• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2002.05a
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• pp.281-287
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• 2002

Nowadays, many components used in machinery industry is required lightness and high strength. Therefore, the effects of compressive residual stress by shot-peening which is method to improve fatigue lift of spring steel (JISG SUP-9), which used in suspension of automobile, on fatigue crack growth characteristics was investigated with considering fracture mechanics. So, we can obtain followings 1. The fatigue crack growth rate on stage II is conspicuous with the size of compressive residual stress and is dependent on Paris equation. 2. Although the maximum compressive residual stress is deeply and widely formed from surface, fatigue life does not improve than when maximum compressive residual stress is formed in surface. 3. The threshold stress intensity factor range is increased with increasing compressive residual stress. 4. In fracture surface of fatigue crack growth it is investigated that compressive residual stress remarkably retards fatigue crack growth.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.348-352
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• 2004

Recently the steel parts used for automiles and trains are required to be used under higher stress than ever before in need of the weight down. However, threr are a lot of problems with developing such of fatigue strength and fatigue life are mainly focused on by adopting residual stress. And got the following characteristics from crack growth test carried out stress ratio. Fatigue life shows more improvement in the Un-peening material. And Compressive residual stress of surface on the Shot-peening processed operate resistance force of fatigue. So we cam obtain fallowings. (1) The fatigue crack growth rate on stage II is conspicuous with the size of compressive residual stress and is dependent of Paris equation. (2) Although the maximum compressive residual stress is deeply and widely formed from surface, fatigue life does not improve than when maximum compressive residual stress is formed in surface. (3) The threshold stress intensity factor range is increased with increasing compressive residual stress.

• 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.

• Proceedings of the KSR Conference
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• 1998.11a
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• pp.553-559
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• 1998

This thesis studied to evaluate the fatigue behavior and propagation of rail steel under welding line. Test of crack growth was performed by all member of rail under constant amplitude loading at the structures laboratory in Hongik University. The effect of the following parameters with initiated crack length on the bottom edge of rail were studied. Here, fracture mechanics mode is opening mode. and Testing Material is KS50N Rail. From analysis and experimented result on the three Point bending in the lab, This paper presented a effect of crack growth , shape and remaining service life. Further more, according to the variable crack length, variable section and the ratio of section the fatigue behavior and propagation were studied.

• Transactions of the Korean Society of Automotive Engineers
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• v.16 no.5
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• pp.1-7
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• 2008

A study of new materials that are light-weight, high in strength has become vital to the machinery of auto industries. But then, there are a lot of problems with developing such materials that require expensive tools, and a great deal of time and effort. Therefore, the improvement of fatigue strength and fatigue life are mainly focused on by adopting residual stress. And Influence of corrosive condition for corrosion fatigue crack was investigated, after immersing in 3.5%NaCl, $10%HNO_3$+3.5%HF, $6%FeCl_3$. The immersion period was performed for 365days. The compressive residual stress was imposed on the surface according to each shot velocity based on shot peening, which is the method of improving fatigue life and strength. Fatigue life shows more improvement in the shot peened material than in the un peened material in corrosion conditions. The threshold stress intensity factor range was decreased in corrosion environments over ambient. Compressive residual stress of surface on the Shot-peen processed operate resistance force of fatigue crack propagation. The fatigue crack growth rate of the Shot-peened material was lower than that of the un peened material. Also m, fatigue crack growth exponent and number of cycle of the shot peened material was higher than that of the un peened material. That is concluded from effect of da/dN.

• Journal of the Korean Society of Safety
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• v.10 no.2
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• pp.120-128
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• 1995

The fatigue failure of rail is a principal source of derailment accidents. The reduction of fatigue failures can be achieved by Intensive track maintenance and periodic safety assessments for the railway. For the safety assessments, it is required to have more accurate knowledge for fatigue behavior such as the crack initiation, propagation, crack growth rate and the remaining service life in rail. In this paper, the mean stress effects for the fatigue behavior of rail steel are studied. For this study, the fatigue test is conducted and some equations for fatigue evaluation are applied and compared. From the results, we can see that the fatigue crack growth rate is the more increased as the men stress Is the more increased, the mean stress effect is represented well by the combination of stress intensity factor range and maximum stress intensity factor and Crooker and Range's equation represented by ${\Delta}K, K_{max}$ is the best fit for fatigue evaluation and safety assessment of rail.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.3
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• pp.505-512
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• 1988

Since the propagation of a short fatigue crack is directly related to the large crack which causes the fracture of bulk specimen, the detailed study on the propagation of the short crack is essential to prevent the fatigue fracture. However, a number of recent studies have demonstrated that the short crack can grow at a low applied stress level which are predicted from the threshold condition of large crack. In present study, the threshold condition for the propagation of short fatigue crack is examined with respect to the microstructure and cyclic loading history. Specimens employed in this study were decarburized eutectoid steels which have various decarburized ferrite volume fraction. Rotating bending fatigue test was carried out on these specimens with the special emphasis on the '||'&'||'quot;critical non-propagating crack length.'||'&'||'quot; It is found that the reduction of the endurance limit of their particular microstructures can be due to the increase of the length of critical non-propagating crack, and the quantitative relationship between the threshold stress .DELTA. .sigma. $_{th}$ and the critical non-propagating crack length Lc can be written as .DELTA. .sigma. $_{th}$, Lc=C where m, C is constant. Further experiments were carried out on the effect of pearlitic structure and cyclic loading history on the length of critical non-propagating crack. It is shown that the length of critical non-propagating crack is closely related to both pearlite interlamellar spacing and cyclic loading history.ory. cyclic loading history.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.8 s.239
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• pp.1078-1084
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• 2005

The relationship between fatigue crack opening behavior and the reversed plastic zone sizes is studied. An elastic-plastic finite element analysis (FEA) is performed to examine the opening behavior of fatigue crack, where the contact elements are used in the mesh of the track tip area. The smaller element size than reversed plastic zone size is used fer evaluating the distribution of reversed plastic zone. In the author's previous results the FEA could predict the crack opening level, which crack tip elements were in proportion to the theoretical reversed plastic zone size. It is found that the calculated reversed plastic zone size is related to the theoretical reversed plastic zone size and crack opening level. The calculated reversed plastic zone sizes are almost equal to the reversed plastic zone considering crack opening level obtained by experimental results. It can be possible to predict the crack opening level from the reversed plastic zone size calculated by finite element method. We find that the experimental crack opening levels correspond with the opening values of contact nodes on the calculated reversed plastic zone of finite element simulation.

• 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.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.6
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• pp.84-90
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• 2004

For the precise assessment of the effect of welding residual stresses on structural strength and fatigue crack growth behavior, new FE analysis algorithms for the estimation of residual stress relaxation due to external load and redistribution due to fatigue crack propagation were proposed in this paper. Initial welding residual stress field was obtained by thermal elasto-plastic analysis considering temperature dependent material properties, and the amount of residual stress relaxation and redistribution were assessed by subsequent elasto-plastic analysis In the analysis of fatigue crack propagation, the applied SIF(Stress Intensity Factor) range was evaluated by $\frac{1}{4}$-point displacement extrapolation method, and the effect of welding residual stresses on crack propagation was considered by introducing the effective SIF concept. The test results of crack propagations were compared with the predicted data obtained by the analysis.