Brilliant LeaderShip

Play Areas in Department Store

There are some excellent indoor play areas for children at Pyongyang Children’s Department Store. Those picturesque play areas that look like a paradise in a fairy tale remind us of one day ...

Mirae Scientists Street

One day in May, Juche 103(2014), the respected Comrade Kim Jong Un visited the construction site of apartment houses for lecturers of Kim Chaek University of Technology. Looking around almost every corner of ...

In all Weathers

It was one nasty rainy day in May when the respected Comrade Kim Jong Un found himself at the August 25 Fishery Station. Regardless of the non-stop nasty rain, he, with an umbrella ...

Never Turn a Blind Eye

One day several decades ago, Chairman Kim Jong Il heard that the residents in an area of Hamhung City were envious of the good-tasting tap water in the city centre. He learnt in ...

Story of Mullet Soup Cooked with Cold Water

One morning in April several decades ago, President Kim Il Sung called an official to acquaint himself with the preparation for a consultant meeting which was due that morning. Calling upon him to ...

Exposed to Rain Bright and Early

Little rain in spring is one of the typical climate features of our country. One year some decades ago, very little rain fell throughout the country until the beginning of April. On April 14 ...

News

Jo May 20, 2024

Recently, aluminum has been widely used in various industries including aerospace, mechanical and wire cable industries, for its good mechanical properties and engineering properties.

Aluminum used in the wire cable industry has relatively low density, low melting point and good corrosion resistance compared to other metallic materials such as copper, steel, etc. However, it has worse mechanical properties.

It is also possible to increase the mechanical properties of aluminum by adding various alloying elements. However, in pure aluminum (more than 99.5%), mixed elements such as Fe, Si, Cu, etc. bring several negative effects. Therefore, it is important to investigate and introduce new ways to improve the mechanical properties of pure aluminum.

Carbon nanotubes have been widely used as reinforcement material for composites because of their mechanical, physicochemical and electrical properties. From the mechanical point of view, the combination of sp2 hybridization mode of carbon nanotubes is one of the very strong chemical bonds. Therefore, carbon nanotubes with ideal structure have very good mechanical properties.

Hence, addition of carbon nanotubes, an ideal reinforced material for composites, can be the most suitable method for achieving grain refinement of the matrix and enhancing the mechanical properties.

Jon Song Won, a researcher at the Faculty of Materials Science and Technology, has investigated the effect of carbon nanotubes on the properties of aluminum-carbon nanotube composites fabricated by the melting method.

He has found that the tensile strength, elongation, bending number and hardness are all high when the carbon nanotube content is 0.3%.

...

Jo May 19, 2024

Rotor system, which plays an important role in modern industries, is used in many machines such as internal combustion and jet engines, various types of motors, generator turbines and so on. High speed and high power are the development direction of modern rotating machinery, and fatigue crack is apt to happen as one of the most serious damages in the heavy loading environment. Therefore, detecting the crack in a timely way is very important for safe operation.

Kim Yu Jong, a researcher at the Faculty of Mechanical Science and Technology, has proposed a finite element model of a hollow shaft with open cracks. She derived the time-varying stiffness matrix of finite element based on the transformation matrix by which node displacements are converted from the rotating coordinates into the fixed ones. She employed the harmonic balance method (HBM) to find the responses of a cracked rotor system.

She analyzed the nonlinear vibration of rotor system and compared the results with some published results, considering the eccentricity of disk, the dimensionless crack depth, the ratio between the inner radius and outer radius of the rotor and the angle between the crack and imbalance directions.

As a result, she drew the following conclusions.

The subcritical rotational speeds corresponding to the first critical forward whirling speeds are not excited in the open cracked hollow rotor system. For the deep crack, the first forward whirling rotational speeds decrease more rapidly than the first backward ones as the dimensionless crack depth increases.

The whirl orbits near the subcritical backward whirl speeds have the same shape and behavior for both breathing and open crack models. However, the variations of the whirl orbits shape with the change in the rotational speeds are slower at the deep crack than at the shallow crack.

...

Jo May 17, 2024

Continuous casting is widely used for production of steel slab. The main problem is that if the stress and strain exceed a threshold value, steel slab will be cracked as the layer of steel slab depends on variable thermodynamic load in the solidification process.

The main cause of steel slab crack is the thermal stress that is generated by uneven temperature distribution during the solidification of molten steel.

Ri Yong Hwa, a researcher at the Faculty of Aerospace Engineering, proposed a method of determining the optimum heat transfer coefficient to prevent thermal crack of slab, on the basis of the identification of the cause of crack by analysis of unsteady thermal stress in the continuous casting of the slab whose width is relatively long for its height.

He analyzed unsteady temperature field and thermal stress of 800×150mm2 steel slab in the continuous casting process by using ANSYS to find out the reasons why steel slab is cracked. The analysis revealed that the reason is that the layer is not thick enough to withstand the thermal stress and the static pressure of the molten steel in the height direction of this band, and that all the thermal stresses on the surface exceed the yield limit of 1.9Mpa.

Then, based on the band analysis data and the optimization algorithm in the continuous casting process, he determined the reasonable coefficient of band heat transfer to prevent the crack of the steel slab by using ANSYS-workbench.

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Students on “Risang” Local Network (1)

“Risang”, homepage for distance education of Kim Chaek University of Technology, has been making the part-time learners realize their ideal to the full.

Nov 11, 2022