• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.1
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• pp.67-75
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• 2013

The flow stress of zircaloy-4 used in the spacer grid supporting a nuclear fuel rod was determined by the Johnson-Cook model, and model parameters were determined using reverse engineering. Parameters such as A, B, n and $\dot{\varepsilon}_0$ were determined by the tensile test result. To obtain the parameters C and m, a slot milling test and numerical simulation were performed. The objective functions were defined as the difference between the experimental and the simulation results, and then, the parameters were determined by minimizing the objective function. To verify the validity of the determined parameters, cross-verification for each case was conducted through a shearing test and simulation. The results tend to show agreement with the experimental results, such as the features of sheared edges and maximum punch force, with the correlation coefficients exceeding at least 0.97.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.9
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• pp.745-751
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• 2014

For the multi-nozzle propulsion, the rupture pressure of nozzle closure has an effect on the initial strain rate of ignition. Moreover, the deviation of rupture pressure for each nozzle closure leads to side forces which can disturb the attitude control of rocket. When designed, it should be considered whether nozzle closures are ruptured equally and exactly in the intented pressure. In this paper, the rupture behavior is analyzed by analytical and experimental methods for plate and "+" notched nozzle closures. The rupture pressure and deviation for operating temperature, whether notched or not and notched directions are analyzed. This paper provides a comparison between rupture pressure prediction of finite elements method which tool is Abaqus/Explicit and results of the rupture test. Jonson-Cook shear failure model which corresponds to the damage initiation criterion were used in this simulation.

• Transactions of the Korean Society of Automotive Engineers
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• v.11 no.2
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• pp.164-171
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• 2003

The side impact behavior has been investigated when the high strength steel 60TRIP(Transformation Induced Plasticity) is replaced for the conventional low-carbon steel for weight reduction of an auto-body. The side impact analysis was carried out as specified in US-SINCAP with the center pillar and the side sill of the conventional steel or 60TRIP. For accurate impact analyses, the dynamic material properties are adopted with the Johnson-Cook model. The analysis results demonstrate that the penetration of the side members is remarkably reduced when 60TRIP is employed for the center pillar and the side sill replacing the conventional steel. The crashworthiness in the side impact is considerably improved with less penetration of the side members and less acceleration of the opposite floor.

• Korean Journal of Comparative Education
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• v.28 no.5
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• pp.47-69
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• 2018

The purpose of this study is to classify the lifelong learning policy implementation process of lifelong learning in Korea according to the policy making decision models and to suggest developmental tasks. Korea's lifelong learning policy came to a full-fledged start with the enactment of the Lifelong Education Act in 2000. The Lifelong Education Act proposed the establishment of an open educational system as a strategy to realize the lifelong learning society. According to the Lifelong Education Act, the Korean government has developed several lifelong education policies such as providing learning opportunity for the underprivileged, facilitating lifelong learning city project, building lifelong learning culture, recognizing of experiential learning result, funding lifelong learning hub university, launching lifelong learning supporting administrative etc. The Korean lifelong system is characterized as Allison's (1971) governmental/bureaucratic, Ziegler and Johnson's (1972) legislative, Griffin's(1987) social control and Green's (2000) state-led models which make policy through the coordination between the government and the parliament and control bureaucratic power and educational qualifications. Lifelong learning policies should be managed in terms of supply and demand at the learning market. In addition, the state has to strengthen lifelong learning through supporting NGOs' activities and adult learners' tuition fee for the disadvantaged group of people.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.13 no.5
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• pp.50-56
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• 2014

A technique based on the finite element method (FEM) is used in the simulation of metal cutting process. This offers the advantages of the prediction of the cutting force, the stresses, the temperature, the tool wear, and optimization of the cutting condition, the tool shape and the residual stress of the surface. However, the accuracy and reliability of prediction depend on the flow stress of the workpiece. There are various models which describe the relationship between the flow stress and the strain. The Johnson-Cook model is a well-known material model capable of doing this. Low-alloy steel is developed for a dry storage container for used nuclear fuel. Related to this, a process analysis of the plastic machining capability is necessary. For a plastic processing analysis of machining or forging, there are five parameters that must be input into the Johnson-Cook model in this paper. These are (1) the determination of the strain-hardening modulus and the strain hardening exponent through a room-temperature tensile test, (2) the determination of the thermal softening exponent through a high-temperature tensile test, (3) the determination of the cutting forces through an orthogonal cutting test at various cutting speeds, (4) the determination of the strain-rate hardening modulus comparing the orthogonal cutting test results with FEM results. (5) Finally, to validate the Johnson-Cook material parameters, a comparison of the room-temperature tensile test result with a quasi-static simulation using LS-Dyna is necessary.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.9
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• pp.740-753
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• 2008

The conventional Split Hopkinson Pressure Bar (C-SHPB) technique with aluminum pressure bars to achieve a closer impedance match between the pressure bars and the specimen materials such as hot temperature degraded POM (Poly Oxy Methylene) and PP (Poly Propylene) to obtain more distinguishable experimental signals is used to obtain a dynamic behavior of material deformation under a high strain rate loading condition. An experimental modification with Pulse shaper is introduced to reduce the nonequilibrium on the dynamic material response during a short test period to increase the rise time of the incident pulse for two polymeric materials. For the dynamic stress strain curve obtained from SHPB experiment under high strain rate, the Johnson-Cook model is applied as a constitutive equation, and we verify the applicability of this constitutive equation to the probabilistic reliability estimation method. The methodology to estimate the reliability using the probabilistic method such as the FORM and the SORM has been proposed, after compose the limit state function using Johnson-Cook model. It is found that the failure probability estimated by using the SORM is more reliable than those of the FORM, and the failure probability increases with the increase of applied stress. Moreover, it is noted that the parameters of Johnson-Cook model such as A and n, and applied stress affect the failure probability more than the other random variables according to the sensitivity analysis.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.12
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• pp.1054-1061
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• 2015

Navigation accuracy, integrity, and safety of commercial Unmanned Aerial Vehicle (UAV) is becoming crucial as utilization of UAV in commercial applications is expected to increase. Recently, the concept of Local-Area Differential GNSS (LADGNSS) which can provide navigation accuracy and integrity of UAV was proposed. LADGNSS can provide differential corrections and separation distances for precise and safe operation of the UAV. In order to derive separation distances between UAVs, modeling of Flight Technical Error (FTE) is required. In most cases, FTE for civil aircraft has been assumed to be zero-mean normal distribution. However, this assumption can cause overconservatism especially for UAV, because UAV may use control and navigation equipments in wider performance range and follow more diverse path than standard airway for civil aircraft. In this research, flight experiments were carried out to understand the characteristics of FTE distribution. Also, this paper proposes to use Johnson distribution which can better describe heavy-tailed and skewed FTE data. Futhermore, Kolmogorov-Smirnov and Anderson-Darling tests were conducted to evaluate the goodness of fit of Johnson model.

• Composites Research
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• v.36 no.3
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• pp.199-204
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• 2023

A combination of a metallic mesh and an adhesive layer of metallic particle/epoxy composite was introduced as an intermediate layer to enhance the adhesion between cold-sprayed particles and fiber-reinforced composites (FRCs). Aluminum was considered for both the metallic particles in the adhesive and the metallic mesh. To predict the mechanical characteristics of the intermediate bond layer under a high strain rate, the properties of the adhesive layer needed to be calculated or measured. Therefore, in this study, the Al particle/epoxy adhesive was homogenized by using a rule of mixture. To verify the homogenization, the penetration depth, and the thickness decrease after the cold spray deposition from the undeformed surface, was monitored with FE analysis and compared with experimental observation. The comparison displayed that the penetration depth was comparable to the diameters of one cold spray particle, and thus the homogenization approach can be reasonable for the prediction of the stress level of particulate polymer composite interlayer under a high strain rate for cold spray processing.