Abstract
This paper reports on a computational study and experimental validation of creep-fatigue crack growth rates at high temperature in two structural materials. The objectives are to develop a methodology to predict creep-fatigue crack growth rates using plasticity-induced crack closure under creep-fatigue loading conditions by characterizing the effect of hold time on crack growth rates during cyclic loading. In this study, the computation of fatigue crack growth rates is based on the crack closure phenomenon. The total crack growth rate during creep-fatigue loading is based on the addition of fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. The study identifies the effects of frequency and shape of loading cycle on crack-tip opening stresses induced by the combined action of the plasticity-induced crack closure and creep relaxation at the crack tip. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions. Elastic-plastic-creep material behavior is considered in these simulations. Closure levels are computed for high temperature structural materials such as 9Cr-1Mo steel and Alloy 709. The numerical predictions provide satisfactory agreement with experimental data of creep-fatigue crack growth rates in modified 9Cr-1Mo and Alloy 709 steels at high temperatures.
Highlights
Innovative structural materials are a keystone in improving the design and development of advanced energy systems
The performance of advanced energy systems, hereby advanced nuclear reactors, require that structural materials to be employed in advanced reactor components go through critical stages of development and qualification
This behavior is due to the fact that the fatigue crack growth rate per cycle is predicting using ΔK values and coefficients A and B that satisfy the fatigue crack growth rates corresponding to a higher load
Summary
Innovative structural materials are a keystone in improving the design and development of advanced energy systems. Due to its superior creep resistance compared to conventional 304 and 316 stainless steels, and due to the cost to produce it, which is estimated to be 2 to 4 times that for 304 SS, and far below the cost of Ni-based superalloys with comparable strength [2], alloy 709 was down-selected as a candidate material for structural applications, namely, for reactor vessels, core supports, and primary and secondary piping components in SFRs. Creep-fatigue (CF) is a dominant loading mode expected in fast reactor structural materials. Fatigue and CF characterization are necessary to construct the creepfatigue interaction diagram required to build the future code case submittal for alloy 709. In this regard, structural integrity assessment of these power generation components requires rigorous creep-fatigue crack growth characterization and predictive computational models for the creep-fatigue crack growth behavior that takes into account creep-fatigue interactions in Alloy 709 at temperatures between 600°C and 700°C
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