Jo Oct 13, 2025
Welding has been widely used in many fields of industry. Its advantages include high efficiency of joining, simple operation, good flexibility and low cost. However, uneven temperature distribution inevitably exists during welding. Such uneven temperature distribution causes rapid thermal expansion, which leads to heterogeneous plastic deformation by thermal constriction in and near the welding joint. Thus, residual stress appears in welding structure after cooling.
In order to accurately predict welding residual stress and deformation, welding temperature distribution should be simulated properly. What is important in welding simulation is to apply suitable heat source models according to the welding methods. The models include spot heat source model, line heat source model (HSM), surface heat source model and volume heat source model. As heat is transferred to workpieces in the form of volume in most welding methods, volume HSM is most widely used. Volume HSM is classified into semi-sphere HSM, elliptical heat source model and double-elliptical heat source model. At present, double-elliptical model is accepted as the most appropriate heat source model for gas metal arc welding (GMAW) with steep temperature slope at the front section of the welding pool and smooth slope at the rear section.
Cha Kwang Jin, a researcher at the Faculty of Materials Science and Technology, has performed a welding temperature simulation by FEM and predicted temperature distribution while varying welding parameters such as welding current.
For the simulation, he employed ANSYS software widely used for welding simulations across the world that offers the most approximate parameters to the desired result by varying several parameters of its own optimization module.
The FEM analysis results and experiment results of welding show that the analysis result is accurate when Gauss-distributed double-elliptical heat source for GMAW process simulation is used.
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Jo Oct 9, 2025
Recently, attempts have been made to use fiber-reinforced plastic composites as aircraft structural materials, but aluminum alloys still have 70% utilization as airborne fuselage materials. In particular, high-alloy duralumin Du16 has been used as the main material for manufacturing aircraft fuselage surfaces and blades.
The body material of an airplane varies in its nature with the speed of flight, so it is very important to determine whether a given alloy can be used as a body material for subsonic and supersonic aircrafts.
Previous studies show that the grain growth of duralumin alloy at annealing temperatures above 300 °C leads to poor mechanical properties, but they failed to deal with annealing at temperatures below that. What is more, they did not cover the changes in the texture and property with temperature when the body of an airplane is heated during the flight.
Therefore, Ri Hyon Song, a researcher at the Faculty of Materials Science and Technology, has measured the fuselage heating temperature range by means of simulation analysis and investigated the changes in the texture and property with annealing temperature.
First, he designed a model of a supersonic aircraft and conducted a simulation analysis of the temperature distribution of the body surface according to flight speed. Then, he performed hardness and microstructure observation experiments.
The results showed that Du16 alloy does not degrade its hardness at 100℃ for 8 hours of annealing, and above 300℃, the hardness is gradually degraded with time.
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Jo Oct 6, 2025
Electrospinning is widely used as a versatile and convenient method for producing nanofibers. However, the drawback of this method is its low productivity. To overcome this drawback, various kinds of electrospinning methods have been developed. They are divided into two broad sorts. One is multi-needle electrospinning, which uses a spinneret consisting of many needles, and the other is needleless electrospinning. Multi-needle electrospinning finds its wide application in the field of advanced materials as it enables controlling the properties of nanofibers as well as producing special kinds of nanofibers such as core-shell nanofibers.
In multi-needle electrospinning, strong electrical interaction between the spinneret needles caused by the increased number of nozzles breaks the uniformity of electric field at the spinneret needle tip, which consequently destabilizes the flow of polymer jet and degrades the quality of fibers. Therefore, keeping the electric field uniform at the spinneret needle tip is a key factor for obtaining good-quality nanofibers.
There have been many studies so far to improve the uniformity of electric field. However, few of them dealt with optimal structure parameters to maximize the uniformity of electric field at the spinneret needle tip. In addition, there are almost no reports on the investigation of uniformity of electric field while varying the parameters of a dielectric medium such as the length, the diameter and the relative permittivity when it is inserted between spinneret needles.
Jang Kum Chol, a researcher at the Institute of Nano Science and Technology, has investigated the effects of geometric parameters of a dielectric medium around the spinneret needle on electric field uniformity in order to obtain uniform electric field distribution at the spinneret needle tip.
He simulated the electric field distribution using COMSOL Multiphysics 5.4 and optimized the dielectric medium parameters to improve the electric field uniformity by using Taguchi method.
As a result, he has confirmed that the electric field uniformity is the best when the diameter, length, arrangement distance and relative permittivity of a dielectric medium are 1.5mm, 5mm, 4mm and 2.3, respectively.
After optimization, the uniformity was over 1.27 times higher than before optimization.
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Jo Oct 2, 2025
Research on temperature field is of great importance for hot-pressing processes of ceramic materials. It affects both the uniformity of materials and the properties of ceramic products. Additionally, non-uniformity causes formation of thermal residual stresses during sintering.
Previous works mostly focused on the sintering of traditional ceramics and very few of them aimed at the hot pressing of modern ceramic materials.
Mun Hyok Chol, a researcher at the Faculty of Chemical Engineering, has proposed a heat transfer model for a system comprised of the mixture of Al2O3 and TiB2 and a graphite mould. Then, he has established a new method of determining the coefficient of heat transmission of ceramic composite by taking into account the change of porosity with sintering temperature, and simulated the temperature field using the COMSOL Multiphysics software.
He set the most suitable heating rate as 0.5K/s by analyzing both material properties and manufacturing efficiency.
The simulation result has shown increased initial relative density of blanks and more uniform temperature distribution of the system during hot pressing, which leads to further improvement of the physical and mechanical properties of ceramic composite materials.
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Jo Sep 30, 2025
On-demand charging schemes have been recently proposed to make efficient charging schedules of mobile chargers by introducing MCDM methods in wireless rechargeable sensor networks (WRSNs). However, most of the existing schemes use paired ratio scale (PRS) for weight assignment of multiple criteria to exaggerate the actual paired difference between them, and in the case of using FCNP of paired interval scale (PIS) for weight assignment of multiple criteria, weight compensation is not considered.
What is more, it is still unknown which is the best method for integrating FCNP with several MCDM approaches.
Rim Ju Song, a student at the Faculty of Communications, with the help of Ri Man Gun, an institute head of the same faculty, has proposed novel CS methods by integrated FCNP-VWA-MCDM(i) called FCVM(i) for solving all these problems.
The proposed methods first assign the weights to multiple criteria discriminating charging request node (cRNs) using FCNP and make compensation of them to be relatively exact weights with VWA. Then, on the basis of these weights, MCDM(i) is used to select the most proper next charging position. Recharging schedule is drawn up in this way and, at the same time, reasonable partial charging time at the selected charging locations are determined using the assigned weights with FCNP-VWA.
The extended experimental results prove that the FCVM(1) using TOPSIS gives the best performance among FCVM(i) methods.
You can find the details in his paper “FCVM(i): Integrated FCNP-VWA-MCDM(i) Methods for On-Demand Charging Scheduling in WRSNs” in “Journal of Data Science and Intelligent Systems” (SCI).
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Jo Sep 29, 2025
Fractal interpolation function (FIF) is a function which interpolates a given data set and whose graph is a fractal set. In 1986, M. F. Barnsley introduced the FIF using an iterated function system (IFS). Ever since, many researchers have studied the FIFs and applied them to a lot of scientific fields such as computer graphics, function approximation, signal processing, metallurgy, earth science, surface physics, geography, geology, medical science, chemistry, etc.
Rakotch contraction is the generalization of Banach contraction, which implies that the use of Rakotch’s fixed point theorem enables modeling of more objects than the use of Banach’s fixed point theorem. Moreover, hidden variable recurrent fractal interpolation functions (HVRFIFs) with Hölder function factors are more general than fractal interpolation functions (FIFs), recurrent FIFs and hidden variable FIFs with Lipschitz function factors.
Ro Chung Il, a researcher at the Faculty of Applied Mathematics, has demonstrated that HVRFIFs can be constructed using the Rakotch’s fixed point theorem, and investigated the analytical and geometric properties of those HVRFIFs.
Firstly, he constructed nonlinear hidden variable recurrent fractal interpolation functions with Hölder function factors on the basis of a given data set using Rakotch contractions. Next, he analyzed the smoothness of the HVRFIFs and showed that they are stable for the small perturbations of the given data. Finally, he obtained the lower and upper bounds for their box-counting dimensions.
If further information is needed, please refer to his paper “Analytical properties and the box-counting dimension of nonlinear hidden variable recurrent fractal interpolation functions constructed by using Rakotch’s fixed point theorem” in “Applied Mathematics and Computation” (SCI).
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