• Steel and Composite Structures
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• 제46권1호
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• pp.1-13
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• 2023

Elastic bending of imperfect functionally graded sandwich plates (FGSPs) laying on the Winkler-Pasternak foundation and subjected to sinusoidal loads is analyzed. The analyses have been established using the quasi-3D sinusoidal shear deformation model. In this theory, the number of unknowns is condensed to only five unknowns using integral-undefined terms without requiring any correction shear factor. Moreover, the current constituent material properties of the middle layer is considered homogeneous and isotropic. But those of the top and bottom face sheets of the graded porous sandwich plate (FGSP) are supposed to vary regularly and continuously in the direction of thickness according to the trigonometric volume fraction's model. The corresponding equilibrium equations of FGSPs with simply supported edges are derived via the static version of the Hamilton's principle. The differential equations of the system are resolved via Navier's method for various schemes of FGSPs. The current study examine the impact of the material index, porosity, side-to-thickness ratio, aspect ratio, and the Winkler-Pasternak foundation on the displacements, axial and shear stresses of the sandwich structure.

• Steel and Composite Structures
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• 제46권3호
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• pp.367-383
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• 2023

In this investigation, an improved integral trigonometric shear deformation theory is employed to examine the vibrational behavior of the functionally graded (FG) sandwich plates resting on visco-Pasternak foundations. The studied structure is modelled with only four unknowns' variables displacements functions. The simplicity of the developed model being in the reduced number of variables which was made with the help of the use of the indeterminate integral in the formulation. The current kinematic takes into consideration the shear deformation effect and does not require any shear correction factors as used in the first shear deformation theory. The equations of motion are determined from Hamilton's principle with including the effect of the reaction of the visco-Pasternak's foundation. A Galerkin technique is proposed to solve the differentials governing equations, which enables one to obtain the semi-analytical solutions of natural frequencies for various clamped and simply supported FG sandwich plates resting on visco-Pasternak foundations. The validity of proposed model is checked with others solutions found in the literature. Parametric studies are performed to illustrate the impact of various parameters as plate dimension, layer thickness ratio, inhomogeneity index, damping coefficient, vibrational mode and elastic foundation on the vibrational behavior of the FG sandwich plates.

• Steel and Composite Structures
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• 제49권4호
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• pp.361-380
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• 2023

A size dependent bending behavior of piezoelectrical flexoelectric layered perforated functionally graded (FG) composite nanobeam rested on an elastic foundation is investigated analytically. The composite beam is composed of regularly cutout FG core and two piezoelectric face sheets. The material characteristics is graded through the core thickness by power law function. Regular squared cutout perforation pattern is considered and closed forms of the equivalent stiffness parameters are derived. The modified nonlocal strain gradient elasticity theory is employed to incorporate the microstructure as well as nonlocality effects into governing equations. The Winkler as well as the Pasternak elastic foundation models are employed to simulate the substrate medium. The Hamiltonian approach is adopted to derive the governing equilibrium equation including piezoelectric and flexoelectric effects. Analytical solution methodology is developed to derive closed forms for the size dependent electromechanical as well as mechanical bending profiles. The model is verified by comparing the obtained results with the available corresponding results in the literature. To demonstrate the applicability of the developed procedure, parametric studies are performed to explore influences of gradation index, elastic medium parameters, flexoelectric and piezoelectric parameters, geometrical and peroration parameters, and material parameters on the size dependent bending behavior of piezoelectrically layered PFG nanobeams. Results obtained revealed the significant effects both the flexoelectric and piezoelectric parameters on the bending behavior of the piezoelectric composite nanobeams. These parameters could be controlled to improve the size dependent electromechanical as well as mechanical behaviors. The obtained results and the developed procedure are helpful for design and manufacturing of MEMS and NEMS.

• Journal of the Korean Academy of Child and Adolescent Psychiatry
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• 제20권3호
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• pp.146-151
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• 2009

Objectives : We conducted this study to investigate the relationship between sleep duration and body mass index (BMI), in Korean children. Methods : We performed a cross-sectional analysis of data collected on 3,639 boys and girls (aged 7-12) in Daegu, Korea. The data included each child's age, sex, weight, height, extracurricular activities, bedtime, wake-up time, sleep latency, total sleep duration, parents' occupations, and parents' educational levels. The relationship between sleep duration and each variable was examined via analysis of variance (ANOVA). Results : The analysis showed an association between short sleep duration and high BMI. Boys showed a graded inverse relationship between sleep duration and BMI. However, there was no significant corresponding result for girls. In the total sample, hours of computer use, time when the computer was turned off, time when the television was turned off, mother's bedtime, and hours of extracurricular activity were associated with longer sleep duration. No association was found between sleep duration and hours of watching television, child's wake-up time, or educational level of the parents. Conclusion : The results of this study show an inverse relationship between a child's sleep duration and BMI；thus, children with shorter sleep duration tend to have higher BMIs.

• Food Quality and Culture
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• 제3권1호
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• pp.27-33
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• 2009

This study evaluates the effectiveness of the "Seoul Sanitation Grading System Evaluation Index" developed earlier and to analyze sanitation management practices in restaurants in Seoul, Korea. The categories evaluated were the food management standard, facilities/equipment standard, and essential checking items specified in the law. These items were graded and classified into A ($100{\sim}90$), B ($89{\sim}80$), C ($79{\sim}70$) and Score (less than 69) based on the criteria set by the present researchers. We randomly selected 56 restaurants in five local cities (Jung-gu, Seocho-gu, Jongno-gu, Songpa-gu and Yeongdeungpo-gu) and investigated each by actually visiting the site of business. The achievement rate for food management standard was 80.8%; as for the specific items in the category, it was the highest in food ingredients at 77.1% and the lowest in food storage at 62.1%. For the facilities/equipment standard, the achievement rate was 77.8%; as for the specific items in the category, it was the highest for vermin at 88.1% and the lowest for operation at 70.8%. The achievement rate for overall individual sanitary management was 70.7% and in the category, the lowest score was seen in hand washing at 57.1%. The overall average score of sanitation management practices using the Seoul Sanitation Grading System Evaluation Index in restaurants in Seoul was 73.7, which fell into the C category. As for the number of restaurants in each grade category, there were 10 (17.9%) in each category of A ($100{\sim}90$), B ($89{\sim}80$) and C ($79{\sim}70$) with 30 (53.6%) scoring higher than 70, whereas those scoring less than 69 included 26 (46.4%). The average scores for those restaurants designated by local governments (exemplary restaurants, general restaurants, best Korean restaurants in Seoul) were not significantly different; however, they were higher in franchises than those small restaurants ran by individuals.

• Safety and Health at Work
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• 제15권2호
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• pp.192-199
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• 2024

Background: Safety data sheets (SDSs) are hazard communication materials that accompany chemicals/hazardous products in the workplace. Many SDSs contain dense, technical text, which places considerable comprehension demands on workers, especially those with lower literacy skills. The goal of this study was to assess SDSs for readability, comprehensibility, and suitability (i.e., fit to the target audience). Methods: The Suitability Assessment of Materials (SAM) tool assessed SDSs for suitability and readability. We then amended the SAM tool to further assess SDSs for comprehensibility factors. Both the original and amended SAM tool were used to score 45 randomly selected SDSs for content, literacy demand, graphics, and layout/typography. Results: SDSs performed poorly in terms of readability, suitability, and comprehensibility. The mean readability scores were Flesch-Kincaid Grade Level (9.6), Gunning Fog index (11.0), Coleman-Liau index (13.7), and Simple Measure of Gobbledygook index (10.7), all above the recommended reading level. The original SAM graded SDSs as "not suitable" for suitability and readability. When the amended SAM was used, the mean total SAM score increased, but the SDSs were still considered "not suitable" when adding comprehensibility considerations. The amended SAM tool better identified content-related issues specific to SDSs that make it difficult for a reader to understand the material. Conclusions: In terms of readability, comprehensibility, and suitability, SDSs perform poorly in their primary role as a hazard communication tool, therefore, putting workers at risk. The amended SAM tool could be used when writing SDSs to ensure that the information is more easily understandable for all audiences.

• 한국전산구조공학회논문집
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• 제29권6호
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• pp.529-538
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• 2016

멱 법칙 및 S 형상 함수를 이용한 점진기능재료(FGM) 판의 정적 및 동적해석을 위해 단순화된 전단변형이 고려된 이론을 정식화 하여 동적 평형방정식을 유도하였다. 단순화된 전단변형 이론은 전단보정계수가 필요없으며 수직 전단변형률과 전단응력의 곡선분포를 고려하였고 판의 상부와 하부에서 0이 된다는 조건을 만족한다. 또한 4개의 변수만으로 평형방정식이 유도되고 합응력, 평형방정식 그리고 경계조건이 고전적 이론과 유사한 형태를 가지게 된다. 점진기능재료의 형태는 멱 법칙 및 S 형상 함수로 두께방향으로 변화가 고려된다. Hamilton 원리를 이용하여 동적 평형방정식을 유도하였고 Winkler-Pasternak 탄성지반 모델을 적용하였다. 단순지지된 점진기능재료 판의 정적 및 자유진동 응답을 계산하였고 비교하였다. 본 연구에서 제시한 결과는 참고문헌과 비교하여 정확하고 관련성을 가진다. 거듭제곱 지수, 탄성지반 계수 그리고 폭-두께비의 변화에 따른 정적 및 자유진동 해석결과를 제시하였다.

• 한국광학회지
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• 제12권2호
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• pp.135-142
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• 2001

최근 플라스틱 광섬유(Plastic Optical Fiber, POF)가 홈 네트워크(Home Network) 및 LAN(Local Area Network)의 새로운 통신채널로 등장하였다. 특히 PMMA 계에 기반을 둔 언덕형 굴절률 분포 플라스틱 광섬유(Graded Index Plastic Optical Feiber)는 500nm와 650nm 근처에서 아주 적은 손실을 가지므로 이 파장영역에서의 광원으로서 극초단 광펄스 생성이 요구되고 있다. 본 연구에서는 650nm InGaAIP FP(fabry perot) LD와 간단한 RF(radio frequency)소자들을 이용한 회로를 구성하고 이득스위칭 기술을 사용하여 1GHz의 펄스 반복율을 가지고 33.3psec 정도의 짧은 반치선폭(FWHM)을 가지는 광펄스를 생성시키는데 성공하였다. 이득스위칭 된 광펄스는 주입되는 직류전류(DC bias current)와 변조되는 주파에 따라 반치선폭은 33.3-82.8 psec, 펄스 에너지는 0.97-9.69pJ의 값을 얻었다. 또한 CW 발진과 이득스위칭 된 광펄스의 스펙트럼 폭은 각각 0.44nm, 150nm로 됨을 측정하였다. 이러한 결과를 가지는 연속적인 광펄스는 직류전류와 VCO에 의한 변조주파수의 적절한 선택에 따라 초고속 근거리 광통신분야 뿐만 아니라 여러 가지 초고속 광소자 및 전자소자의 특성평가 등에 응용될수 있을 것으로 예상된다.

• 한국환경보건학회지
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• 제46권4호
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• pp.444-456
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• 2020

Objectives: This study developed an index for the indoor air quality management of city buses to allow the provision of indoor air quality information to city bus users. Methods: Nine city buses in Seoul were measured for PM₁₀, PM_2.5, CO₂, temperature, and relative humidity through IoT sensors. Big data collected through the sensors was analyzed to identify indoor air quality on city buses and graded through the index. Results: As a result of dividing the measured city bus data into five grades through the IAQ index, PM₁₀ was rated "good" for 30.4% of the total measured values, and 9.2% were rated "risky". For PM_2.5, 67.7 percent were rated "good" and 0.4 percent were rated "risky". For CO₂, 0.9% were 'good' and 1.1% were 'risky'. The results of the classification through the IAQ index for city buses showed that the impact of good, normal, sensitive, bad, and dangerous were 2.7, 38.8, 46.0, 12.4, and 0.1%, respectively. According to the analysis by measurement area, Seocho-gu, Gangnam-gu, Seongdong-gu, Gwangjin-gu, and Dobong-gu are "normal" and other areas (Seodaemoon-gu, Jongno-gu, Yongsan-gu, Jung-gu, Seongbuk-gu, Dongdaemun-gu, Junggye-gu, Gangbuk-gu, and Nowon-gu) are all rated "sensitive". Conclusions: When analyzing cases where PM₁₀ and CO₂ indices are in the "bad" zone, the concentration is generally found to increase during rush hour, during which there are a large number of passengers. It is expected that indoor air quality management in vehicles will be necessary during rush hour.

• Current Research on Agriculture and Life Sciences
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• 제31권3호
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• pp.200-208
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• 2013

본 연구는 사전 예방적 국립공원 관리정책 도입과 과학적인 근거중심의 공원관리를 위해 탐방로 중심의 세부지표를 추출하여, 국립공원 주요 탐방구간별 스트레스 지수개발과 지수를 활용한 단계별 공원관리 정책 수단을 마련하고자 수행되었다. 탐방구간별 스트레스 지수의 개발을 위하여 국립공원 탐방로 총 144개 구간의 자료를 이용하였으며, 국립공원 스트레스 지수 개발을 위하여 델파이 설문 조사와 요인분석을 실시한 다음, 공원 탐방로별 스트레스지표 요인들의 표준화 점수를 이용한 스트레스 지수를 산출하였다. 그 결과 8개(탐방객 수, 단체 탐방객 수, 정상탐방객 수, 탐방로 훼손정도, 탐방로 훼손정도, 샛길 총 연장, 쓰레기 발생량, 생물종 다양도 등)의 스트레스 지표가 선정되었으며, 스트레스 지수 산정을 위한 요인은 탐방량, 탐방영향, 탐방로 훼손, 생물종 다양도 등 4개로 추출되었다. 요인별 가중치 산정은 2차 델파이 조사에서 평가한 지표별 중요도 값을 기준으로 산정하였으며, 5점 척도 평균값을 요인별 순 가중치로 산정하였다. 다음으로 앞서 산출된 표준화점수에 따른 누적확률을 5등급화(20점 미만: 5등급 매우 건전, 20~40점 미만: 4등급 건전, 40~60점 미만: 3등급 보통, 60~80점 미만: 2등급 심각, 80점 이상: 1등급 매우 심각) 하였다. 구간별 스트레스 지수 산정 결과, 지수값이 80점 이상인 1등급 구간은 6개 공원(지리산, 북한산, 덕유산, 내장산, 설악산, 주왕산) 20개 구간으로 나타났다.