• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.4
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• pp.349-355
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• 2003

Material degradation due to corrosion fatigue was evaluated nondestructively using backward radiated Rayleigh surface wave. h corrosion fatigue test was carried out for the specimens made of thermo-mechanically controlled process steel in 3.5wt.% NaCl solution at $25^{\circ}C$. The backward radiation profile, which is the amplitude variation of backward radiated ultrasound according to the incident angle, of the specimens were measured in water at room temperature after the corrosion fatigue test. The velocity of Rayleigh surface wave, determined from the incident angle at which the profile of the backward radiated ultrasound became maximum, decreased for the specimen that had the large number of cycles to failure in the corrosion fatigue test. This fact implies that the corrosion degradation occurred at specimen surface in this specific test is dominantly dependant on the me exposed to corrosion environment. The result observed in the present work demonstrates the high potential of backward radiated Rayleigh surface wave as a tool for nondestructive evaluation of corrosion degradation of aged materials.

• Proceedings of the KWS Conference
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• 2006.05a
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• pp.21-23
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• 2006

The microstructural investigation and thermo-mechanical reliability evaluation of the Sn-3.0Ag-0.5Cu solder bumped flip chip package were carried out during the thermal shock test of the package. In the initial reaction, the reaction product between the solder and Cu mini bump of chip side was Cu6Sn5 layer, while the two phases which were (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 were formed between the solder and Ni-P layer of the package side. The cracks were occurred at the corner solder joints after the thermal shocks of 400 cycles. The primary failure mechanism of the solder joints in this type of package was confirmed to be thermally activated solder fatigue failure.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2009.06a
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• pp.185-186
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• 2009

• International Journal of Automotive Technology
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• v.5 no.4
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• pp.297-302
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• 2004

This paper presents the low cycle thermal fatigue of the engine exhaust manifold subject to thermo-mechanical cyclic loading. As a failure of the exhaust manifold is mainly caused by geometric constraints of the less expanded inlet flange and cylinder head, the analysis is based on the exhaust system model with three-dimensional temperature distribution and temperature dependent material properties. The result show that large compressive plastic deformations are generated at an elevated temperature of the exhaust manifold and tensile stresses are remained in several critical zones at a cold condition. From the repetition of these thermal shock cycles, maximum plastic strain range (0.454%) could be estimated by the stabilized stress-strain hysteresis loops. It is used to predict the low cycle thermal fatigue life of the exhaust manifold for the thermal shock test.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.8
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• pp.71-76
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• 2002

In this study low cycle fatigue (LCF) behavior of 12Cr steel at high temperature are described. Secondly, comparisons between predicted lives and experimental lives are made for the several sample life prediction models. Two minute hold period in either tension or compression reduce the number of cycles to failure by about a factor of two. Twenty minute hold periods in compression lead to shorter lives than 2 minute hold periods in compression. Experiments showed that life predictions from classical phenomenological models have limitations. More LCF experiments should be pursued to gain understanding of the physical damage mechanisms and to allow the development of physically-based models which can enhance the accuracy of the predictions of components. From a design point-of-view, life prediction has been judged acceptable for these particular loading conditions but extrapolations to thermo-mechanical fatigue loading, for example, require more sophisticated models including physical damage mechanisms.

• Journal of the Korean Society of Safety
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• v.29 no.4
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• pp.28-33
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• 2014

Recently, the actuator worked by the driving recovery-force of the thermo elastic martensitic transformation of shape memory alloys(SMA) has been studied. This paper presents a study on the fatigue life of shape memory alloy (SMA) actuators undergoing thermally induced martensitic phase transformation under various stress levels. shape memory recoverable stress and strain of Ti-44.5at.%Ni-8at.%Cu alloys were by means of constant temperature tensile tests. Differential scanning calorimetry (DSC) was employed in order to investigate the transformation characteristics of the alloy before the tests. the results were summarized as follows. The martensite inducing stress incerased with the increasing of the Cu-contents. The fatigue life decreased with the increasing of the test load and the Cu-content. The data acquired will be very useful during the design process of an SMA NiTiCu element as a functional part of an actuator.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.9
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• pp.1227-1233
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• 2010

In the hot section of a gas turbine, the turbine blades were protected from high temperature by providing a thermal barrier coating (TBC) as well as by cooling air flowing through internal passages within the blades. The cooling air then passed through discrete holes on the blade surface, creating a film of cooling air that further protects the surface from the hot mainstream flow. The holes are subjected to stresses resulting from the lateral growth of thermally grown oxide, the thermal expansion misfit between the constituent layers, and the centrifugal force due to high-speed revolution; these stresses often result in cracking. In this study, the deformation and cracks occurring near a hole on a heat-resistant alloy subjected to thermo-mechanical cycling were investigated. The experiment showed that cracks formed around the hole depending on the applied stress level and the number of cycles. These results could be explained by our analytic solution.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1843-1850
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• 2016

This study focused on predicting the fatigue life of an insulated gate bipolar transistor (IGBT) power module for electric locomotives. The effects of different wiring technologies, including aluminum wires, copper wires, aluminum ribbons, and copper ribbons, on solder fatigue life were investigated to meet the high power requirement of the IGBT module. The module's temperature distribution and solder fatigue behavior were investigated through coupled electro-thermo-mechanical analysis based on the finite element method. The ribbons attained a chip junction temperature that was 30℃ lower than that attained with conventional round wires. The ribbons also exhibited a lower plastic strain in comparison with the wires. However, the difference in plastic strain and junction temperature among the different ribbon materials was relatively small. The ribbons also exhibited different crack propagation behaviors relative to the wires. For the wires, the cracks initiated at the outmost edge of the solder, whereas for the ribbons, the cracks grew in the solder layer beneath the ribbons. Comparison of fatigue failure areas indicated that ribbon bonding technology could substantially enhance the fatigue life of IGBT modules and be a potential candidate for high power modules.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.1
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• pp.1-6
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• 2002

The automatic turbine startup system provides turbine control based on thermal stress. During the startup, control system monitors and evaluates main components of turbine using damage mechanism and life assessment. In case of valve chest, the temperature of inner/outer wall is measured by thermo-couples and the safety of these values are evaluated by using allowable △T limit currie during the startup. Because allowable ΔT limit curve includes life assessment, it is possible to apply this curve to turbine control system. In this paper, low cycle fatigue damage, combined rupture and low cycle fatigue damage criterion were proposed for yielding the allowable ΔTf limit curve of CV(control valve) chest. To calculate low cycle fatigue damage, the stress analysis of valve chest has been performed using FEM. Automatic turbine startup to assure service life of CV was achieved using allowable ΔT limit curve.

• Journal of the Microelectronics and Packaging Society
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• v.12 no.2 s.35
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• pp.135-140
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• 2005

Although it was recently observed that severe fatigue damage was formed in Al or Cu interconnects due to the cyclic temperatures generated by Joule heating of the metal lines by the passage of alternating currents (AC), AC loading frequency effect on the damage evolution characteristics are not known so far. This work focused on the effect of AC loading frequency (100 Hz vs. 10 kHz) on the thermo-mechanical fatigue characteristics by using polycrystalline sputtered Cu lines with temperature cycles with amplitudes from 100 to $300^{\circ}C$. It was consistently observed that higher loading frequency accelerated damaged grain growth and led to earlier failure irrespective of Cu grain sizes. The frequency effect is believed to result from differences in the concentration of defects created by the deformation-induced motion of dislocations to the grain boundaries.