• Bulletin of the Society of Naval Architects of Korea
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• v.12 no.2
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• pp.43-58
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• 1975

There are many reports on fatigue crack of metallic materials but most of them relate crack propagation rate to stress intensity factor. The problem of crack propagation is not yet clarified, especially the bridge between micro and macro phenomena In this experiment rotating bending fatigue tests have been carried out with smoothed specimen of rolled steel plates including 0.2% carbon under application of three stress conditions to investigate the slip band and the crack propagation behaviour. The results obtained are as follows; 1) The length of cracks which have grown at initial crack tips can be expressed as follows; $l=Ae^{BNr}$(A,B: constant, $N_r$: cycle ratio) $\frac{dl}{dN}=\frac{AB}{N_f}{\cdot}e^{BNr}$($N_f$:fatigue life) 2) The ratio of slipped grain number to total grain number is $S_f=7{\sigma}-5.6$-5.6{\sigma}_c$($\sigma$: stress amplitude) (${\sigma}_c$: fatigue limit) 3) When the fatigue process transfers from Stage I to Stage II, the crack which propagates into specimen changes its direction from that of the maximum shear stress to the direction of perpendicular to principal stress and this is same in the circumferential direction of specimen. the crack propagation behaviors of both sides of a crack are different each other when they approach to the grain boundary.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.11 no.3
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• pp.29-36
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• 1991

This experimental research evaluated the length of crack extension with the measured compliances as the mutual comparison factors instead of the method proposed in ASTM E561-80. And this research measured the R-curves with the application to the concept of the strain energy release rate that was formulated from the inelastic energy absorbed during the crack growth. With the interpretation of R-curves, this research obtained the starting point of the unstable crack growth, and compared the values of critical fracture toughness with each other, and then examined the effects of variations of the maximum size of coarse aggregate and the thickness of specimen on the values of the critical fracture toughness.

• Korean Journal of Materials Research
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• v.6 no.6
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• pp.561-567
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• 1996

The grain size effect on grain boyndary cracking in Pb-Sn eutectic during isothermal fatigue was investigated. Fatigue experiments were confined to two conditions : (1) 0.4% total strain range(approximetely 0.2% plastic strain range), 1.67$\times$10$^{-3}$/s frequency; and (2) 1.5% total strain rante(approximately 1.2% plastic strain range), 8.33$\times$10$^{-4}$/s frequency. Fatigue specimens were cross-sectioned to monitor the depth of crack growth continuosly and then, the maximum crack depths in units of the number of boundaries were plotted as functions of number of cycles for these two different strain ranges. The results revealed that the rate of crack growth(per cycle at fixed rate of crosshead motion) can be expressed as dc/dN=($\Delta$$\varepsilon$$_p$)$^n$c where n is typically 2, c is the crack length, $\Delta$$\varepsilon$$_p$ is the plastic strain range, and A is a "constant" that depends on whether the crack is deeper or shallower than its first triple point of the grain boundary, A decrdases by about a factor of three after the crack hits the first triple point, indecating that the fatigue crack is trapped at the triple point of the grain boundaries.

• Journal of Welding and Joining
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• v.14 no.6
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• pp.48-57
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• 1996

The weldability of high purity aluminum-lithium binary alloys has been investigated using the Varestraint test. Autogenous GTAW (gas-tungsten-arc-welds) were run along specimens of different lithium concentration using three sets of welding parameters. Welding voltage was held constant at 10 volts. Welding current (70∼100 amps) and travel speed (23∼33 cm/min) were the parameter varied. Hot-tearing susceptibility varied with lithium content and exhibited a steep peak at 2.6 weight percent lithium. Depth of penetration increased with increasing heat input and lithium concentration. The susceptibility is influenced by the wettability of dendrites by the interdendritic eutectic liquid as well as the time available for back-Siting by eutectic liquid. The welding condition of welding current 70A and travel speed 23 cm/min was showed good resistance to cracking in aluminum-lithium alloys. Suggestions for improving weld cracking resistance are also provided.

• Proceedings of the KSR Conference
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• 2008.11b
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• pp.874-882
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• 2008

In a domestic, HSR-350x which has the maximum speed 350km/h was developed and then next, the next generation high speed train which has the maximum speed 400km/h has still been developing. With developing the next generation high speed railway, there need to be a general plan to make sure of dynamic safety though the a study on the crack and failure of rail by rolling contact fatigue. Therefore, this study investigated occurring stress of rail according to the track quality, train velocity, wheel radius, track stiffness, distance between sleepers, axial force using Eisenmann's equations. For the more, via the finite element method, it investigated shear force on the rail head which could be changed by the early crack length, angle and temperature. As a result, this study confirmed the main elements which effect on the fatigue life cycle of rail.

• Journal of Welding and Joining
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• v.33 no.2
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• pp.62-68
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• 2015

This study investigates the appropriate range of reverse bending load for the CTOD test of thick weld by observing improvement of pre-crack shape and determination of the limit applicable load. In order to do it, the effect of the amount of the reverse bending load on the maximum deviation of the pre-crack length was investigated by the extensive tests, and the variation of plastic zone size in way of the crack tip under reverse bending load were evaluated by FEA. With the results obtained by the experiments and FEA, the proper range of reverse bending load was suggested. The effectiveness of the reverse bending method was verified by examining the pre-crack straightness after CTOD tests of thick weld specimens with various thickness and strength.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.8 s.239
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• pp.1118-1122
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• 2005

The direct current potential drop (DCPD) method and the unloading compliance (UC) method with a crack opening displacement gauge were applied simultaneously to the Zr-2.5Nb curved compact tension (CCT) specimens to determine which of the two methods can precisely determine the crack initiation point and hence the crack length for evaluation of their fracture toughness. The DCPD method detected the crack initiation at a smaller load-line displacement compared to the UC method. As a verification, a direct observation of the fracture surfaces on the curved compact tension specimens was made on the CCT specimens experiencing either 0.8 to 1.0 mm load line displacement or various loads from $50\%\;to\;80\%$ of the maximum peak load, or $P_{max}$. The DCPD method is concluded to be more precise in determining the crack initiation and fracture toughness, J in Zr-2.5Nb CCT specimens than the UC method.

• Journal of the Korean Society for Precision Engineering
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• v.8 no.2
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• pp.27-35
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• 1991

We propose the crack growth rate equation which applied over three regions (threshold region, stable region, unstable region) of fatigue crack propagation. Constant stress amplitude fatigue tests are conducted for four materials under three stress ratios of R=0.05, R=0.2 and R=0.4. Materials which have different mechanical properties i.e. stainless steel, low carbon steel, medium carbon steel and aluminum alloy are used. The fatigue crack growth rate equation is given by $da/dN={\beta} (1-R)^{\delta}\({\DELTA}K-{\DELTA}K_t)^{\alpha} / (K_{cf}-K_{max})$${\alpha}, {\beta}$ , and ${\delta}$ are constants, and ${\Delta}K_t$ is stress intensity factor range at low ${\Delta}K$ region. The constants are obtained from nonlinear least square method. $K_{ef}$is critical fatigue stress intensity factor. The relation between half crack length and number of cycles obtained by integrating the crack growth rate equation is in agreement with the experimental data. It is also experimented with constant maximum stress and decreasing stress ratios, and the fatigue growth rate of each material is in accord with the proposed equation.

• Structural Engineering and Mechanics
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• v.55 no.6
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• pp.1087-1097
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• 2015

In this paper, the effect of functionally graded material (FGM) coatings on the fracture behavior of semi-elliptical cracks in cylinders is assessed. The objective is to calculate the stress intensity factor (SIF) of a longitudinal semi-elliptical crack on the wall of an aluminum cylinder with FGM coating. A three-dimensional finite element method (FEM) is used for constructing the mechanical models and analyzing the SIFs of cracks. The effect of many geometrical parameters such as relative depth, crack aspect ratio, FG coating thickness to liner thickness as well as the mechanical properties of the FG coating on the SIF of the cracks is discussed. For a special case, the validity of the FE model is examined. The results indicated that there is a particular crack aspect ratio in which the maximum value of SIFs changes from the deepest point to the surface point of the crack. Moreover, it was found that the SIFs decrease by increasing the thickness ratio of the cylinder. But, the cylinder length has no effect on the crack SIFs.

• Structural Engineering and Mechanics
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• v.35 no.1
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• pp.1-18
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• 2010

Bonding of carbon fiber reinforced polymer (CFRP) composites has become a popular technique for strengthening concrete structures in recent years. The bond stress between concrete and CFRP is the main factor determining the strength, rigidity, failure mode and behavior of a reinforced concrete member strengthened with CFRP. The accurate evaluation of the strain is required for analytical calculations and design processes. In this study, the strain between concrete and bonded CFRP sheets across the notch is tested. In this paper, indirect axial tension is applied to CFRP bonded test specimen by a four point bending tests. The variables studied in this research are CFRP sheet width, bond length and the concrete compression strength. Furthermore, the effect of a crack- modeled as a notch- on the strain distribution is studied. It is observed that the strain in the CFRP to concrete interface reaches its maximum values near the crack tips. It is also observed that extending the CFRP sheet more than to a certain length does not affect the strength and the strain distribution of the bonding. The stress distribution obtained from experiments are compared to Chen and Teng's (2001) analytical model.