PSI - Issue 7

F. Fomin et al. / Procedia Structural Integrity 7 (2017) 415–422 Fedor Fomin and Nikolai Kashaev/ Structural Integrity Procedia 00 (2017) 000–000

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7. Summary and outlook In this work, the mechanism of the fatigue failure of the laser beam welded machined Ti-6Al-4V butt joints was studied. The fish-eye pattern of the fracture surface and rough ODA in the vicinity of the crack origin were found by fractographical observations. The model for ODA formation, based on the plastic zone size at the crack tip, was in accordance with the quantitative fractographical analysis. An analytical fracture-mechanics model for fatigue life assessment, taking into account internal defects and physically short cracks effect, was presented. Good agreement between the model and experimental S-N data for the as-welded and heat-treated conditions was achieved. It was shown that up to 95% of the total life crack is propagating within the ODA. Cracks started from internal pores were found even in the unbroken specimens after 10 7 cycles. Further work is required to more carefully study the material properties within the FZ and verify the conditions for crack arrest. Acknowledgements The authors would like to thank Mr. Falk Dorn, Mr. Kai Erdmann, Dr. Volker Ventzke, Mr. Stefan Riekehr and Dr. Vasyl Haramus from the Helmholtz-Zentrum Geesthacht for their technical support and assistance in the experimental work. The authors are particularly grateful to Dr. Uwe Zerbst and Dr. Mauro Madia from the BAM-Federal Institute for Materials Research and Testing for their valuable suggestions on the modelling of physically short cracks. References Anderson, T.L., 2005. Chapter 10: Fatigue crack propagation, in “Fracture Mechanics: Fundamentals and Applications” , 3 rd ed. CRC Press, Boca Raton, USA, pp. 451-509. El Haddad, M.H., Topper, T.H., Smith, K.N., 1979. Prediction of non propagating cracks. Engineering Fracture Mechanics 11, 573-584. Fomin, F., Ventzke, V., Dorn, F., Levichev, N., Kashaev, N., 2017. Effect of Microstructure Transformations on Fatigue Properties of Laser Beam Welded Ti ‐ 6Al ‐ 4V Butt Joints Subjected to Postweld Heat Treatment, in “Study of Grain Boundary Character” , Tański, T. (Ed.), InTech, DOI: 10.5772/66178. Green, D.J., 1980. Stress intensity factor estimates for annular cracks at spherical voids. Journal of the American Ceramic Society 63, 342-344. Gregory, J.K., 1996. Fatigue crack growth of titanium alloys, in “ ASM Handbook. Fatigue and Fracture, Volume 19” . ASM International, Materials Park, USA, pp. 845-853. Kashaev, N., Ventzke, V., Fomichev, V., Fomin, F., Riekehr, S., 2016. Effect of Nd:YAG laser beam welding on weld morphology and mechanical properties of Ti-6Al-4V butt joints and T-joints. Optics and Lasers in Engineering 86, 172–180. Madia, M., Zerbst, U., 2016. Application of the cyclic R-curve method to notch fatigue analysis. International Journal of Fatigue 82, 71-79. Murakami, Y., 2002. Chapter 15: The Mechanism of Fatigue Failure of Steels in the Ultralong Life Regime, in “Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions” , 1 st ed. Elsevier, Oxford, UK, pp. 273-303. Ritchie, R.O., Peters, J.O., 2001. Small fatigue cracks: mechanics, mechanisms and engineering applications. Materials Transactions 42(1), 58-67. Tanaka, K., Akiniwa, Y., 1988. Resistance-curve method for predicting propagation threshold of the short fatigue cracks at notches. Engineering Fracture Mechanics 30(6), 863-76. Wagner, L., Lütjering, G., 1987. Microstructural influence on propagation behavior of short cracks in an (α + β) Ti alloy. Zeitschrift für Metallkunde 78, 369-375. Xiulin, Z., Hirt, M.A., 1983. Fatigue crack propagation in steels. Engineering Fracture Mechanics 18(5), 965-973. Zerbst, U., Madia, M., 2015. Fracture mechanics based assessment of the fatigue strength: approach for the determination of the initial crack size. Fatigue & Fracture of Engineering Materials & Structures 38, 1066-1075. Zerbst, U., Vormwald, M., Pippan, R., Gänser, H., Sarrazin-Baudoux, C., Madia, M., 2016. About the fatigue crack propagation threshold of metals as a design criterion – A review. Engineering Fracture Mechanics 153, 190-243. Zhao, A., Xie, J., Sun, C., Lei, Z., Hong, Y., 2011. Prediction of threshold value for FGA formation. Materials Science and Engineering A 528, 6872 – 6877.