13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Results from Fig. 5, and especially the different levels of plastic strains for a given stress that can be achieved in WPS experiments compared to isothermal tests, suggest that integrating over the active plastic zone may not be enough to account for the necessity of plastic strains. This may explain why considering strain-controlled germination of microcracks as in the Bordet model leads to better estimate of fracture toughness after prestressing. 4. Conclusion In this study, the warm prestress effect on 16MND5 RPV steel was studied, focusing on transient loadings in which the load increases during cooling. The existence of a critical slope under which no propagation occurred is confirmed for this steel. Looking at the evolution of the active plastic zone gives some insights into the physical explanation of this critical slope: in the last part of the transient where the loading path is above the Master Curve, plasticity is confined to the vicinity of the crack tip and is much smaller than the size of the plastic zone at prestressing. Local approach to fracture models give predictions in good agreement with experimental data, a simplified version of the model proposed by Bordet et. al. leading to more accurate predictions The difference between the two models investigated is explained by looking at the evolution of stress, strain and triaxiality close to the crack tip: compared to an isothermal test, cumulated plastic strain can be larger for a given stress for WPS tests, suggesting that accounting explicitly for the necessity of plastic strain – as in the Bordet model – is required. Acknowledgements Financial and technical supports from CEA program RSTB and project MASOL is acknowledged. B. T. would like to thank P. Wident from SRMA/LC2M and his Master students N. Mozzani and J. Li who have been involved in this research project. References [1] B. Pickles, A. Cowan, A review of warm pre-stressing studies. Int. J. Pres. Ves. And Piping, 14 (1983) 95-131 [2] D. Smith, S. Garwood, The significance of prior overload on fracture resistance: A critical review. Int. J. Pres. Ves. And Piping, 41 (1990) 225-296 [3] K. Wallin, Master Curve implementation of the warm pre-stress effect. Eng. Fract. Mech. 70 (2003) 2587-2602 [4] B. Tanguy, C. Bouchet, S.R. Bordet, J. Besson, A. Pineau, Toward a better understanding of the cleavage in RPV steels: Local mechanical conditions and evaluation of a nucleation enriched Weibull model and of the Beremin model over a large temperature range. In 9th European Mechanics of Materials Conference, Local approach to fracture, Moret-sur-Loing (2006) [5] ASTM E 1921- Standard test method for determination of reference temperature T0 for ferritic steels in the transition range [6] S. Bordet, B. Tanguy, J. Besson, S. Bugat, D. Moinereau, A. Pineau, Cleavage fracture of RPV steel following warm-prestressing: micromechanical analysis and interpretation through a new model. Fatigue Fract. Engng. Mater. Struct. 29 (2006) 799-816 [7] T. Yuritzinn, L. Ferry, S. Chapuliot, P. Mongabure, D. Moinereau, A. Dahl, Illustration of the WPS benefit through batman test series: Tests on large specimens under WPS loading
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