• Journal of Industrial Technology
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• v.23 no.A
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• pp.3-7
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• 2003

Effect of nitriding on fatigue crack initiation and growth rate has been studied on SNCM. Specimens were nitrided for 15 hr at $860^{\circ}C$. The fatigue limit of nitrided specimens was superior to that of annealed($860^{\circ}C$, 15 hr) specimens. Based on detailed observations of slip band and micro crack initiation, it is concluded that the excellent fatigue limit of nitrided specimens is attributed to improved slip initiation resistance by nitriding. The characteristic of fatigue crack growth rate of nitrided specimens was investigated and compared with those of annealed specimens. It was found that by nitriding the crack growth rate was markedly decreased and the threshold stress intensity factor range was improved. It is concluded that the excellent fatigue limit of nitrided specimens is also attributed to improved fatigue crack growth rate and threshold stress intensity factor range by nitriding.

• Journal of Korean Society of Steel Construction
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• v.10 no.4 s.37
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• pp.589-600
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• 1998

This study was planned to verify the usefulness of Latin square design method in fatigue tests of cables and to see the axial fatigue behavior of wire ropes being used as hangers in suspension bridges. Three parameters : mean stress, stress range. and specimen length, were adopted for verification. The effects of these parameters are in argument except for stress range. Three classes in each parameter were used. Triple replication was performed in each cell to increase the number of replication (or degree of freedoms). The major cause of fatigue failure was fretting fatigue at trellis contact point. Three chosen parameters were proved to be significant. It was verified that the effect of stress range was in agreement with expectation, but the effect of specimen length was contrary to the expectation. It was also observed that the effect of mean stress depended upon the chosen level. Therefore Latin square design method is effective for verifying the parameters that affect fatigue behaviour under orthogonality conditions.

• 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.

• Journal of Ocean Engineering and Technology
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• v.9 no.1
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• pp.92-100
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• 1995

The effect of different anisotropy and stress ratio on fatigue crack propagation behavior was investigated under various stress ratio(R=-0.4, -0.2, 0.2, 0.2, 0.4) using pure titanium sheet used in aerospace, chemical and food industry. The rack closure behavior under constant load amplitude fatigue crack propagation test was examined. Fatigue crack propagation rate da/dN was estimated in terms of effective stress intensity factor range, $\Delta$K$_{eff}$, regardless of various stress ratio but was influenced by anisotropy. Also, it was found that the effect of anisotropy was considerably decreased but still not negligible when he da/dN was evaluated by a conventional parameter, $\Delta$$K_{eff}$/E and when the modified da/dN.$\sqrt{\varepsilon}_f$ was evaluated by $\Delta$$K_{eff}$/E. On the other hand, da/dN could be evaluated uniquely by effective new parameter, $\Delta$K$_{eff}$/$sigma_{ys}$, regardless of anisotropy, as int he following equation da/dN=C''[\frac{{\Delta}K_{eff}}{{\sigma}_{ys}}]^{n''}. And effective stress intensity factor range ratio, U was estimated by the following equation with respect to the ratio of reversed plastic zone size, $\Delta r_{p}$ to monotonic plastic zone size, $r_p$ regardless of stress ratio and anisotropy. U=-4.45$(\Delta r_{p}/r_{p})^{2}$+4.1$(\Delta r_{p}/r_{p})$+0.245_{p})$+0.245

• Journal of Ocean Engineering and Technology
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• v.14 no.1
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• pp.23-28
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• 2000

The fatigue crack growth behavior of the cold-rolled STS 304 steel developed for membrane material of LNG storage tank was examined experimentally at 293K, 153K and 111K. The fatigue crack growth rate(do/dN) tends to increase as the stress ratio (R) increases over the testing temperature when compared at the same stress intensity factor range($\Delta$K). The effect of R on do/dN is more explicit at low temperatures than at room temperature. The resistance of fatigue crack growth at low temperatures is higher compared with that at room temperature which is attributed to the extent of strain-induced martensitic transformation at the crack tip. The temperature dependence of fatigue crack growth resistance is gradually vanished with an increase in $\Delta$K which correlates with a decreasing fracture toughness with decreasing temperature. Fractographic examinations reveal that the differences of the fatigue crack growth characteristics between room and low temperature are mainly explained by the crack closure and the strengthening due to the martensitic transformation.

• Journal of Ocean Engineering and Technology
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• v.7 no.2
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• pp.103-113
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• 1993

X-ray diffraction analysis technique was used for the fatigue damage analysis and fatigue life prediction in Al 7075-T651 alloy. The tensile test, fatigue strength and fatigue crack propagation test with change of stress ratio were carried out. As a result, half-value breadth was increased with the plastic deformation in the specimen increasint at all test conditions. In particular, half-value breadth at the surface of the specimens fractured by fatigue was increased as stress intensity factor range and effective stress intensity factor range were increased. In addition, the good relationship between half-value breadty and diffraction pattern was shown.

• Journal of Mechanical Science and Technology
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• v.17 no.6
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• pp.796-804
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• 2003

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3$\leq$a/W$\leq$0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K$\_$I/ and $_{4}$ (SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.6 no.4
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• pp.80-88
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• 1997

An experimental investigation of the fatigue through crack growth behavior under simple stepped variable loading condition has been performed using Al7075-T651. Experiments were carried out by using cantilever bending type specimens, with chevron notches on a small electro-magnetic test machine. Tensile overloads have a retarding effect on the fatigue crack growth rates, therefore tensile overloads were used for the beneficial effect on the fatigue life. While in most cases compressive overloads have only a vanishing effect on crack growth rates, some experiments with single edge crack tension specimens reveal a marked growth retardation. The stress ratios used in this investigations varies from R=0.32 to 0.81, from R=0.04 to 0.76, from R=-0.15 to 0.73, and from R=-0.33 to 0.68 and the peak load for each case was not varied. The crack growth and crack closure were measured by Kikukawa's compliance method with a strain gauge mounted on the backside of each specimens. The results obtained are as follows. When the stepped variable load was applied, the smaller the stress ration was, the larger the delayed retardation of the crack growth rate was. The fatigue crack growh rate data obtained for through cracks were plotted well against the effective stress intensity factor range from 4.0 to 20.0MP{a^{SQRT}m}. It was found that the effective stress intensity factor range ratio was related well to the opening stress intensity factor, the maximum stress intensity factor, and crack length.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.5
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• pp.797-804
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• 2001

To obtain representative fatigue crack growth characteristic curve in residual stress field, fatigue crack growth test was carried out at various stress ratio and fatigue crack growth characteristic curve was represented using crack closure concept. Obtained results are as follows;K(sub)op/K(sub)max was independent of K(sub)max when R was lower than 0.5 and crack closure phenomenon was not observed when R is higher than 0.5. therefore neglecting crack closure behaviour, actual fatigue crack growth rate can be underestimated. Thus, considering crack closure phenomenon, fatigue crack growth characteristics curve of A 106 Gr B Steel weldment can be effectively estimated.

• Transactions of the Korean Society of Automotive Engineers
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• v.3 no.1
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• pp.1-12
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• 1995

Isothermal and thermal-mechanical fatigue characteristics of 12Cr heat resisting steel used for high temperature applications were investigated including hold time effects. Isothermal low cycle fatigue test at $600^{\circ}C$ and in-phase, out-of-phase thermal-mechanical fatigue test at 350 to $600^{\circ}C$ were conducted using smooth cylindrical hollow specimen under strain-control with total strain ranges from 0.006 to 0.015. Regardless of thermal-mechanical and isothermal fatigue tests, cyclic softening behavior was observed and much more pronounced in the thermal-mechanical fatigue tests with hold times due to the stress relaxation during the hold time. The phase difference between temperature and strain in thermal-mechanical fatigue tests resulted in significantly shorter fatigue life for out-of-phase compared to in-phase. The differences in fatigue lives were dependent upon the magnitudes of plastic strain ranges and mean stresses. During the hold time in the strain-controlled fatigue tests, the increase in the plastic strain range and the stress relaxation were observed. It appeared that the increase in plastic strain range per cycle and the introduction of creep damage made important contributions to the reduction of thermal-mechanical fatigue life with hold time, and the life reduction tendency was more remarkable in the in-phase than in the out-of-phase thermal-mechanical fatigue. Isothermal fatigue tests performed under the combination of fast and slow strain rates at $600^{\circ}C$ showed that the fatigue life decreased as the strain rate and frequency decreased,especially for the low strain ranges.