PSI - Issue 7

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Mirco D. Chapetti/ Struc ural Int grity Procedia 00 (2017) 000–000

Mirco D. Chapetti / Procedia Structural Integrity 7 (2017) 229–234

0 2 4 6 8 Threshold, ∆ K th [MPa m 1/2 ] 10 12 14 16 18

Chapetti, exp (5) Murakami, exp (10)

∆ K thR

∆ K dR

d

0

0,5

1

1,5

] [ a mm Root of Crack Length,

Fig. 5. Estimated threshold curve for S45C steel reported by Tokaji et al (1998).

It is important to emphasize that to estimate the threshold as a function of crack length, only the plain fatigue limit ∆σ eR , the microstructural characteristic dimension and the threshold for long cracks, ∆ K thR , are needed. Here we proposed a simple way to estimate the fatigue limit by using Vickers hardness H V and d . Besides, It is also possible to estimate the fatigue threshold for long crack propagation by using tensile strength of Vickers hardness (see, for instance, Chapetti 2011). 3. Concluding remarks The plain fatigue limit or endurance of the material can be associated with the minimum intrinsic resistance to micro-crack propagation (microstructural threshold, ∆ K dR ) for a given load ratio R, so that it can be defined by the same plain (intrinsic) fatigue limit, ∆σ eR , and the distance from the surface d of the strongest microstructural barrier. In this work, a simple expression to estimate the microstructural threshold ∆ K dR by using the Vickers Hardness and the microstructural dimension d is proposed. The proposed expression seems to work reasonably well for low and medium strength (ferritic-pearlitic and martensitic) steels and load ratio R = -1. Results show that effort should be made to add data of H V and d in future publications on fatigue in order to improve this expression for different configurations (material and load ratio). Acknowledgements Author wish to express his gratitude to the funding provided by Agencia Nacional de Promoción Científica Tecnológica, Argentina (PICT2010 Nro.0379). References Chapetti, M.D. (2003). International Journal of Fatigue, Vol. 25, p. 1319-1326. Kitagawa, H. and Takahashi, S. (1976). Applicability of fracture mechanics to very small cracks in the early stage. In: Proceedings of the Second International Conference on mechanical behavior of materials ASM, pp.627-31. Kunio, T., Shimizu, M., Yamada, K. and Enamoto, M. (1979). Fatigue Fract Engng Mat Struct, Vol. 2, pp.237-249. Kunio, T., Shimizu, M., Yamada, K. and Tamura, M. (1984). Fatigue 84, Proceedings of the 2 nd International Fatigue Conference, Birmingham, UK, 3-7 September. EMAS Publishing.

Miller, K.J. (1993). Fat Fract Engng Mater Struct Vol.16, No.9, pp.931-939. Murakami Y. and Endo M. (1994). Int. J. Fatigue Vol.16, pp.163-182. Tokaji, K., Ogawa, T. and Osako, S. (1988). Fatigue Fract Engng Mat Struct, Vol. 11, No. 5, pp.331-342.