PSI - Issue 35

M. Faruk Yaren et al. / Procedia Structural Integrity 35 (2022) 98–105 Yaren M. F. et al / Structural Integrity Procedia 00 (2021) 000 – 000

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Next, crack growth lives under block loading were calculated using models existing in the literature. Wheeler and its modifications Sheu (2001), Xiaoping (2008), Yuen&Taheri (2006), Willenborg and Generalized Willenborg (Gallagher 1974) models were used for calculation of crack growth life. 13 kN overload block was applied with “n” cycles between repeated 7 kN constant amplitude fatigue load blocks with 1000 cycles in accordance with the experiments. The comparisons of experimental results with the calculated lives are given in Fig. 3. As can be seen from the figure, the results obtained from the Wheeler model and its modifications agree well with those of experiments. 5. Conclusion Significant changes in crack propagation behavior were experimentally observed with the changing cycle size of the overload block. A crack propagation life of more than three times was observed by applying the short size of the overload block, compared to the constant amplitude test under 7 kN. As the size of block overload increases, the difference between crack propagation life of block overload test and that of constant amplitude test decreases. If the cycle size of the overload block is too large, the crack propagation life will be shorter than the life under constant amplitude. The new plastic zone created in the first cycle of the overload block is traveled through by the crack tip faster due to more number of available remaining overload cycles in the block. The experimental results agree with Porter's (1972) study, which was performed under block loading using 7075 aluminum alloy. Also, Xiaoping H. et al. (2008) obtained test data from Porter (1972) and numerically validated their model. Although Porter (1972) obtained the data using center-notched thin specimens, a similar trend to the mentioned studies was seen in the results of the current work, in which the specimens used are under plane strain conditions. In addition to experimental studies, crack growth lives were also calculated with different models from the literature, and the calculated results were presented comparatively in graphs. The experimental results are closely predicted when the Wheeler Model and its modifications are used. Life calculations using the Willenborg model and its modifications generally did not match with the experimental results. Acknowledgments The financial support by The Scientific and Technological Research Council of Turkey (TUBITAK) under Project Number: 217M609 for this study is gratefully acknowledged. References ASTM International. 2015. “E647 - 15: Standard Test Method for Measurement of Fatigue Crack Growth Rates.” West Conshohocken, PA, USA. Forman, R.G., 1972. Study of Fatigue Crack Initiation from Flaws Using Fracture Mechanics Theory. Engineering Fracture Mechanics 4 (2): 333 – 45. https://doi.org/10.1016/0013-7944(72)90048-3. Gallagher, J. P., 1974. A generalized development of yield zone models (No. AFFDL-TM-FBR-74-28). Air Force Flight Dynamics Lab Wright Patterson Afb Oh.,. Xiaoping, H., Torgeir, M., Cui, W., 2008. An Engineering Model of Fatigue Crack Growth under Variable Amplitude Loading. International Journal of Fatigue 30 (1): 2 – 10. https://doi.org/10.1016/j.ijfatigue.2007.03.004. Paris, P., and Erdogan, F., 1963. A Critical Analysis of Crack Propagation Laws. Journal of Basic Engineering 85 (4): 528 – 33. https://doi.org/10.1115/1.3656900. Porter, T. R., 1972. Method of Analysis and Prediction for Variable Amplitude Fatigue Crack Growth. Engineering Fracture Mechanics 4 (4): 717 – 36. https://doi.org/10.1016/0013-7944(72)90011-2. Song, P. S., Sheu, B. C., Chang L., 2001. A Modified Wheeler Model to Improve Predictions of Crack Growth Following a Single Overload. JSME International Journal Series A Solid Mechanics and Material Engineering. Wheeler, O. E., 1972. Spectrum Loading and Crack Growth. Journal of Fluids Engineering, Transactions of the ASME 94 (1): 181 – 86. https://doi.org/10.1115/1.3425362. Willenborg, J., Engle, R. M., Wood, H. A., 1971. A Crack Growth Retardation Model Using an Effective Stress Concept. Technical Memorandum 71-1-FBR, no. January 1971: 1 – 22. https://doi.org/https://doi.org/10.21236/ADA956517. Yaren, M. F., Ayhan, A. O., Iriç, S.,. 2019. Evaluation of Different Crack Growth Retardation Models Under Single Overloads Using Al-7075