• 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.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.1
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• pp.78-85
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• 1986

This study has been made to investigate Behavior of the fatigue crack propagation for the purpose of taking into consideration the fatigue behavior Which initiate and propagate in tip of defect of the defected specimens, Which Contain the micro-hole or micro-slit. Especially, the specimens have been conducted with high-freguency heat treatment of 850.deg. C, 1050.deg. C to consider strength elevation of defected specimens. The results of this study are as follow; (1) The case of the same in the length of crack, the fatigue crack propagation rate of the micro-slit is always faster than that of micre-hole. But, the first step of the fatigue crack propagation it is not always so. (2) Fatigue crack propagation rate of specimens with micro-slit or micro-hole which have been treated with high-frequency heat treatment satisfy the following formula between the fatigue crack propagation rate and nominal stress; dl/dN .var..sigma.$^{m}$ *l$^{n}$ .

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.856-860
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• 2002

The main objective of this study is to consider the effect of fatigue crack behavior on the variable depth of micro hole defects in SM20C at the symmetric position. The fatigue crack propagation test is performed by rotary bending fatigue test machine. The relationship between crack length(2a), cycles(N) and crack growth rate(da/dN) are investigated in this study. The result from the rotary bending fatigue test under the applied stress at 250MPa turned out that the fatigue life illustrated almost constant when the depth of symmetric micro hole deflects is both part A and B at the hope depth(h) = 0.5mm.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.5
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• pp.842-849
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• 1997

The aim of this study is an investigation of the interaction of two micro hole defects affecting fatigue crack initation life and propagation behavior. The locatio of two micro hole defects was considered as an angle of alignment and the distance between the centers of two micro hole defects. The fatigue cracking behavior is experimented under bending. When micro defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of local plastic strain strongly played a role in the fatigue crack initiation lives. Therefore we introduce a parameter which contains the plastic deformation area at stress concentrations and propose a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two micro hole defects are studied experimentally.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.2
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• pp.278-285
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• 1988

The Critical size of artificially induced micro-holes in 0.17%, 0.36% Carbon steel Specimens with Spheroidized Cementite and in 0.17% carbon steel specimens with martensite structure is compared with annealed pre-crack in order to discuss the physical meaning of the fatigue limit and evaluation of the tolerant micro flaw size at the stress level of the fatigue limit. Results obtained were summarized as follows; (1) In this study, non-propagating crack length of Smooth specimen and critical pre-crack length (lc) is coincide. (2) In the carbon steels with spheroidized cementite structure, critical pre-crack length (lc) and allowable micro-hole size (dc) is coincide each other at the fatigue limit level. (3) It has been published that there exists a particular size of micro-hole which has no effect on the fatigue limit. In this study, the micro-hole of critical size can be regarded as equivalent to a tolerant micro flaw which would not reduce the fatigue limit.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.5 s.176
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• pp.1288-1296
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• 2000

In this study, fatigue crack behavior between arbitrarily located defects was investigated by experiment. Especially, stress interaction between micro hole defects and fatigue cracking, and fatigue crack initiation life following the variation of location of micro hole defects were considered. In addition, crack initiation position by micro hole stress interaction and the relationship between stress concentration factor and fatigue initiation life are studied in detail.

• 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.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.2
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• pp.141-144
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• 1984

The fatigue limits of crack initiation and propagation on the edge elliptical notched semi-infinite plate under completely reversed fatigue stress are determined theoretically. Assuming that the crack initiation and propagation occur when stress intensity factors of notched plate reach the critical values obtained from critical micro-crack length under plain fatigue limit loading and the threshold stress intensity factory, respectively, the fatigue limits of crack initiation and propagation are obtained. The induced theoretical fatigue limit of crack initiation is expressed in terms of plain fatigue limit, critical micro-crack length and notch shape. The one of crack propagation is in terms of threshold stress intensity factor, plain fatigue limit and notch shape. These theoretical results are showed to be in good agreement of Frost's experimental data.

• Journal of Mechanical Science and Technology
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• v.14 no.10
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• pp.1021-1027
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• 2000

In this study, fatigue tests were performed to examine the effects of micro drill hole on fatigue limit of as cast and austempered ductile cast iron (ADI) using the rotary bending fatigue tester. As results, micro drill holes ($diameter{\leq}0.4mm$) did not influence the fatigue limit of ADI, compared to annealed ductile cast iron; the critical defect size of crack initiation, in ADI was larger than as cast. If the ${\sqrt{area}}$ of micro drill hole and graphite nodule in ADI are comparable, crack initiates at the graphite nodule. When the ruggedness developes through austempering treatment process, microstructure on crack initiation at micro drill hole is tougher than that of as cast ductile cast iron.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.2
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• pp.150-157
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• 1985

In terms of behavior of fatigue cracks propagated after build-up around the artificial drilled miro-hole, this study has been made of the build-up process of slips and micro cracks, behavior of micro-crack propagation and the definition of fatigue limit under the rotating bending stress with low carbon steel. The results of this study are as follows: (1) The fatigue limit is the repropagating critical stress for the nonpropagating cracks which have grown to some limit around the micro-hole in regard of the magnitude of micro-hole. (2) Behavior of the slips and micro-cracks initiation are occurring simultaneously in front and in rear of micro-hole tips in the view of the rotational direction, regardless of the magnitude of micro-hole. (3) Behavior of fatigue crack propagation is different from magnitude of micro-hole, its behavior is propagation of single crack about respectively large hole, but about respectively small hole, fatigue crack propagated joining phenomena of micro-cracks. (4) The behavior of fatigue fracture is affected by the factor of its defects in the view of magnitude of micro-hole when the diameter of the micro-holes are smaller than 50.mu.m, and this is also affected with the size effect of micro-hole diameter.