PSI - Issue 2_A

Jia-nan Hu et al. / Procedia Structural Integrity 2 (2016) 934–941 J. Hu et al./ Structural Integrity Procedia 00 (2016) 000–000

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3. Micromechanical aspects of damage development In this paper we have used a simple Kachanov damage variable  , empirically defined as the ratio between the normal creep separation and a prescribed critical separation (Eq. 3), to describe damage development within the interface. Laha, Chandravathi et al. (2012) have described the microstructural changes that occur at the interface leading up to failure. They demonstrate that, for both the P91/Inco82 and P22/Inco82 systems, carbides nucleate and grow at the interface throughout the life. These provide preferential sites for cavities to nucleate, grow and coalesce to cause separation at the interface. Cocks and Ashby (1982) and Cocks and Ponter (1989) have explored the relationship between micromechanical and empirical Kachanov models and demonstrate that if a single process dominates the life it is possible to map from one description to the other. In situations where multiple processes determine the life, simple single damage parameter models may not be able to adequately capture the constitutive response. It remains to be determined whether the simple model presented here can capture the important features of damage development and crack growth in dissimilar metal welds. In situations where the models are deficient the approach described here can be readily extended to more elaborate multi-state variable micromechanical models. 4. Conclusion Creep rupture at 823 K with failure at dissimilar weld interfaces of two dissimilar metal weld (DMW) systems (P91/Inco82 and P22/Inco82) has been investigated, adopting a cohesive zone modelling approach in ABAQUS. A Kachanov damage model has been developed to describe the degradation of elastic-creep properties and to simulate interface failure. Creep parameters have been calibrated against the interface failure data obtained by Laha, Chandravathi et al. (2012). Difference in the captured damage accumulation along the interface of the two systems has been attributed to the mismatch in creep properties of the bulk materials. Microstructural models of the evolution of precipitates and cavities at the interface can be employed to inform a more detailed physical model of damage accumulation. Acknowledgement Jianan Hu and Alan Cocks would like to acknowledge the financial support of MHI. References Clark, J.W.G., McCartney, D.G., Saghafifar, H. and Shipway, P.H., 2014. Modelling Chemical and Microstructural Evolution across Dissimilar Interfaces in Power Plants. Proceedings of the Asme Power Conference, 1 Cocks, A.C.F., Ashby, M.F., 1982. Creep Fracture by Coupled Power-Law Creep and Diffusion under Multiaxial Stress. Metal Science 16(10), 465-474. Cocks, A.C.F., Ponter, A.R.S., 1989. Creep Deformation and Failure under Cyclic Thermal Loading. Nucl Eng Des 116(3), 363-387. DuPont, J.N., 2012. Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds. Int Mater Rev 57(4), 208 234. Kachanov, L.M., 1999. Rupture time under creep conditions. Int J Fracture 97(1-4), Xi-Xviii. Kumar, Y., et al., 2016. Study of creep crack growth in a modified 9Cr-1Mo steel weld metal and heat affected zone. Mat Sci Eng a-Struct 655, 300-309. Laha, K., et al., 2012. A Comparison of Creep Rupture Strength of Ferritic/Austenitic Dissimilar Weld Joints of Different Grades of Cr-Mo Ferritic Steels. Metall Mater Trans A 43A(4), 1174-1186. Mvola, B., Kah, P., Martikainen, J., 2014. Dissimilar Ferrous Metal Welding Using Advanced Gas Metal Arc Welding Processes. Rev Adv Mater Sci 38(2), 125-137. Park, K., Paulino, G.H., 2012. Computational implementation of the PPR potential-based cohesive model in ABAQUS: Educational perspective. Eng Fract Mech 93, 239-262. Wen, J.F., Tu, S.T., Gao, X.L., Reddy, J.N., 2013. Simulations of creep crack growth in 316 stainless steel using a novel creep-damage model. Eng Fract Mech 98, 169-184. Yamazaki, M., Watanabe, T., Hongo, H., Tabuchi, M., 2008. Creep rupture properties of welded joints of heat resistant steels. Journal of Power and Energy Systems 2(4), 1140-1149. Yu, C.H., et al., 2012. Effects of grain boundary heterogeneities on creep fracture studied by rate-dependent cohesive model. Eng Fract Mech 93, 48-64.