• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.4
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• pp.425-433
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• 1983

Fatigue tests by axial loading (R=0.1) were carried out to investigate fatigue crack growth properties of small surface cracks in mild steel at room temperature, 250.deg. C and 400.deg. C, by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present tests are determined as a function of the stress intensity factor range, so that the applicability of liner fracture mechanics to the fatigue crack growth of surface cracks at elevated temperatures is investigated and discussed in comparison with the data of type 304 stainless steel at room temperature and elevated temperature. The obtained results are as follows: 1) Relations of both surface fatigue crack length and its depth to cycle ratio fall within a narrow scatter band in spite of different stress levels. 2) The .DELTA. .sigma. .root. .pi. a-da/dN relation of surface fatigue crack growth at room temperature is independent of the stress level and can be plotted as a straight line at log-log diagram, but the relation at 400.deg. C depends partly on the stress level. 3) Relations of the fatigue crack growth into depth d(2b)/dN and is stress intensity factor range .DELTA. $K_{I}$, accounted for the aspect ratio variation, fall within a narrow scatter band for wide range of the applied stress levels. And .DELTA. $K_{I}$E-d(2b)/dN relations of mild steel at different stress level coincide relatively well with the data of type 304 stainless steel. 4) The value of aspect ratio obtained by a beach mark method and a temper coloring method approaches about 0.9 in common with crack growth and it is independent of stress level and temperatures. 5) The equi-crack length curve is parallel to S-N$_{f}$ curve at elevated temperatures.s.s.s.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.11a
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• pp.345-348
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• 2004

For practical applications, the evaluation of reliability or endurance of HTS conductors is necessary. The mechanical properties and the critical current, $I_c$, of multifilamentary Bi-2223 superconducting tapes, externally reinforced with stainless steel foils, subjected to high cycle fatigue loading in the longitudinal direction were investigated at 77K. The S-N curves were obtained and its transport property was evaluated with the increase of repeated cycles at different stress amplitudes. The effect of the stress ratio, R, on the $I_c$ degradation behavior under fatigue loading was also examined considering the practical application situation of HTS tapes. Microstructure observation was conducted in order to understand the L degradation mechanism in fatigued Bi-2223 tapes.

• Journal of Welding and Joining
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• v.7 no.1
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• pp.28-35
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• 1989

The fatigue crack growth behavior in GTA butt welded joints of Al-Alloy 5052-H38 was examined using Single Edge Notched(SEN) specimens. It is well known that welding residual stress has marked influence on fatigue crack growth rate in welded structure. In the general area of fatigue crack growth in the presence of residual stress, it is noted that the correction of stress intensity factor (K) to account for residual stress is important for the determination of both stress intensity factor range(.DELTA.K) and stress ratio(R) during a loading cycle. The crack growth rate(da/dN) in welded joints were correlated with the effective stress intensity factor range(.DELTA.Keff) which was estimated by superposition of the respective stress intensity factors for the residual stress field and for the applied stress. However, redistribution of residual stress occurs during crack growth and its effect is not negligible. In this study, fatigue crack growth characteristics of the welded joints were examined by using superposition of redistributed residual stress and discussed in comparison with the results of the initial welding residual stress superposition.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.05a
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• pp.306-311
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• 2003

A growing fatigue crack is known to be retarded on application of an overload cycle. The retardation may be characterized by the total number of cycles involved during retardation and the retarded crack length. The overload ratio plays an important role to influence the retardation behavior. The objective of the present investigation is to study the effect of different overload ratio on the retardation behavior. For DENT(double edge notched tension) specimens and ESET(eccentrically-loaded single edge crack tension) specimens, fatigue crack growth tests are conducted under cyclic constant-amplitude loading including a single tensile overloading with different overload ratios. The proposed crack retardation model predicts crack growth retardation due to a single tensile overloading. The predictions are put into comparison with the experimental results to confirm the reliability of this model.

• Journal of Welding and Joining
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• v.26 no.1
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• pp.69-75
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• 2008

Spot welding which use the main process for side block production of stainless steel railway vehicle is legged behind in laser welding about a quality and productivity. Although the laser welding has many potential advantages such as low heat input and aspect ratio of weld bead, its application to a new structural component still is required many engineering data including mechanical properties such as tensile, fatigue strength, etc on. Therefore, experimental analysis was carried out to understand the fatigue phenomena of different thickness stainless steel overlap joining panels by Nd:YAG laser welding. The fatigue life curves were obtained through fatigue tests with the various levels of applied load. The fatigue life is related with the parameters such as gap size and penetration depth through experiment. As the results, tensile strength and fatigue life were proportional in heat input level and gap was identified the major factor for fatigue life. Also we could know that deferent a-ferrite content at HAZ depend on welding heat input was important factor to determine a formation of initial crack and total fatigue life cycle.

• Journal of Ocean Engineering and Technology
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• v.1 no.1
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• pp.104-110
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• 1987

Fatigue tests by axial loading (R = 0.05) were carried out to investigate fatigue crack growth characteristics of small surface cracks in 2 1/4 Cr-1 Mo steel at room temperature by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present test are determined as a function of the stress intensity factor range about a semi-elliptical crack, so that the application of linear fracture mechanics to the surface fatigue crack growth and to the fatigue crack growth into depth, and all the data obtained from tests were discussed in comparison with the data of Type 304 stainless steel and two type of mild steel under the same test conditions. The obtained results are as follows: 1)When the cycle ratios are same, surface fatigue crack length and its depth are almost same and fall within a narrow scatter band in spite of different stress levels. 2)Relations of the surface fatigue crack growth rate (da/dN) and fatigue crack growth rate into depth (db/dN) to its stress intensity factor range ($\Delta K_{Ia}, \Delta K_{Ib}$) can be plotted as a straight line at log-log diagram without dependence of stress level and coincide with the data of part-through crack in various steels.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.2
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• pp.186-192
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• 2002

The fatigue crack propagation characteristics were investigated on dissimilar friction weld of two kind of heat-resisting steels (STR3 and STR35) commonly used in valve materials for vehicles. A small circular artificial defect was machined to induce fatigue crack at bonded line, heat affected zone and base metal of the weld on the surface of the specimens. From the results of the experiment, the fatigue limits of the materials STR3 and STR35 were obtained to be 429.0MPa and 409.4MPa respectably. The STR35 base metal and 1.0mm HAZ specimens showed 190% and 82% higher fatigue life than STR3 base metal. And the fatigue life of 1.0mm HAZ specimen was shown 99% on STR3 and 29% on STR35 higher than that of their base metal. But the fatigue life of weld interface specimen was shown 18% on STR3 and 72% on STR35 lower than that of their base metal because of the weld interface separation.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.8
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• pp.923-928
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• 2005

The estimation of fatigue life at the design stage of the compressed air tank on board is very important in order to arrive at feasible and reliable solutions considering the total lifetime of the tank. In this paper the compressed air tank on board was selected as a model and the change of inside pressure of the tank during normal navigation period was measured and the cycle of fluctuation stress was presumed statistically based on this. Also the effect of stress concentration with the FEM analysis on the longitudinal weld and the mean stress effect on the fatigue strength of compressed air tank were discussed.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.10
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• pp.84-91
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• 2002

The shaft with the circular cross section have symmetric structural combination parts to keep the rotating balance. Hence the crack usually emanate from symmetric combination parts due to the stress concentration of these parts. In this study to estimate the fatigue behavior of symmetric cracks, the fatigue test was performed by using rotary bending tester and the specimen with symmetric defects in circular cross section. From the facts the characteristics of crack initiation and propagation on the symmetric surface cracks in circular cross section was examined. Also we observed the internal crack using oxidation coloring and investigated the fatigue behavior using the relationship between surface crack and internal crack. As a result of fatigue lift of symmetric cracks was reduced to 35% compared to single crack’s. We examined the characteristics of fatigue behavior in element with symmetric cracks using internal crack propagation rate and maximum stress intensity factor range obtained from approximation method.

• Journal of Ocean Engineering and Technology
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• v.10 no.1
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• pp.53-64
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• 1996

In this paper, rotating bending fatigue tests have been carried out to investigate the growth behabiors of surface fatigue crack initiated from a small artificial surface defect, that might exist in real structures, on 2024-T3 and 6:4 brass. The test results are analysed in the viewpoints of both strength of materials and fracture mechanics, it can be concluded as follows. The effect of a small artificial surface defect upon the fatigue strength is very large. The sensitivity of 2024-T3 on the defect is higher than that of 6:4 brass. The growth behavior of the surface fatigue crack of 2024-T3 is different from that of 6:4 brass. The growth rate of the surface fatigue crack of 2024-T3 is considerably rapid in the early stage of the fatigue life and apt to decrease in the later stage. It was impossible to establish a unifying approach in the analysis of crack growth begabior of 2024-T3 and 6:4 brass using the maximum stress intensity factor because of their dependence on stress level. But if the elastic strain and cyclic total strain intensity factor range were applied to obtain the growth rate of surface fatigue cracks of the materials, the data were found to be nearly coincided.