• Journal of the Korean Ceramic Society
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• v.33 no.5
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• pp.602-615
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• 1996

The purpose of the present study is to investigate the microstructural effect on the R-curve behavior in three aluminas with different grain size distributions by analyzing the bridging stress distribution. The crack opening displacement (COD) according to the distance behind the stationary crack tip was measured using an in situ SEM fracture method. The measured COD values in the fine-grained alumina agreed well with Wiederhorn's sollution while they deviated from Wiederhorn's solution in the two coarse-grained aluminas because of the increase of the crack closure due to the grain interface bridging in the crack wake. A numerical fitting procedure was conducted by the introduction of the power-law relation and the current theoretical model together with the measured COD's in order to obtain the bridging stress distribution. The results indicated that the bridging stress function and the R-curve computed by the current model were consistent with those computed by the power-law relation providing a reliable evidence for the bridging stress analysis of the current model. The strain-softening exponent in the power-law relation n, was calculated to be in the range from 2 to 3 and was closely related to the grain size distribution. Thus it was concluded from the current theoretical model that the grain size distribution affected greatly the bridging stress distribution thereby resulting in the quantitative analysis of microfracture of polycrystalline aluminas through correlating the local-fracture-cont-rolling microstructure.

• Journal of Ocean Engineering and Technology
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• v.2 no.2
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• pp.61-70
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• 1988

In order to consider the concept of the fitness for purpose'in fatigue design of offshore structure, fracture mechanics is applied to evaluate initial or weld defects. Generally, linear elastic fracture mechanics has been applied to tstimate initial fatigue crack propagation rate as well as long fatigue crack propagation rate. But, initial fatigue crack propagation rate in elasto-plastic notch field may not be characterized by application of stress intensity factor range .DELTA. K, because plastic effect due to stress concentration of notch may contribute to initial crack propagation. Therefore, to introduce the plastic effect into fatigue crack driving force, in this studty, the evaluating method of J-integral range .DELTA. J, was developed by willson was modified for application to notch field. In calculation of .DELTA. J obtained from the modified J-integral, stress gradient and crack closure behavior in the notch field were considered. The initial crack propagation rates in the notch fields of mild steels and high tensile strength steels were correlated to .DELTA. J. As the result, it was cleared that the present .DELTA. J is applicable to charachterize the fatigue crack propagation rates in both the elastic and elasto-plastic notch fields.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.10
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• pp.1703-1711
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• 2003

An elastic-plastic finite element analysis is performed to investigate detailed closure behaviour of fatigue cracks in residual stress fields and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using contact elements can predict fatigue crack closure behaviour. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. Specially, the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point can precisely predict the opening level. By using the concept of the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point, the opening level of fatigue crack can be determined very well.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.37-42
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• 2003

The problem of an orthotropic material with a central crack is studied. The material is subjected to uniform biaxial loading along its boundary. The normal stress ratio theory is applied to predict fracture strength behavior in cracked orthotropic material. The dependence of the critical stress with respect to the biaxial loading and the crack orientation is discussed. Our analysis shows significant effects of biaxial loading on the critical stress. The additional tenn in the asymptotic expansion of the crack tip stress field appears to provide more accurate critical stress prediction.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1996.03a
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• pp.165-169
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• 1996

In this study residual stress in weldment was considered about the effect on the fatigue propagation and about the effect of redistribution of residual stress. Then, fatigue tests were conducted by the center notched specimens machined with welded plate. The residual stress and its redistribution after the crack growth were measured by the magnetizing stress indicator and hole-drilling method. Fatigue crack propagation was estimated by the specimens having residual stress redistributed after the cracks growth and having the effects of crack closure. Crack growth rates were predicted and compared with experimental results. It had been found that the predicted crack propagation rates have a good agreement with experimental results when the redistribution of residual stress was considerd.

• International Journal of Precision Engineering and Manufacturing
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• v.2 no.3
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• pp.19-26
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• 2001

In order to investigate the effect of single tensile overload on the short crack growth behavior under the out-of-plane cyclic bending moment, crack opening stresses were continuously measured by an elastic compliance method using strain gages. The characteristics of short crack growth after the single tensile overload are analyzed by the effective stress range ratio. Futhermore, the investigation was carried out with respect to various fatigue crack growth behaviors such as the plastic zone size effect on crack retardation, the retarded crack length and the number of cycles.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.187-192
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• 2001

Cracks in mechanical joints is generally under mixed-mode and there is the critical inclined angle at which mode I stress intensity factor becomes maximum. In order to evaluate the fatigue life of cracks in mechanical joints, horizontal crack normal to the applied load and located on minimum cross section is major concern but critical inclined crack must also be considered. In this paper mixed-mode fatigue crack growth test is performed for horizontal crack and critical inclined crack in mechanical joints. Fatigue crack growth path is predicted by maximum tangential stress criterion using mode I and mode II stress intensity factors obtained from weight function method, and fatigue crack growth rates of horizontal and inclined crack are compared.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1994.10a
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• pp.820-825
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• 1994

It is improtant to examine life or crack propagation behavior of structures because of its safety evaluation. The purpose of this study is to investigate the effect of crack-crack interaction to evaluate fatigue life and crack behavior. In this study, the behavior of the interaction of two cracks is studied by experiment. The vertical distance of two cracks is varied to make different interaction stress field. In addition, the effect of plastic zone is considered to examine crack propagation path and propagation rate.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.7 s.178
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• pp.1841-1846
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• 2000

To examine the effect of acid fog on the corrosion fatigue behavior in structural steel, fatigue tests under acid fog atmosphere were carried out in comparison with distilled water. The corrosive c omponents contained in acid fog pile up the corrosion products on crack face and show a crack branching and crack tip blunting. Therefore, due to these workings crack growth rate was reduced by decreasing the effective stress range in crack tip rather than under distilled water. Also the effect of sulfuric acid, which is the main component of acid fog, and testing speed on fatigue crack growth were examined. It was found that corrosion behavior was remarkably dependent upon pH and Hz rather than components of acid fog. According as pH and testing speed decrease below a specific value, crack growth was accelerated in comparison with distilled water. This reveals that due to liquid having strong acidity and slow speed of test the crack face dissolution was promoted, so crack closure was disturbed in the process of stress descent.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.5
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• pp.938-946
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• 1989

Using the CT specimen of carbon steel(SM45C), we estimated the fatigue crack propagation behavior in stable crack propagation range. Furthermore the fatigue crack propagation rate, Acoustic Emission(AE) count rate, and fractography characteristics were also compared among others. The following results were confirmed by experimental observation. Near-threshold stress intensity factor range(.DELTA. $K_{th}$) is influenced by stress ratio but not at the upper limit of stable crack propagation range. As stress intensity factor range(.DELTA.K) and(or) stress amplitude increase (s), both crack propagation rate(da/dN) and AE count rate(dn/dN) increase. Effective stress intensity factor range(.DELTA. $K_{off}$) determined from the crack closure point measurement by AE method is useful for the evaluation of fatigue crack propagation rate. Fractography in stable crack propagation range showed striation, and agreed with the crack propagation rate obtained either by experiment of by the results of microscopic measurements.s.