• 대한기계학회논문집A
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• 제36권4호
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• pp.443-449
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• 2012

Ti-6Al-4V 재에 다양한 표면처리방법을 통하여 나노스킨화된 피로시험편을 제작하고 회전굽힘피로 시험과 축하중 인장압축피로시험을 수행하였다. 특히 S-N 곡선에서 초음파나노표면개질(UNSM)법은 국내특허기술로 다른 처리법들보다 우수한 결과를 보였고, UNSM 처리된 직경 5 mm 재는 6 mm 티타늄재를 대치 가능할 성능을 얻었다. UNSM 처리된 $10^6$ 사이클 이상의 장수명영역에서 피로강도가 크게 향상되는 시험결과를 얻었다. 균열발생 패턴의 분석을 통하여 Ti-6Al-4V 재는 응력의 작용형태에 따라 표면균열발생형(surface originating crack type)과 내부균열발생형(interior originating crack type)인 어안균열(fish eye crack)이 발생하는 메커니즘을 분석하였다.

• 대한공간정보학회지
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• 제17권1호
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• pp.105-111
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• 2009

최근 각종 건설현장에서 최신측량기술을 이용한 다양한 정보 취득을 통해 시공성 및 안전성을 높이는 연구가 활발히 진행되고 있다. 디지털 영상은 영상 취득이 간편할 뿐만 아니라 영상으로부터 다양한 정보를 취득할 수 있기 때문에 최근 영상처리기술의 발전과 더불어 많은 활용성이 기대되고 있다. 본 연구에서는 터널과 같은 지하공간의 경우 일반렌즈로는 영상 촬영이 어려운 단점을 극복하기 위하여 어안렌즈를 활용한 영상 취득을 제안하였으며, 터널벽면을 모자이크 영상으로 매핑하는 프로그램을 개발하였다. 터널벽면에 대한 모자이크영상은 터널현장에서 절리면의 검출과 콘크리트 라이닝의 균열, 누수, 백화, 박리 등의 이상을 검측하고 분석하는데 활용될 수 있다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2004년도 학술대회지
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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.

• 한국해양공학회지
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• 제22권2호
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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.

• 한국해양공학회지
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• 제25권6호
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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.

• 대한기계학회논문집A
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• 제35권9호
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• pp.1119-1129
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• 2011

쇼트피닝(shot-peening) 처리를 한 베어링강의 고사이클 피로거동을 연구하였다. 열처리한 베어링강(JISSUJ2) 시험편과 열처리 후 쇼트피닝 표면가공을 추가한 시험편을 사용해 회전굽힘피로시험을 수행하고 파단 된 시험편의 파면을 분석하였다. 시험편의 피로파괴는 표면 결함에서 비롯된 표면 파괴와, 내부의 비금속개 재물에서 비롯된 내부형 파괴의 두 종류로 발생하였다. 내부 파괴의 경우 비금속 개재물 주변에 어안(fisheye) 모양의 파괴양식이 관찰되었다. 개재물은 표면으로부터의 깊이와 형상에 따라 피로 수명에 큰 영향을 주었다. 쇼트피닝 가공을 한 베어링강은 저사이클 하중영역 및 고사이클 하중영역에서 모두 피로수명이 향상되었으며, 하중이 작아질수록 피로수명의 개선은 더욱 커졌다. 또한 2 모수 와이블(Weibull) 분포를 이용하여 베어링강의 확률-응력-수명 (P-S-N) 곡선을 구하여 신뢰도 높은 피로수명 예측을 하였다.

• 한국정밀공학회지
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• 제18권9호
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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.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집A
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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.