• Composites Research
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• 제35권3호
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• pp.188-195
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• 2022

본 논문은 멀티스케일 공정-구조 해석의 방법론을 제안하고 단섬유층과 직물층으로 이루어진 배터리 하우징 파트에 적용한다. 특별히 마이크로스케일 대표체적요소(RVE: Representative Volume Element)안 기지의 불확정성을 고려하였다. 마이크로스케일의 RVE내 기지 물성의 랜덤한 공간내 분포는 KLE(Karhunen-Loeve Expansion)을 통해 구현하였다. 공간상 랜덤분포된 기지 물성을 갖는 RVE의 유효 물성을 전산균질화를 통해 얻어 매크로스케일 유한요소 모델에 매핑하였다. 또한 하이브리드 공정해석을 통해 압축 성형 해석으로부터 얻은 잔류응력과 섬유배향을 매핑한 유한요소 모델과 드레이핑 공정 해석결과로부터 얻어진 섬유배향을 매핑한 모델을 결합하였다. 본 연구에 제안된 방법은 배터리 하우징 뿐만 아니라 다양한 재료 구성을 갖는 복합재료의 공정-구조해석을 통해 설계요구도를 엄밀하게 평가할 수 있을 것이라 기대된다.

• 마이크로전자및패키징학회지
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• 제29권3호
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• pp.43-48
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• 2022

본 연구에서는 재료의 이방성 점탄성 거동을 고려한 해석 기법을 개발하여 휨(Warpage) 해석의 정합성을 개선하고자 하였다. 먼저, 이방성 점탄성 거동 구현을 위해 구리 패턴(Cu trace) 및 범프(Bump)가 존재하는 패키지를 모델링 하였다. 복잡한 형상의 범프 영역은 대표체적요소 모델을 기반으로 등가 이방성 점탄성 물성 및 열 팽창계수를 도출하였다. 도출된 물성을 기반으로 패키지에 0~125도의 열 주기(Thermal cycle)를 가하였으며, 열 주기에 따른 패키지의 휨 경향을 확인하였다. 해석 결과의 검증을 위해 해석 모델과 동일한 패키지를 제작하였고, 쉐도우 모아레 간섭계(Shadow Moire interferometer)를 통해 열 주기에 따른 실제 패키지의 휨 정도를 측정하였다. 결과적으로 구리 패턴, 범프 등의 요소가 고려된 등가 이방성 점탄성 해석 기법의 적용으로 5 ㎛ 이내의 오차로 패키지의 휨 정도를 계산하고 휨의 형태를 예측할 수 있었다.

• Composites Research
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• 제35권5호
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• pp.334-339
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• 2022

본 연구에서는 복합재료 제조 공정 과정 중 수지의 유동 저항성을 대변하는 인자인 투수 계수를 예측하는 알고리즘을 개발하였다. 단방향 연속 섬유 복합재료 내부에서 섬유와 수직인 방향 투수 계수의 정확한 예측을 위해 대표 체적 요소의 단면 형상을 고려하였다. 섬유의 유체 유동 저항성을 정량화하기 위한 인지로 섬유 간 간격이 사용되었고, 등가 길이는 섬유 배열에 따른 수지의 유로 변화를 나타내는 인자로 사용되었다. 전기-유압 유사성을 접목하여 투수 계수 예측 알고리즘을 개발하고 그 타당성을 확인하였다. 알고리즘은 Matlab과 Python으로 구성되고, 타당성 검증을 위해 FLUENT를 통해 예측된 투수 계수와 비교하였다. 알고리즘과 수치 해석을 통해 얻은 투수 계수가 거의 일치함을 확인하여 알고리즘을 검증하였으며, 소요 시간은 수치 해석 대비 약 1/450로 감소하였다.

• Composites Research
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• 제36권5호
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• pp.329-334
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• 2023

기존의 멀티스케일 유한요소법(Multiscale finite element, FE² )은 거시 스케일의 모든 적분점에서 대표 체적요소(representative volume element, RVE)의 미시 경계치 문제를 반복적으로 계산하기 때문에 긴 해석 시간과 많은 데이터 저장 공간을 필요로 한다. 이를 해결하기 위해 본 연구에서 평균장 균질화 데이터 기반 멀티스케일 해석 기법을 개발하였다. 데이터 기반 전산역학(data-driven computational mechanics, DDCM) 해석은 변형률-응력 데이터 셋을 직접적으로 사용하는 모델-프리(model-free)접근 방식이다. 멀티스케일 해석을 수행하기 위해, 평균장 균질화(mean-field homogenization)를 활용하여 복합재의 미세구조에 대한 변형률-응력 데이터베이스(database)를 효율적으로 구축하고, 이를 기반으로 데이터 기반 전산역학 시뮬레이션을 수행하였다. 본 논문에서는 개발한 멀티 스케일 해석 프레임워크(framework)를 예제에 적용하여, 초탄성(hyperelasticity) 복합재의 미세 구조를 고려한 데이터 기반 전산역학 시뮬레이션 결과를 확인하였다. 따라서, 데이터 기반 전산역학 접근 방식을 활용한 멀티스케일 해석기법은 다양한 재료 및 구조에 적용될 수 있으며, 멀티스케일 해석 연구 및 응용 가능성을 열어줄 것으로 기대된다.

• Composites Research
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• 제31권5호
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• pp.251-259
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• 2018

본 논문에서는 섬유강화복합재의 멀티스케일 해석을 위해 필요한 대규모/소규모 통계적 체적요소 모델을 개발하였다. 미시영역모델의 크기효과를 최소화하기 위해서 섬유를 최대한 포함한 거대모델을 구성하였다. 이를 위해 국부 영역의 요소 절단법을 이용하여 전체 유한요소 크기에 상관없이 신속한 격자 섬유/기지의 모델링이 가능한 요소생성기를 구성하였다. 이를 통해 대규모 통계 체적 모델을 도출하여 체적모델의 크기에 따른 국부하중 공유의 차이를 고찰하고, 섬유방향의 연속체손상역학모델을 BSVEs 모델 해석으로부터 도출 하였다. BSVEs 모델을 보편적인 RVE모델과 비교 검증하였다.

• Archives of Plastic Surgery
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• 제38권5호
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• pp.621-626
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• 2011

Purpose: Implant-based breast reconstruction has multiple advantages such as decreased morbidity, shorter operative time and faster recovery. However, postoperative infection with tissue expander increases medical cost and causes a delay in concurrent antineoplastic treatment. To reduce tissue expander infection, it is important to identify related risk factors and minimize them when possible. Methods: A retrospective review of patient records in a single breast cancer center was performed. Eighty-six tissue expanders were placed in 80 women for postmastectomy breast reconstruction. Variables including patients'age, body mass index (BMI), preoperative breast volume, operation time, drain indwelling time, postoperative seroma/hematoma formation, chemotherapy, and radiation therapy were evaluated. Infection was defined as the status that shows any symptom of local inflammation and identification of pathogens. Representative values were compared through Student's t-test and univariate and multivariate analyses. Results: We examined 86 postmastectomy tissueexpanders which were placed between June 2004 and April 2010. Seven cases of tissue expander infection (8.1%) were identified. The infected tissue expander was removed in three of the cases. The relationship between BMI, and preoperative breast volume and that between infection and non-infection groups were significant ($p$ <0.05). Univariate analysis showed significant association between BMI ($p$=0.023) and preoperative breast volume ($p$=0.037). Multivariate analysis revealed that BMI and preoperative breast volume were independent variables regarding tissue expander infection. Conclusion: Certain characteristics of implant-based breast reconstruction patients increase infection rate of tissue expander. These risk factors should be monitored and evaluated before surgeries for more successful outcome.

• Interaction and multiscale mechanics
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• 제1권2호
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• pp.279-301
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• 2008

This paper develops a 3D homogenization based continuum damage mechanics (HCDM) model for fiber reinforced composites undergoing micromechanical damage under monotonic and cyclic loading. Micromechanical damage in a representative volume element (RVE) of the material occurs by fiber-matrix interfacial debonding, which is incorporated in the model through a hysteretic bilinear cohesive zone model. The proposed model expresses a damage evolution surface in the strain space in the principal damage coordinate system or PDCS. PDCS enables the model to account for the effect of non-proportional load history. The loading/unloading criterion during cyclic loading is based on the scalar product of the strain increment and the normal to the damage surface in strain space. The material constitutive law involves a fourth order orthotropic tensor with stiffness characterized as a macroscopic internal variable. Three dimensional damage in composites is accounted for through functional forms of the fourth order damage tensor in terms of components of macroscopic strain and elastic stiffness tensors. The HCDM model parameters are calibrated from homogenization of micromechanical solutions of the RVE for a few representative strain histories. The proposed model is validated by comparing results of the HCDM model with pure micromechanical analysis results followed by homogenization. Finally, the potential of HCDM model as a design tool is demonstrated through macro-micro analysis of monotonic and cyclic damage progression in composite structures.

• Nuclear Engineering and Technology
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• 제42권1호
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• pp.9-16
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• 2010

Experimental examinations for evaluating fracutres were conducted by using transparent replicas of a single fracture in order to obtain the fracture data to contribute to the methodlogy on how to improve the definitaion of representative parameter values used for a parallel plate fracture model. Quantitative aperture distribution and quantitative tracer concentration data at each point in time were obtained by measuring the attenuation of transmitted light through the fracture in high spatial resolution. the representative aperture values evaluated from the multiple different measurement methods, such as arithmetic mean of aperture distribution measured by the optical method, transport aperture evaluated from the tracer test, and average aperture evaluated from the fracture void volume measurement converged to a unique value that indicates the accuracy of this experimental study. The aperture data was employed for verifying the numerical simulation under the assuption of Local Cubic Law and showed that the calculated flow rate through the fracture is 10%-100% larger than hydraulic test results. The quantitative tracer concentration data is also very valuable for validating existing numerical code for advection dispersion transport in-plane heterogeneous fractures.

• 한국환경과학회지
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• 제23권2호
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• pp.193-205
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• 2014

The direct-runoff of South Korea's representative dams (Soyanggang, Chungju, Andong, Daecheong, and Seomjingang) and precipitation were analyzed mainly with the evenly distributed spring rainfall events across the country for the last five years. For precipitation, an increasing was presented during the period 2008-2011, but did not continue to increasing 2012. The average precipitation of the five dams displayed a similar trend. Except for Chungju and Andong Dams, the trend of runoff was similar to the one shown in the precipitation. Despite the precipitation of 2009 increased, the runoff volume decreased for Andong and Chungju Dams. In addition, Chungju Dam remarkably showed a bigger runoff volume compared to other dams. As for the Sumjingang Dam, the runoff volume was the smallest, and the difference is as great as over 15-fold when compared to other runoff values. After the result of analyzing the relation between a single runoff event and synoptic weather patterns, pattern 4 contributed to the greatest impact on this event and weather patterns. The total runoff volume of the five dams for spring rain event for the last five years that exhibited this characteristic was estimated at 5.68 billion tons($10^6m^3$). Lastly, the value of this estimation was assessed as approximately 273.1 billion KRW.

• 한국물환경학회지
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• 제20권5호
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• pp.415-421
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• 2004

Litter wastes on highway runoff are gradually being considered one of the major pollutants of concern in protecting the integrity of receiving waters for beneficial use. The California State Water Resources Control Board has identified in their 303(d) list at least 36 water bodies where trash or litter is considered a pollutant of concern. The first TMDL adopted by the Region 4 (Los Angeles area) of the California State Water Quality Control Board was for trash in the Los Angeles River. The first flush characteristic study was developed to obtain first flush water quality and litter data from representative stormwater runoff from standard highway drainage outfalls in the Los Angeles area. Total captured gross pollutants in stormwater runoff were monitored at six Southern California highway sites over two years. The gross pollutants were 90% vegetation and 10% litter. Approximately 50% of the litter was composed of biodegradable materials. The event mean concentrations show an increasing trend with antecedent dry days and a decreasing trend with total runoff volume or total rainfall. Event mean concentrations were ranged 0.0021 to 0.259g/L for wet gross pollutants and 0.0001 to 0.027g/L for wet litters. The first flush phenomenon was evaluated and the impacts of various parameters such as rainfall intensity, drainage area, peak flow rate, and antecedent dry period on litter volume and loading rates were evaluated. First flush phenomenon was generally observed for litter concentrations, but was not apparent with litter mass loading rates. Litter volume and loading rates appear to be directly related to peak storm intensity, antecedent dry days and total flow volume.