• Journal of the Korean Society for Precision Engineering
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• v.9 no.1
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• pp.106-117
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• 1992

In this study, the propagation behaviour and the closure phenomena of physically small surface cracks were investigated by the techinque of the Kikukawa-unloading elastic compliance method using a back face strain gage. The surface cracks initiated and propagated from notched specimens under constant amplitude bending load. The crack shape (aspect ratio) with approximately semi-circular at the early stage was changed to semi-elliptical as the cracks grew larger. The crack depth (a) could be expressed uniquenly by the crack length (c). The dependence of the crack propagation rate on the stress ratio R was strongly related in the lower ${\Delta}K$ range. The deceleration of the surface crack propagation rate was prominent in lower R during the crack length was small. When the propagation rate was rearranged with the effective stress intensity factor range ${\Delta}$K_{eff} the dependence of the crack propagation rate on the stress ratio R was found to be diminshed. These were caused by the crack closure phenomena that was most prominent at the lower propagation rate. The mechanism of crack closure phenomena was dominated by the plasticity-induced mechanism.

• 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.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.2
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• pp.188-197
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• 1986

With respect to structural carbon steel(SM 22C), it was studied how the overstress or the understress has effects on fatigue inside crack propagation curve of a two level stress. Obtained results are summarized as follows. (1) The overstress or the understress, at a slip band occurrence stage, does not change the inside crack propagation curve because the crack closure and opening phenomena do not happen. (2) The overstress, at a crack propagation stage, does not change the inside crack propagation curve because the crack closure of overstress in compressive state is nearly same that of base stress in compressive state. (3) The understress, at a crack propagation stage, give rise to an acceleration of crack growth because the crack closure of understree in compressive state is more open than that of base stress in compressive state and the phenomenon is the essential increase of the actual applied stress of the specimen.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.3
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• pp.316-325
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• 1986

Characteristics of fatigue crack propagation in martensite-ferrite duel phase steels have been investigated. In low .DELTA.K region, fatigue crack propagation resistance increases with increasing volume fraction of martensite, but the difference of crack propagation resistance resulted from the volume fraction decreases with increasing .DELTA.K. Also, threshold stress intensity factor range .DELTA.K$_{th}$ increases with increasing volume fraction of martensite, But fatigue crack propagation rates of dual-phase steels in terms of .DELTA.K$_{eff}$ are independent to volume fraction of martensite. These phenomena can be explained by the roughness induced crack closure due to crack deflection.n.n.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.1839-1851
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• 1991

Fatigue crack behavior is studied through the two-level rotary bending test with the deep non-through radial holed notch specimens of low carbon steels(SM22C). The main factors investigated are the effects of the damage zone size around crack tip and phenomena of closing or opening of the crack tip. Obtained results are summarized as follows. Fatigue crack behavior in second level stressing slightly lower than fatigue limit is closely related to the size of damage zone produced by the first level stress higher than fatigue limit and to the phenomena of crack closing and opening for the second level stress. The non-propagating crack limit condition depends upon the crack length l$_{1}$ propagated under the first level stress and the magnitude of second level stress .sigma.$_{2}$ lower than the fatigue limit. The non-propagating crack limit condition is expressed by following eq. $\sigma_2^{6.1}{\times}l_{1}=7.35{\times}10^{6}[(kg_{f}mm^{6.1}(mm)]$

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.4
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• pp.670-679
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• 1989

The constant amplitude loading and 100% single overloading fatigue studies of domestic high tensile 7075-T73 aluminum alloy were performed to exmine the effect of specimen thickness and its mechanisms on fatigue crack growth behavior. The stage II fatigue crack growth rates tend to increase with decreasing specimen thickness under constant amplitude loading condition and this has relation with stress intensity factors and plastic zone size. The amount of retardation by an overload increased with decreasing specimen thickness when the crack depth and baseline stress intensity factors were constant. The crack depth is one of major factors which affect retardation phenomena by an overload and the amount of retardation increase with decreasing the crack depth. Its main mechanisms are crack closure and decreasing of K at the crack tip by branching and deflection of crack. And they are affected by near surface more severely than central portion of specimen.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.2
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• pp.271-277
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• 1988

The threshold fatigue crack growth and the stable crack propagation behaviors were studied in 7017 T 651, 7020 T 651 and 5083 H 115 aluminum alloys. The threshold (.DELTA. K $_{th}$) fatigue crack growth can be expressed by the equation .DELTA. $K_{th}$) = .DELTA. $K_{tho}$(1-R)$^{r}$ , where R is stress ratio, .DELTA. $K_{tho}$ is .DELTA. K at R = 0 and r is material constant. The stable crack growth rate against stress intensity factor range .DELTA. K exhibits the trilinear form with two transitions and results of investigation on crack closure phenomena showed that the crack opening stress intensity factor $K_{op}$ is approximately equal to R $K_{max}$. + .DELTA. K $_{th}$.th/.