Design and numerical simulation of thermal fatigue tests for high-temperature and high-pressure water pipelines
[Objective]During the startup,shutdown,and regular operation of nuclear power plant pressure vessels,temperatures and pressures vary within specific limits.Excessive fluctuations can lead to significant fatigue damage to equipment and pipelines.Thermal fatigue cracks have been detected in primary loop recirculation pipes and other components within nuclear power plants.Elucidating the growth rate of these fatigue cracks in pipeline structures is crucial for assessing safety after the detection of defects during in-service structural inspections.Additionally,conducting thermal fatigue behavior tests and research evaluations on nuclear power plant pipelines is crucial to provide necessary data support for the safe operation and maintenance of China's domestically developed third-generation nuclear power plants.[Methods]A high-throughput thermal fatigue test method for pressure pipelines has been developed using a double-loop testing device.This device operates within a subcritical water vapor environment to assess structural materials and is positioned at the high-temperature and high-pressure corrosion experimental station of the Beijing University of Science and Technology.The specimens used for the thermal fatigue test comprise 304 austenitic stainless steel stepped pipes with prefabricated defects.Through the establishment of both a program control system and a temperature control system,the temperature within the test pipe can be adjusted from 325℃to 38℃,with an internal pipeline pressure maintained at 15 MPa.Regarding this experimental method,this study investigated the effect of transient water temperature changes on fatigue crack growth in pipelines.Additionally,the finite element analysis(FEA)software ABAQUS and the new generation crack analysis software FRANC3D were used to simulate the thermal fatigue crack propagation of 304 stainless steel stepped tubular structures.First,the physical modeling of the stepped tubular structure was performed using ABAQUS software.This analysis involved inputting material constitutive and thermophysical parameters and setting corresponding boundary conditions to calculate changes in the temperature field.Subsequently,the temperature field served as the initial condition,and a temperature-displacement coupling solver was used to determine the thermal deformation of the stepped pipe without prefabricated cracks.The FEA file was imported into the crack growth analysis software FRANC3D,with the initial prefabricated crack parameters defined for the model.FRANC3D automatically completed mesh redivision and then calculated the stress intensity factor.Finally,the stress field of fatigue crack propagation was obtained using ABAQUS software,and fatigue life can be determined in FRANC3D.[Results and Conclusions]The results indicated that the cooling rate significantly influenced the thermal fatigue crack growth rate in stepped pipes.As the stepped pipe specimen moved further from the inlet of hot and cold water,the fatigue crack growth rate gradually decreased due to the reduced cooling rate.Moreover,an exponential relationship was observed between the fatigue crack growth rate and the cooling rate.After 1 226 thermal fatigue cycles,the maximum crack length reached 170 µm.Through the integration of finite element thermo-mechanical coupling simulation with three-dimensional crack propagation analysis software,the thermal fatigue crack propagation life of stepped pipes was successfully predicted.A Paris model was developed to describe the relationship between the thermal fatigue crack propagation rate and the stress intensity factor of stepped pipes.This model was calibrated using the results obtained from thermal corrosion fatigue experiments.
nuclear power plantthermal fatiguefatigue crack growthfinite element methodlife prediction model
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