• 한국산업융합학회 논문집
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• 제26권6_2호
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• pp.1011-1017
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• 2023

In this study, the harmless crack size (a_hml) by double shot peening (DSP) using shot balls with different diameters was evaluated on carburized, quenched-tempered SCM822H steel. The minimum crack size (a_NDI) detectable by non-destructive inspection was also evaluated. The relationship between the crack size (a_25,50) that reduces the fatigue limit by 25% and 50% and a_hml was evaluated. The residual stress of DSP was greater in SP_(0.6+0.08) than SP_(0.8+0.08) and appeared deeper in the depth direction. In addition, the hardness below the surface appeared larger. The fatigue limit of DSP increased 2.07 times and 1.95 times compared to non-SP. All a_hml of the DSP specimen was determined at the depth (a). The compressive residual stress distribution affects a_hml, and the a_hml of SP_(0.6+0.08), which has a large compressive residual stress and a high fatigue limit, appeared large. a_hml of SP_(0.6+0.08) introduced deeper than the residual stress of SP_(0.8+0.08) is larger in the range of As=1.0-0.3. Since the residual stress in the thickness direction has a greater effect on a_hml than the residual stress at the surface, it is necessary to introduce it more deeply. The relation of a_hml, a_25,50, and a_NDI were evaluated in the point for safety and reliability.

• Geomechanics and Engineering
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• 제8권4호
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• pp.541-557
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• 2015

This research aims to analyze the fracture coalescence characteristics of brittle sandstone specimen ($80{\times}160{\times}30mm$ in size) containing various flaws (a single fissure, double squares and combined flaws). Using a rock mechanics servo-controlled testing system, the strength and deformation behaviours of sandstone specimen containing various flaws are experimentally investigated. The results show that the crack initiation stress, uniaxial compressive strength and peak axial strain of specimen containing a single fissure are all higher than those containing double squares, while which are higher than those containing combined flaws. For sandstone specimen containing combined flaws, the uniaxial compressive strength of sandstone increase as fissure angle (${\alpha}$) increases from $30^{\circ}$ to $90^{\circ}$, which indicates that the specimens with steeper fissure angles can support higher axial capacity for ${\alpha}$ greater than $30^{\circ}$. In the entire deformation process of flawed sandstone specimen, crack evolution process is discussed detailed using photographic monitoring technique. For the specimen containing a single fissure, tensile wing cracks are first initiated at the upper and under tips of fissure, and anti-tensile cracks and far-field cracks are also observed in the deformation process; moreover anti-tensile cracks usually accompanies with tensile wing cracks. For the specimen containing double squares, tensile cracks are usually initiated from the top and bottom edge of two squares along the direction of axial stress, and in the process of final unstable failure, more vertical splitting failures are observed in the ligament region. When a single fissure and double squares are formed together into combined flaws, the crack coalescence between the fissure tips and double squares plays a significant role for ultimate failure of the specimen containing combined flaws.

• 한국군사과학기술학회지
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• 제2권1호
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• pp.101-109
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• 1999

본 논문은 항공기 구조물의 체결용 홀(hole)을 냉간가공(cold working)할때 홀주변에 생성되는 잔류응력이 균열발생 수명과 균열성장에 미치는 영향을 연구한 내용이다. 항공기용 재료로 많이 사용되는 AL7075-T6 및 AL2024-T3 시편으로 측정된 계수를 Morrow의 수명예측식에 적용하여 수정된 냉간가공(cold working)에 관한 수명예측식을 제안하였다. 수정된 계산식으로 얻어진 수명예측값과 실험에 의하여 이미 알려진 값이 비교적 일치함을 보여 이러한 재료에 대하여 수명예측이 가능함을 보였다. 균열성장 예측을 위해 역시 AL7075-T6 재료에 대하여 가중함수(weight function)방법으로 잔류응력 세기계수를 구하여 Forman의 균열성장 예측식을 수정하여 계산 한 결과 이 또한 알려진 실험값과 거의 일치함을 보여 실험에 사용한 재료의 구조물 해석에 유용 할 것으로 보인다. 본 연구는 항공기용 구조물의 홀주변을 냉간가공 (cold working)할 때 생성되는 잔류응력의 영향을 연구하는 기초적인 단계 일 뿐이며, 향후 실제 정비현장에서 적용 할 수 있는 대상분야와 연구 방향에 대한 보다 심층적 연구가 필요함을 보인다.

• Structural Engineering and Mechanics
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• 제66권3호
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• pp.343-351
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• 2018

Brazilian disc test is one of the most widely used experiments in the literature of geo-mechanics. In this work, the pre-holed concrete Brazilian disc specimens are numerically modelled by a two-dimensional discrete element approach. The cracks initiations, propagations and coalescences in the numerically simulated Brazilian discs (each containing a single cylindrical hole and or multiple holes) are studied. The pre-holed Brazilian discs are numerically tested under Brazilian test conditions. The single-holed Brazilian discs with different ratios of the diameter of the holes to that of the disc radius are modelled first. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured and the crack propagation mechanism around the wall of the ring is investigated. The crack propagation and coalescence mechanisms are also studied for the case of multi-holes' concrete Brazilian discs. The numerical and experimental results show that the breaking mechanism of the pre-holed disc specimens is mainly due to the initiation of the radially induced tensile cracks which are growth from the surface of the central hole. Radially cracks propagated toward the direction of diametrical loading. It has been observed that for the case of disc specimens with multiple holes under diametrical compressive loading, the breaking process of the modelled specimens may occur due to the simultaneous cracks propagation and cracks coalescence phenomena. These results also show that as the hole diameter and the number of the holes increases both the failure stress and the crack initiation stress decreases. The experimental results already exist in the literature are quit agree with the proposed numerical simulation results which validates this simulation procedure.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2004년도 춘계 학술발표회 제16권1호
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• pp.632-635
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• 2004

In this paper, material property of fiber reinforced concrete(FRC) according to the steel fiber, glass fiber and carbon fiber blended ratio. The fiber reinforced concretes are increased mechanical strength, because the fibers are dispersed with randomly direction and disturb crack progression in concretes. Adhesive fracture is occurred slowly at interface between fiber and concrete, and the fracture energy is absorbed due to softening phenomenon.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2004년도 추계학술발표대회 논문집
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• pp.167-170
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• 2004

In order to investigate the impact behaviors of 3-D braided glass/epoxy composites, the energy profiles and damage area were compared to the laminates of similar volume fraction and composition. The energy profiles showed different characteristics from each other which indicates they have distict energy absorption mechanisms. The image analysis on the damage projections visualized the crack propagation paths along the fiber direction.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2005년도 추계학술발표회
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• pp.324-325
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• 2005

• Composites Research
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• 제32권6호
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• pp.327-334
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• 2019

층간분리는 복합재 적층판에서 발생하는 특수한 파손 모드이다. 유한요소해석기법을 활용하여 균열 성장방향이 확실히 예측되는 일방향 복합재 적층판의 층간분리 거동과 관련된 많은 수치적 연구가 수행되었다. 반면에 여러 방향으로 적층된 복합재 적층판의 층간분리는 층간 균열 뿐 만 아니라 기지재료 파손 및 섬유 브릿징을 수반하는 층내 균열이 발생한다. 또한 층간 균열과 층내 균열이 불규칙적인 비율로 나타나고 층내 균열도 임의의 각도로 성장한다. 이러한 직교적층 복합재 적층판의 균열 성장 방향에 대한 예측은 확정론적 해석 방법 보다는 확률론적 해석 방법이 유리하다. 본 논문에서는 직교적층 탄소섬유/에폭시 복합재 적층판에 모드 I 하중이 가해질 때 균열 경로를 분석하여 향후 확률론적 해석의 기반 자료로 사용할 수 있는 확률 데이터를 수집하고 분석하였다. 직교이방성 재료의 균열선단에서의 응력장 해석결과를 활용하여 균열 성장 방향을 분석할 수 있는 두 가지 기준을 제안하였다. 제안한 방법을 이용하여 직교적층 탄소섬유/에폭시 복합재 적층판의 균열 성장 방향을 정성적, 정량적으로 분석하고 실험값과 비교분석하였다.

• 한국전기전자재료학회논문지
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• 제14권7호
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• pp.578-583
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• 2001

The bushing for high voltage and large and power should endure weight of itself and force of pushing from contact with circuit breaker. Like this, epoxy mold bushing has to be strong without fault. However, the external circumstances and internal factors was caused by partial discharge, flashover and dielectric breakdown. Therefore, to remove external factor of defect and to prevent the internal cracks and cavity generated from the contraction on interface of Cu-Epoxy, we should form semi-conductive layer on Cu bar by carbon. Then, the PD properties and the insulation qualities of epoxy mold type bushing was able to improved by roles of cushions for the direction of diameter and by effects fo natural sliding like as separated from conductor for the direction of length. So, in this work, we could prove the method of semi-conductive layer in making the long conductor.

• 한국세라믹학회지
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• 제36권7호
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• pp.711-717
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• 1999

Biaxial fracture behavior of alumina specimens with the diameter of 20mm and four kinds of thickness of 1.9, 2.3, 2.6 and 2.8mm was studied by the ball-on-3-ball test and the fracture results were analyzed by he analysis of variance (ANOCA), The strength measured with the down speed prescribed in ASTM showed that the measured strength was not dependent on the thickness of the specimens. Equivalent radius and crack-braching number were observed to increase lineraly with the thickness of the specimens. The jog direction was observed to study the effect of grinding direction on surface flaws. It is though that the surface finishing with #600 grit diamond wheel did not affect the surface flaws of the specimens.