• 대한조선학회지
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• 제21권1호
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• pp.35-42
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• 1984

In the present study, rotating bending fatigue tests have been carried out in three kinds of carbon steel specimens; an annealed low carbon steel, an annealed high carbon steel and quenched-tempered high carbon steel; with a small artificial surface defect that might exist in real structures. Fatigue crack lengths have been observed by a method of replication in order to investigate the growth characteristic of fatigue crack in the viewpoints of strength of materials and fracture mechanics. The main results obtained are as follows: 1) The effect of a small surface defect upon the reduction of fatigue limit is considerably large, and the rate of fatigue limit reduction grows in the following order; annealed low carbon steel(mild steel), annealed high carbon steel, quenched-tempered high carbon steel. 2) When the growth rate of surface crack length(2a) was investigated in the viewpoints of fracture mechanics based upon $ ${\Delta}K_{\varepsilon}$, the dependence of stress level and of surface defect size disappear, and there exists a linear relationships between d(2a)/dN and ${\Delta}K_{{\varepsilon}t},\;\Delta_{{\varepsilon}t}\sqrt{{\pi}a}$, on log. plot, i.e, $d(2a)/dN={C{\cdot}{\Delta}K_{\varepsilon}}^3_t$, where ${\Delta}_{{\varepsilon}t}\sqrt{{\pi}a}$ a is the cyclic total strain intensity factor range.

• 대한기계학회논문집A
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• 제22권4호
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• pp.779-789
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• 1998

In this paper, the thermal shock responses of collinear cracks in a layered medium are investigated based on the uncoupled, quasi-static plane thermoelasticity. The medium is modeled as a bonded structure composed of a surface layer and a semi-infinite substrate. Between these two dissimilar homogeneous constituents, a functionally graded interfacial zone exists with the nonhomogeneous features of continuously varying thermoelastic properties. Three cracks are assumed to be present in the layered medium, one in each one of the constituent materials, aligned collinearly normal to the nominal interfaces. A system of singular integral equations is solved, subjected to the forcing terms of equivalent transient thermal tractions acting on the locations of cracks via superposition. Main results presented are the transient thermal stress intensity factors to illustrate the parametric effects of various geometric and amterial combinations of the medium with the thermoelastically graded interfacial zone and the collinear cracks.

• 전기의세계
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• 제29권8호
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• pp.524-531
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• 1980

In order to investigate the effect of inorganic dielectric fine particle mixed in Low Density Polyethylene on the deterioration by treeing, a comparative study for initiation and development of the tree has been carried out between the pure thin film specimen and the same geometrical specimen mixed with a constant weight percent by a defiend particle size of $Al_{2}$O$_{3}$ and SiO$_{2}$, having larger dielectric constants than that of the base material. According to the results, it has been observed that as increasing dielectric constant, the initiation of tree is expedited, however, the development of the tree reached at the surface of filler particles shows the suppressive trends. From these facts, a reasonable interpretation may be possible by considering the effect of intensified electrical field around the tip in the presence of filler particles, that the initiation and the development of tree are a mechanical break down process caused by Maxwell stress due to the concentration of electrical field at the tip. This suppressive effect is specifically suggestive for the reason that a discharge route must be constructed around the particle surface because of the intensified field strength near filler, which, in turn, reduces the geometrical curvature of the tip so that the local intensity of electrical field can be relaxed. Further more an experimental evidence for this assumption was able to observe in this investigation.

• IMMUNE NETWORK
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• 제5권2호
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• pp.117-123
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• 2005

The immune response to any stimulus is complex, requiring coordinated action by several types of cells in a tightly regulated sequence. Thus, a physical stress such as exercise may act at any number of points in the complex sequence of events collectively termed the immune response. Although exercise causes many propound changes in parameters of immune function, the nature and magnitude of such changes rely on several factors including the immune parameters of interest; type, intensity, and duration of exercise; fitness level or exercise history of the subject; environmental factors such as ambient temperature and humidity. Although regular moderate exercise appears to be important factor for increasing immunity, Athletes are susceptible to illness, in particular upper respiratory track infection, during periods of intense training and after competition. In addition, in elite athletes, frequent illness is associated with overtraining syndrome, a neuroendocrine disorder resulting from excessive training. Through this paper, we want to investigate the effects of exercise on the immunosuppression such as exercise induced lymphopenia, asthma, anaphylaxis, URT (upper respiratory track), and TB (tuberculosis) infection. and also, we want to suggest a direct mechanism, protection and therapy of exercise induced immunosuppression.

• 대한간호학회지
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• 제39권3호
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• pp.376-385
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• 2009

Purpose: The purposes of this study were(a) to identify smoking behavior by following a cohort representative of the population of 4th grade elementary schoolers in South Korea over a four-year period(2004-2007), and(b) to explore predictors of smoking initiation among non-smokers in Wave 1. Methods: Secondary data, the Korea Youth Panel Study, was analyzed in this study. First, frequencies or percentages were calculated to identify smoking behavior(i.e., smoking initiation, smoking intensity, and smoking duration). Second, binary logistic regression analysis was performed to examine significant factors related to smoking initiation. Results: Smoking initiation and daily smoking were more pronounced when the participants entered middle school. In bivariate analysis, statistically significant predictors of smoking initiation were loneliness at school, self-control, delinquent behavior, depressive symptoms, and stress. However, after controlling for other factors, only a high level of risk-taking tendency and a greater number of delinquent behaviors remained statistically significant. Conclusion: Based on greater involvement in smoking among first-year middle schoolers, smoking prevention strategies should be provided to elementary schoolers rather than middle schoolers. A risk-taking tendency and delinquent behaviors should be considered as proxy measures to detect the high-risk group for smoking initiation.

• 대한치과보철학회지
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• 제31권1호
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• pp.39-50
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• 1993

The purpose of this study was to investigate the association between characteristics of temporomandibular joint sound and possible contributing factors of temporomandibular joint sound. 1487 high school students (798 men and 689 women) aged from 16 to 18 were asked by questionnaires which was made for this study by T.M.J. clinic, dental infirmary, School of Dentistry, Chosun University, and data obtained from questionnaires were analyzed by chi-square test. The following results were obtained: 1. The prevalence of temporomandibular joint sound were found in 35.7% of all the subjects. 2. Temporomandibular joint sound could be unchanged, reduced and increased frequencey over time dependent on the individual. 3. The intensity of temporomandibular joint sound could be unchanged and increased during the chewing of hard food dependent on the individual. 4. The unilateral chewing and bruxism were not considered as possible contributing factors upon the occurring of temporomandibular joint sound. 5. Statistical significant association was found between clenching, stiffness, stress and tempore-mandibular joint sound, respectiely.(P<0.001, P<.01, P<0.05).

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2005년도 춘계학술발표대회 논문집
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• pp.80-83
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• 2005

In this work, effects of oxygen plasma on surface characteristics of carbon fibers were investigated in mechanical properties interfacial of carbon fibers-reinforced composites. The surface properties of the carbon fibers were determined by acid/base values, FT-IR, and X-ray photoelectron spectroscopy (XPS). Also, the mechanical properties of the composites were studied in and critical stress intensity factor ($K_{IC}$) and critical strain energy release rate mode II ($G_{IIC}$) measurements. As experimental results, the $O_{lS}/C_{lS}$ ratio of the carbon fiber surfaces treated by oxygen plasma was increased compared to that of untreated ones, possibly due to development of oxygen-containing functional groups. The mechanical properties of the composites, including $K_{IC}$ and $G_{IIC}$ had been improved in the oxygen plasma on fibers. These results could be explained that the oxygen plasma was resulted in the increase of the adhesion of between fibers and matrix in a composite system.

• 대한기계학회논문집A
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• 제25권12호
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• pp.2091-2099
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• 2001

Fatigue crack growth behavior of inclined cracked Al 6061-T6 thick aluminum plate(6mm) repaired with the bonded composite patch was studied. A 0°inclined crack bonded reinforced composite patch and 15°, 30°, 45°, 60°inclined crack plates were tested. The effect of patch and inclined angle were studied and compared to each other. Also we investigated to the crack propagation direction and debonding behavior during the fatigue crack growth test. In this paper. a study was con(ducted to get an fatigue life, fatigue crack growth ratio and crack growth direction. Finally, the effectiveness of composite patch on inclined cracked plate was investigated. The results demonstrated thats there was a definite variation in fatigue life and fatigue crack growth behavior depending on the inclined crack angle.

• 대한기계학회논문집A
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• 제24권2호
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• pp.370-377
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• 2000

This paper presents a mixed-mode type ER(electro-rheological) engine mount, and its vibration control performance for a passenger vehicle is presented. The field-dependent yield stress of a transfo rmer oil-based ER fluid is empirically distilled in both shear and flow modes. This is then incorporated with the governing equation of motion of the proposed mixed-mode(shear mode plus flow mode) type engine mount. The damping force is analyzed with respect to the intensity of the electric field and design parameters such as electrode gap. Subsequently, the ER engine mount which is equivalent to the conventional hydraulic engine mount in terms of the damping level is designed and manufactured. Both computer simulation and experimental test are undertaken in order to evaluate vibration isolation performance. In addition, this performance is compared with that of the conventional hydraulic engine mount.

• 대한기계학회논문집
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• 제7권1호
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• pp.1-10
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• 1983

The photoelastic determination of S.I.F. in Fracture mechanics has been regarded as one of the most effective and practical experimental methods in which stresses are read directly, except a few shortcomings involved in the process of experiment; the difficulties of making a sharp crack tip similar to the practical one and nearly impossibilities of carving an arbitrarily shaped crack on the test plate, etc. To eliminate flaws mentioned above, recently, Kitagawa and Watanabe of Tokyo Univ.developed a new method named"Teflon Insert Method". which has improved experimental accuracy to a considerable extent byt remaining still room for further improvement, that is, the elimination of bonding boundary scars which render photoelastic fringes obscure. In this paper, a newly exploited"Teflon Molding Method" was attempted for the completion of teflon-epoxy experimental method. The experimental results obtained by this method are compared with existent theoretical and experimental values to evaluate its accuracy. As result, 1-6% of margin of errors were appeared in a series of photoelastic experiments which defied any other conventional method in terms of experimental accuracy.perimental accuracy.