• Journal of Mechanical Science and Technology
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• v.16 no.8
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• pp.1109-1116
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• 2002

To investigate the effect of nitriding layer on both fatigue crack initiation and fatigue life, turbine rotor steel ( IC.- 1Mo-0.25V steel) specimens were nitrided by the nitemper method and then put to a rotary bending fatigue test at room and elevated temperatures. In nitriding, temperature and time were controlled to obtain a different nitrided thickness. Microstructure analysis, micro-Victors hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at inclusion near the interface between nitrided layer and substrate, which showed fish-eye type appearance in fractograph. The fatigue life of nitrided specimens at every temperature was prolonged compared to that of the non-nitrided. However, there was not observable improvement in fatigue characteristics with the increase of a nitrided thickness.

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

• Journal of the Korean Society for Precision Engineering
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• v.18 no.9
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• pp.122-130
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• 2001

Induction surface hardening is widely used to enhance local strength and hardness. However, most research is only to have a focus on fatigue life and fatigue behavior is not so much studied. So, in this study, Cr-Mo steel alloy(SCM440) was used to show the effect of residual stress and micro hole on the fatigue strength fur base metal and induction surface hardened specimen. In addition, the fatigue characteristic between surface hardened and fully hardened steel is somewhat different. It is caused by hardness distribution, residual stress and inclusions etc.. The modification of prediction equation of fatigue strength is proposed and predicted results show very good accuracy. A $textsc{k}$, which is calculated 1.46, is introduced to consider the effect of stationary crack with defect. A new method of modifying residual stress is proposed to examine the mean stress effect under fatigue loading.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.05a
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• pp.370-375
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• 2004

Fatigue tests were carried out to find the fatigue characteristics in the super-long life range by using a cantilever type rotational bending fatigue test machine. Three kinds of specimen in bearing steels with the quenched and tempered in air (A and B, B: shot peened after heal treatment) and under vacuum conditions(C:non-shot peened)were tested in this study. S-N curves obtained from fatigue tests of C specimen tend to come dawn again in the super-long life range due to fish-eye type cracking, while most of A and B specimens fractured by surface defects such as scratches and slip lines. This duplex S-N behavior for the high strength steels have to be reviewed by the change of fracture modes.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.333-338
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• 2000

Induction surface hardening is widely used to enhance local strength and hardness. However, most research is only to have a focus on fatigue life and fatigue behavior is not so much studied. So, in this study, Cr-Mo steel alloy(SCM440) was used to show the effect of residual stress and micro hole on the fatigue strength for base metal and induction surface hardened specimen. In addition, the fatigue characteristic between surface hardened and fully hardened steel is somewhat different. It is caused by hardness distribution, residual stress and inclusions etc.. Crack origins are generally micro inclusions for the high strength steel. So, the distribution of inclusions is analyzed statistically.

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

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.256-261
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• 2007

The purpose of this study is to investigate the effect of shot peening on the fatigue strength and fatigue life of two kinds of aluminum alloys. The fatigue strength behavior of aluminum alloys were estimated by the stress ratio and shot velocities. The fatigue life and strength increased with increasing the test shot velocity. However, at the shot velocity range between 50m/s and 70m/s, the compressive residual stress phenomena were observed in test conditions of different shot velocity. The optimal shot velocity is acquired by considering the peak values of the compressive residual stress, dislocations, brittle striation, slip, and fisheye on the fracture surface of test specimen. It was observed from the SEM observation on the deformed specimen that the brittle striation, fisheye were showed in the intergranular fracture structure boundaries at the this velocities. Therefore, fatigue strength and fatigue life would be considered that shot velocity has close relationship with the compressive residual stress.

• Journal of Ocean Engineering and Technology
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• v.22 no.2
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• pp.99-105
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• 2008

Ultrasonic nanocrystal surface modification (UNSM) is a method to induce severe plastic deformation to a material surface, so that the structure of the material surface becomes a nanocrystal structure from the surface to a certain depth. It improves the mechanical properties, namely hardness, compressive residual stress, and fatigue characteristics. Specimens of SKD61 were tested to verify the effects of the variation of UNSM static load level on fatigue characteristics. The results were as follows: the grain size of SKD61 treated with UNSM became very fine from the material surface to a $100{\mu}m$ depth. The surface hardness of SKD61 was increased up to 37% after UNSM. And fatigue strength at $10^7$ cycles was increased by 8.3, 11.2, and 17.9% respectively, when the static load levels of UNSM were 4, 6, 8 kgf.