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이승학,송형용,김동건,임성욱,박은영,현규 한국고분자학회 2017 Macromolecular Research Vol.25 No.4
The effects of the curing and blowing agent concentration in an epoxy resin were investigated using rheological, mechanical, and optical methods. The curing time from the time sweep test decreased with increasing amount of curing agent at the fixed blowing agent concentration. On the other hand, with increasing blowing agent at a fixed curing agent concentration, the curing time showed a local minimum value. Minimization of the curing time is very useful for reducing the processing time. Axial normal forces as a function of time showed a relationship between the contraction force by the curing process and the expansion force by foaming process. From the axial normal force measurements, it could be categorized quantitatively into three parts: the curing dominant region (negative axial normal force development), transition region, and the foaming dominant region (positive axial normal force development). At the transition region, the axial normal force development was delayed because the foaming process was disturbed by the contraction. Mechanical and structural analysis were conducted for the fully cured and foamed epoxy resin. The completely developed epoxy foams with the high curing agent concentration become brittle. On the other hand, they contained well-distributed unit cell foams inside. This is because the fast curing process interrupts the coalescence of the closed foams. Overall, the optimal curing and blowing agent concentration for the epoxy resin could be determined from rheological analysis during the process and mechanical and structural analysis for fully cured and foamed epoxy resin.
Duffy 방법을 이용한 임의 형상 도체의 전자파 산란 해석
이승학,김채영,이창원 한국전자파학회 2002 한국전자파학회논문지 Vol.13 No.8
임의 모양의 완전도체의 전자파 산란을 해석하기 위하여 모멘트 방법을 이용하였다. GID(Graphic Interface Design)툴을 이용하여 설계된 금속 도체의 표면은 서로 다른 모양의 삼각패치로 모델링 되었다. 도체 표면전류는 삼각패치의 벡터 기저함수로 확장되었다. 관측점과 전원점이 동일 삼각패치에 위치할 때, 삼각패치의 특이점 적분은 Duffy 방법을 이용하여 특이점이 없는 적분으로 변환이 가능하였고, 변환된 특이점 항들은 1차원의 가우시안 구적법을 이용하여 간단하게 계산되어질 수 있었다. 특이점을 제외한 적분들은 2차원 가우시안 구적법으로 계산되었다. The method of moment is applied to the analysis of electromagnetic scattering from an arbitrarily-shaped conductor. The conducting surface is discretized into triangular patches using a GID tool. Surface currents on a conductor are expanded with a vector triangle basis function. By using the Duffy's method, the singular integration appeared in a triangle patch can be transformed into the non-singular integral form suitable for one dimensional Gaussian quadrature integration method. Mutual and self integration extracted singular terms are evaluated by two dimensional Gaussian quadrature techniques.
Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions
이승학,송형용,현규 한국유변학회 2016 Korea-Australia rheology journal Vol.28 No.2
In this study, the effects of adding silica nanoparticles to PVA/CuNW suspensions were investigated rheologically, in particular, by small and large amplitude oscillatory shear (SAOS and LAOS) test. Interesting, the SAOS test showed the complex viscosities of CuNW/silica based PVA matrix were smaller than those of PVA/CuNW without silica. These phenomena show that nano-sized silica affects the dispersion of CuNW in aqueous PVA, which suggests small particles can prevent CuNW aggregation. Nonlinearity (third relative intensity ≡ I3/1) was calculated from LAOS test results using Fourier Transform rheology (FT-rheology) and nonlinear linear viscoelastic ratio (NLR) value was calculated using the nonlinear parameter Q and complex modulus G*. Nonlinearity (I3/1) results showed more CuNW aggregation in PVA/CuNW without silica than in PVA/CuNW with silica. NLR (= [Q0(ϕ)/Q0(0)]/[G*(ϕ)/G*(0)]) results revealed an optimum concentration ratio of silica to CuNW to achieve a well-dispersed state. Degree of dispersion was assessed through the simple optical method. SAOS and LAOS test, and dried film morphologies showed nano-sized silica can improve CuNW dispersion in aqueous PVA solutions.
Assessment of Dysarthria Using One-Word Speech Recognition with Hidden Markov Models
이승학,감민제,서한길,오병모,이강표,이자호 대한의학회 2019 Journal of Korean medical science Vol.34 No.13
Background: The gold standard in dysarthria assessment involves subjective analysis by a speech–language pathologist (SLP). We aimed to investigate the feasibility of dysarthria assessment using automatic speech recognition. Methods: We developed an automatic speech recognition based software to assess dysarthria severity using hidden Markov models (HMMs). Word-specific HMMs were trained using the utterances from one hundred healthy individuals. Twenty-eight patients with dysarthria caused by neurological disorders, including stroke, traumatic brain injury, and Parkinson's disease were participated and their utterances were recorded. The utterances of 37 words from the Assessment of Phonology and Articulation for Children test were recorded in a quiet control booth in both groups. Patients were asked to repeat the recordings for evaluating the test–retest reliability. Patients' utterances were evaluated by two experienced SLPs, and the consonant production accuracy was calculated as a measure of dysarthria severity. The trained HMMs were also employed to evaluate the patients' utterances by calculating the averaged log likelihood (aLL) as the fitness of the spoken word to the word-specific HMM. Results: The consonant production accuracy reported by the SLPs strongly correlated (r = 0.808) with the aLL, and the aLL showed excellent test–retest reliability (intraclass correlation coefficient, 0.964). Conclusion: This leads to the conclusion that dysarthria assessment using a one-word speech recognition system based on word-specific HMMs is feasible in neurological disorders.
이승학,오병모,천성민,이정찬,민유선,방상흠,김희찬,한태륜 대한재활의학회 2013 Annals of Rehabilitation Medicine Vol.37 No.3
Objective To evaluate the accuracy of the swallowing kinematic analysis.Methods To evaluate the accuracy at various velocities of movement, we developed an instrumental model of linear and rotational movement, representing the physiologic movement of the hyoid and epiglottis, respectively. A still image of 8 objects was also used for measuring the length of the objects as a basic screening, and 18 movie files of the instrumental model, taken from videofluoroscopy with different velocities. The images and movie files were digitized and analyzed by an experienced examiner, who was blinded to the study.Results The Pearson correlation coefficients between the measured and instrumental reference values were over 0.99 (p<0.001) for all of the analyses. Bland-Altman plots showed narrow ranges of the 95% confidence interval of agreement between the measured and reference values as follows: 0.14 to 0.94 mm for distances in a still image, -0.14 to 1.09 mm/s for linear velocities, and -1.02 to 3.81 degree/s for angular velocities.Conclusion Our findings demonstrate that the distance and velocity measurements obtained by swallowing kinematic analysis are highly valid in a wide range of movement velocity.