• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.8
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• pp.1269-1276
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• 2001

In case of contact fatigue, the accurate calculation of surface tractions and subsurface stress is essential to the predication of crack initiation life. Surface tractions influencing shear stress amplitude have been obtained by contact analysis based on influence function. Subsurface stress has been obtained by using rectangular patch solutions. In this study, to simulate asperity contact under sliding condition, the tip of asperity was simulated by sphere and to calculate crack initiation life in the substrate, dislocation pileup theory was used.

• Journal of the Korean Society for Railway
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• v.7 no.4
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• pp.345-349
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• 2004

This study investigates the fatigue lives of SM490A material(base metal) specimens and fillet weld specimens, which are made same material and weld method for the railway vehicle. These fatigue lives have a difference, the fatigue lives of weld specimen are shorter than those of base metal. We measured the strains on the weld positions of the specimens during the fatigue test for investigation of crack initiation and crack growth. In these result, we could find the information of the crack initiation position on weld bead and the history of crack growth. Also we knew that the fatigue crack initiation cycles and the changes of the strain which were affected the fractured surface roughness and morphology.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.10
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• pp.1091-1098
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• 2009

In this paper, the procedure to estimate fatigue crack initiation life has been established by considering fretting wear and multiaxial stress states on the contact surface of press-fitted shafts. And a method to calculate the local friction coefficient during the running-in period of fretting wear process has been proposed. The predicted result of worn surface profile in the press-fitted shaft with non-linear local friction coefficient can avoid excessive wear depth estimation compared with that for the case of constant local friction coefficient. Furthermore, the predicted fatigue crack initiation lives based on Smith-Watson-Topper model considering the fretting wear are in good agreement with the experimental data. Consequently, the present method is valid not only for predicting worn surface profile, but also for assessing fatigue crack initiation lives considering the fretting wear during the running-in period in press fits.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.6
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• pp.1350-1357
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• 1988

A computer software was developed for predictions of fatigue crack initiation life of notched members under variable loadings. The software was constructed based on a new fatigue life prediction method utilizing modified .epsilon.-N curves, which can account for the stress interaction effect. The effect of mean plastic strain on low-cycle fatigue life was also incorporated in the software. The software can be utilized for the first step approximation when fundamental data of material fatigue properties are not available.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.1
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• pp.76-82
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• 2002

In this study, the rotary bending fatigue test was carried out with low carbon steel(SM25C). The specimens were heat-treated in order to change the grain size, and investigated items are fatigue limit, small crack initiation, fatigue crack propagation behavior and possibility of fatigue life prediction according to the different grain size. The summarized result are as follows ; Fatigue limit of the smooth specimen was dependent upon the grain size. The fatigue crack initiation of the small grain size specimen was delayed more than that of the large grain size specimen. And the small cracks of small grain size specimen were distributed in the narrow region of the main crack circumference contrary to the large grain size specimen. The main crack was grown along the grain boundary having co-alliance with small cracks. The experiment material has quantitatively disclosed the possibility of fatigue life prediction because the fatigue crack propagation behavior is dependent upon the grain size.

• Journal of Mechanical Science and Technology
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• v.16 no.8
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• pp.1109-1116
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• 2002

To investigate the effect of nitriding layer on both fatigue crack initiation and fatigue life, turbine rotor steel ( IC.- 1Mo-0.25V steel) specimens were nitrided by the nitemper method and then put to a rotary bending fatigue test at room and elevated temperatures. In nitriding, temperature and time were controlled to obtain a different nitrided thickness. Microstructure analysis, micro-Victors hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at inclusion near the interface between nitrided layer and substrate, which showed fish-eye type appearance in fractograph. The fatigue life of nitrided specimens at every temperature was prolonged compared to that of the non-nitrided. However, there was not observable improvement in fatigue characteristics with the increase of a nitrided thickness.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.61-66
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• 2000

The fatigue crack initiation life is studied on automotive spot weldment made from cold rolled carbon steel(SPC) sheet by using DCPDM and local strain approach. It can be found that the fatigue crack initiation behavior in spot weldment can be definitely detected by DCPDM system. The local stresses and strains are estimated by elastic-plastic FEM analysis and the alternative approximate method based on Neuber's rule were applied to predict the fatigue life of spot weldment. A satisfactory correlation between the predicted life and experimental life can be found in spot weldment within a factor of 4.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.2
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• pp.264-274
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• 2001

An integrated fatigue strength assessment system was computerized. The system developed consists of 9 modules: user interface, cycle counting, load history construction, data searching, fatigue properties estimation, fatigue data analysis, true stress and strain analysis, expert system for crack initiation life prediction, fatigue crack initiation and propagation life prediction. Fatigue strength database also was included in this system. The fatigue expert system helps a beginner to predict a fatigue crack initiation life in fatigue strength assessment. The expert system module in this system is developed on the personal computer by using C language and UNiK, an expert system developing tool. To evaluate the system, the results of test under variable loading of SAE and failure data from a field were analyzed. The evaluation show that the system provided fatigue life prediction within 3-scatter band and gave reasonable predictions. To get more accurate predictions of fatigue life without fatigue properties, we recommend utilizing the system along with the fatigue strength database.

• Journal of the Korean Society of Safety
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• v.17 no.1
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• pp.33-38
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• 2002

SS41 material is welded automatically and is investigated some effects of the welding residual stress on the growth and propagation of fatigue crack, so as to study the fatigue behaviour in the welding residual stress field. The summarized results are as follows; 1) In case of the load amplitude is constant, as the stress ration is changing to 0.1, 0.33 and 0.5 the propagation life is constant but the initiation life decreases. And than, when maximum load or minimum load is constant, s the stress ration increases the initiation life and propagation life. 2) It was shown that the fatigue crack propagation ratio da/dn was almost constant regardless of the stress ratio change at constant load amplitude and that the larger stress ratio, the slower was the fatigue crack propagation ratio. 3) The opening ratio U is influenced by $K_{max}$ but it isn't only the function of $K_{max}$ because data range is very large. 4) The fatigue life of the specimens on tensile compressive residual stress field was decreased and increased respectably more than that of the base metal.

• Tribology and Lubricants
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• v.20 no.5
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• pp.252-258
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• 2004

In this study, using high cycle fatigue (HCF) criteria, the simulation of rolling contact fatigue is conducted under elliptical contact. The HCF criteria fall into three categories: the critical plane approach, the stress invariant approach and the approach based on the mesoscopic scale. The accurate calculation of contact stresses and subsurface stresses is essential to the prediction of crack initiation life. Contact stresses are obtained by contact analysis of a semi-infinite solid based on the use of influence functions and the subsurface stress field is obtained using rectangular patch solutions. The simulation results show that the critical load is decreasing rapidly and the site of crack initiation also moves rapidly to the surface from the subsurface when the friction coefficient exceeds a specific value for all of three fatigue criteria.