• Journal of Ship and Ocean Technology
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• 제11권2호
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• pp.26-33
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• 2007

For the assessment of the retardation of fatigue crack propagation behavior due to overload, new FE analysis algorithms considering compressive residual stress redistribution near crack tip was proposed in this paper. The size of plastic zone near crack tip was obtained by elasto-plastic analysis and it was compared with Irwin's equation. The amount of residual stress redistribution was assessed by subsequent elasto-plastic analysis, and the difference of residual stress distributions between constant amplitude load and overload was obtained. In the analysis of fatigue crack propagation, the applied SIF range was evaluated by ASTM E647, and the effect of residual stresses on crack propagation was considered using the effective SIF concept. The test results of crack propagations were compared with the predicted data obtained by the analysis.

• Structural Engineering and Mechanics
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• 제15권3호
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• pp.367-378
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• 2003

The magnitude of the plastic zone around the crack tip of DENT (Double Edge Notched Tension) specimen and the crack growth length under cyclic loading were measured by ESPI (Electronic Speckle Pattern Interferometry) system. The measured magnitude of plastic zone was compared with the proposed by Irwin and calculated by a nonlinear static method of MSC/NASTRAN. The measured crack growth length by ESPI system was also compared with the obtained data by the image analysis system. From the study, it is confirmed that the plastic zone and crack growth length can be measured accurately with the high-tech equipment (ESPI System).

• International Journal of Precision Engineering and Manufacturing
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• 제7권3호
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• pp.30-34
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• 2006

We investigated fatigue crack growth behavior in laser-welded sheet metal caused by a single applied overload The fatigue specimens were made using butt jointed cold rolled sheet metal that was welded with a $CO_2$ laser, The effects of the specimen thickness and overload ratio were determined from fatigue crack propagation tests, These tests were performed in such a way that the fatigue loading was aligned parallel to the weld line while the crack propagated perpendicular to the weld line, Overload ratios of 1.0, 1.5, and 2. 0 were applied near the tip of the fatigue crack at points located 6, 4, and 2 mm from the weld line. The specimens were either 0.9 or 2.0 mm thick. The size of the plastic zone at the crack tip due to the single applied overload was also determined using finite element analysis.

• 한국정밀공학회지
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• 제13권8호
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• pp.80-87
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• 1996

The present paper will give some results of the fatigue behavior of typical axi-symmetric forward extrusion die. The extrusion process is analyzed by rigid-plastic FEM and the deformation analysis of extrusion die is conducted by elasto-plastic FEM. To approach the crack problem LEFM (Linear Elastic Fracture Mechanics) is introduced. Using special element in order to conside the sigularity of stress/ strain in the vicinity of the crack tip, stress intensity factor and the effective stress intensity factor is calculated. Applying proper fatigue crack propagation criterion such as Paris/Erdogan fatigue law and maximum principal criterion to these data, then, the angle and the direction of fatigue crack propagation is simulated. In result, it is proved that the simulated fatigue crack propagates in the zigzag path along the radial direction and fatigue life of the extrusion die is evaluated by using the computed crack growth rate.

• 대한기계학회논문집A
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• 제27권4호
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• pp.577-584
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• 2003

A crack with growth in ferroelectric ceramics under purely electric loading is analyzed. The crack tip stress intensity factor for the growing crack under small-scale conditions is evaluated by employing the model of nonlinear domain switching. The crack tip stress intensity factor increases or decreases with crack growth, depending on the electrical nonlinear behavior and the direction of an applied electric field. It is shown that the ferroelectric material can be either toughened or weakened as the crack grows. The steady state crack growth in ferroelectric ceramics is also discussed.

• 한국자동차공학회논문집
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• 제13권6호
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• pp.63-69
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• 2005

For the case that center crack is surrounded by four small cracks which are symmetrically distributed around center crack, the same values of normalized stress intensity factor of center crack according to the position of the tip of small cracks are located on the smooth curve. And the stress intensity factor according to any position of small cracks can be sufficiently obtained from this curve. The plastic zones between distributed cracks are also investigated by changing the positions of nearly small cracks. The occurrence of plastic zone due to the interaction between center crack and small cracks are analyzed by finite element method. The mechanical behavior at the vicinity of crack tips is investigated by plastic areas. The changes of plastic zones according to positions of distributed cracks are drawn schematically. The safety of materials is also analyzed.

• 대한기계학회논문집A
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• 제20권4호
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• pp.1177-1185
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• 1996

The crack growth under creep condition is one of the major damage mechanisms which determines remaining life of the component operating at high temperatures. In this paper, the creep crack growth by grain boundary cavitation is studied, which is frequently observed failure mechanism for creep brittle materials. As a result of diffusive growth of creep cavities, it is shown that the crack-tip stress field is modified from the original stress distribution by the amount of singularity attenuation parameter which is function of crack growth rate and material properties. Also, the stress relaxation at crack-tip results in the extension of cavitating area by the load dump effect to meet the macroscopic force equilibrium conditdion.

• 비파괴검사학회지
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• 제29권5호
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• pp.473-478
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• 2009

본 연구에서는 비파괴법을 이용하여 압력용기 강 다층용접부에서 채취한 표준 샤르피 시험편의 균열선단에서 형성되는 소성변형 거동을 평가하였다. 모재, 후열처리재 및 용접재를 대상으로 시험편에 기계적인 노치를 가공한 후 예균열(pre-crack)을 낸 다음 4점 굽힘실험과 음향방출(acoustic emission: AE)실험을 동시에 실시하였다. 균열선단은 용접재외 후열처리재의 경우 용융선 근처에 위치하도록 하였다. 하중이 가해짐에 따라 균열선단에서 형성되는 소성영역의 크기는 응력확대계수를 이용하여 구했으며, 각 시험편에 대한 소성역의 크기와 음향방출 특성과의 관계는 축적된 AE 에너지 관점에서 고찰하였다. 시험편에 관계없이 탄성역역에서는 거의 AE신호가 감지되지 않았으며, 대부분의 AE신호는 소성변형 과정에서 발생하였다. 또한, 용접재가 모재와 후열처리재에 비해 AE신호가 훨씬 많이 발생하였다. 모재와 후열처리재 및 용접재의 균열선단에서 소성영역 크기와 축적된 AE에너지와의 관계는 현저히 다르게 나타났으며, 용접재의 AE counts는 모재와 후열처리재에 비해 많이 발생하였다.

• 대한기계학회논문집A
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• 제22권3호
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• pp.611-623
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• 1998

The delamination fracture of multidirectional carbon-fiber/epoxy laminates under the Mode I condition has been studied using the modified beam analysis for a fracture mechanics approach. It was found that the variation of fracture energy $G_IC$ with increasing length of the propagating crack exhibited a minimum for the pure interlaminar fracture and a maximum for the intraply fracture,i.e. a rising "R-curve", which was strongly affected by the degree of fiber bridging and crack-tip splitting arising in the global delamination. The maximum $G_IC$ value was significantly dependent on such types of delamination as no crack jumping, crack jumping into the adjacent ply and edge-delamination. It was shown also that the value of "effective flexural modulus" estimated from the modified beam analysis increased much with the development of fiber bridging behind the crack tip.ehind the crack tip.

• International Journal of Concrete Structures and Materials
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• 제18권3E호
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• pp.191-198
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• 2006

The objective of this study is to examine fracture characteristics of concrete at early ages, i.g. critical stress intensity factor, critical crack-tip opening displacement, fracture energy, and bilinear softening curve based on the concepts of effective-elastic crack model and cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By experimenting with various strengths and ages, load-crack mouth opening curves were obtained, and the results were analyzed by linear elastic fracture mechanics and FEM(finite element method). The results from the test and analysis showed that critical stress intensity factor and facture energy increased while critical crack-tip opening displacement decreased with concrete aging from 1 day to 28 days. Four parameters of bilinear softening curve from 1 day to 28 days were obtained from a numerical analysis. The obtained fracture parameters and bilinear softening curves at early ages from this study are to be used as a fracture criterion and an input data for the finite element analysis of concrete at early ages.