PSI - Issue 13

Valentin Tkachenko et al. / Procedia Structural Integrity 13 (2018) 1396–1401 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The value of SIF is assumed constant within the segment length. After, based on kinematic diagram the crack growth rate is determined to each segment. It is assumed that fatigue crack increments at a given crack growth rate is in the normal direction to a given segment. The number of cycles for crack increments is corresponded to experimental data. The examples of crack front predictions of fracture pattern, fig.3-b, at different stages of crack growth are given in fig.4.The present curves of crack front shape are close to each other except some deviations at the edge zones. 4. Discussions and conclusions Analysis of obtained results, fig.4, shows that mathematical modeling gives appropriate results of fatigue crack front predictions in the case of curvilinear front line. Initially, fig.4-1 and 4-b, the mathematical modeling gives a lower increment of fatigue crack growth by the specimen surface. Further, at later stages of fatigue growth the mathematical modeling is the same within an error position of fatigue crack front. This result is similar to development of torsion fatigue crack observed by Shiozawa et al. (2014), when crack growth in depth was replaced by growth on the surface and this sequence was repeated again. The result of SEM observations, fig. 5.

(a) (b) Fig. 5. Example of fracture surface of crack growth specimens (a) overview (b) magnification of crack front.

As a conclusion it can be outlined, that proposed mathematical model for curvature crack front prediction in titanium alloy is suitable for qualitative and quantitative prediction of fatigue crack evolution under ultrasonic loading. Acknowledgements Authors thanks Institute of Mechanics and Engineering (ENSAM site Bordeaux) in the person of Prof. Thierry Palin-Luc for fruitful cooperation in performance the crack growth tests and Aviation Register of Russian Federation in the person of Prof. Andrey Shanyavskiy for realizing the fracture surface observations. References Nicholas T., 1999. Critical issues in high cycle fatigue. Int.J.Fatigue. Vol.21, pp. 221 -231. Bathias C., Paris C.P., 2004. Gigacycle fatigue in mechanical practice. NY.: Dekker. 328 p. Mayer H., Fitzka, Schuller R., 2013. Constant and variable amplitude ultrasonic fatigue of 2024-T351 aluminium alloy at different load ratios. Ultrasonics. Vol.53, pp.1425 - 1432. McEvely A.J., Nakamura T., Oguma H., Yamashita K., Matsunaga H., Endo M., 2008. On the mechanism of very high cycle fatigue in Ti-6Al 4V. Scripta Materialia. Vol. 59, pp. 1207 - 1209. Nikitin A., Palin-Luc T., Shanyavskiy A., 2016. Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes. International Journal of Fatigue. Vol. 93, pp. 318–325. Shanyavskiy A.A., 2014. Very-High-Cycle-Fatigue of in-service air-engine blades, compressor and turbine. Science China Physics, Mechanics and Astronomy. Vol.57, pp. 19- 29.