TEU specimens is 21% and 17% higher than that of CU specimens in terms of nominal and hot spot stress, respectively. Meanwhile, the development of fatigue cracks has been measured using the strain gauges installed along the welded joint. It is found that such the crack remains almost in semi-elliptical shape during the initiation and propagation. For the further application of TEUs, the design curve under the specific survival rate is required for the RD joints using TEUs. Since the fatigue strength of welded joints is highly scattered, the design curves derived by using the limited test data only are not reliable enough to be used as the reference. On this ground, an experiment-numerical hybrid approach is employed. Basing on the fatigue test, a probabilistic assessment model has been established to predict the fatigue strength of RD joints. In the model, the randomness in material properties, initial flaws and local geometries has been taken into consideration. The multiple-site initiation and coalescence of fatigue cracks are also considered to improve the accuracy. Validation of the model has been rigorously conducted using the test data. By extending the validated model, large-scale databases of fatigue life could be generated in a short period. Through the regression analysis on the generated database, design curves of the RD joint have been derived under the 95% survival rate. As the result, FAT 85 and FAT 110 curves with the power index m of 2.89 are recommended in the fatigue evaluation on the RD joint using TEUs in terms of nominal stress and hot spot stress respectively. Meanwhile, FAT 70 and FAT 90 curves with m of 2.92 are suggested in the evaluation on the RD joint using CUs in terms of nominal stress and hot spot stress, respectively.

• 한국지능시스템학회논문지
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• 제13권2호
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• pp.200-208
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• 2003

본 연구에서는 효율적인 학습규칙의 신경망 기반 독립성분분석기법을 이용한 영상신호의 분리와 특징추출을 제안하였다. 제안된 학습규칙은 할선법과 모멘트를 이용한 조합형 고정점 학습알고리즘이다. 여기서 할선법은 독립성분 상호간의 정보를 최소화하기 위한 목적함수의 최적화 과정에서 요구되는 1차 미분에 따른 계산을 간략화하기 위함이고, 모멘트는 최적화 과정에서 발생하는 발진을 억제하여 보다 빠른 학습을 위함이다. 제안된 기법을 $512\times512$의 픽셀을 가지는 10개의 영상을 대상으로 임의의 혼합행렬에 따라 발생되는 혼합영상의 분리에 적용한 결과, 뉴우턴법에 기초한 기존의 알고리즘과 할선법만에 기초한 알고리즘보다 각각 우수한 분리률과 빠른 분리속도가 있음을 확인하였다. 또한 $256\times256$ 픽셀의 10개 지문상과 $480\times225$ 픽셀의 지폐영상에서 선택된 각각 10,000개의 3가지 영상패치들을 대상으로 적용한 결과, 제안된 기법은 뉴우턴법이나 할선법의 알고리즘보다도 빠른 특징추출 속도가 있음을 확인하였다. 한편 추출된 $16\times16$ 펙셀의 160개 독립성분 기저벡터 각각은 영상 각각에 포함된 공간적인 주파수 특성과 방향성을 가지는 경계 특성이 잘 드러나는 국부적인 특징들임을 확인하였다.

• Composites Research
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• 제29권4호
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• pp.145-150
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• 2016

Polyvinylidene fluoride (PVDF)는 압전성을 나타내는 대표적인 고분자로 1960년대부터 많은 연구가 진행되어 왔다. PVDF는 반결정의 고분자로써 5가지의 결정 구조(${\alpha}$, ${\beta}$, ${\gamma}$, ${\delta}$, 그리고 ${\varepsilon}$형)로 구성되어 있다. ${\alpha}$형과 ${\delta}$형 결정은 전기적으로 반응하지 않는 무극성 결정구조이나 ${\beta}$형, ${\gamma}$형 그리고 ${\varepsilon}$형은 전기적으로 반응하는 극성 결정구조이다. 그 중에서도 ${\beta}$형 결정구조는 트랜스 형 분자 쇄가 평행으로 충진 된 형태로서 PVDF 단위체가 갖는 영구 쌍극자가 모두 한 방향으로 배열되어 있는 구조이기 때문에 자발 분극이 커지게 되고 압전성을 나타내게 된다. 일반적으로 ${\beta}$형 결정구조는 연신을 통한 ${\alpha}$형 결정구조의 변환을 통하여 얻을 수 있고, 연신 후 후처리 공정을 통해 그 양을 증가시킬 수 있다. 습식방사로 제조된 PVDF 섬유는 응고욕에서 극성 용매의 확산 메커니즘에 의해 ${\beta}$형 결정구조가 형성되는 장점을 가지고 있지만 극성 용매가 빠져나감과 동시에 섬유 고화가 진행되기 때문에 용매의 확산 경로가 섬유 내부 기공으로 남게 되는 단점을 가지고 있다. 이 기공은 폴링(Poling) 공정에서 전기장에 의한 분극을 방해하여 그 효과를 감소시키는 역할을 한다. 또한, PVDF 섬유가 압전 특성을 필요로 하는 응용분야에 사용되기 위해서는 섬유 가공 후에 전극이 반드시 부착되어야 하는데 섬유 형태로 제조된 PVDF에 전극을 형성하기는 매우 어렵다. 본 연구에서는 압전성을 갖는 PVDF 섬유를 습식 방사와 건식 방사의 혼합 공정으로 제조하여 기공 문제를 해결하였고, 전극이 섬유 내부에 삽입된 Core/Shell 형태의 PVDF 섬유를 제조하여 까다로운 전극형성 문제를 해결하였다.

• Composites Research
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• 제31권5호
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• pp.177-185
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• 2018

섬유금속적층판은 금속과 섬유 강화 복합소재를 함께 적층한 하이브리드 재료 중 하나다. 섬유금속적층판은 계면의 접착층이 파괴되는 층간분리 현상이 발생할 수 있기 때문에 계면의 접착층에 대한 한계응력과 에너지 해방률을 실험적으로 도출해야만 한다. 하지만, 온도에 따른 에너지 해방률을 실험적으로 도출하는 과정에서 측정 장비의 사용 온도에 대한 제약을 받는다. 따라서, 본 연구에서는 Levenberg-Marquardt 기법을 기반한 역해석 기법을 사용하여 접착층에 대한 모드 I 한계응력과 에너지 해방률에 대한 예측 가능성을 확인하는 것이 목표다. 먼저, 한계응력은 접착층의 인장강도와 같다고 가정하였으며, 에너지 해방률은 DCB 시험(double cantilever beam test)을 수행하여 정의하였다. 또한, 유한요소법 기반 모델을 적용하여 한계응력과 에너지 해방률을 수치해석적으로 예측할 수 있는 지 확인하였다. 그 후, Levenberg-Marquardt 기법을 유한요소법 기반 모델에 적용하여 모드 I 한계응력과 에너지 해방률을 수치해석적으로 예측하였다. 아울러, 본 연구에서 사용한 역해석 기법의 수렴성을 확보하기 위하여 두 가지 경우의 초기 매개변수에 대한 역해석을 추가적으로 수행하였다. 결과적으로, 본 연구에서 사용한 역해석 기법은 모드 I 한계응력과 에너지 해방률을 효과적으로 예측할 수 있음을 보였다.

• Macromolecular Research
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• 제10권3호
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• pp.158-167
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• 2002

In order to endow with new bioactive functionality from small intestine submucosa (SIS) powder as natural source to poly (L-lactide) (PLA) and poly (lactide-co-glycolide) (PLGA) synthetic biodegradable polymer, porous SIS/PLA and SIS/PLGA as natural/synthetic composite scaffolds were prepared by means of the solvent casting/salt leaching methods for the possibility of the application of tissue engineered bone and cartilage. A uniform distribution of good interconnected pores from the surface to core region was observed the pore size of 40~500 ${\mu}{\textrm}{m}$ independent with SIS amount using the solvent casting/salt leaching method. Porosities, specific pore areas as well as pore size distribution also were almost same. After the fabrication of SIS/PLA hybrid scaffolds, the wetting properties was greatly enhanced resulting in more uniform cell seeding and distribution. Five groups as PGA non-woven mesh without glutaraldehyde (GA) treatment, PLA scaffold without or with GA treatment, and SIS/PLA (Code No.3 ; 1 : 12 of salt content, (0.4 : 1 of SIS content, and 144 ${\mu}{\textrm}{m}$ of median pore size) without or with GA treatment were implanted into the back of nude mouse to observe the effect of SIS on the induction of cells proliferation by hematoxylin and eosin, and von Kossa staining for 8 weeks. It was observed that the effect of SIS/PLA scaffolds with GA treatment on bone induction are stronger than PLA scaffolds, that is to say, in the order of PLA/SIS scaffolds with GA treatment > PLA/SIS scaffolds without GA treatment > PGA nonwoven > PLA scaffolds only with GA treatment = PLA scaffolds only without GA treatment for the osteoinduction activity. The possible explanations are (1) many kinds of secreted, circulating, and extracellular matrix-bound growth factors from SIS to significantly affect critical processes of tissue development and differentiation, (2) the exposure of SIS to GA resulted in significantly calcification, and (3) peri-implant fibrosis due to covalent bonding between collagen molecule by crosslinking reaction. In conclusion, it seems that SIS plays an important role for bone induction in SIS/PLA scaffolds for the application of tissue engineering area.

• Corrosion Science and Technology
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• 제9권4호
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• pp.147-152
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• 2010

Conventional paints for conversion coating applications in steel production derived mainly from water-based polymer dispersions containing several additives actually show good general performance, but suffer from poor scratch and abrasion resistance during use. The reason for this is because the relatively soft organic binder matrix dominates the mechanical surface properties. In order to maintain the high quality and decorative function of coated steel sheets, the mechanical performance of the surface needs to be improved significantly. In fact the wear resistance should be enhanced without affecting the optical appearance of the coatings by using appropriate nanoparticulate additives. In this direction, nanocomposite coating compositions (Nanomer$^{(R)}$) have been derived from water-based polymer dispersions with an increasing amount of surface-modified nanoparticles in aqueous dispersion in order to monitor the effect of degree of filling with rigid nanoparticles. The surface of nanoparticles has been modified for optimum compatibility with the polymer matrix in order to achieve homogeneous nanoparticle dispersion over the matrix. This approach has been extended in such a way that a more expanded hybrid network has been condensed on the nanoparticle surface by a hydrolytic condensation reaction in addition to the quasi-monolayer type small molecular surface modification. It was expected that this additional modification will lead to more intensive cross-linking in coating systems resulting in further improved scratch-resistance compared to simple addition of nanoparticles with quasi-monolayer surface modification. The resulting compositions have been coated on zinc-galvanized steel and cured. The wear resistance and the corrosion protection of the modified coating systems have been tested in dependence on the compositional change, the type of surface modification as well as the mixing conditions with different shear forces. It has been found out that for loading levels up to 50 wt.-% nanoparticles, the mechanical wear resistance remains almost unaffected compared to the unmodified resin. In addition, the corrosion resistance remained unaffected even after $180^{\circ}$ bending test showing that the flexibility of coating was not decreased by nanoparticle addition. Electron microscopy showed that the inorganic nanoparticles do not penetrate into the organic resin droplets during the mixing process but rather formed agglomerates outside the polymer droplet phase resulting in quite moderate cross linking while curing, because of viscosity. The proposed mechanisms of composite formation and cross linking could explain the poor effect regarding improvement of mechanical wear resistance and help to set up new synthesis strategies for improved nanocomposite morphologies, which should provide increased wear resistance.

• Restorative Dentistry and Endodontics
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• 제31권6호
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• pp.445-451
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• 2006

이 연구에서는 미세인장결합강도 실험과 주사전자현미경을 이용하여 unground enamel에 대한 all-in-one adhesive의 1회 또는 2회 적용에 따른 효과를 평가하였다. 발거된 하악 대구치의 설측 치관부를 사용하여 3개씩 5개의 군으로 분류하였다. SE군은 Clearfil$^{tm}$ SE Bond를 적용하였다. LP1 군, LP2 군은 Adper Prompt L-Pop$^{tm}$을 각각 1회 , 2회 적용하였다. XN1 군, XN2 군은 Xeno$^{R}$III를 각각 1회, 2회 적용하였다. 그런 다음 복합 레진 Z100을 적층 충전하였다. 법랑질 표면의 접착제를 아세톤으로 제거하고 주사 전자 현미경을 이용하여 산 부식 양상을 관찰하였다 미세인장결합강도 측정을 위해 표본을 절단하여 레진과 치아의 접착 계면이 1 mm$^2$이 되도록 하여 실험하였다. 실험의 결과는 One-way ANOVA를 사용해 분석한 다음 Duncan's post-hoc test로 사후 검정 하였다. 본 연구의 결과 all-in-one adhesive의 1회와 2회 적용간에는 유의한 차이가 나타나지 않았으며 two-step self-etching adhesive와 all-in-one adhesive의 산 부식 양상 모두 얕고 불규칙 하였다.

• Restorative Dentistry and Endodontics
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• 제25권1호
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• pp.123-132
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• 2000

To overcome problems of conventional glass ionomers, resin components have been added to glass ionomers. On a continuum between glass ionomers and composites are a variety of blends, employing different proportions of acid-base and free radical reactions to bring about cure. Popular groups defined between the ends are resin-modified glass-ionomers(RMGIs), polyacid-modified composite resins(Compomers) and ionomer modified resins. These groups show different clinical properties, and in selecting these materials for a restoration, one should sufficiently understand these different setting properties. In this study, some difference in the setting characteristics of different groups of hybrid ionomers were examined. Two RMGIs (Fuji2 LC,GC / Vitremer, 3M), three Compomers (Dyract AP, Dentsply / F2000, 3M / Elan, Kerr) were involved in this study. The identification of the setting characteristics of different groups was achieved by a two-stage study. First, thermal analysis was performed by a differential scanning calorimeter, and then the hardness of each group at different depth and time were measured by a micro-hardness tester. Thermal analysis was performed to identify the inorganic filler content and to record the heat change during setting process. The setting process was progressed for each material by chemical set mode and light-cured mode. In the hardness test, samples of materials were prepared with a 6mm-diameter metal ring, and the hardness was measured at the top, and 1mm, 2.5mm, 4mm below at just after a 40 second-cure, and after 10 minutes, 24 hours, and 7 days. Statistical analysis was performed by Mann-Whitney rank sum test to assess significant differences between set modes and types of materials, and by ANOVA and T-test to evaluate the statistical meanings of data at different times and depths of each materials. Followings are findings and conclusions derived from this study. Thermal analysis; 1. Compomers show no evidence of chemical setting while RMGIs exhibit heat output during the process of chemical setting. 2. Heat of cure of RMGIs exceed Compomers. 3. The net heat output of RMGIs through light-cured mode is higher than through chemically set mode. Hardness test; 1. Initial hardness of RMGIs immediately after light cure is relatively low, but the hardness increases as time goes by. On the contrary, Comomers do not show evident increase of the hardness following time. 2. Compomers show a marked decrease of setting degree as the depth of the material increases. In RMGIs, the setting degree at different depths does not significantly differ.

• 마이크로전자및패키징학회지
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• 제29권2호
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• pp.99-105
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• 2022

칩사이즈가 작아짐에 따라 선폭 또한 미세화되면서 인터커넥션의 밀집정도가 증가하고 있다. 그로 인해 캐패시터 층과 전기전도층의 저항 차이로 인해 RC delay가 문제되고 있다. 이를 해결하기 위해서는 높은 전기전도도의 전극과 낮은 유전율의 유전체 개발이 요구된다. 본 연구에서는 PCB (Print Circuit Board)의 회로를 외부요인으로부터 보호하는 상용 PSR (photo solder resist)과 우수한 내열 및 저유전 특성을 보유한 PI (polyimide)를 혼합하여 저유전체 잉크 개발을 진행하였다. 그 결과 PSR과 PI를 10:3으로 혼합한 잉크가 가장 우수한 결과를 보였으며 20 GHz와 28 GHz에서 각각 유전 상수 약 2.6, 2.37을 보였고, 유전손실은 약 0.022, 0.016으로 측정되었다. 차후 어플리케이션 적용 가능성 검증을 위해 테프론에 제작된 다양한 선폭의 전송선로에 평가하였으며 그 결과, PSR만 사용했을 때보다 PI와 혼합한 저유전체 잉크를 사용한 전송선로의 손실이 S21에서 평균 0.12 dB 덜 감소한 결과를 보였다.