Abstract
The design and development of the next generation of nuclear power plants require innovative engineered structural materials able to withstand higher temperatures and pressures. These new operating conditions will increase load and temperature burdens for longer periods of time, challenging the performance of structural materials. In fact, representative operational cycles of power plants entail periods of start-up, continuous operation at high temperatures, and shut-down periods. These in-service characteristics lead to crack growth under a combination of creep and fatigue loads. Consequently, predictive computational models for creep-fatigue crack growth (CFCG) behavior able to incorporate creep-fatigue interactions at high temperatures are relevant to life assessment and determination of inspection intervals of structural components used in energy systems. Creep-fatigue crack growth rates at high temperatures, i.e. 550, 600 and 700°C and plasticity-induced crack closure are predicted by performing two-dimensional and three-dimensional finite element simulations. The finite element analyses compute crack growth under cyclic and time-dependent loading conditions, with the consideration of the elastic-plastic-creep deformation of the material at the crack tip and crack front in the case of 3D simulations. These simulations quantify the effect of hold time on opening load induced by the combined action of plasticity-induced crack closure and creep-induced stress relaxation at the crack tip. Through these studies, it is shown that increasing the hold time during a creep-fatigue cycle results in a decrease of crack-tip opening load, thus increasing the crack growth rate during the next cyclic loading. The finite element simulations produce predictions of creep-fatigue crack growth rates that are in agreement with experimental values for Alloy 709.
Finite element simulations are performed to predict fatigue crack growth rates (FCGR). 2D finite element analyses of compact tension specimens are performed to simulate fatigue crack growth, considering elastic-plastic deformations at the crack tip. The simulations quantify the effect of the load ratio R on crack opening loads induced by plasticity. The finite element simulations predicted FCGR in agreement with the experimental data of Alloy 709. Finally, the effect of load frequency on plasticity-induced crack closure is investigated under creep-fatigue conditions in austenitic and martensitic steels in the temperature range of 550-700 °C. The study explores the effects of load cycle frequency and shape on the crack-tip opening stresses, which are driven by the combination of plasticity-induced crack closure and creep relaxation at the crack tip. Triangular and trapezoidal waveforms with different hold times at constant load amplitude were considered for the analysis.
Finite element simulations of 2D and 3D compact-tension specimens with propagating through-thickness cracks were performed to compute crack-tip opening stresses under time-dependent conditions. Experimental tests were also performed, and the results compared well with those from the finite element simulations. This study provides valuable experimental and numerical modelling insights on how load frequency effects can be incorporated into predictive models of crack growth in alloys at high temperatures.