• Nuclear Engineering and Technology
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• 제55권4호
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• pp.1250-1264
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• 2023

This paper presents approximate in-depth residual stress and plastic strain profiles for laser-peened alloy 600 surface via FE analysis. In approximations, effects of the initial welding residual stress and the number of shots are quantified. Based on FE analysis results, residual stress profiles are quantified by two variables; the maximum difference in stress before and after LSP, and the depth up to which the compressive residual stress exists. Plastic strain profiles are quantified by one variable, the maximum equivalent plastic strain at the surface. The proposed profiles are validated by comparing with published LSP experimental results for welded plates. Effects of the initial welding residual stress and the number of shots on these variables are discussed. The proposed profile can be directly applied to predict the mitigation effect of LSP on PWSCC and to efficiently perform structural integrity assessment of laser peened nuclear components.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2003년도 추계학술발표회초록집
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• pp.134-135
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• 2003

The dental implant materials required good mechanical properties, such as fatigue strength, combined with a high resistance to corrosion. For increasing fatigue resistance and delaying onset of stress corrosion cracking, shot peening has been used for > 50 years to extend service life of metal components. However, there is no information on the electrochemical behavior of shot peened and hydroxyapatite(HA) coated Ti-6Al-4V alloys. To increase fatigue strength, good corrosion resistance, and biocompatibility, the electrochemical characteristics of Ti/TiN/HA coated and shot peened Ti-6Al-4V alloys by electron beam physical vapor deposition(EB-PVD) have been researched by various electrochemical method in 0.9%NaCl. Ti-6Al-4V alloys were prepared under the condition of hydrogen and vacuum arc furnace. The produced materials were quenched at 1000$^{\circ}C$ under high purity dried Ar atmosphere and were hold at 500$^{\circ}C$ for 2 hrs to achieve the fatigue strength(1140㎫) of materials. Ti-6Al-4V alloys were prepared under the condition of hydrogen and vacuum arc furnace. Shot peening(SP) and sand blasting treatment was carried out for 1, 5, and 10min. On the surface of Ti-6Al-4V alloys using the steel balls of 0.5mm and alumina sand of 40$\mu\textrm{m}$ size. Ti/TiN/HA multilayer coatings were carried out by using electron-beam deposition method(EB-PVD) as shown Fig. 1. Bulk Ti, powder TiN and hydroxyapatite were used as the source of the deposition materials. Electrons were accelerated by high voltage of 4.2kV with 80 - 120mA on the deposition materials at 350$^{\circ}C$ in 2.0 X 10-6 torr vacuum. Ti/TiN/HA multilayer coated surfaces and layers were investigated by SEM and XRD. A saturated calomel electrode as a reference electrode, and high density carbon electrode as a counter electrode, were set according to ASTM GS-87. The potentials were controlled at a scan rate of 100 mV/min. by a potentiostat (EG&G Co.273A) connected to a computer system. Electrochemical tests were used to investigate the electrochemical characteristics of Ti/TiN/HA coated and shot peened materials in 0.9% NaCl solution at 36.5$^{\circ}C$. After each electrochemical measurement, the corrosion surface of each sample was investigated by SEM.

• 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.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 추계학술대회 논문집
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• pp.106-111
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• 2003

This study made an experiment On fatigue crack propagation da/dn, stress intensity factor range ${\Delta}K$ respectively in room temperature and in low temperature. And we got the following characteristics from fatigue crack growth test carried Out in the environment of room temperature and law temperature at $25^{\circ}C$, $-60^{\circ}C$, $-80^{\circ}C$, and $-100^{\circ}C$ in the range of stress ratio of 0.3 by means of opening made displacement. The threshold stress intensity factor range ${\Delta}Kth$ in the early stage of fatigue crack growth (Mode I) and stress intensity factor range ${\Delta}K$ in the stable of fatigue crack growth (Made II) was decreased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at law temperature and high temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region.

• Journal of Welding and Joining
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• 제33권2호
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• pp.40-46
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• 2015

This study was performed to investigate the effect of ultrasonic nano crystal surface modification (UNSM) on residual stress mitigation after Weld Inlay repair for butt dissimilar metal weld with Alloy 82/182 in reactor vessel In/Outlet nozzle. As-welded and Weld Inlay specimens were made in accordance with design standard of ASME Code Case N-766, and two planes of their weld specimens were peened by the optimum UNSM process condition. Peening characteristics for weld specimens after UNSM treatment were evaluated by surface roughness and Vickers hardness test. And, residual stress for weld specimens developed from before and after UNSM treatment was measured and evaluated by instrumented indentation technique. Consequently, it was revealed that the mitigation of residual stress in weld metal after Weld Inlay repair of reactor vessel In/Outlet nozzle could be possible through UNSM treatment.

• 대한기계학회논문집A
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• 제26권12호
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• pp.2647-2655
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• 2002

This research presents an analytical model to investigate the stability due to the ball bearing waviness i n a rotating system supported by two ball bearings. The stiffness of a ball bearing changes periodically due to the waviness in the rolling elements as the rotor rotates, and it can be calculated by differentiating the nonlinear contact forces. The linearized equations of motion can be represented as a parametrically excited system in the form of Mathieu's equation, because the stiffness coefficients have time -varying components due to the waviness. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as the simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving the Hill's infinite determinant of these algebraic equations. The validity of this research is proved by comparing the stability chart with the time responses of the vibration model suggested by prior researches. This research shows that the waviness in the rolling elements of a ball bearing generates the time-varying component of the stiffness coefficient, whose frequency is called the frequency of the parametric excitation. It also shows that the instability takes place from the positions in which the ratio of the natural frequency to the frequency of the parametric excitation corresponds to i/2 (i=1,2,3..).

• Journal of Advanced Marine Engineering and Technology
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• 제23권3호
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• pp.331-337
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• 1999

Cracks on the cylinder block of diesel engines will often happen due to cyclic load and thermal stress. According to the Classification Societies' rules welding reparis of cylinder block made of cast irons are generally not permitted. However such welding repairs became inevitable taking enormous cost and time for their renewal into consideration. In this study repair welding methods for the clinder blocks made of gray cast irons were reviewed and the tests of their welds were carried out in order to purpose the repair welding meth-ods of packing seat and o-ring seat of cylinder block and apply them to the practice. The following conclusions are botained :1 The tensile strength of weld of cast iron more than that of base metal can be obtained by means of preheating keeping temperature above $100^{\circ}C$ between welding pass-es preventing slag inclusion peening and cramping weld metal by studs. 2. The suspected crack by a magnetic particle test due to different magnetic permeability can be identified which are not associated with a mechanical discotinuity.

• 한국재료학회지
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• 제17권11호
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• pp.580-586
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• 2007

To enhance the reliability of a newly designed reciprocating compressor applied in a domestic compressor, accelerated life tests were developed using new definitions of the sample size and the $B_1$ life index. In $1^{st}$ accelerated life testing, the compressor was locked due to the fracture of the suction reed valve. The failure modes and mechanisms of the suction reed valve in the accelerated tests were found to be similar to that of the failed product in the field. The root cause of the failure was the overlap between the suction reed valve and the valve plate in the suction port. The missing parameters in the design phase were modified by expanding the trespan size, introducing tumbling process, changing the material and thickness for the valve, introducing a ball peening and brushing process for the valve plate. In $2^{nd}$ accelerated life testing, the compressor was locked due to the interference between the crank shaft and thrust washer. The corrective plan was to heat treat the crank shaft. The $B_1$ life of the compressor improved from 1.5 to 12.9 years.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2017년도 춘계학술대회 논문집
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• pp.111-111
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• 2017

쇼트피닝 기술은 크게 피로강도 향상을 위한 쇼트피닝(shot peening), 재료의 청정 및 도장 품질 향상을 위한 쇼트블라스트(shot blast) 그리고 쇼트피닝 시 판재의 변형되는 성질을 이용한 핀포밍(peen forming) 등으로 구분할 수 있다. 그 중 본 연구에서는 해양산업 분야에서 널리 사용되는 회주철의 효과적인 내구성 향상을 위해 쇼트피닝 기술을 적용하였다. 그러나 쇼트피닝 기술 적용에 있어서 가장 중요한 것은 제품의 균일성, 정확성, 신뢰성을 확보하기 위해 쇼트피닝 강도를 제어하는 여러 가지 변수들에 대하여 최적 상태를 유지하는 것이다. 따라서 회주철에 대한 최적 쇼트피닝 분사조건 규명작업은 반드시 쇼트피닝 가공 전에 수반되어야만 한다. 그 일환으로 실험은 쇼트피닝 분사시간과 분사압력을 변수로 하여 회주철 표면에 적용하였으며, 기계적 특성 평가를 통해 최적의 쇼트피닝 조건을 규명하고자 하였다. 쇼트피닝 분사조건에 따른 회주철의 내구성을 평가하기 위해 캐비테이션 실험을 실시하였으며, 경도 측정, 횡단면 관찰 및 표면의 3D 현미경 관찰 등을 통해 기계적 특성을 분석하였다. 캐비테이션 실험은 ASTM G32 규정에 의거하여 천연해수 내 $30^{\circ}C$에서 $50{\mu}m$의 진폭으로 실시하였다. 실험 후에는 주사전자현미경으로 손상표면을 관찰하였으며, 손상 정도를 비교하기 위해 무게 감소량을 계측하여 상호 비교/분석하였다.

• 한국산업융합학회 논문집
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• 제23권1호
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• pp.73-81
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• 2020

This study analyzed the causes of failure of SCM435 bolts that fix the springs of automobile air brakes that have been fractured during use. The cause of failure was analyzed using SEM, EDS, metallogical microscope and Vickers hardness tester. In the fracture, the ratchet mark began at the outer boundary of crack origin, and the grains at crack origin were found to have clear intergranular corrosion. One SCM435 bolt was subjected to a stress of 398 MPa, it's a stress of about 80% of the fatigue limit. As a result of such a large applied stress, cracks occurred at the corrosion origin and were fractured. In order to prevent the SCM435 bolt from fracture, it is necessary to use the correct composition, the accuracy of heat treatment, preventing damage by external impact, preventing corrosion of the damage part by moisture, and introduction a compressive residual stress by peening.