Dependence of Fatigue Life of Low-Pressure Die-Cast A356 Aluminum Alloy on Microporosity Variation

이충도,Suk Jong Yoo 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.4

The aim of the present study was to investigate the dependence of the high cycle fatigue property on themicroporosity variation of a low-pressure die-cast A356 alloy. Also, it aimed to describe quantitatively therelationship between the fatigue property and monotonic tensile strength using modified Basquin’s equationwhich takes into account the microporosity variation. The fatigue life of the A356 alloy can be described by anexponential dependence on the variation of the fractographic porosity, in terms of the modified Basquin’s equationwhich is composed of the defect susceptibility of fatigue life to microporosity variation and the maximum tensilestrength achievable in the defect-free condition. Using a modified form of Basquin’s equation, the maximumvalues of the fatigue strength coefficient and exponent in the defect-free condition are 341.5 MPa and-0.076, respectively, even though the nominal values of fatigue strength coefficient and exponent withoutconsideration of microporosity variation are 237.6MPa and -0.048, respectively. Also, the difference betweenthe maximum tensile strength and the fatigue strength coefficient on modified Basquin’s equation is about120MPa, and it arises from variation in the deformation behavior due to the difference of loading conditionbetween the monotonic and cyclic test modes such as the strain rate, Bauschinger effect, cyclic work hardeningand damage accumulation on loading condition.

Defect Susceptibility of Sensile Strength to Microporosity Variation in As-Cast Magnesium Alloys with Different Grain Sizes

이충도 대한금속·재료학회 2010 METALS AND MATERIALS International Vol.16 No.4

The variability in the tensile strength of as-cast AM60 and AZ91 alloys was investigated in terms of the defect susceptibility to the variation in grain size and microporosity. The microporosity was measured from the quantitative fractography analysis through scanning electron microscopy (SEM) observation on fractured surface after tensile test. The ultimate tensile strength (UTS) of both alloys can be characterized as a power law relationship to microporosity variation in terms of the defect susceptibility and maximum strength achievable in the defect-free condition. The defect susceptibility of tensile strength to microporosity variation is decreased remarkably with grain refinement. The defect susceptibility of AZ91 alloy to microporosity variation exhibits more sensitive dependence on the variation in grain size than AM60 alloy. Also, the dependence of UTS on the variation in grain size is described as a power law relationship for various levels of microporosity. The variation on effective void area fraction by the damage evolution of Mg17Al12 phase may introduce a practically significant decrease of load bearing capacity, less than by microporosity variation. The Hall-Petch relation of both alloys in the defect-free condition could be suggested as maximum values of friction stress and locking parameter.

A356합금의 품질지수에 미치는 미소기공율의 영향

이충도 한국주조공학회 2018 한국주조공학회지 Vol.38 No.5

The dependence of the tensile properties on variations in the porosity of A356 aluminium alloys was investigated in terms of the quality index of the tensile properties based upon the ultimate tensile strength and elongation as well as the variation of the strength coefficient and strain-hardening exponent with regard to a T6 treatment. The test specimens were prepared by low-pressure die-casting and a subsequent T6 treatment, and the experimental results of a tensile test carried out at room temperature were compared to the theoretical description using a modified constitutive model. The nominal value of the quality index of A356 alloys increases gradually with a lapse of the ageing time upon a T6 treatment, despite the fact that this value is temporarily decreased during the initial stage of ageing from a solutionised condition. Additionally, the quality index depends practically upon the porosity variation with a power law relationship without regard to whether in solutionised or artificial aged conditions. The theoretical description indicates that the strength coefficient directly determines the nominal level of the quality index. Moreover, the overall dependence of the quality index on the porosity variation is remarkably weakened with an increase in the tensile strain, whereas the quality index depends sensitively upon the porosity variation with a low value of the strain-hardening exponent.

원심주조법에 의한 주철-Babbitt Metal 복합관 제조에 관한 연구

이충도,강춘식 ( Chung Do Lee,Choon Sik Kang ) 한국주조공학회 1993 한국주조공학회지 Vol.13 No.1

N/A Conventional manufacturing process for cast iron-babbitt metal composite is complicate and bimetallic bonding by centrifugal casting is also difficult because their melting point is largely different and nonmetallic inclusion exists on outer shell. This study is aiming to simplify multistage process by adding Cu-powder as insert metals during cast iron solidification. The variables on fabrication of composite pipe are mold rotating speed and inner surface temperature of outer metal. The optimum temperature range for fusion bonding between cast iron and Cu-layer was 1100℃-1140℃ in case of mold rotating speed was 700rpm. When the inner surface of Cu-layer was at 900℃, the value of interfacial hardness between Cu-layer and babbitt metal were higher than Cu-matrix by forming diffusion layer, interfacial products between Cu-layer and babbitt metal are proved to be Cu_6Sn_5(η)by XRD.

7. 極地에서의 海洋建設

李忠東(이충동) 대한토목학회 1986 대한토목학회지 Vol.34 No.6

Effect of Strain Rate on Elastic-Plastic Deformation in High Cycle Fatigue Properties

이충도 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.6

The aim of the present study was to investigate the contribution of the strain rate to the transition of elastic-plastic deformation behavior and the difference between the fatigue strength coefficient and monotonictensile strength in terms of the modified Basquin’s relation, which considers microporosity variation. Thetransition phenomena of elastic-plastic deformation were evaluated through comparison of the overall contour ofthe hysteresis loops measured in high cycle fatigue tests of a low-pressure die-cast A356 alloy. The increase ofthe alternating stress amplitude at a given excitation frequency causes a variation in the strain rate per unit timeinterval that depends fundamentally on the excitation frequency. In addition, the transition of elastic-plastic deformationbehavior is induced by variation of strain rate in high cycle fatigue test, i.e., typically, a variation ofthe elastic modulus and extension of the elastic deformation region, compared with monotonic deformationunder a very slow strain rate. The dependence of the elastic modulus on the strain rate due to the variation of thestress amplitude can be described in an exponential form of strain rate. The modified Basquin’s equation whichincludes the contribution of the strain rate and microporosity to the fatigue strength coefficient and fatigue lifewas re-established, including the dependence of the elastic modulus and transition of elastic-plastic deformationon the variation of the strain rate.

산업용 로봇 시스템 기술

이충동 대한기계학회 2011 기계저널 Vol.51 No.2

1950~60년대 아동문학단체장1의 계보학적 고찰

이충일(Lee, chung-il) 한국아동청소년문학학회 2013 아동청소년문학연구 Vol.- No.12

일본의 성년후견제도의 이용실태를 통한 우리나라에의 시사점 -전문직 후견인의 부정행위를 중심으로-

이충은 제주대학교 법과정책연구원 2015 法과 政策 Vol.21 No.3

노령과 질병으로 인해 판단능력이 부족한 자를 보호하고 일상적인 생활을 영위할 수 있도록 하는 것이 성년후견제도이다. 성년후견제도는 기존의 무능력자 제도와는 달리 의사능력이 결여된 사람에 대한 낙인효과를 방지하고 자기결정권을 최대한 보장함으로써 피후견인의 복리를 실현시키겠다는 취지를 갖고 있다. 우리나라는 이미 2000년에 고령화 사회에 접어들었으며, 앞으로도 고령 인구의 비율이 지속적으로 증가할 것이 예상되는 만큼, 성년후견제도가 향후 이루고 있는 역할은 매우 클 것으로 보인다. 따라서 성년후견제도가 건전한 형태로 기능을 발휘함과 동시에 안정적으로 조기에 정착할 수 있도록 면밀한 검토가 이루어져야 할 것이다. 올해로 시행 15년을 맞이한 일본은 성년후견제도가 실시된 이래 후견제도의 이용건수가 해마다 증가하고 있다. 이러한 점에서 성년후견제도의 입법목적을 달성한 것으로 보이지만, 문제는 이용건수만 증가한 것이 아니라 성년후견인의 배임과 착복 등 부정행위까지도 함께 증가하였다는 것이다. 현재까지 발각된 사건이 전부가 아닐 것이고, 성년후견제도를 비즈니스의 영역으로 인식하게 됨으로써 부정한 의도를 가진 성년후견인이 발생하고 있다는 점에서 그 문제는 날로 심각해 질 것으로 보인다. 특히, 피후견인의 연령이 점차 고령화되고, 경제적 불황 등으로 인해 유혹에 빠지기 쉬운 사회적 상황으로 발전되어 가고 있으며, 부정행위가 즉시 발각되는 것이 아니라 일정 시간이 지난 후에 발각되는 경우가 많다는 점에서 그 유혹에 더욱 쉽게 빠져들게 될 것이다. 우리나라의 경우도 일본과 마찬가지로 성년후견 사건의 수가 지속적으로 증가하고 있으며, 이로 인한 배임과 착복 등 부정행위의 발생도 함께 증가할 것으로 예상되는 바, 본 논문에서는 일본의 성년후견제도 이용실태를 면밀히 분석하여 사전 방지책을 강구함으로써 안정적이고 성공적인 성년후견제도의 정착에 기여하고자 한다. The adult guardianship system is to protect those with weaker assessment ability due to advanced age and illness and help them lead a normal daily life. The purpose of the system, unlike the existing scheme for the incompetent, is to prevent any social stigma on people lacking a decision making ability and ensure their right to make a decision on their own to the maximum extent with a view to realize the wellbeing of the wards. South Korea already became an aging society in 2000 and its elderly population is predicted to increase continuously. In this situation, the adult guardianship scheme will play a huge role in the future as well. Therefore, the scheme should be closely investigated for its sound functionality provision and stable early settlement in the society. The adult guardianship system of Japan marks 15th year of implementation this year and since its execution, the number of cases dealt with under the scheme has grown every year. In this aspect, it may seem that the system achieved its legal purposes. But the problem is that what has increased is not only the number of cases but also relevant fraudulent behaviors of adult guardians such as malpractices and embezzlements. It is certain that not all of such abusive behaviors have been uncovered so far. And ill-intended adult guardians increase as they gradually recognize the guardianship system as a business. For this reason, the problems seem to grow more serious. Especially in a situation where the age of wards grows older and the social conditions such as slow economy make more people easily succumb to evil temptations as fraudulent behaviors tend not to be detected instantly but later on, after a certain time. Just like Japan, South Korea experiences the continuously rising number of cases dealt with under the adult guardianship scheme. Together with this trend, also relevant embezzlement, misappropriation, and other violations are expected to move up as well. in this recognition, the study seeks to contribute to the stable and successful landing of adult guardianship system by scrutinizing the implementation status of Japan’s adult guardianship system and exploring corresponding preventive measures.

Time Series Changes in Sea-surface Temperature, Chlorophyll a, Nutrients, and Sea-wind in the East/Japan Sea on the Left- and Right-hand Sides of Typhoon Shanshan’s Track

이충일,박미옥 한국해양과학기술원 2010 Ocean science journal Vol.45 No.4

Time series changes in sea surface temperature (SST), chlorophyll a (Chl a), nutrients (PO4, NO3), and sea winds, which correlated with the passage of Typhoon Shanshan in the East/Japan Sea (EJS), are illustrated using satellite data for Chl a, SST, sea winds, and in situ data for nutrients and water temperature. The sea-surface cooling (SSC) effect by the passage of the typhoon was higher at stations nearer to the center compared to stations further from the center. The SSC effect at stations in the colder water region (on the left side of the typhoon’s track) was higher than at stations in the Tsushima Warm Current region (on the right side of the typhoon). The SSC effect continued for approximately 10 days after the passage of the typhoon. The Chl a concentration at all stations increased after the passage of the typhoon. This increase continued for a period of approximately 10 days, but the duration period at each station varied with distance from the typhoon center. Changes in Chl concentrations at stations within a 2° distance on both sides from the typhoon’s center were higher than that at other stations. The changes in Chl a by the passage of the typhoon were measured at approximately 0.3-1.0 mg/㎥ along the moving path of the typhoon. Phosphate and nitrate changes were inversely correlated with the water temperature changes; the nutrient concentration increased with the passage of the typhoon. Like the changes in SST, changes in nutrient concentrations on the left side of the typhoon’s track were higher compared to those at the center and the right side.