Jo Sep 25, 2023
Mechanical power loss in pneumatic hammers is caused by the friction between parts in relative motion, and wear is among failure mechanisms of the top hammer. Therefore, it is important for a high performance and a longer service life of pneumatic hammers to reduce the friction force between parts in relative motion.
Yang Un Hyok, a section head at the Faculty of Mining Engineering, has presented a novel approach to quantitatively determine the friction force and considered it in the simulation model of a pneumatic hammer.
First, the friction force between a piston and a cylinder in a small pneumatic hammer was measured using an experimental setup at different inlet pressures. Then, numerical analysis of a small pneumatic hammer performance was performed by a model with the friction force in consideration using AMESim software.
The result showed that the friction force was about 0.8N under the horizontal installation when there was no pressure supply, but it increased significantly, 20 times greater than that without pressure supply, due to aerodynamic action by compressed air leaking from the annular gaps between the cylindrical matching surfaces of the components. In addition, it indicated that friction increased from 10.27 to 16.7 N due to the increase in inlet pressure and mechanical power loss in the pneumatic hammer was about 10% of impact energy.
The proposed approach could significantly reduce errors between the simulated and the measured values for the impact energy due to the ignoring of friction force.
The details of this are found in his paper “Numerical analysis of a small pneumatic hammer performance based on evaluation method of friction force” in “Shock and vibration” (SCI).
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Jo Sep 24, 2023
Offshore structures such as oil platforms, wind turbines and marine renewable energy devices are constantly exposed to a harsher environment, which requires a deep understanding of the wave interaction with marine structures such as bottom-mounted ones to make them reliable, safe and cost-effective.
Rim Un Ryong, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has considered the interaction between a monochromatic incident wave and multiple bodies mounted on an undulated seabed using an exact Dirichlet-to-Neumann (DtN) boundary condition on an artificial circular cylindrical surface by which the fluid domain is divided into an interior subdomain and an exterior one.
The proposed method was validated by precedent results with a bottom-mounted circular cylinder and showed good agreement. Then, it was extended to the cases of a single rounded-rectangular cylinder, a 2*2 array and a 2*1 array with a paraboloidal shoal.
The proposed model can be applied to the study of the wave interaction with multiple bottom-mounted marine structures with irregular configurations whether the seabed is flat or undulated.
If further information is needed, please refer to his paper “Wave interaction with multiple bodies bottom-mounted on an undulated seabed using an exact DtN artificial boundary condition” in “Journal of Ocean Engineering and Marine Energy” (EI).
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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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