Jo Sep 19, 2023
Ti2AlNb-based alloys are under continuous development as promising candidates for advanced automotive and aerospace applications due to their good creep resistance, low density and optimal balance of strength and elongation at the elevated temperature.
Ti2AlNb-based alloys can be deformed not only into sheet or rod, but also into complex types of parts such as turbine engine blades. However, deformation processing of Ti2AlNb-based alloys is inevitably performed at high temperatures because of their limited plasticity at room temperature.
Therefore, investigation into the flow behavior and reasonable deformation criteria of Ti2AlNb-based alloys is quite crucial for design and control of industrial hot working processes.
In recent years, some scholars have developed constitutive models and processing maps of as-cast Ti2AlNb-based alloys and powder metallurgy Ti2AlNb-based alloys. These powder metallurgy Ti2AlNb-based alloys showed microstructure with coarse grain due to higher sintering temperature and longer sintering time.
Unfortunately, few researchers have endeavored to construct the constitutive model and processing maps of the fine-grained (FG) Ti-22Al-25Nb alloy by mechanical alloying (MA) and subsequent spark plasma sintering (SPS) using elemental powders.
Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has established suitable constitutive models to predict the high–temperature flow behavior of FG Ti–22Al–25Nb alloy fabricated by MA and subsequent SPS. He has also developed processing maps to describe the reasonable deformation criteria for hot working of FG Ti–22Al–25Nb alloy.
First, in order to obtain true stress-strain curves of FG Ti-22Al-25Nb alloy, isothermal uniaxial compression tests were conducted at different deformation conditions of 950 – 1 070℃ and 0.001 – 1 s-1.
Second, constitutive models for FG Ti–22Al–25Nb alloy were developed by using the modified Johnson–Cook model and the strain–compensated Arrhenius type model based on the corrected experimental data.
Finally, DMM–based processing maps were constructed to determine reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy.
Conclusively, the following conclusions were drawn:
(i) The modified Johnson–Cook model for the FG Ti–22Al–25Nb alloy showed good prediction accuracy at the reference temperature and strain rate. However, the predictability was lowered in other deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 9.78 % and 0.985 8, respectively.
(ii) The strain–compensated Arrhenius type model for FG Ti–22Al–25Nb alloy exhibited excellent predictability under most deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 4.19 % and 0.992 7, respectively. Compared with the modified Johnson–Cook model, the strain–compensated Arrhenius type model is more suitable for describing the high–temperature flow behavior of FG Ti–22Al–25Nb alloy.
(iii) Based on the processing maps of FG Ti–22Al–25Nb alloy, flow instability was predicted to occur at temperatures lower than 990℃ and strain rates higher than 0.1s–1. The reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy were in the temperature range of 1 020 – 1 070℃ and the strain rate range of 0.001 – 0.32s–1.
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Jo Sep 18, 2023
A sharp rise in electric power transmitting capacity and distance makes it important to ensure the security of power transmission lines.
The major cause of most cutting accidents of transmission lines is vibration by wind. Transmission lines vibrate continuously owing to the Kalman effect by wind. This effect generates endurance fracture in the cross sections of transmission lines, resulting in the decrease in the life time of transmission lines.
Therefore, a lot of researchers installed anti-vibrators on transmission lines to reduce vibration amplitude largely.
Recent researches are not enough with vibration of a tower-transmission line-dampers system as a whole.
Choe Sun Bok, a researcher at the Faculty of Mechanical Science and Technology, has studied an analytical method for determining the correct solution for the forced vibration of a power transmission line-anti-vibrators system with flexible supports by wind, regarding a steel tower as a flexible body. On this basis, she has performed numerical calculations and compared the results with experimental values to verify the validity of the presented method.
Her method can be applied to determination of design parameters and installing places of anti-vibrators.
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Jo Sep 17, 2023
Jong Kum Sil, a lecturer at the Faculty of Distance Education, has developed an online education aid and consultation system for assessing course designs from evaluation of students' cognition, real-time check and virtual board check of their drawings, and application program practice guidance by screen sharing.
As the whole course of evaluation is done via a real-time dialogue between a lecturer and a student online and real-time drawing check and screen sharing improve the effectiveness of evaluation, the system will make a contribution to the improvement of practical education.
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Jo Sep 16, 2023
A research team led by Ri Jin Gwang, a researcher at the Faculty of Distance Education, has made a study of face anti-spoofing by spoof cues learning to give a solution to the problems arising in face authentication, one of the common problems in distance education.
Generally, typical face anti-spoofing methods used intrinsic features of attack mediums such as a printed photo or video replay, and selecting discriminative features between live samples and spoof ones became automatic with the development of deep learning. As a result, spoofing detection became a binary classification problem for distinguishing between false and true.
However, the existing methods might not get enough discriminative features or tend to overfit predefined datasets, which leads to some problems with generalization. Limited generalization capacity of FAS is attributable to the diversity of spoof samples including unknown ones.
Therefore, the research team has introduced spoof cues to improve the generalization capacity of a learning model and the correctness of FAS.
As the proposed method has been introduced for learner identification of distance education on the mobile network, face anti-spoofing caused by printed photos or video replay is not a problem any more. It means the practicability and scientific accuracy of online education are fully guaranteed.
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Jo Sep 15, 2023
A research team led by Song Yong Dok, a researcher at the Faculty of Distance Education, has developed a new analysis system based on face-image recognition for evaluating students’ study attitude.
The system can analyze students’ emotions by recognizing, detecting and verifying their face-images. Emotion affects study attitude to such a great degree that emotion analysis plays a big part in analysis of study attitude.
The system consists of image input module, image recognition module, emotion detection module, expression detection module, attitude analysis module, etc. In the image input module, images or real-time images taken by an IP camera and a Web camera are fed. In the image recognition module, one or more students are identified and feature points on their faces are extracted. In the emotion detection module, students’ main feelings are detected in 8 kinds including happy, sad and surprised and they are presented on value-charts. In the expression detection module, expressions like smile, laugh, blink, opened mouth, frowning, etc. are detected.
This system can put education management on a scientific basis by analyzing students’ study-attitude and automatically checking their attendance.
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Jo Sep 14, 2023
Mun Chol Su, a researcher at the Faculty of Metal Engineering, has developed an analytical method and analytical instrument for analyzing rapidly and accurately group of pitch binder that used to take a long time.
Rapid and accurate evaluation of the quality of pitch binder used for electrode production is important for scientific production and active and timely quality control of products.
Group composition is a very important parameter for evaluating the properties of binders used for electrode production and for determining the appropriate process conditions, but long analysis time limits their wide application on production sites.
In order to reduce the time spent on group composition analysis, he employed a reduced pressure filtration method, and a method of simultaneous analysis with different solvents for one sample, thus greatly shortening the analysis time.
Then, he designed and manufactured an instrument for rapid analysis of the group composition to minimize errors made by unskilled analyzers and to prevent environmental pollution inside analytical chambers by organic solvents.
The instrument consists of a thermostatic dissolver, a suction filter, and an exhaust fan, and temperature measurement and time adjustment in each section are automatically controlled by a microprocessor.
The thermostatic dissolver is designed so that solvents and samples can be heated in a water bath at a specified temperature, and temperature measurement and temperature control are carried out by an intelligent PID thermostat.
The suction filter is designed to allow rapid filtration of dissolved pitch samples and solutions in the thermostatic dissolver under reduced pressure.
The exhaust fan is placed inside the instrument to discharge volatile organic solvent to the outside and to work all the time of analysis.
Now, it only takes about five hours for one analyzer to perform a group analysis that used to take over 50 hours, which means reduction in the consumption of labour and power.
The analytical method and instrument might help the factories and enterprises that use pitch as binder quickly and actively control their production processes in accordance with the changing conditions of raw material supply.
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