• Journal of the Korean Society for Railway
• /
• v.12 no.2
• /
• pp.212-218
• /
• 2009

In order to investigate the thermal cracking of the rolling stock brake disc, finite element analysis was conducted on the temperature distribution and thermal stress of the disc during braking. In case of initial vehicle speed of 90, 106, 120km/h, the maximum temperature on the disk surface due to braking was $135.9^{\circ}C,\;157.9^{\circ}C,\;178.7^{\circ}C$, respectively. And, the maximum von-Mises stress at the disc surface was 42.4, 50.3, 57.1MPa at a speed of 90, 105, 120km/h, respectively, indicating that the stress increases with an increment in the speed. Damage fraction due to braking during one year running on the Seoul - Busan line was determined as 14.6%.

• Journal of the Society of Naval Architects of Korea
• /
• v.44 no.5
• /
• pp.517-525
• /
• 2007

This paper presents how to approximate an optimal shape of roll bending process in the fabrication of a curved shell plate. The roll bending process usually makes the cylindrical or conic shape from an initial flat plate. It means that the final shape is developable or its surface representation has zero Gaussian curvature. The fabrication shape is important in order to find process parameters of roil bending. An optimal concept is used to determine the developable fabrication shape which is in the closest proximity to the design surface or the given shell plate and is subject to developability. The results and the efficiency of this algorithm are evaluated by applying to some shell plates. Furthermore, the fabrication shape will be fundamental information for other process parameters of roll bending such as the vertical displacement of the center roller and the rolling directions.

• Textile Coloration and Finishing
• /
• v.30 no.4
• /
• pp.288-293
• /
• 2018

This study reported the dual functional self-cleaning PET fabrics prepared from $TiO_2$ and hexadecyltrimethoxysilane treatment, which have photodegradation and superhydrophobicity on the fabric surface. Phodegradation and superhydrophobicity of the resulting fabric were compared with $TiO_2$ or silane treated fabrics. The dual functional self-cleaning PET fabric showed less photodegradation than the $TiO_2$ treated fabric. However, the dual functional self-cleaning fabrics showed superior superhydrophobicity to silane treated fabric with increased water contact angle and significantly decreased roll-off angle. This suggested that the dual functional PET fabric has a great potential to be the commercialized self-cleaning fabric because it is repellent to soil or dust and even if soil or dust is adsorbed on the fabric surface, it can be removed by water rolling off on the surface or photodegradation by the photocatalyst.

• Journal of Ocean Engineering and Technology
• /
• v.25 no.6
• /
• pp.42-48
• /
• 2011

The present study makes three original contributions to nanoskinned Ti-6Al-4V materials. The nanoskins were fabricated on Ti-6Al-4V material using various surface treatments: deep rolling (DR), laser shot peening (LSP), and ultrasonic nanocrystal surface modification (UNSM). These surface treatments are newly developed techniques and are becoming more popular in industrial fields. A fatigue strength comparison at up to 106 cycles was conducted on these nanoskinned Ti-6Al-4V materials. Fatigue tests were carried out using MTS under axial loading tension-compression fatigue (R = -1, RT, 5 Hz, sinusoidal wave). The analysis of the crack initiation patterns in the nanoskinned Ti-6Al-4V materials found an interior originating crack pattern and surface originating crack type. Microscopic observation was mainly used to investigate the fatigue fractured sites. These surface modification techniques have been widely adopted, primarily because of the robust grade of their mechanical properties. These are mainly the result of the formation of a large-scale, deep, and useful compressive residual stress, the formation of nanocrystals by the severe plastic deformation (SPD) at the subsurface layer, and the increase in surface hardness.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.35 no.5
• /
• pp.369-374
• /
• 2007

Unsteady aerodynamic characteristics of the wing with flaperon flying over nonplanar ground surface are investigated using a boundary-element method. The time-stepping method is used to simulate the wake shape according to the motion of the wing and flaperon over the surface or in the channel. The aerodynamic coefficient according to the periodic motion of the flaperon is shown as the shape of loop. The rolling moment coefficient of the wing flying in the channel is same as that of the wing flying over the ground surface. The variation range of pitching moment is wider when the wing flies in the channel than over the ground surface. The present method can provide various aerodynamic derivatives to secure the stability of superhigh speed vehicle flying over nonplanar ground surface using the present method.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.36 no.4
• /
• pp.443-449
• /
• 2012

Nanoskins were fabricated on a Ti-6Al-4V material by carrying out various surface treatments, i.e., deep rolling, laser shot peening, and ultrasonic nanocrystal surface modification (UNSM). These surface treatments are newly developed techniques and are becoming more popular for industrial applications. Fatigue tests were carried out using material test system (MTS); these tests included the axial loading tension-compression fatigue test (R = -1, RT, 5 Hz, sinusoidal wave) and rotating bending fatigue test (R = -1, RT, 3200 rpm). The analysis of the crack initiation pattern in the UNSM-treated material indicated that the crack was interior originating in the axial loading tension-compression cycle, and was surface originating in the bending fatigue test. UNSM treatment significantly improved the fatigue strength for the regime of above $10^6$ cycles that S-N curve of rotating bending stress clearly show the performance of a 5 mm titanium specimen after UNSM treatment is similar to that of an untreated 6 mm titanium specimen.

• Korean Journal of Materials Research
• /
• v.28 no.7
• /
• pp.391-397
• /
• 2018

This study deals with the microstructure and tensile properties of 700 MPa-grade high-strength and seismic reinforced steel bars. The high-strength reinforced steel bars (600 D13, 600 D16 and 700 D13 specimens) are fabricated by a TempCore process, while the seismic reinforced steel bar (600S D16 specimen) is fabricated by air cooling after hot rolling. For specimens fabricated by the TempCore process, the 600 D13 and 600 D16 specimens have a microstructure of tempered martensite in the surface region and ferrite-pearlite in the center region, while the 700 D13 specimen has a microstructure of tempered martensite in the surface region and bainite in the center region. Therefore, their hardness is the highest in the surface region and shows a tendency to decrease from the surface region to the center region because tempered martensite has a higher hardness than ferrite-pearlite or bainite. However, the hardness of the 600S D16 specimen, which is composed of fully ferrite-pearlite, increases from the surface region to the center region because the pearlite volume fraction increases from the surface region to the center region. On the other hand, the tensile test results indicate that only the 700 D13 specimen with a higher carbon content exhibits continuous yielding behavior due to the formation of bainite in the center region. The 600S D16 specimen has the highest tensile-to-yield ratio because the presence of ferrite-pearlite and precipitates caused by vanadium addition largely enhances work hardening.

• Tribology and Lubricants
• /
• v.22 no.6
• /
• pp.314-319
• /
• 2006

In this paper, the friction and wear characteristics of contact type sealing unit far a water turbine have been presented. The sealing unit for a small hydropower generation is to stop a leakage of circulating water from an outside of an impeller to an inside of a rolling bearing. The friction heating between a seal ring and a seal seat may radically increase a surface temperature in which increase a power loss and wear on the rubbing surface. The surface wear strongly affect to the seal life of a mechanical face seal. In this study, the hardness of a stainless steel in which is a heat-treated is 892.8 in Victors hardness and the hardness of silicone carbide of SiC is 714.1 in Victors hardness. The surface hardness of a heat-treated stainless steel is 25% high compared with that of a ceramic material of SiC. The contact modes of rubbing surfaces are a dry friction, a water film friction and a mixed friction that is contaminated by a dust, silt, and moistures, etc. These two factors of a contact rubbing modes and a material property are very important parameters on the tribological performance such as a friction and wear between a seal ring and a seal seat in primary sealing unit. The experimental result shows that the surface hardness of a seal material is very important on the friction coefficient and a wear volume. Thus, the results recommend higher hardness of a seal material, which may reduce a friction loss and increase a wear life of primary seal components.

• Applied Microscopy
• /
• v.42 no.2
• /
• pp.95-103
• /
• 2012

The cell surface sculpture of the plant epidermis has received great interest recently. It has also been an active area of research, as the biological microstructures of the surface, such as papillae and waxes, exhibit several unique properties, including self-cleaning character; namely the "Lotus effect" first described in the leaves of the lotus, Nelumbo nucifera. The Lotus effect is the phenomenon in which the super-hydrophobic and water-repellent nature of lotus leaves allow water drops to run off easily on the surface in a rolling and sliding motion thereby facilitating the removal of dirt particles. It is well-known that surface roughness on the micro- and nanoscale is a primary characteristic allowing for the Lotus effect. This effect is common among plants and is of great technological importance, since it can be applied industrially in numerous fields. In the present study, Nelumbo nucifera leaf and stem epidermal surfaces have been examined with a focus on the features of papillae and wax crystalloids. Both young and mature Nelumbo nucifera leaf epidermis demonstrated the Lotus effect on their entire epidermal surface. The central area of the upper epidermis, in particular, formed extremely papillose surfaces, with an additional wax layer, enabling greater water repellency. Despite the presence of wax crystalloids, epidermal surfaces of the lower leaf and stem lacking papillae, were much more easily wetted.

• Tribology and Lubricants
• /
• v.34 no.6
• /
• pp.254-261
• /
• 2018

In order to reduce resistance torque and energy loss, minimizing friction between race surface and rolling elements of a bearing is necessary. Recently, to reduce friction in bearing element, solid lubricant coating for the bearing raceway surface has been receiving much attention. Considering the operating conditions of real bearings, verifying the effect of solid lubricant coatings under extreme conditions of high load that is more than 1 GPa is necessary. In this study, we evaluated the friction and wear characteristics of SUJ2 bearing steels deposited by carbon-based coatings (Si-DLC, ta-C), $MoS_2$ and graphite. In case of $MoS_2$ and graphite coatings, different surface treatments were applied to the coatings to verify the effect of surface treatment. A pin-on-disc type tribotester was used to evaluate the tribological characteristics of the coatings. It was possible to quantitatively estimate the friction and wear characteristics of solid lubricant under dry and lubrication conditions. The carbon-based coatings improved the friction and wear properties of SUJ2 bearing steels under the high load condition, but $MoS_2$ and graphite coatings were not suitable for high load conditions due to its low hardness. Different friction and wear behaviors were found for different substrate surface treatment method. Also, it was confirmed that solid lubricant coatings had a more positive effect than just applying the lubricant for improving the tribological characteristics.