Jo Mar 13, 2024
Laser cutting, which is one of the laser machining technologies, has been widely used in industrial production due to its good processing flexibility, high quality, high cutting speed, high precision and high production rate.
In order to ensure high quality in laser cutting, not only the appropriate process parameters (laser power, focal position, cutting speed, type and pressure of assist gas, etc.) must be set, but the stand-off distance between a work piece and a cutting head has to be kept at a fixed distance.
An Chol Min, a section head at the Faculty of Physical Engineering, has built a precise mathematical model of a truncated cone-shaped capacitive sensor based on the theory related to the electrostatic field and the properties of conductors and dielectrics in the field. He has also proposed limitations and suppositions to get a unique solution of the capacitance while analyzing the model by MATLAB, and verified the correctness of the model through experiments.
The theoretical analysis and the experiments show that the truncated cone sensor has much higher sensitivity than a parallel-plate sensor in laser cutting.
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Jo Mar 12, 2024
Electron density is a key parameter in designing and constructing a pumping system for high power transversely excited atmospheric (TEA) pressure CO2 laser. It is necessary to ensure uniform volumetric discharge in order to enhance the output power and the efficiency of gas laser. It is, however, very hard for TEA-CO2 laser to achieve electron density of more than 1014cm-3 necessary for volumetric discharge as the pressure of active gas of the laser is higher than atmospheric pressure.
Recent-developed external pre-ionization systems such as those based on high-energy field emission, ultraviolet or X-ray have enabled atmospheric pressure volumetric discharge in a short time of about 1㎲ and raised the output power of pulsed laser up to the level of GW. The main factor of getting higher power (over tens of GW) is to ensure that electron avalanche caused by inelastic collisions between CO2 (or N2 and He) molecule and cold electrons with energy of hundreds of keV is spatially uniform, and that non-self-sustained main discharge is conducted as a uniform volumetric one. It means that finding out the characteristics of change of electron density is of primary importance in developing high power TEA-CO2 laser.
The model of electron density for uniform volumetric discharge is a nonlinear model describing microphysical processes such as pre-ionization, avalanche ionization, recombination, attachment and so on. So far, there have been no general solutions of this model but only special solutions for some extreme cases.
Yun Tu Hon, a researcher at the Faculty of Physical Engineering, has proposed a method of general solutions for the nonlinear model and verified the accuracy and generality of his method by comparing the results with the data reported in the literature. He also reported some new special solutions unknown in the literature and the experimental results for them.
The proposed method will prove to be valuable for high power TEA-CO2 laser of hundreds of MW.
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Jo Mar 10, 2024
“Muyedobothongji”, printed from wood blocks in 1790, is an illustrated book of martial arts by Ri Tok Mu, Pak Je Ga, Paek Tong Su et al. where the traditional martial arts of our nation are comprehensively systemized.
The book illustrates 24 kinds of martial arts.
It is surmised that the illustrations were offered by Kim Hong Do, a famous painter of those days. Preceding books on martial arts were “Muyebo” descriptive of six kinds of martial arts written by Han Kyo in the sixteenth century and a book with addition of twelve more kinds to the six martial arts in the mid-eighteenth century.
“Muyedobothongji” has another addition of six kinds to them.
The book illustrates detailed moves of several fighting skills like swordsmanship, spearsmanship, archery, etc. on the ground and on a horse.
The book has a full collection of data related to Korean boxing that started in Ancient Joson, was improved in Koguryo age and was completed during the Koryo and feudal Joson dynasties.
This book is of weighty importance in showing the long history and superiority of the national martial arts created in our country, the cradle of Taekwon-Do.
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Jo Mar 7, 2024
Submerged arc welding process with high productivity has been one of the widely-used surfacing methods for recycling worn components or fabricating bimetal. Surfacing with a lay-down plate electrode is a new variation of submerged arc surfacing, in which metal is deposited on the surface of a workpiece by arc heat between the workpiece and a plate laid down on it.
Both direct and alternate current can be used as welding power source. Surfacing with a lay-down plate shows the possibility of reducing the cost for restoring worn components like large-scale straight teeth gears with large module, and of improving the productivity and quality of surfacing.
Temperature distribution resulted by spontaneous movement of electric arc has a big effect on the stability and quality of surfacing with a lay-down plate. However, to the best of our knowledge, there have not been many discussions regarding the spontaneous movement of electric arc and the analysis of temperature field during surfacing with a lay-down plate electrode.
In order to make an optimal surfacing process condition, Pak Sang Yun, a researcher at the Faculty of Materials Science and Technology, has modeled the spontaneous movement orbit of arc during surfacing with a lay-down plate electrode and investigated its temperature distribution by ANSYS.
He has drawn the following conclusions.
First, the periodic and stable movement of electric arc, which is self-controlled in the channel during surfacing, results from periodic variation of forces acting on electric arc.
Second, the dimension and shape of the weld pool varies with time, but the boundary of solidus temperature is constant regardless of time. Thus, the temperature field remains in quasi-stationary state during surfacing with a lay-down plate electrode.
Third, the distribution of quasi-stationary temperature field depends on build-up technical conditions and the width of plate electrodes.
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Jo Mar 6, 2024
Our people have regarded dressing themselves up as decorum for themselves and others from olden times, paying special attention to it.
When they needed to go out, they never forgot to examine their clothing, and they never loosed their breast-ties however hot the weather was.
In the house, too, they kept their clothes tidy, and they regarded it as politeness to tidy themselves when they were expecting seniors or guests.
When they visited other people’s houses, they shook dust or anything dirty away from their clothing, tidied them up, and placed their shoes squarely before going in.
However busy and tired they were, they always washed their clothes well regularly and ironed wrinkles out of them.
In addition, they were dressed appropriately in different circumstances, i.e. seasons, ages, happy and sad occasions, etc.
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Jo Mar 5, 2024
The Monument to the Great Victory in the Northern Area, a historic relic in Rimmyong ri, Kim Chaek City, was erected in 1708 in commemoration of the signal victory that Jong Mun Bu’s Volunteer Army won by annihilating the Japanese invaders who had made an inroad into the districts of Hamgyong Province during the Japanese invasion of Korea in 1592.
The first part of the inscription defines the position of the struggle of Jong Mun Bu’s Volunteer Army as a deed no less significant than the battles on Hansan Island or those in the Fort on Mt. Haengju and Yonan Fort, and mentions its organization and major battles. The main part says that volunteers who rose up against the aggression of Japanese invaders fought bravely, and thus the enemies were expelled from the northern part of the country and the people there could do farming safely, and that their exploits should be remembered forever.
The monument is a precious historical relic of our country that has a great significance in studying the history of our ancestors who courageously fought against the invasion of the Japanese foes and in educating our people in the Korean-nation-first spirit.
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