• Proceedings of the KSME Conference
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• 2004.11a
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• pp.214-219
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• 2004

Crack tip behavior of single mode loading condition(mode I)depend on tensile loading component but one of mixed mode loading condition(mode I+II) have influenced on shear loading component like the practical structure. Because crack closure is caused by shear loading component under mixed-mode loading a research on the behavior in the stage of crack initiation and propagation require to be evaluate about crack closure effect by fatigue crack surface friction. For that reasion we examined the behavior at the crack tip by direct measuring method. Measured behavior at the crack tip was analyzed through vector crack tip displacement. As a result, crack propafation equation was corrected by considering with crack closure effect. In addition we compared fatigue fracture crack surface and crack closure level.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.4
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• pp.432-440
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• 2007

In this study, we have investigated the closure behavior of fatigue cracks in SAPH440 steel plates under a mixed-mode I+II loading. A crack image capturing system as a direct measuring method was used to measure the closure levels at a crack tip. The crack closure levels in the fluctuation and stable sections were increased with the increase of the mode mixture ratio. The mode mixture ratio independent fatigue crack propagation rates equation was calculated by considering mixed-mode crack closure levels. The equation was examined according to the application method of crack opening ratio. The fracture surface analysis by C-scan method was also performed in order to investigate the closure mechanism and propagation mode of crack under the mixed-mode I+II loading. The crack closure under the mixed mode I+II is confirmed as a surface roughness closure by the quantitative analysis of fracture surface using the proposed surface roughness parameter.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2001.04a
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• pp.365-372
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• 2001

This study is concerned with the application of an analytical model of cyclic crack growth that includes the effects of crack closure. The crack closure model is based on the Dugdale model and the strip model, considering the plasticity-induced closure which is caused by residual plastic deformation remaining in the wake of an advancing crack. This study is performed to get the relation between crack growth and crack opening stress with the constant stress ratio, and the relation between stress ratio and crack opening stress with the constant maximum stress under constant-amplitude loading. Under constant-amplitude loading, the crack opening stress is conversed the constant value as a crack grows and is proportion to both the stress ratio and the maximum stress. The crack closure effect, however, is decreased in the positive stress ratio and disappeared at about 0.7. The crack growth analysis using the crack closure model shows that the influence of stress ratio is minimized in the relation between crack growth ratio and effective stress intensity range specially at the negative stress ratio.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.5
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• pp.797-804
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• 2001

To obtain representative fatigue crack growth characteristic curve in residual stress field, fatigue crack growth test was carried out at various stress ratio and fatigue crack growth characteristic curve was represented using crack closure concept. Obtained results are as follows;K(sub)op/K(sub)max was independent of K(sub)max when R was lower than 0.5 and crack closure phenomenon was not observed when R is higher than 0.5. therefore neglecting crack closure behaviour, actual fatigue crack growth rate can be underestimated. Thus, considering crack closure phenomenon, fatigue crack growth characteristics curve of A 106 Gr B Steel weldment can be effectively estimated.

• Journal of the Korean Society for Precision Engineering
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• v.14 no.7
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• pp.82-90
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• 1997

When a structure is made by the process of forging, it has the different mechanical properties from those it has before the process. This study is based on the crack closure phenomenon of the crack growth behavior of forged AI7050-T7452. The specimens were prepared in three kinds of forging ratio in order to find out the effects of crack closure on the forged material and compare the crack growth behavior with not-forged aluminum. COD method and strain gage method were used in measuring the crack closure stress and the results from those methods were compared each other. FEM analysis was applied to verify the effective stress intensity factor range by the superposition of the crack closure load to the crack tip. In the result of this study, the crack closure stress decreased with increasing the forging ratio due to the finer grain size and the brittle manner.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.6
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• pp.898-905
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• 1986

In order to obtain the microstructure improving fatigue crack propagation resistance of steels, fatigue crack propagation behavior of martensite-ferrite dual phase steels is investigated in terms of crack deflection and crack closure. The results obtained are as follows; (1) .DELTA.K$_{th}$ and fatigue crack propagation resistance in low .DELTA.K region increases with increasing hardness of second phase. But the difference of this crack propagation resistance decreases with increasing .DELTA.D. (2) In low .DELTA.K region, crack closure increases with increasing hardness of second phase, when the materials have all the sam volume fractionof second phase, or when yield strengths are similar in all materials. (3) These crack closure can be explained by fracture surface roughness due to crack deflection.n.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.79-84
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• 2000

Crack closure and growth behavior of physically short fatigue cracks under random loading are Investigated by performing narrow- and wide-band random loading tests for various stress ratios. Artificially prepared two-dimensional, short through-thickness cracks are used. The closure behavior of short cracks under random loading is discussed, comparing with that of short cracks under constant-amplitude loading and also that of long cracks under random loading. Irrespective of random loading spectrum or block length, the crack opening load of short cracks is much lower under random loading than under constant-amplitude loading corresponding to the largest load cycle in a random load history, contrary to the behavior of long cracks that the crack opening load under random loading is nearly the same as or slightly higher than constant-amplitude results. This result indicates that the largest load cycle in a random load history has an effect to enhance crack opening of short cracks.

• Transactions of the Korean Society of Automotive Engineers
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• v.13 no.3
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• pp.178-185
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• 2005

Crack tip displacement is originated by tensile stress component, s and shear stress component, t on pure Mode I and pure Mode II. The crack tip displacement(CTD) depends on combined types of different two stress components under mixed-mode loading conditions (MMLC). Thus, the analysis of crack tip displacement must be CTD vector, dv which is composition of ds and dt under MMLC. In this paper, various effects of MMLC on the crack closure are studied experimentally. The crack closure magnitude is calculated from the information of crack tip displacement under MMLC. This information has been obtained from the high resolution optical microscope in direct observations of crack displacement behavior at the crack tip. Observed crack tip displacement is analyzed by using CTD vector to determine crack opening load. The various effects of MMLC on the crack closure are explained using crack opening ratio with crack length and mode mixture. The effective stress intensity factor considering crack closure is also discussed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.243-252
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• 1987

The propagation and closure behavior of surface crack initiated at a sharply notched specimens were investigated in 5083-H113 aluminium alloy under constant amplitude of tension load by the unloading elastic compliance method. The crack shape (aspect ratio) was found to be approximately semicircular during the crack was being small and to be changed to semi-elliptical during it was being long. The propagation rate of a surface crack initiated from notch root decelerated with increasing crack length when the crack was small and then accelerated when it was large. The effect of stress ratio was large in lower .DELTA.K range, but the effective stress intensity factor range .DELTA.K$_{eff}$ was found to diminish the difference of the crack propagation rate. By considering the increase in crack closure stress with crack length and examining the microphotographs, plasticity-induced and roughness-induced crack closure mechanisms were predominant in the range of this study.y.

• Journal of Mechanical Science and Technology
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• v.18 no.11
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• pp.1989-1995
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• 2004

Finite element analysis(FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.