Jo Aug 25, 2024
During hot working, material exhibits a complex nonlinear relationship between flow stress and thermo-mechanical processing parameters, due to the simultaneous occurrence of work hardening and softening. In order to express more accurately the nonlinear relationship between flow stress and thermo-mechanical processing parameters, a large number of constitutive models have been developed, which can be divided into three categories: a phenomenological constitutive model, a physically-based model and an artificial neural network model.
Phenomenological constitutive models are widely used to predict the hot deformation behavior of metallic materials including alloy steels and titanium alloys, due to their simple equations and small computational quantities. Physically-based constitutive models consider not only the thermo-mechanical processing parameters, but also the physical mechanism such as dislocation movement and thermal activation during hot working process. Compared with phenomenological constitutive models, physically-based models are more complex to establish. However, due to their high prediction accuracy, physically-based models are widely used in many commercial finite element analysis programs.
The Ti-6Al-2Sn-2Zr-3Mo-1.5Cr-2Nb (TC21) alloy is a 1 100MPa damage tolerance titanium alloy, with ultimate tensile strength of 1 130MPa, yield strength of 1 020MPa, elongation of 12%, toughness value of 80MPa·m1/2, and crack extension rate of 1.47×10-5 mm·cycle-1. Due to a good combination of strength and toughness, the application value of TC21 alloy is much higher than that of TC4 alloy.
So far, there have been many studies on the hot deformation behavior of TC21 alloy. However, few studies have been conducted or reported on other constitutive models for TC21 alloy including physically-based constitutive models.
Pak Hun, a student at the Faculty of Materials Science and Technology, has constructed phenomenological and physically-based models for predicting more accurately the hot deformation behavior of TC21 alloy, and compared their predictability. To this end, he carried out isothermal uniaxial compression tests at different strain rates in α + β two-phase and β phase fields. Based on the experimental data (flow stress curves), he developed a modified Johnson-Cook (m-JC) model, a strain-compensated Arrhenius type (sc-AT) model and a modified Zerilli-Armstrong (m-ZA) model for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy. In addition, he compared the prediction accuracies of the three constitutive models using standard statistical parameters.
The results showed that the proposed m-ZA model is the preferred constitutive model to predict the hot deformation behavior of Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy.
You can find the details in his paper “Comparisons of phenomenological and physically-based constitutive models for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy” in “Applied Physics” (SCI).
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Jo Aug 23, 2024
Today, electricity is not only the main power of industry but also the main energy source indispensable for the people’s lives, and the demand is in a steady rise. Therefore, it is important for the power industry to reliably ensure the operation of power systems in a scientific and technological way.
The trend of global power system development is to establish and operate a flexible AC transmission system (FACTS) that can rapidly control the main characteristics of power system like voltage, frequency, active and reactive power, etc.
One of the important issues in establishing and operating such FACTS is to reduce the total harmonic distortion rate in power system equipment including reactive power compensators, for power quality.
Ri Tong Chol, a researcher at the Faculty of Electrical Engineering, has established a fuzzy logic controller for controlling the DC bus voltage of the converter to improve the response in a distribution grid static synchronous compensator (DSTATCOM) which is widely used in FACTS and to ensure normal operations of the compensator under asymmetric conditions and grid voltage variations, and verified its effectiveness through computer simulation tests.
For effective operation of DSTATCOM, the DC-side capacitor voltage of the voltage-type converter should be kept constant, and for this purpose, a fuzzy PI controller was used.
The simulation model of DSTATCOM was designed in a three-phase distribution system using SIMULINK tool in MATLAB environment and the performance of DSTATCOM with fuzzy logic control algorithms was simulated and analyzed under distorted PCC voltage due to unbalanced nonlinear load.
The harmonic distortion factors (THD) of the power supply current, load current and PCC voltage on A were measured to be 3.6%, 26.0% and 4.4%, respectively, using the proposed fuzzy logic controller of DSTATCOM.
The results confirmed that the controller performance improved with a power supply current of 3.6%, while the THD of PCC voltage was 4.4%.
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Jo Aug 22, 2024
While water consumption increases steadily due to various socio-economic factors such as population concentration in cities, increase in industrial wastewater along with the rapid development of industry and the improvement of people’s living and cultural level, a large amount of water resources are contaminated, so it is vital to increase the amount and efficiency of wastewater treatment at sewage treatment plants.
The trend in the development of global wastewater treatment processes is the introduction of a large number of aeration facilities that provide biochemical oxidation of organic pollutants to the plant. The problem is that with the increase of sewage treatment plants, its operating cost, especially the power consumption of the fan, increases. Aeration is one of the most energy-intensive processes in the wastewater treatment plants and any improvement in it is likely to enhance the efficiency of the overall process.
Of course, 15~20% of electric power consumption can be reduced by controlling the blast volume according to the influx of sewage and its composition, but upgrading of the aerating facilities for improvement of the oxidation effect is more stable and effective.
Ji Chol Hyok, a researcher at the Nano Physics Engineering Institute, has proposed a sewage clarification method by micro-bubble aeration for improving the clarification efficiency and reducing its cost, and compared its effectiveness with conventional ones, thus demonstrating its superiority.
He chose an 80㎥ tank of a pig farm as an experimental reaction tank and measured some sewage treatment parameters.
The results showed that the proposed method ensured up to 90.8% of removal efficiency of COD (chemical oxygen demand) while the conventional ones guaranteed only 86.4%, and that the overall removal efficiency is over 20% higher than bottom aeration when the temperature is low.
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Jo Aug 20, 2024
The main purpose of laser surface melting is to obtain great strength of alloy surface and to ensure high productivity. The laser surface melting of AZ31B Mg alloy is very important in determining the microstructures and strength of alloy surface. The laser power and laser scanning speed in the laser surface melting play important roles in the temperature change and the morphology of the melting region. However, massive amounts of data on laser surface melting process take a lot of time and cost for detailed experiments of investigation.
To date, there has been an impressive amount of research on this topic, especially to investigate the melting flow and its thermal transfer by laser.
Kim Yong Il, a researcher at the Nano Physics Engineering Institute, has estimated the effects of laser power and laser scanning speed on the temperature distribution of AZ31B Mg alloy using COMSOL Multiphysics 5.3.
Firstly, based on finite element analysis software, he built a finite element model of laser surface melting Mg alloy. Then, he conducted a numerical computation with the variables used in the experiments of previous works. The numerical computation by simulation was done with different laser powers at the laser scanning speed of 360mm/min, and with different laser scanning speeds at the laser power of 2 000W.
The simulation results seem to be in good agreement with the experimental results based on infrared temperature measurement.
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Jo Aug 18, 2024
Determination and description problems are two basic problems of Formal Concept Analysis (FCA). Since most methods of determining the concept lattice are based on generation of non-neighbor concepts, the process of analyzing the neighboring relation between generated concepts is required to solve the description problem.
Pak Chol Hong, a researcher at the Faculty of Applied Mathematics, has proposed some new efficient algorithms for simultaneously describing the concept lattice and its hierarchy-matrix without this process. The hierarchy-matrix is a successful description of the concept lattice, by which any software can autonomously understand the information of hierarchy of the concepts. The focus of his algorithms is on generation of neighbor concepts tested for canonicity and registering the neighbor information in the hierarchy-matrix sequentially.
He drew the following conclusions.
First, the concepts of subcontexts induced by a given concept are lower (or upper) ones of the concept and all lower (or upper) neighbor concepts of the concept are denoted by the antitone Galois connections on the subcontexts.
Second, all lower (or upper) neighbor concepts of a given concept are generated by the antitone Galois connections on the subcontexts and restricted by the rank of the subcontex-matrices.
Third, the key aspects of the efficiency of the concept lattice and its hierarchy-matrix based on the generation of lower neighbor concepts (BCLHMLN) and the concept lattice and its hierarchy-matrix based on the generation of upper neighbor concepts (BCLHMUN) are generation of neighbor concepts on the subcontext, the equivalence class of the object or the attribute with respect to adjoin mappings of Galois connections, the design of canonicity test and the utilization of the histories of generated lower (or upper) concepts.
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Jo Aug 16, 2024
In recent years, a number of modal identification methods based on continuous wavelet transform (CWT) have been developed. The time variation of the instantaneous amplitude and phase of each mode component within measured signals can be considered by time-frequency representation. In the time-frequency plane, wavelet analysis decomposes the measured signals into a series of curves called ridges which directly express the amplitude and phase of each mode component within the measured signals. Thus, by the extraction of ridges and the value of CWT along the ridges, modal parameters are identified from the measured signals. The measured signals are free-decay responses of mechanical structures which are processed by time-frequency transform. In this process, the challenge is to determine ridges exactly. When the frequency window of CWT includes closely spaced natural frequencies, the ridge is distorted and a pseudo mode is generated.
Existing methods for modal identification by CWT give correct results when the frequency window includes one natural frequency. Thus, it is necessary to build a theoretical basis for correctly determining closely spaced natural frequencies by considering the distortion of ridges and determining the reasonable bandwidth when the frequency window includes two closely spaced natural frequencies.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a formula for mathematically considering the distortion of characteristics of a ridge when the frequency window includes two close mode components, and conducted a numerical simulation on MATLAB to verify it.
The simulation verified the accuracy of the proposed formula.
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