PSI - Issue 2_B

Balázs Fekete et al. / Procedia Structural Integrity 2 (2016) 2164–2172 Author name / Structural Integrity Procedia 00 (2016) 000–000

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It was demonstrated that in most cases the accuracy of presented stored-energy based model was more than that of the widely accepted and used Coffin-Manson model and the classical strain energy based models as can be seen in Fig. 7.

Fig. 7. Accuracy of the prediction of low cycle fatigue models considered (a) 15Ch2MFA (b) 08Ch18N10T

4. Conclusions The low cycle fatigue behavior of 15Ch2MFA and 08Ch18N10T reactor steels was investigated for different temperature regimes under isothermal and thermomechanical conditions. The following conclusions can be drawn from the results:  Microstructural changes that took place during thermomechanical fatigue tests in both steels related to the observed mechanical behavior. Measurement of dislocation was completed using TEM and XRD. The course of dislocation density and its dependence on usage factor was similar for both steels. At the beginning of cyclic loading, the total dislocation density increased and then gradually decreased with the increasing number of cycles. However, the nature and distribution of dislocations were different between the individual steels and this resulted in different mechanical behavior.  The macrocracks formed nets of small cracks and plastically deformed strips at sites where slip bands cross the surface of the test specimen. The distances between striation lines initially increased with increasing crack length and then became saturated.  A new low cycle fatigue prediction model based on the strain energy was developed as an attempt to account only the part of the strain energy which is stored in the material’s microstructure and which causes the fatigue damage. The suggested model had no limitations and could be applied for two types of reactor steels, under both isothermal and thermo-mechanical conditions. References Fekete, B., Szekeres, A. 2015. Investigation on partition of plastic work converted to heat during plastic deformation for reactor steels based on inverse experimental-computational method. European Journal of Mechanics - A/Solids. Vol. 53, pp. 175–186 Kettunen, P.O., Kuokkala, V.-T., 2003. Plastic deformation and strain hardening, Materials Science Foundations, Vol. 16-18, Trans Tech Publications Ltd., pp. 304–393 Mayer, T., Balogh, L., Solenthaler, C., Gubler, E. M., Holdsworth, S. R., 2012. Dislocation density and sub-grain size evolution of 2CrMoNiWV during low cycle fatigue at elevated temperatures, Acta Materialia, Vol. 60 pp. 2485–2496 Mughrabi, H., Ackermann, F., Herz,K., 1979. Persistant slip bands in fatigued face-centered and body-centered cubic crystals. In: Fatigue mechanism, J.T. Fong Eds., ASTM-STP 675, American Society for Testing Materials, pp. 69–105 Nagesha, A., Valsan, A. M., Kannan, R., Bhanu Sankara Rao, K., Bauer, V., Christ, H.-J., Singh, V., 2009. Thermomechanical fatigue evaluation and life prediction of 316L (N) stainless steel, International Journal of Fatigue, Vol. 31, pp. 636–643