• Journal of the Korean Society for Precision Engineering
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• v.16 no.12
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• pp.143-151
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• 1999

The shape of surface micro-crack is very irregular due to nonhomogeneous microstructure but is very important in respect to qualitative estimation of fatigue life. Fractal geomety can quantify the shape of surface mciro-crack. Fractal dimension is measured for surface micro-cracks with coast line and box counting method and estimates cycle ration in Al 2024-T3. The average fractal dimension $D_{favg}$ of surface micro-cracks has 3-parameter weibull distribution and location parameter is nearly constant but shape parameter decreases as cycle ration increases. The fractal dimension by coast line method is measured for individual surface micro-crack but the fractal dimension by box countin method is measured for all the surface micro-cracks under sampling area. Therefore, This paper shows fractal dimension $D_{fb}$ can predict cycle ratio $N/N_f$ more convenient than fractal dimension $D_{favg}$.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.521-527
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• 1997

Irregular shapes and growth behavior of surface micro-crack showed very complex and nonlinear propeties and many investigators have performed theoretical analysesand experiments on them to characterize fatigue strength. They had difficulties in estimating fatigue life due to random distribution, growth and coalescence of surface micro-cracks. The straightness of crack growth along intergranular and transgranular was prevented from irregular microstructure and precipitates. Euclid geometry can't quantify shape of surface micro-crack but ftractal geometry can. Therefore, it is suggested that average fractal dimension of surface micro-cracks is able to estimate fatigue life but fractal dimension of maximum surface micro-crack is not in Al 2024-T3 alloy.

• Journal of Ocean Engineering and Technology
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• v.2 no.2
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• pp.104-111
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• 1988

This paper deals with the micro-surface-cracks behavior on the unnotched smooth specimens of Type 304 stainless steel at $593^{\circ}C$ in air under creep and creep-fatigue conditions that have 10 mim and 1 min load holding times respectively. The behaviors of the micro-surface-cracks have been visualized by means of surface replica method and optical micro-photography. The quantitative characteristics of initiation, growth and coalescence of micro-surface-cracks have been investigated by observing and measuring the crack growth behaviors. some of the important results are as follows: Main crack initiates at grain boundary in the early stage(10 to 20%)of its life time and grows through coalescence and finally leads to fracture. The distribution of micro-surface-crack length, 2a, can be plotted against the composite Weibull distribution. The growth rate of the main crack can be plotted against the stress intensity factor, crack tip opering displacement and J integral.

• Journal of the Semiconductor & Display Technology
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• v.14 no.3
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• pp.23-26
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• 2015

A seam carving based micro-crack detection method is proposed which aims at detecting the micro-crack regions in heterogeneously textured surface of polycrystalline solar cells. By calculating the seam which is a connected path of low energy pixels in the image, the micro-crack regions can be detected. Experimental results show that the proposed seam carving based micro-crack detection method has superior efficiency in detecting the micro-crack without background noise pixels and the algorithm's computation time is less than the conventional algorithm.

• Journal of Ocean Engineering and Technology
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• v.14 no.1
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• pp.29-36
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• 2000

Surface micro-crack grows along intergranular or transgranular region of crystal grains. But if it meets the barrier such as sessile dislocation and precipitates it loses straightness and deflects. Investigators had many difficulties in estimating fatigue life of smooth specimen because of the random distribution growth and coalescence of surface micro-cracks. The path of surface micro-crack has irregularity due to nonhomogeneous microstructure. Euclidian geometry can't quantify the shape of surface micro-crack but fractal geometry can. Therefore in this paper fractal dimension is measured at various stage of cycle ratio and estimated cycle ratio in 2024-T3 aluminium, alloy.

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.31-37
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• 2000

When a crack is produced in a concrete structure, a micro crack zone of fracture process zone (FPZ) appears at the crack tip. To investigate the behaviour of this the micro crack zone, nonlinear fracture mechanics (NLFM) must be applied. However, when a massive concrete structure such as a concrete gravity dam is considered, the micro crack zone can be neglected and the structure can be assumed to have linear elastic fracture mechanics (LEFM) behaviour. This study is divided into two main topics : (1) Calculating stress intensity factor (SIF) at the crack tip by surface integral method and (2) Investigating the propagation of the initial crack. If the initial crack propagates, the angle of the propagation is calculated by using maximum circumferential tensile strength theory. This study, also, contains the effects of body forces and water pressures on the crack face.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.9
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• pp.1746-1752
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• 2002

Fatigue fracture in machine components is produced by surface micro-crack from stress concentration area such as notch and material defect. It is difficult to predict the remaining fatigue lift of mechanical components because the surface micro-crack on critical area initiates and grows with statistical distribution. Plane bending fatigue tests were carried out on the plain specimen of Al 2024-T3 and the initiation and growth behavior of surface micro cracks were observed. The statistical distribution of surface length of multiple micro cracks and their maximum length were investigated. The maximum surface crack length distributions were analyzed on the basis of the statistics of extremes in order to examine the prediction of remaining life.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.6
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• pp.1320-1326
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• 1988

This paper deals with the behavior of surface crack growth and the characteristics of surface micro-crack distribution under creep and creep-fatigue with 1 min. and 10 min. of load holding times at 593.deg.C, in air. The test specimen is a plate type with a small artificial defect of type 304 the small defect has been carried out by the surface replica method and optical microphotography. The experimental results have been interpreted from the view-point of fracture mechanics. It can be concluded that the longer the hold time the longer the total life time. Most of surface micro-cracks initiate at grain boundaries before the specimen reaches 20% of its total life time, a few of them lead to fracture by coalescence with the main crack.

• Journal of Ocean Engineering and Technology
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• v.1 no.2
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• pp.74-82
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• 1987

This study has been made to investigate into the behavior of fatigue limit, of fatigue crack initiation, and of fatigue crack propagation under the condition of rotating bending stress; specifically on the independency of stress field as well as the crack behavior of surface micro hole defect, which is made artificially through the specimen. The results obtained can be summarized as followa; 1) For the single micro hole defect, initiation of fatigue crack is occurred at both tips of microhole defect simultaneosly along the slip which are produced in the range of maximum principal stress arround micro hole defect independent of the size of micro hole defect. 2) For the neighbored deuble micro hole defects with equal size, in the range ($\frac{L}{r}$)ratio $\gtrsim$ 3 defined as the size of micro hole defect(2r) to the distance between the centers of micro hole defects (2L), the crack behavior of the micro hole defects is same as single one. However, for the range of $\frac{L}{r}$<3, the interference effect becomes significant as the ratio approaches to 1.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2001.04a
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• pp.1052-1055
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• 2001

In micro-drilling of brittle materials including glass, cracks occur at the exit surface. In drilling glass, the main type of crack is cone crack. Cone crack is generated by thrust force acting at the bottom surface of the workpiece. Cone crack size could be reduced by changing cutting conditions, but cone crack still existed. Two methods were proposed to prevent crack formation and perfect hole shapes were obtained. One method is attaching two glass plates with water and the other method is constraining two glass plates. The proposed methods eliminated tensile stress acting on the exit surface of glass and prevented crack propagation.