Jo Jun 22, 2022
Cast aluminum alloys are quite widespread and they find more and more applications in modern industry such as aerospace, navigation and railway transport thanks to their good mechanical property, castability, corrosion resistance, etc.
Quality indices are a measure to quantify quality of the alloy on the basis of its mechanical properties.
The indices are Q, QR, QC, QD(Q0), QE, etc. Each one has its own unique features and individual one cannot display the overall performance of cast aluminium alloys.
The most representative indices to evaluate their overall performance can be chosen by two methods ― method of correlation analysis between quality indexes and mechanical properties and method of correlation analysis and rank deviation analysis between overall performance scores obtained from quality indices and MCDM method.
The analysis result showed that Q0 proved to be the most representative one of all the indices Q, QR, QC and Q0, and that Q0 can be used instead of MCDMs for evaluation of overall performance of cast aluminum alloys and selection of optimal cast aluminum alloys.
Detailed information about this can be found in the paper “ Method to evaluate overall performance of cast aluminum alloys using most representative quality index instead of MCDM method” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Jounal “International Journal of Metalcasting”.
...
Jo Jun 14, 2022
Kim Ryul, a researcher at the Faculty of Electrical Engineering, has realized selection of crack insulators from a pile of 10kV pin insulators by data processing of thermal shock sample breaking test and a corona short-circuit nondestructive test device.
Compared with previous selection methods by electric penetration breakdown test, this method provides more safety and lower production cost. In addition, as it uses nondestructive electric tests to sort out crack insulators, the reliability of production pile of insulators can be further improved.
This method is constructed with several stages: determination of size of 10kV pin insulator piles and samples, thermal shock sample destructive test, state decision of pass or pass failure of production piles by processing test data, determination of acceptance probability by quality control software and of the number of faulty insulators, and selection of crack pin insulators by the corona short-circuit nondestructive selection device.
This thermal shock test costs less and provides 1.4 times higher accuracy than the previous electric penetration breakdown test. To estimate cracks in 10kV pin insulator piles, previous binomial distribution was replaced with noncentral t-distribution, which reduced the number of samples to half and improved the accuracy of acceptance probability of piles and the calculation of the number of crack insulators by 1.2 times. Instead of electric penetration breakdown test devices, corona nondestructive test devices were used, which made it possible to select 100% of crack insulators from passed piles.
...
Jo May 27, 2022
NTC (Negative Temperature Coefficient) thermistors are used in many electrical and electronic products including mobile phones, PCs and their peripherals, secondary batteries, LCDs, etc. for measurement, adjustment, compensation and time delay of their temperature, and voltage adjustment and noise reduction.
For most temperature sensing applications the NTC thermistors are made of spinel manganite.
Thick film thermistors are of several types such as sandwich, multilayer, segmented and interdigitated, and the methods of calculating resistivity and B value for these types are different.
Multilayer NTC thermistors are made up of inner metal electrodes and paralleled layers of NTC thermistor ceramics. In the process of manufacturing, stresses are produced between the components of multilayer NTC thermistors due to mechanical, thermal and electrical loads.
These stresses in multilayer electronic components can be simulated through the finite element method (FEM) and calculated by analytical models and measured by X-ray diffraction (XRD).
Compared to other methods, FEM proves to be more convenient for analysis of more detailed overall stresses in multilayer electronic components under various loads.
Up to now, scientists have often used two-dimensional models for considering the details of multilayer thermistors, but these models cannot fully display the distribution of stresses of multilayer thermistors.
Therefore, 3D finite element analyses (FEA) are more suitable for further study into the effects of the design parameters of multilayer thermistors on the stresses.
The research results show that manipulation of the length of the lateral margin most significantly influences the maximum principal stress experienced in multilayer thermistors during soldering process. In addition, the number of inner electrodes also contributes to the tensile stress that occurs in the soldering process.
More information about this can be found in the paper “The Simulation Study on Internal Stress in Multilayer Thermistors during Soldering Process” presented by Yu Nam Chol, a researcher at the Science Engineering Institute, to the SCI Journal “Solid State Electronics Letters”.
...
Jo May 24, 2022
Materials selection is a multi-criteria decision-making (MCDM) whereby materials designers and engineers have to select optimal material among two or more alternatives based on two or more criteria.
Many MCDM methods are applicable to the materials selection. These include rank sum ratio (RSR), simple additive weighting (SAW), weighted product method (WPM), analytic hierarchy process (AHP), technique for order preference by similarity to ideal solution (TOPSIS), VIse Kriterijumska Optimizacija Kompromisno Resenje (VIKOR) method, elimination and et choice translating reality (ELECTRE) method, preference selection index (PSI) method, preference ranking organization method for enrichment evaluations (PROMETHEE), grey relational analysis (GRA), complex proportional assessment (COPRAS) method, range of value method (ROVM), EXPROM, etc.
As materials selection may vary with the weights of materials selection criteria, the weights of materials selection criteria play an important role in materials selection by the MCDM methods.
Up to now, analytic hierarchy process (AHP) and entropy weighting method have widely been used to determine the weights of materials selection criteria. The AHP method is a subjective method while the entropy weighting method is an objective method. While objective weighting methods such as the entropy method fail to reflect the opinions of the materials designers and engineers, the AHP method can do it.
This is why the AHP method has mostly been applied to determine the weights of the materials selection criteria in many materials selection problems.
However, it still has some drawbacks. The major drawbacks to the AHP are as follows:
First, it is difficult to meet the consistency requirement of the pairwise comparisons.
Second, the pairwise comparison matrix (PCM) can hardly be constructed.
The solution is the introduction of a PCM construction method based on a new simplest questionnaire and a new modifying method of inconsistent PCM based on CR decrements.
Three indices are introduced to evaluate the effectiveness of the modifying method of inconsistent PCM.
• CR decrement ΔCR(A,B):
• Deviation index DI(A,B):
• Consistency ratio improving rate CRIR(A,B):
The results of analysis show that the simplest questionnaire helps materials designers and engineers to perform pairwise comparison judgments and the construction of PCM simply, easily and concisely without confusion, even if they have no knowledge and experience about the AHP method and that the modifying method based on CR decrements improves the consistency of inconsistent PCM better and faster by modifying a smaller number of elements with a smaller amount of modification.
This indicates that the simplest questionnaire and the modifying method could be widely used for calculating the weights of materials selection criteria or materials property indices in materials design and applications such as materials selection and optimization.
You can find more information about this in the paper “Materials selection criteria weighting method using analytic hierarchy process (AHP) with simplest questionnaire and modifying method of inconsistent pairwise comparison matrix” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Journal “Proc IMechE Part L: Journal of Materials: Design and Applications” 2022, Vol. 236(1) 69–85.
...
Jo May 21, 2022
Ho Tong Chol, a researcher at the Faculty of Electrical Engineering, has developed DC-DC chopper experimental equipment, a part of a power electronics integrated laboratory table. It is designed to enable experiments on typical DC-DC converters ― Buck, Boost and Buck-Boost.
It can easily get an operational waveform at any point thanks to the terminals placed on the front-plate, it is supported by AC 220V as main power source, and the circuit diagrams of the front-plate are intuitive and plain enough to provide users with convenient environment for experiments on a certain converter.
This equipment, composed of power supply, control block, driving circuit, main circuit, load and filter, is useful for raising or dropping static DC 12V and for measuring some parameters such as duty cycle, voltage and current ripple rate, instantaneous value, mean value, etc.
The control block using current typed PWM integrated circuit SG3525 compares the carrier in 32.5kHz with DC voltage(0~3.3V) before it generates a PWM pulse at that comparison position. This process is observed at the terminals.
Additionally, since it includes a soft-starting circuit, you can increase the duty cycle smoothly so that no shock current flows into the main switch even when it starts on the maximum duty cycle.
The control range of duty cycle is 0.2 to 0.8.
The control block is also available for the control of a transformer isolated DC-DC converter since it has two ground separated power supplies, which is useful for the experiment on a half-bridge DC-DC converter.
The driving circuit guarantees a reliable and safe switch of the main device. The driving circuit is fed from unipolar power source.
The main circuit consists of a main switching device, a reactor, FRD, a capacitor and a current sensing resistor. The load is a resistor in 200Ω and it can also include a filter when necessary.
The newly-developed experimental equipment is useful to students, researchers and teachers who major in power electronics.
...
