• Proceedings of the Korean Society of Precision Engineering Conference
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• 1991.11a
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• pp.21-31
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• 1991

대부분의 구조물은 정하중뿐아니라 동하중을 받고있으며, 반복되는 동하중에 의해 구조물은 피로를 받게되고 가장 취약한 부위에 작은 미시균열이 발생된다. 미시균열이 성장, 확산되어 기술적으로 인지되는 길이가 0.5 mm 이상이 될때 이를 거시균열이라 하는데, 거시균열의 균열성장은 파괴역학적 해석 방법의 도입으로 많은 공학적 재료들이 광범위하게 연구되었으며, 거시균열의 확산과 연관되어지는 균열닫힘과 미시구조의 관계가 연구되었다. 최근에는 거시균열의 해석과 같이 응력강도가 미시균열에서의 확산거동에 미치는 영향을 설명하는데 많은 관심을 가지게 되었다.(중략)

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.9
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• pp.2145-2152
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• 1993

The objective of this paper is to examine the detection limit, growth characteristics and notch curvature radius in short crack problem. Measurement techniques such as ultrasonic method and back-face strain compliance method were adopted. The fatigue crack growth rate of the short crack is slower than that of a long crack for a notched specimen. The characteristic of crack growth and crack closure is same as the case of a delay of crack growth caused by constant amplitude load for an ideal crack or single peak overload for a fatigue crack. The short crack is detected effectively by ultrasonic method. A short surface crack occurs in the middle of specimen thickness and is transient to a through crack depth is larger than the notch curvature radius.

• Journal of Korean Society of Steel Construction
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• v.12 no.3 s.46
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• pp.239-250
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• 2000

This study investigates the mechanical behavior on trough rib to crossbeam joint in orthotropic steel plate decks, specially emphasizing on the effect of diaphragm inside trough rib on the fatigue behavior of slit by static and fatigue tests. In particular, the effects of diaphragm on in-plane stress and out-of-plane stress, stress concentration, propagation of fatigue cracks at the silt are studied. With the result of experiment and numerical analysis, we have estimated the fatigue strength using the nominal stress and hot-sport stress. The details with diaphragm have occurred about 50% stress reduction at trough rib part of trough rib to crossbeam joint than the detail without diaphragm, however, the lower parts of crossbeam have occurred much more stress. Initial crack size or slit have an considerable influence on the propagation of fatigue cracks due to V-notch. The fatigue strength category of the details without diaphragm has higher value than fatigue limit, whereas that of the details with diaphragm is estimated lower than fatigue limit.

• Journal of the Korean Ceramic Society
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• v.40 no.7
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• pp.662-666
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• 2003

Hertzian cyclic fatigue properties of the alumina-glass dental ceramics were evaluated in exact in vitro environment at contact loads from 200 N to 1000 N and up to 10$\^$6/ cycles to investigate the indentation damage and strength degradation. At 200 N, no strength degradation and crack generation was observed up to 10$\^$6/ contact cycles. As load increased from 200 N to 1000 N, the drastic reduction in strength was found when the damage transition from ring to radial crack occurred. The extent of strength degradation was more pronounced in vitro environments probably due to chemical reaction of artificial saliva with glass phase through radial cracks introduced during large numbers of contacts.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.9
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• pp.1185-1191
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• 2010

Aged aircraft have several cracks as a results of long-term service, and these cracks affect the safety and decrease the rate of operation of the aircraft. To solve these problems, crack propagation analysis should be performed to determine the service life at fatigue critical location(FCL). It is, however, almost impossible to obtain the stress spectrum, which is crucial for crack propagation analysis of the FCLs of wing structure of aged aircraft. In this study, to analyze the fatigue crack propagation behavior at the FCL of an aged aircraft, first finite element analysis is performed for a 3D geometry model of the aircraft wing structure, which is obtained using CATIA based on the paper drawings. Then, the transfer function and stress-spectrum of the FCL are derived using the load factor data and the FEA results. Finally, the crack propagation rates of the FCL are evaluated using the commercial software, NASGRO 6.0.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.6 s.261
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• pp.701-709
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• 2007

To clarify the characteristics of surface damage due to fretting in press-fitted shaft, experimental methods were applied to small scale specimen with different bending load condition. Fatigue tests and interrupted fatigue tests of press-fitted specimen were carried out by rotate bending fatigue test. Macroscopic and microscopic characteristics were examined using scanning electron microscope (SEM), optical microscope or profilometer. It is found that small fatigue cracks are nucleated early in life regardless of bending stress, and thus the most portion of fatigue life on press fits can be considered to be crack propagation process. Most of surface cracks are initiated near the contact edge, and multiple cracks are nucleated and interconnected. Furthermore, the fretting wear rates at the contact edge are increased rapidly at the initial stage of total fatigue life. It is thus suggested that the fatigue crack nucleation and propagation process is strongly related to the evolution of surface profile by fretting wear in press fits.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.7
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• pp.637-646
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• 2016

This study is to investigate the influence of hydrogen attack and UNSM on fatigue behaviors of the Inconel 718 alloy. The decrease of the fatigue life between the untreated and the hydrogen attacked material is 10-20%. The fatigue lives of hydrogen attacked specimen decreased without a fatigue limit, similar to those of nonferrous materials. Due to hydrogen embrittlement, about 80% of the surface cracks were smaller than the average grain size of $13{\mu}m$. Many small surface cracks caused by the embrittling effect of hydrogen attack were initiated at the grain boundaries and surface scratches. Cracks were irregularly distributed, grew, and then coalesced through tearing, leading to a reduction of fatigue life. Results revealed that the fatigue lives of UNSM-treated specimens were longer than those of the untreated specimens.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.4
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• pp.819-827
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• 1992

Investigations of the fatigue damage of roughing mill roll and experimentally. By the computer simulation for analysing the stresses on the roll surface and experimental hot rolling, the following results were drawn : The crakcs observed on the roll surface were initiated thermally in the initial stage of the rolling and propagated by repeated thermal and bending stresses. The size of the roll surface cracks smaller than 4.87mm could avoid the occurrence of tiny scab, surface defect of hot steel strip. Since the size of surface cracks observed on the roughing mill roll was very small, the fatigue damage of roll surface was found not to be the major factor for the formation of the scab.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.11
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• pp.1137-1143
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• 2015

The stage II fretting fatigue crack growth and branching, i.e., the process of fretting fatigue crack growth starting in an inclined direction and then changing to the normal direction, is analyzed using the finite element method. The fretting fatigue experiment data of A7075-T6 are used in the analysis. The applicability of maximum tangential stress intensity factor, maximum tangential stress intensity factor range, and maximum crack growth rate as the crack growth direction criteria is examined. It is revealed that the stage II crack growth before and after the branching cannot be simulated with a single criterion, but can be done when different criteria are applied to the two stages of crack growth. Moreover, a method to determine the crack length at which the branching occurs is proposed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.1
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• pp.8-17
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• 2003

The butt lap joint structures which are usually designed by the concept of slow crack growth damage tolerance, show frequently the behaviors of multiple site fatigue crack growth around the fastener hole edges due to the fretting between the two jointed parts. In this paper, experimental tests of fatigue crack growth have been performed of a bolted butt lap joint structure having an initial corner crack at the fastener hole edge, with different fretting conditions under a flight load spectrum. The obtained test results were reviewed to investigate the effects of fretting fatigue cracks on the damage tolerance crack growth life. Computations of corner crack growth were also carried out using an existed model to compare with test results.