• Journal of the Korean Society for Heat Treatment
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• v.28 no.2
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• pp.68-74
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• 2015

The low cycle fatigue (LCF) and creep-fatigue (hold time tension fatigue, HTTF) tests were performed on the modified 25Cr-13Ni cast stainless steel, which was selected as a candidate material for exhaust manifold in automotive engine. The exhaust manifold is subjected to an environment in which heating and cooling cycle occur due to the running pattern of automotive engine. Several types of fatigue behaviour such as thermal fatigue, thermal mechanical fatigue and creep-fatigue are belong to the main failure mechanisms. High temperature tensile test was firstly carried out to compare the sample with the traditional cast steel for the component. The low cycle fatigue and HTTF tests were carried out under the strain controlled condition with the total strain amplitude from ${\pm}0.6%$ to ${\pm}0.7%$ at $800^{\circ}C$. The hysteresis loops of HTTF tests showed significant stress relaxation during tension hold time. With the increase of tension hold time, the fatigue life was remarkably deceased which caused from the formation of intercrystalline crack by the creep failure mechanism.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.121-126
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• 2000

Metal/Ceramic structures have many attractive properties, with great potential for applications that demand high stiffness, as well as chemical and biological stability, thermal and electrical insulation. They are currently in use for mechanical and thermal protection in cutting tool and engine parts. With all their great advantage, ceramics suffer from one major problem they are brittle, and are especially susceptible to cracking from surface contacts. Delamination at the interfaces with adjacent layers is a particularly disturbing problem, and can cause premature failure of a composite system. so determination of adhesive properties of coating is one of the most important problems for the extension of the use of coated materials. In this work, mechanical characteristics of Interface of ceramic/Metal composites are evaluated by means of hardness test, indentation test apparent interfacial toughness and bonding strength test. The interface indentation test provides a relation between the applied load(P) and the length of the crack(a) created at the interface between the coating and the substrate.

• Tribology and Lubricants
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• v.27 no.1
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• pp.19-24
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• 2011

Metallic sintered brake pads are often applied to mid/high speed train due to their high strength and thermal characteristics. Imbalance contact between discs and pads can greatly influence the life span, one sided wear, discs attack/crack and threat the safety of the train during operation. In this research, we analyzed pressure/temperature distribution between brake pads and disks. Analyzed data had been verified and modified to conduct further tests of flexible brake pads with small/full-scale dynamo test. Flexible brake pads were installed to high speed train to conduct further tests to identify the differences between rigid brake pads and flexible brake pads. In result, Flexible brake pads showed outstanding disk thermal stability, one sided wear, noise and wear rate than rigid brake pad.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.3 s.258
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• pp.225-233
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• 2007

The growth of GaN on the patterned substances has proven favorable to achieve thick, crack-free GaN layers. In this paper, numerical modeling of transport and reaction of species is performed to estimate the growth rate of GaN from tile reaction of TMG(trimethly-gallium) and ammonia. GaN growth rate was estimated through the model analysis including the effect of species velocity, thermal convection and chemical reaction, and thermal condition for the uniform deposition was to be presented. The effect of shape and construction of microscopic pattern was also investigated using a simulator to perform surface analysis, and a review was done on the quantitative thickness and shape in making GaN layer on the pattern. Quantitative analysis was especially performed about the shape of reactor geometry, periodicity of pattern and flow conditions which decisively affect the quality of crystal growth. It was found that the conformal deposition could be obtained with the inclination of trench ${\Theta}>125^{\circ}$. The aspect ratio was sensitive to the void formation inside trench and the void located deep in trench with increased aspect ratio.

• Korean Journal of Materials Research
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• v.4 no.3
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• pp.334-338
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• 1994

This report details the microscopic aspects of grain boundary cracking in Pb-Sn eutecticduring displacement-controlled mechanical tests performed over a range of low frequency ($10^{-3}-10^{-5}$/s)and moderate strain range (0.2 - 1 %) where is the most technologically relevant to solder jointssubjected to thermal cycling. It is shown that intergranular cracking begins with the appearance ofcrack-like features (CLF's), which can be seen due in part because they are associated with grainboundary sliding, and is able to be described by certain stages of isolated crack growth. In the initialstages CLF's are not ture cracks but instead what I shall call "proto-cracks" where grain boundarysliding begins to damage the gram boundary at the surface. At some point during the initiation stagesonce proto-cracks become ture cracks, they develop into isolated cracks and the growth of isolatedcracks is eventually accomplished by coalescence, resulting in 5 stages of cracking.ing in 5 stages of cracking.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.147-151
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• 2004

In this study, an optical fiber assembly directly coupled with a laser diode or a photo diode is designed to confirm high reliable optical coupling efficiency of optical transmitter(Tx) and receiver(Rx). The optical fiber assembly is fabricated by soldering an optical fiber and a Kovar ferrule using a glass solder after inserting an optical fiber through a Kovar ferrule. The Kovar which has good welding characteristics is applied to introduce laser welding technique. The glass solder has excellent thermal characteristics such as thermal shift delamination compared with PbSn, AuSn solder previously used usually. Furthermore, the glass solder doesn't need fiber metalization and this enables low cost fabrication. However, the glass soldering is high temperature process over 35$0^{\circ}C$ and the convex shape after solidification due to surface tension causes the stress concentration on optical fiber. The stress concentration on the optical fiber increases the optical insertion loss and possibility of crack formation. The shape of glass solder was designed referring to 2-D Axi-symmetric FEM simulation. To test the mechanical reliability, mechanical vibration test and shock test were done according to Telcorida GR-468-Core protocol. After each test, the optical loss of the stress distributed fiber assembly didn't exceed 0.5 dB, which passes the test.

• Journal of the Korean Ceramic Society
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• v.45 no.11
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• pp.725-733
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• 2008

In this study the preparation method of the spherical ADU droplets, intermediate compound of a HTGR nuclear fuel, was detailed-reviewed and then, the characteristics on an ageing and a washing steps among the wet process and the thermal treatment process on the died-ADU${\rightarrow}UO_3$ conversion with the high temperature furnaces were studied. The key parameters for spherical droplets forming are a precise control of feed rate and a suitable viscosity value selection of a broth solution. Also, a harmony of vibrating frequency and amplitude of a vibration dropping system are important factor. In our case, an uranium concentration is $0.5{\sim}0.7mol/l$, viscosity is $50{\sim}80$ centi-Poise, vibration frequency is about 100Hz. In thermal treatment for no crack spherical $UO_3$ particle, the heating rate in the calcination must be operated below $2^{\circ}C$/min, in air atmosphere.

• Journal of Applied Reliability
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• v.13 no.2
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• pp.109-116
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• 2013

The solder is any of various fusible alloys, usually tin and lead, used to join metallic parts that provide the contact between the chip package and the printed circuit board. Solder plays an important role of electrical signals to communicate between the two components. In this study, two kinds of Ag-free solder as sample is made to conduct the thermal shock test and the high humidity temperature test. Low resistance is measured to estimate crack size of solder, using daisy chain. The low speed shear test is also performed to analyze strength of solder. The appropriate degradation model is estimated using the result data. Depending on the composition of solder, lifetime estimation is conducted by adopted degradation model. The lifetime estimated two kinds of Ag-free solder is compared with expected lifetime of Sn-Ag-Cu solder. The result is that both Ag-free composition are more reliable than Sn-Ag-Cu solder.

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.941-947
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• 2008

In the braking of a railroad car, mechanical brake systems using wheel tread and brake disk are applied as well as electrical brake systems by regenerator and rheostat. During disk braking, kinetic energy of the vehicle is converted into thermal energy through friction between disk and brake pad. And it causes high temperature concentration and generates thermal crack on the brake disk surface. In this study, comparative test process for heat-resistance of candidate materials was designed for development of brake disk materials having high heat-resistance. We also verified the efficiency of the process by experiments using conventional brake disk materials.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.653-656
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• 2008

The main purpose of this research was to enhance the durability in both the design and construction of dams. Especially, in case of rockfill dams, the durability of face slab concrete in a concrete-faced rockfill dam(CFRD) is achieved by optimizing the fly ash replacement for cement and application of blended fiber. The effect on durability and thermal property corresponding to the increasing replacement of fly ash and application of blended fiber was evaluated, and the optimum value of fly ash replacement and blended fiber application was recommended. The results show that 15% of fly ash replacement and 0.9kg/m3 of blended fiber application was found to be an optimum level and demonstrated excellent performance in durability and thermal property.