• International Journal of Precision Engineering and Manufacturing
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• 제3권3호
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• pp.76-81
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• 2002

In order to study the fatigue crack and retardation mechanism in variable loading, the effects of crack tip branching in crack growth retardation were examined. The characteristics of crack tip branching behavior were considered with respect to microstructure and crack tip branching angle was examined. Crack tip branching was observed along the grain boundary of finite and pearlite structure. It was found that the branching angle ranges from 25 to 53 degrees. Using the finite element method, the variable of crack driving farce to branching angle was examined. The effective crack driving farce (K$\_$eff/) decreased as the branching angle increased. The rate of decrease was 33% for kinked type and 29% for forked one. It was confirmed that the effect of crack tip branching is a very important factor in crack growth retardation. Therefore, crack branching effect should be considered in building the hypothetical model to predict crack growth retardation.

• 한국생산제조학회지
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• 제8권2호
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• pp.126-136
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• 1999

We studied on crack growth retardation in single overloading condition. Crack tip branching which as the second mechanism on crack growth retardation was examined. Crack tip branching was observed to kinked type and forked type. It was found that the branching angle range was from 25 to 53 degree. The variations of crack driving force with branching angle were calculated with finite element method The variation of {{{{ KAPPA _I}}}}, {{{{ KAPPA _II}}}} and total crack driving force(K) were examined respectively So {{{{ KAPPA _I}}}}, {{{{ KAPPA _II}}}} and K mean to mode I, II and total crack driving force. Present model(Willenborg's model) for crack growth retardation prediction was modified to take into consideration the effects of crack tip branching When we predicted retardation with modified model. it was confirmed that predicted and experimental results coincided with well each other.

• 한국정밀공학회지
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• 제14권6호
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• pp.83-89
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• 1997

In order to study on fatigue crack retardation and retardation mechanism in variable loading, the effects of crack tip branching in fatigue crack growth retardation were examined. The characteristics of crack tip banching behavior was considered to micro structure. It was examined that the variation of crack tip branching angle. Crack tip branching was observed along the grain boundary of ferrite and pearlite structure. It was found that the abanching angle ranges from 25 to 53 degrees. Using the finite element method, the variable of crack driving force to branching angle was examined. The effective crack driving force ( $K_{\eff}$ ) decreased as the braching angle increases. The rate of decrease was 33% for the kinked type and 29% for the forked one. It was confirmed that the effect of crack tip branching is a very important factor in fatigue crack growth retardation. Therefore, crack branching effect should be considered building the hypoth- etical model to predict crack growth retardation.

• 대한기계학회논문집
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• 제19권2호
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• pp.451-462
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• 1995

Single overload tests performed to examine the crack retardation behavior for the specimen thickness and overload ratios. Delayed crack length was tend to increase in small thickness and big overload ratio but was difference between delayed crack length and plastic zone size that expected in specimen thickness. So retardation behavior that estimated in plastic zone size, was not sufficient. Crack tip branching and striation distribution, secondary mechanisms that effected in retardation behavior, was examined by experiment and finite element analysis. Crack tip branching was affected by micro structure, and appeared the more complicatedly according to increasing damage by overload and decreasing crack driving force in base line stress level. And crack tip branching the branching angle decreased crack driving force in the crack tip. And a characteristic of the fractography on retardation zone was that striation distribution did not appear due to decreased crack driving force.

• 한국전산구조공학회논문집
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• 제24권6호
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• pp.637-643
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• 2011

본 논문에서는 결합 기반 페리다이나믹스 해석법을 사용하여 동적취성 파괴시뮬레이션을 수행하였다. 페리다이나믹스 모델은 분기 균열, 균열 불안정성, 균열 경로의 비대칭성, 연쇄 분기 균열, 2차 균열 전파 등 다양한 동적취성 파괴현상을 잘 해석해 낼 수 있다. 본 논문에서는 분기 균열의 분기 각도와 균열 전파속도에 대한 응력파의 영향에 대해 연구하였다. 극한 시점에 도달한 균열은 둘 이상으로 분기되어 전파되고 그 전파속도는 기존 균열의 전파속도와 크게 달라지지 않는다는 사실이 여러 실험을 통해서 입증이 되었다. 페리다이나믹스로 해석된 분기 균열은 실험을 통해 제안된 균열 전파현상들과 잘 부합되는 것을 확인할 수 있었다.

• Structural Engineering and Mechanics
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• 제75권3호
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• pp.339-356
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• 2020

In this article, a developed bond-based peridynamic model for functionally graded materials (FGMs) is proposed to simulate the dynamic fracture behaviors in FGMs. In the developed bond-based peridynamic model for FGMs, bonds are categorized into three different types, including transverse directionally peridynamic bond, gradient directionally peridynamic bond and arbitrary directionally peridynamic bond, according to the geometrical relationship between directions of peridynamic bonds and gradient bonds in FGMs. The peridynamic micromodulus in the gradient directionally and arbitrary directionally peridynamic bonds can be determined using the weighted projection method. Firstly, the standard bond-based peridynamic simulations of crack propagation and branching in the homogeneous PMMA plate are performed for validations, and the results are in good agreement with the previous experimental observations and the previous phase-field numerical results. Then, the numerical study of crack initiation, propagation and branching in FGMs are conducted using the developed bond-based peridynamic model, and the influence of gradient direction on the dynamic fracture behaviors, such as crack patterns and crack tip propagation speed, in FGMs is systematically studied. Finally, numerical results reveal that crack branching in FGMs under dynamic loading conditions is easier to occur as the gradient angle decreases, which is measured by the gradient direction and direction of the initial crack.

• 대한기계학회논문집A
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• 제39권11호
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• pp.1137-1143
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• 2015

프레팅 피로균열의 2단계 성장과 분지, 즉 균열이 경사방향으로 성장하다가 방향을 전환하여 수직방향으로 성장하는 과정을 유한요소법으로 해석하였다. 해석에서 A7075-T6의 프레팅 피로실험자료를 이용하였다. 균열성장방향을 결정하는 기준으로 최대 접선응력확대계수, 최대 접선응력확대계수범위, 최대 균열성장속도의 적용 가능성을 검토하였다. 하나의 기준으로는 분지 전후의 균열성장방향을 모사할 수 없고, 분지 전후에 다른 기준을 적용하면 모사가 가능하였다. 또한 분지가 발생하는 균열 길이를 결정하는 방법도 제시하였다.

• 한국전산구조공학회논문집
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• 제28권3호
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• pp.309-316
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• 2015

상태 기반 페리다이나믹 모델은 일반적인 재료 구성 모델을 구현할 수 있고 비국부 영역 내에서 연결된 모든 결합의 변형을 통해 각 절점의 재료 응답이 결정되기 때문에 체적 및 전단 변형을 모두 표현할 수 있다. 따라서 상태 기반 모델은 복잡한 동적 취성 파괴 현상(분기균열, 2차 균열, 계단균열, 균열 유착 등)을 해석하는데 유용하다. 본 논문에서는 평면응력 탄성체에 대해 2차원 상태 기반 페리다이나믹 모델을 적용하고 에너지해방율과 페리다이나믹 에너지 포텐셜로부터 손상 모델을 구성하였다. 페리다이나믹 파괴 해석 모델을 통해 취성 유리 재료에 대해 균열 면에 평행한 압축 응력파가 균열 분기 패턴에 미치는 영향에 대해 조사하였다. 실험을 통해 관찰된 동적 균열 진전 및 분기 패턴에 대한 주요 특성들이 페리다이나믹 해석을 통해 확인되었다. 또한 강한 인장 하중 하의 계단균열과 이차균열 등이 상태 기반 페리다이나믹 시뮬레이션을 통해 잘 모사가 되는 것을 확인할 수 있었다.

• Journal of Welding and Joining
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• 제6권1호
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• pp.46-52
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• 1988

Retardation or delay in fatigue crack growth due to overloads are important for the accurate prediction of fatigue lives of structural materials. In this study, retardation of fatigue crack growth in Al 6061-T6 weldments and heat affected zones (HAZ) after single overload cycle had been investigated. Retardation in both weldments and HAZ was observed. It was concluded that retardation in both weldment and HAZ was greater than in base metal due to microstructural change and crack branching and crack closure were major governing factor in retardation.

• Structural Engineering and Mechanics
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• 제71권1호
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• pp.65-75
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• 2019

In this article, the dynamic fracture instability characteristics, including dynamic crack propagation and crack branching, in PMMA brittle solids under dynamic loading are investigated using the discrete element method (DEM) simulations. The microscopic parameters in DEM are first calibrated using the comparison with the previous experimental results not only in the field of qualitative analysis, but also in the field of quantitative analysis. The calibrating process illustrates that the selected microscopic parameters in DEM are suitable to effectively and accurately simulate dynamic fracture process in PMMA brittle solids subjected to dynamic loads. The typical dynamic fracture behaviors of solids under dynamic loading are then reproduced by DEM. Compared with the previous experimental and numerical results, the present numerical results are in good agreement with the existing ones not only in the field of qualitative analysis, but also in the field of quantitative analysis. Furthermore, effects of dynamic loading magnitude, offset distance of the initial crack and initial crack length on dynamic fracture behaviors are numerically discussed.