Jo Jul 14, 2024
The fracture occurring in many metallic structures made of steel is mainly fatigue fracture. Fatigue fracture is a dangerous one that decreases the safety of structures. Fatigue fracture occurring in metallic structures is predicted by fracture mechanics method.
Paris suggested a propagation equation for fatigue crack and many researchers found other crack propagation equations by correcting and supplementing Paris’s equation.
However, very few studies have focused on the prediction of the fatigue life of structures and components from the characteristic data of structures comprising metallic materials. For example, the relationship between the metallographic parameters of steel like hardness, volume ratio, grain size and distribution of non-metallic inclusion, etc., and its fatigue life was not quantified.
Kim Mun Chol, a researcher at the Faculty of Materials Science and Technology, simulated the fatigue procedure of a steel structure using a crack-tip sliding model based on continuous distribution dislocation theory and calculated the rest fatigue life of an inertia train structure.
Through the simulation, he found that the hardness of microstructure exerts decisive effects on the fatigue life of steel structures of ferrite-pearlite structure while the grain size and volume ratio hardly affect it, and slip-band crack sharply decreases as inclusion grain size decreases below 7㎛.
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Jo Jul 12, 2024
Foam glass has been used as insulation material since 1930s. The most important properties of foam glass are light weight, mechanical stability, low thermal conductivity, freeze-thaw cycle resistance, nonflammability and chemical inertness.
There are two methods of producing foam glass according to the kinds of raw materials. One uses glass while the other uses waste glass as raw material. The second method is very simple and consumes less electricity and fuel.
Anthracite fine, graphite powder, soot, calcite, calcium carbide and silicon carbide can be used as a foaming agent. The quantity of foaming agent is 1~7% of finished products.
Previous papers considered thermo-physical properties of foamed glass using a single-system foaming agent.
Pae Kyong Ho, a researcher at the Faculty of Metal Engineering, conducted experiments under different conditions by using a mixture of calcite and hard coal in order to produce low density foam glass with both closed and open porosities and compressive strength of up to 1.8MPa.
The results show that addition of 5wt.% of calcite powder and 2wt.% of anthracite powder ensures the density of foam glass of below 240kg/m3, the compression strength of over 1.8MPa and the heat conductivity of below 0.21W/(m·K).
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Jo Jul 10, 2024
Lagoon Monggum is a scenic spot on the coast in Ryongyon County, South Hwanghae Province.
Blue waves lapping against the shore, white silvery sands stretching on the beach, thick pine woods and red sweet briers in full bloom on the sands blend well to produce spectacular scenery. The name Lagoon Monggum (where mong means ‘dream’ and gum means ‘gold’) is derived from an old tale about having a dream on this beautiful beach.
Its original names were Paeksachongsong (white sands and green pine trees), Paeksajong (port near the white sands) and Kumsasipri (10 ri stretch of golden sands).
In the south of the lagoon are rocky cliffs and rocks of fantastic shape like Elephant Rock, a living monument, which add to the scenic beauty.
The sands of Lagoon Monggum are of so good quality as to be well known to the world. The sandy beach runs up to the width of two kilometers and the length of eight kilometers.
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Jo Jul 9, 2024
Superhydrophobic materials find their applications in self-cleaning, resistance reduction, corrosion protection, and anti-fogging due to their unique hydrophobicity. Lotus, a natural superhydrophobic material (SHP), has a contact angle (CA) of 162° because it has teat-like microstructures with a diameter of 5-9㎛ and nanostructures with a diameter of 124nm. One of the interesting applications of SHP materials is their self-cleaning ability to prevent and remove contamination of material surface by itself.
Superhydrophobic materials can be prepared by various methods, such as physical and chemical decomposition, chemical etching, anode oxidation, laser etching, chemical vapor deposition, sol-gel method, etc. The sol-gel method is a promising method for large-scale applications (metal, glass, ceramics and plastics), which can improve the mechanical durability of superhydrophobic materials and reduce fabrication cost.
Hollow ash is 10-20% in fly ash, a waste produced by coal-fired power plants. Hollow ash has excellent thermal insulation properties due to its porosity in the particles, so it is widely used as an insulating material.
Kim Jong Ho, a researcher at the Faculty of Chemistry, has formed a thermal barrier layer by using a composite solution of MMA-BA co-polymers and hollow ash, and investigated the functional coatings with high thermal insulation efficiency, self-cleaning ability and anticontamination performance. Based on the literature, he prepared mixed solutions of MMA-BA co-polymers and silica sol by using methyl methacrylate, butyl acrylate, methacrylate-β-hydroxyethyl ester and o-xylene. To maximize the thermal barrier properties and superhydrophobicity of the coatings, he carefully studied the effects of various factors such as the ratio of raw materials, coating thickness, and morphology.
Thermal characterization of the prepared films showed that the thermal insulation effect of the films was enhanced when hollow ash was added to the MMA-BA co-polymers solution.
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Jo Jul 7, 2024
Among the precious cultural legacies created by the Korean nation is Nine-storey Tower at Anguk Temple. This stone tower of the Koryo age is at Anguk Temple in Ponghak-dong, Phyongsong City.
The tower is built of granite. It is 6.23 meters tall with the square plane.
The tower consists of platforms, body and head. Every side of the lower platform is engraved with two lotus patterns the bottoms of which are decorated with bracken leaf patterns in the middle.
The upper platform is a lot higher than the lower one. All the sides of the platforms have three pillars each to be divided into two parts.
The roofing stones are pretty tall and three layers of eaves supports are overhanging.
The middle parts of the eaves edge are hollow and the both ends are smoothly raised. The vestiges of windbells are found at the end of the angle rafters.
On the top of the tower is a globular stone.
The whole tower feels like shooting up without much ascending reduction of distances between the roofing stones but with fair height of platforms, bodystone and roofing stones.
This stone tower is a precious legacy that demonstrates the skills of the Korean people in building stone towers. It is in good preservation and care thanks to the policy of the Workers’ Party of Korea on protecting the national inheritance.
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Jo Jul 5, 2024
Flexibility is a key concept in the management of modern manufacturing systems. The principal motivation is to achieve rapid response to manager’s demands by improving the efficiency of a job-shop while retaining its flexibility. To achieve this goal, the term flexible manufacturing system (FMS) is defined. FMS is an integrated computer controlled complex of automated material handling devices and numerically controlled machine tools that can process medium-sized volumes of a variety of part types. Flexible manufacturing systems have many potential advantages including high flexibility and high machine utilization rate.
As scheduling is the core of this control system, it plays a decisive role in achieving a goal. FMS scheduling problems are more difficult than those of conventional production systems because of a number of reasons such as machine setup times, part routing and operations scheduling. Flexible manufacturing system scheduling problems are very difficult, so mathematical modeling methods need to be improved to solve them.
Pak Myong Chol, a researcher at the Robotics Institute, designed a scheduler based on a self-adaptive genetic algorithm and obtained the optimal solution for the job-shop schedule. Then, he constructed a flexible production simulation system with the scheduling module and Flexsim and conducted a simulation based on the optimized part scheduling.
The simulation results verified that it is possible to reduce the total machining completion time, increase machine utilization and realize process optimization.
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