Ki-Hoon Shin

A method for modifying a surface model with non-uniformly scattered displacement constraints for shoe sole design

This paper presents a method for modifying a surface model with non-uniformly scattered displacement constraints. When displacement vectors are given as constraints to feature points on the original surfaces, those of the other points should be computed to construct deformed surfaces. To maintain the overall appearance and smoothness of the original surfaces, the proper relationship should be formulated between the displacement vectors of feature points and those of the other points. For this purpose, a 3D vector field, a set of displacement vectors, is constructed based on the given constraints. Multi-level B-spline approximation technique is also used to circumvent a tradeoff between smoothness and accuracy. The technique uses coarse-to-fine hierarchy of control lattices. The proposed method is implemented as a module of the shoe sole design system and applied especially for shoe grading.

Fabrication of Functionally Graded Materials Between P21 Tool Steel and Cu by Using Laser-Aided Layered Manufacturing

With the development of layered manufacturing, thermally conductive molds or molds embedding conformal cooling channels can be directly fabricated. Although P21 tool steel is widely used as a mold material because of its dimensional stability, it is not efficient for cooling molds owing to its low thermal conductivity. Hence, the use of functionally graded materials (FGMs) between P21 and Cu may circumvent a tradeoff between the strength and the heat transfer rate. As a preliminary study for the layered manufacturing of thermally conductive molds having FGM structures, one-dimensional P21-Cu FGMs were fabricated by using laser-aided direct metal tooling (DMT), and then, material properties such as the thermal conductivity and specific heat that are related to the heat transfer were measured and analyzed.

Utilization of Finite Element Analysis in Design and Performance Evaluation of CFRP Bicycle Frames

With the continuing demand for lightweight bicycles, carbon fiber composite materials have been widely used in manufacturing bicycle frames and components. Unlike general isotropic materials, the structural characteristics of composite materials are strongly influenced by the staking directions and sequences of composite laminates. Thus, to verify the design process of bicycles manufactured using composites, structural analysis is considered essential. In this study, a carbon-fiber-reinforced plastic (CFRP) bicycle frame was designed and its structural behavior was investigated using finite element analysis (FEA). By measuring the failure indices of the fiber and matrix under various stacking sequences and loading conditions, the effect of the stacking condition of composite laminates on the strength of the bicycle structure was examined. In addition, the structural safety of the bicycle frame can be enhanced by reinforcing weak regions prone to failure using additional composite laminates.

Estimation of Mechanical Properties of Sn-xAg-0.5Cu Lead-free Solder by Tensile Test

SnAgCu lead-free solder alloy is considered as the best alternative to eutectic tin-lead solder. However, the detailed material properties of SnAgCu solder are not available in public. Hence, this paper presents an estimation of mechanical properties of SnAgCu lead-free solder. In particular, the weight percent of Ag was varied as 1.0wt%, 2.5wt%, 3.0wt%, and 4.5wt% in order to estimate the effect of Ag in the Sn-xAg-0.5Cu ternary alloy system. For this purpose, four types of SnAgCu bars were first molded by casting and then standard specimens were cut out of molded bars. Micro-Vickers hardness, tensile tests were finally performed to estimate the variations in mechanical properties according to the weight percent of Ag. Test results reveal that the higher the weight percent of Ag is, the higher the hardness, yield strength, and ultimate tensile strength become. More material properties will be further investigated in the future work.

FEA-Based Design of Heterogeneous Objects

Finite Element Analysis (FEA) is an important step for the design of structures or components formed by heterogeneous objects such as multi-materials, Functionally Graded Materials (FGMs), etc. The main objective of the FEA-based design of heterogeneous objects is to simultaneously optimize both geometry and material distribution over the design domain (e.g., Homogenization Design Method). However, the accuracy of the FEA-based design wholly depends on the quality of the finite element models. Therefore, there exists an increasing need for generating finite element models adaptive to both geometric complexity and material distribution. This paper introduces a method for FEA-based design of heterogeneous objects. At the design stage, a heterogeneous solid model is first created by referring to the libraries of primary materials and composition functions that are already available in the field of material science. The heterogeneous solid model is then discretized into an object model onto which appropriate material properties are mapped. Discretization converts continuous material variations inside an object into stepwise variations. Next, the object model is adaptively meshed and converted into a finite element model. The meshing algorithm first creates nodes on the iso-material curves (or surfaces) of heterogeneous solid models. Triangular (or tetrahedral) meshes are then generated inside each iso-material region formed by iso-material curves (or surfaces). FEA using commercial software is finally performed to estimate stress levels. This FEA-based design cycle is repeated until a satisfactory solution is obtained. If the design objective is satisfactory, the object model is fed to the fabrication system where a process planning is performed to create instructions for LM machines. An example (FGM pressure vessel) is shown to illustrate the entire FEA-based design cycle.Copyright

Adaptive Mesh Generation for Finite Element Analysis of Functionally Graded Materials

Finite Element Analysis (FEA) is an important step for the design of structures or components formed by heterogeneous objects such as multi-material objects, Functionally Graded Materials (FGMs), etc. The main objective of the FEA-based design of heterogeneous objects is to simultaneously optimize both geometric shapes and material distributions over the design domain (e.g., Homogenization Design Method). However, the accuracy of the FEA-based design wholly depends on the quality of the finite element models generated. Therefore, there exists an increasing need for developing a new mesh generation algorithm adaptive to both geometric complexity and material distributions. In this paper, a two-dimensional adaptive mesh generation algorithm is proposed based on the discretization by which continuous material variation inside an object is converted into step-wise variation. The proposed algorithm first creates nodes on the iso-material contours of the discretized solid models. Triangular meshes are then generated inside each iso-material region formed by iso-material contours. Current implementation considers two-dimensional problems and thus needs to be extended to include three-dimensional problems in the near future.Copyright

Modeling of Cooling Channels of Injection Mould using Functionally Graded Material

The cycle time in injection moulding greatly depends on the cooling time of the plastic part that is controlled by cooling channels. Cooling channels are required to facilitate the heat transfer rate from the die to the coolant without reducing the strength of the die. Employing layered manufacturing techniques (LMT), a die embedding conformal cooling channels can be fabricated directly while conventional cooling channels are usually made of straight drilled hole. Meanwhile, H13 tool steel is widely used as the die material because of its high thermal resistance and dimensional stability. However, H13 with a low thermal conductivity is not efficient for certain part geometries. In this context, the use of functionally graded materials (FGMs) between H13 and copper may circumvent a tradeoff between the strength and the heat transfer rate. This paper presents a method for modeling of conformal cooling channels made of FGMs.

Estimation of the Overhaul Cycle Time for KTX Wheelset Bearing by RCF Test

KTX wheelset bearings have thus far been maintained in accordance with the maintenance system of French national railway company, SNCF. The overhaul cycle time (OCT) of KTX wheelset bearings is now 1.4 million km in mileage. This value, however, has not been validated in Korea that has much different railway environments with France. In fact, it is impossible to validate OCT of wheelset bearings directly because they are disassembled and cleaned only when some faults are detected. In this paper, the accuracy of the current OCT value was evaluated indirectly by investigating the effect of grease lubricants on the bearing life. Five grease samples (one new and four aged greases) were used in this study. Four aged greases of different conditions were obtained from four wheelset bearings whose mileages were about 0.3, 1.2, 1.3, and 1.8 million km, respectively. Each grease sample was then injected into the RCF (Rolling Contact Fatigue) tester and fatigue lifetime was experimentally estimated. In addition, the wt% of Fe in each grease sample was analysed. The experiment results reveal that the bearing lifetime is inversely proportional to the mileage of grease sample while the wt% of Fe increases with the mileage of grease sample. Based on the experimental results, it can be concluded that the current OCT value is appropriate for the first overhaul of wheelset bearings. However, further validation is required to determine the second and third OCT values.

A Study on the Accelerated Life Test of Rubber Specimens by using Stress Relaxation

Rubber parts are widely used in many applications such as dampers, shock absorbers, and seals used in railway and automotive industries. Much research has thus far been conducted on property estimation and life prediction of rubber parts. To predict the service life of rubber parts at room temperature, most prior work adopts the well-known Arrhenius model that needs the accelerated life test in high-temperature conditions. However, they may not reflect the actual conditions of use that rubber parts are usually used under a specific strain condition during long period of time. In this context, we propose a method for the life prediction of rubber parts in actual conditions of use. The proposed method is based on the accelerated life test using stress relaxation during which three relatively high elongation percentages (100%, 200%, and 300%) are applied to the rubber specimens. Rubber specimens were prepared in accordance with KS M 6518 standard and three stress relaxation testers were fabricated for actual experiments. Finally, a inverse power model for life prediction was derived from experimental results. The predicted life was compared with the actual test life for validation.

Property Estimation of Functionally Graded Materials Between M2 Tool Steel and Cu Fabricated by Powder Metallurgy

의 형상적응형 냉각회 Key Words: Functionally Graded Materials(FGMs, 기능성 경사 복합재), M2 Tool Steel(M2 공구강), Powder Metallurgy(분말야금), Thermal Conductivity(열전도도), Coefficient of Thermal Expansion(CTE,열팽창계수) 초록: 형상적응형 냉각회로를 구비한 사출금형 및 히트 싱크를 가지는 절삭공구 (혹은 냉각 장치) 등과 같은 많은 응용 분야에서 기능성 경사 복합재(FGM)를 사용하여 필요한 강성을 약화시키지 않으면서 열전도 특성을 향상 시킬 수 있을 것으로 기대된다. 본 논문에서는 M2 공구강과 Cu 간의 FGM 히트 싱크를 가지는 절삭 공구 제작을 위한 기초연구로, M2 와 Cu 를 각각 100:0, 80:20, 60:40, 40:60, 20:80, 0:100 wt% 비율로 사전에 혼합한 금속분말을 분말야금법으로 가압성형 및 소결 제작 하였다. 각 시편의 단면을 광학현미경으로 관찰하여 소결 상태를 분석하였으며, 열전도도, 비열 및 열팽창계수 등 열전달 관련 물성을 측정하고 분석하였다. Abstract: The use of functionally graded materials (FGMs) may enhance thermal conductivity without reducing the desired strength in many applications such as injection molds embedding conformal cooling channels and cutting tools with heat sinks (or cooling devices). As a fundamental study for cutting tools having FGM heat sinks between M2 tool steel and Cu, six FGM specimens (M2 and Cu powders were premixed such that the relative compositions of M2 and Cu were 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100 wt%) were fabricated by powder metallurgy in this study. The cross sections of these specimens were observed by optical microscopy, and then the material properties (such as thermal conductivity, specific heat, and coefficient of thermal expansion) related to heat transfer were measured and analyzed.