• Journal of the Korean Society for Precision Engineering
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• v.21 no.4
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• pp.161-169
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• 2004

In this study, we investigated fatigue crack growth behavior of laser welded sheet metal due to a single overload. Fatigue specimens were made using butt joint of cold rolled sheet metal that was welded by $CO_2$ laser. The fatigue crack propagation tests were performed in such a way that fatigue loading was parallel to the weld line while crack propagation was perpendicular to the weld line. Single overload was applied when fatigue crack tip was arrived near the weld line. The distances between the crack tip and the weld line at which a single overload was applied were 6, 4 and 2mm. The effect of specimen thickness and overload ratio on the fatigue behavior was determined. The plastic zone size of crack tip due to the single overload was determined from the finite element analysis. For investigating fatigue crack growth behavior, we used different thickness specimen 0.9mm and 2.0mm, and variable overload ratio applied fatigue crack propagation test. Also we used finite element analysis for investigating the plastic zone size of crack tip when single overload applied

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.9 no.6
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• pp.82-88
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• 2000

The effect of the welding for the offshore structure in the TMCP steel on the fatigue crack propagation rate and crack opening-and-closure behavior was examined. The welding anisotropy of the TMCP steel and crack propagation characteristics of the excessive loading were reviewed. (1) It seemed that a heat which was generated by the welding made a compressive residual stress over the base metal, so fatigue crack propagation rate was placed lower than in case of the base metal. (20 In the base metal, an effect of the anisotropy which has an effect of fatigue crack propagation rate of the excessive load and the constant amplitude laos was not found but in the welding material case, fatigue crack propagation rate of the excessive load in the specimen of the width direction was located in the retard side as compared with a specimen rolling direction. (3) A crack opening ratio of the used TMCP stel in this study was not changed after excessive loading but a retard phenomenon of crack propagation was observed. Consequently, it was thought that all of the retard phenomenon of crack propagation did not only a cause of the crack opening-and-closure phenomenon.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.3
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• pp.15-22
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• 2003

The fatigue crack growth model is derived and the retardation model is proposed. The fatigue crack growth model considers the residual plastic stretch on the crack surface which results from the plastic deformation at the tip of fatigue crack. The fatigue crack growth rate is calculated by using the cumulative fatigue damage and plastic strain energy in the material elements at the crack tip. This model gives the crack growth rate in reasonable agreement with test data for aluminum alloy AL6061-T651 and 17-4PH casting steel. The fatigue crack growth retardation model is based on the residual plastic stretch produced from a tensile overload which reduced the plastic strain range of the following load cycles. A strip-yield model of a crack tip plasticity is used for the calculation of a plastic zone size. The proposed retardation model characterized the observed features and delayed retardation of the fatigue crack growth under tensile overload.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.2
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• pp.451-462
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• 1995

Single overload tests performed to examine the crack retardation behavior for the specimen thickness and overload ratios. Delayed crack length was tend to increase in small thickness and big overload ratio but was difference between delayed crack length and plastic zone size that expected in specimen thickness. So retardation behavior that estimated in plastic zone size, was not sufficient. Crack tip branching and striation distribution, secondary mechanisms that effected in retardation behavior, was examined by experiment and finite element analysis. Crack tip branching was affected by micro structure, and appeared the more complicatedly according to increasing damage by overload and decreasing crack driving force in base line stress level. And crack tip branching the branching angle decreased crack driving force in the crack tip. And a characteristic of the fractography on retardation zone was that striation distribution did not appear due to decreased crack driving force.

• Journal of the Korea Institute of Military Science and Technology
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• v.5 no.4
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• pp.67-77
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• 2002

This paper introduces the calibration results of the fatigue crack growth models for damage tolerance analysis of the aircraft structures. Generalized Willenborg model and Wheeler model are calibrated with experimental data tested under the load spectrum of a trainer. The retardation factors such as, shut-off ratio in Generalized Willenborg model and shaping exponent in Wheeler model, are evaluated for aluminum alloys AL2024-T3511, AL7050-T7451 and AL7075-T73511. It is shown that the retardation effect of the crack growth rate depends on the yield strength of material and the maximum stress in the load spectrum. Generalized Willenborg model and Wheeler model give satisfactory prediction of crack growth life but the calibration of the experimental parameters with test is required.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.11a
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• pp.928-932
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• 1996

Single overload fatigue tests with overload sizes ranging from 50% and 100% have been performed to investing ate the fatigue crack growth retardation behavior. A modified and experimental method of Willenborg's model for prediction of crack growth retardation behavior has been developed, based on evaluations of equivalent plastic zone size (EPZS) changing its size along the overload plastic zone boundary. The minimum crack growth rates of each overload size are linearly decreased with overload size increasing, but fatigue lives extended by single overload are increasing much more unlike the crack growth rates. Comparisons of crack growth behavior predicted by EPZS model and Willenborg model have shown that the EPZS model accounts for overload effects better than Willenborg model. These effects include delayed retardation, large retardation region, minimum crack growth rate, and the increase rate of crack growth rate in the region crack growth rate recovered.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.6
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• pp.109-116
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• 2004

In this study, when variable-amplitude load with various applying mode acts on the pre-crack tip, we examined how fatigue cracks behave. Hence aspects of the deformation caused by changing the applying mode of single overload and propagation behavior of fatigue crack were experimentally examined: What kinds of the deformation would be formed at pre-crack and its tip\ulcorner What aspects of the residual plastic deformation field would be formed in front of a crack\ulcorner How aspects of the plastic zone could be evaluated\ulcorner As applying mode of single overloading changes, the deformation caused by tensile and shear loading variously showed in each applying mode. The different aspects of deformation make influence on propagation behavior of cracks under constant-amplitude fatigue loading after overloading with various modes. We tried to examine the relationship between aspects of deformation and fatigue behavior by comparing the observed deformation at crack and crack propagation behavior obtained from fatigue tests.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1993.04b
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• pp.328-332
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• 1993

Single overload tests are carried out for SS41. Specimens are used hollow type and solid type that stress condition prevail plane stress and plane strain at surface crack. According to the crack initiation and propagation period, single overload applies to three regime and overload ratios change into 1.5, 2.0. Crack retardation zones at initation range aren't different in both specimens respectively, however at propagation range come into large scale in hollow specimen. Delayed load cycles come into large scale in solid type. And the more the overload ratio increase, the larger the retardation zone increase but the magnitude doesn't exactly equal to the expected from the overload ratio.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.4
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• pp.670-679
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• 1989

The constant amplitude loading and 100% single overloading fatigue studies of domestic high tensile 7075-T73 aluminum alloy were performed to exmine the effect of specimen thickness and its mechanisms on fatigue crack growth behavior. The stage II fatigue crack growth rates tend to increase with decreasing specimen thickness under constant amplitude loading condition and this has relation with stress intensity factors and plastic zone size. The amount of retardation by an overload increased with decreasing specimen thickness when the crack depth and baseline stress intensity factors were constant. The crack depth is one of major factors which affect retardation phenomena by an overload and the amount of retardation increase with decreasing the crack depth. Its main mechanisms are crack closure and decreasing of K at the crack tip by branching and deflection of crack. And they are affected by near surface more severely than central portion of specimen.

• Journal of Korean Society of Steel Construction
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• v.9 no.3 s.32
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• pp.301-307
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• 1997

In field fabrication of steel members, the oversize hole is frequently required due to reaming and mismatching. But, there are no provisions and investigations about oversize hole in the Korean specifications. So, in this study, the tension test of friction type high-tention bolted joints is performed with parameters of bolt hole size, surface treatment and tightening force, and investigate the effect of slip behavior with those parameters. From the results, the enough tightening force is needed to obtain some degree slip load in shot blast treatment case, although tightening force is reduced somewhat, it is no problem to guarantee slip load in zincrich primer case. The slip behavior of joints with oversize hole(26mm) is similar to the slip behavior of joints with hole of nominal size.