• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.4
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• pp.760-780
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• 1988

Using the fatigue test results obtained in the SAE Fatigue Cumulative Damage Test Program, prediction methods of fatigue crack initiation life for notched members undergoing random loaming histories were discussed in detail. Conventional fatigue life predictions based on so-called modified Miner's rule were found to be apt to give nonconservative estimate, due to lack of sufficient consideration for stress-interaction effect. A modified .epsilon.-N curve concept was proposed to account for the stress-interaction effect. The predicted fatigue life based on the modified .epsilon.-N curve concept was in good agreement with the experimental results of SAE Test Program. Specifically for the cases when fatigue data was not available at hand, was proposed a procedure to give conservative estimate of fatigue life.

• Transactions of Materials Processing
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• v.30 no.1
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• pp.5-15
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• 2021

The effects of heat treatments (T6 and T73) on the microstructure, mechanical properties, and high cycle fatigue behavior of modified AA7075 alloys were investigated. A modified 7075 alloy was manufactured using modified-Mg (Mg-Al2Ca) instead of the conventional element Mg. Based on the microstructure, the average grain size was 4.5 ㎛ (T6) and 5.2 ㎛ (T73). Regardless of heat treatment, the modified AA7075 alloys consisted of Al matrix containing homogeneously distributed Al2CuMg and MgZn2 phases with reduced Fe-intermetallic compound. Room temperature tensile tests showed that the properties of modified 7075-T6 (Y.S.: 622MPa, T.S: 675MPa, elongation: 15.4%) were superior to those of T73 alloy (Y.S.: 492MPa, T.S: 548MPa, elongation: 12.8%). Experimental data show that the fatigue life of T6 was 400 MPa, about 64% of its yield strength. However, the fatigue life of T73 alloy was 330 MPa and 67%. Irrespective of the stress level, all crack initiation points were located on the specimen surface, and no inclusions acting as stress concentrators were seen. Superior mechanical properties and high cycle fatigue behavior of modified AA7075-T6 alloy are attributed to the fine grains and homogeneous distribution of small second phases such as MgZn2 and Al2CuMg, in addition to reduced Fe-intermetallic compounds.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.11
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• pp.3424-3432
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• 1996

For mechanical system operating at high temperature, damage due to the interaction effect of creep and fatigue plays an important role. The objective of this paper is to develop a modified creep-fatigue damage model which separately analyzes the pure creep damage for hold time and the creep-fatigue interaction damage during startup and shutdown period. The creep damage was calculated by the general creep damage equation and the creep-fatigue interaction damage was calculated by the modified equation which is based on the frequency modified strain range method with strain rate term. In order to verify the proposed model, a service of high temperature low cycle fatigue tests were performed. The test specimens were made from inconel-718 superalloy and the test parameters were wave shape and hold time. A good agreement between the predicted lives based on the proposed model and experimentally obtained ones was observed.

• International Journal of Highway Engineering
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• v.2 no.3
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• pp.129-144
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• 2000

This paper presents the characteristics of viscoelastic properties and fatigue performance of SBS modified asphalt mixtures depending on the aggregate gradation. Dynamic shear rheometer (DSR) and uniaxial tensile creep tests are performed to analyze the thermomechanical behavior of asphalt binders and mixtures, respectively. Uniaxial tensile fatigue tests for seven different asphalt mixtures are conducted to evaluate the effects of aggregate gradations and SBS modifier on the fatigue performance of the mixtures. DSR and uniaxial tensile creep tests results show that the SBS modified asphalt mixtures have better rutting resistance than the unmodified mixtures at high temperatures regardless of the aggregate gradations used. Fatigue factor $G^*sin\delta$ in Superpave binder specification may not be adequate for evaluating the fatigue Performance of asphalt mixtures. It is observed from uniaxial tensile fatigue tests that SBS modified asphalt mixtures compared to unmodified mixtures have ten times longer fatigue lives regardless of the aggregate gradations(dense, SMA, and Superpave gradations) used in the mixtures. The better fatigue performance of the SBS modified mixtures is observed even after long-term aging process. The effect of aggregate gradations on the fatigue performance is not as significant as the SBS modifier. The cellulose fiber added in the SMA mixture has negligible effects on the viscoelastic Properties and fatigue performance of the mixture, but is effective in reducing draindown. Although the SBS modified asphalt binder is used, it may be necessary to add the cellulose fiber into the SMA mixture to prevent the draindown.

• Journal of the Korean Society for Precision Engineering
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• v.17 no.5
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• pp.160-167
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• 2000

In this research, the fatigue behaviors of carbonitrided Cr-Mo(SCM415) steel are examined. From fatigue test results, we propose a modified fatigue strength evaluation model and modified formulae. That is, we develop improved models fur evaluating and predicting the fatigue strength of carbonitrided SCM415 by modifying specific terms in the Murakami and Endo's model which is the representative fatigue strength evaluation formulae fur high strength steel.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.6
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• pp.791-798
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• 2004

Fatigue crack initiation lives of round cylindrical notch specimen were investigated. Firstly, local strain approximation methods, such as the modified incremental Neuber's rule and the modified incremental Glinka's equivalent strain energy density(ESED) rule, were used to get multiaxial stress and strain state components at the notch tip. Based on the history of local stress and strain, multiaxial fatigue models were used to obtain fatigue crack initiation lives. Because the solution of Neuber's rule and Glinka's ESED rule make the upper and lower bound of local strain approximations, fatigue crack initiation lives are expected to place between life predictions by two local strain approximations. Experimental data were compared with the fatigue crack initiation life prediction results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.2
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• pp.596-603
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• 1991

Fatigue life prediction of pressure vessel is studied analytically using cumulative damage models and linear elastic fracture mechanics method. The stresses are analyzed by finite element method. During operation, the maximum stress occurs at the outside of neck region while fatigue analysis indicates that the bottom of nozzle part has the shortest fatigue life. Previously proposed fatigue life prediction equation and cumulative damage model are modified successfully by introducing reference fatigue modulus. It is found that the modified life prediction equation and damage model are useful for lower stress level application.

• Journal of Ship and Ocean Technology
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• v.10 no.3
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• pp.17-26
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• 2006

Spar platforms have been installed as a competitive alternative offshore production structure for deepwater oil field. Since the first spar platform was constructed, its configuration has evolved to the so-called the truss spar and then the cell spar. This paper describes the dynamic analysis and fatigue life assessment of steel catenary riser (SCR) connected to cell spar platform. Two different cell spar platforms are considered herein; the original cell spar and the modified one. The original cell spar was modified by introducing an additional damping plate at its bottom in order to reduce wave-frequency motions. Firstly the wave-frequency motions of cell spar platforms are calculated based on the potential theory. Then, the dynamic responses of SCR induced by platform motions are computed. Finally the fatigue life of SCR is estimated by spectral method and the performance of two spar platforms are compared in terms of the fatigue life. Through the present study, it is found that the fatigue life of the modified cell spar increases only slightly.

• Journal of the Korean Society of Safety
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• v.14 no.4
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• pp.20-27
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• 1999

The standard of the allowed stress in designing components such as shafts used in machinery and aircraft should be determined. It is very important to estimate fatigue strength of a component analytically, because service stress of the components is usually under the level of its fatigue limit. Improving the fatigue strength of each component in machinery is essential for high performance, lightness, and endurance. Surface treatment of components is widely used for these purpose. In this research, the fatigue behaviors of SCM415 steel and carbonitried SCM415 steel are examined. In the analysis of the fatigue behaviors of the carbonitrided SCM 415 steel, we propose a modified fatigue strength evaluation model and modified formulae.

• Journal of the Korean Society of Safety
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• v.26 no.2
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• pp.1-5
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• 2011

In order to evaluate fatigue endurance for an ultra-light weight inline skate frame, FEM analysis was performed. Tensile properties and a S-N curve were determined through tensile and fatigue tests on a modified Al-7075+$S_c$ alloy. The yield and ultimate tensile strengths were 553.3 MPa and 705.5 MPa, respectively. The fatigue endurance limit of this alloy was 201.2 MPa. For evaluating the fatigue endurance of the inline skate frame, the S-N data were compared with the stress analysis results through FEM analysis of the frame. The maximum Von-Mises stress of the frame was determined 106 MPa through FEM analysis of the frame, assuming that the rider weight is 75 Kg. Conclusively, on the basis of fatigue limit, the inline skate frame has a safety factor of approximately 2.0.