• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.187-192
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• 2013

Fatigue crack was detected from a temperature change around surface crack using the thermographic technique. Thermal gradient across the crack decreased very much due to thermal resistance of contact surface in the crack. Heat diffusion flow passing through the discontinuity was visualized in temperature by infrared camera to find and locate the crack. A fatigue crack specimen(SM-45C), which was prepared according to KS specification and notched in its center to initiate fatigue crack from the notch tip, was heated by halogen lamp at the end of one side to generate a heat diffusion flow in lateral direction. A abrupt jump in temperature across the fatigue crack was observed in thermographic image, by which the crack could be located and sized from temperature distribution.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.2
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• pp.402-407
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• 1995

The effects of room temperature and strain aging treatment are discussed on the critical condition for the onset of growth of non-propagating cracks on 0.15% C low carbon steel, with special emphasis on the length of the critical non-propagating crack and on the crack opening displacement(COD) at the crack tip. It is found from the experimental analysis that room temperature and strain aging of a fatigue pre-cracked specimen introduced the closure of a crack tip of the pre-crack and the reduction of crack opening displacement at the wake of crack, together with an improvement in crack growth resistance of the microstructure. This may cause an increase in the endurance limit of the specimen, through the enhancement of effective stress for the onset of growth of the critical non-propagating crack.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.8
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• pp.1493-1502
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• 1992

The purpose of this study is to identify the effects of loading variables on fatigue crack opening behavior, using structural steel SS41. To use various stress ratios, three kinds of the methods varying stress ratio were used; .DELTA.P const., P$_{max}$ const., and P$_{min}$ const.. To measure the opening load, the direct measuring method which measures the distance between two micro indentations 20 .mu.m behind crack tip and the elastic compliance method were used. As the results of present study, the following conclusions are obtained. First, using the direct measuring method, it was possible to measure the COD at any location behind crack tip. Second, as measuring point becomes farther from crack tip, opening load becomes smaller. Third, the acceleration of da/dN near notch is due to crack opening behavior. Finally, opening ratio is a function of not only R, but .DELTA.K.K.K.

• 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 KWS Conference
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• 1991.11a
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• pp.118-122
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• 1991

• Journal of Power System Engineering
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• v.6 no.1
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• pp.61-67
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• 2002

15Cr-5Ni 석출강화 스테인리스강 3종류의 피로균열 발생과 성장 특성 및 파괴인성에 대하여 노치함수로서 연구하였다. 3종류강의 열처리 조건은 $482\;^{\circ}C,\;579\;^{\circ}C$ 및 $621\;^{\circ}C$이다. $621\;^{\circ}C$에서 4시간동안 열처리한 시험편 C는 약 $280\;MPa\;\sqrt{m}$의 가장 높은 파괴인성을 보였으며, 3종류에서 피로균열 성장이 가장 늦었다. $482\;^{\circ}C$에서 1시간 열처리한 시험편 A에서, 피로균열발생한계, ${\Delta}k{\rho}$, 는 노치반경0.3 mm에서 약 $280\;MPa\;\sqrt{m}$의 가장 높은 값을 보였다. 시험편 A는 시험편 B와 C보다 피로균열 성장이 빨랐지만, 피로균열 발생이 늦었다. 예 하중에 의한 노치선단의 압축잔류응력은 노치 시험편의 피로강도 향상에 유용한 방법이었다.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.6
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• pp.133-141
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• 2002

This paper describes a versatile finite element technique which has been used to investigate wide range of structural defects of practical importance. The procedure automatically remeshes the three-dimensional finite element model during the stages of crack growth. Problems include the surface cracks in leak-before-break situations, the shape development of multiple surface defects.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.7
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• pp.1097-1105
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• 1997

Because of the existence of stress interaction field made by other defects and propagating cracks, the structure may be weakened. Therefore in this study, the crack behavior in the interaction field made by two different cracks is studied experimentally. In the experiment, vertical distance between two cracks and applied stress are varied to make different stress interacted field. In addition, the effect of plastic zone is used to examine crack propagation path and rate. Three types of crack propagation in the interacted field were found, and crack propagating path and rate of two cracks were significantly changed according to different applied stress as each crack propagates. And the results are attributed to the effect of the size and shape of the plastic zone.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.46-50
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• 1993

The sensitivity study to evaluate the effects of parameters on the fatigue crack propagation life under the constant loadings is executed in the previous study of the authors. It is shown that the effect of the crack opening ratio is large comparatively. The purpose of this paper is to evaluate the effects of parameters on the fatigue crack propagation life under the random loadings. A new method of evaluation of the effective stress under the random loadings is developed. The sensitivity study of parameters on the fatigue crack propagation life under the random loadings is executed by using it.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.6 no.6
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• pp.531-535
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• 2005

This study investigates the behavior of fatigue crack propagating between holes or holes lilted with another materials. It is experimentally and analytically confirmed that the center crack stops when its tip reaches near the center line of the holes and a small crack is initiated from the boundaries of holes or the holes filled with another materials and it propagates to final fracture. The mechanical behaviors of center crack near the another materials are also investigated. The phenomenon that this crack propagates to fracture is investigated by compliance method.