| Fuel cladding is the first barrier to reactor safety,which coats with nuclear fuel,storage fission products,and directly contact with liquid metal coolant.Many factors may lead to the fatigue failure of fuel cladding,including hot and cold cycles caused by reactor power surges,mechanical vibrations caused by the pump,flow induced vibration and other alternating loads caused by coolant circulation.These alternating loads are likely to cause low cycle fatigue(LCF)failure of the cladding.At the same time,the cladding also needs to withstand the high temperature and corrosion issues.Therefore,the study of LCF behavior in liquid lead-bismuth eutectic(LBE)environment for cladding materials is very important both for science research and engineering application in the reactor design,optimization and safety analysis.Among all the candidate materials of fuel cladding for lead-based reactor,15-15Ti has been proven to be the main candidate material due to its good radiation resistance and swelling properties.In this thesis,the LCF behavior of the 15-15Ti cladding material under 550? oxygen-controlled LBE environment was studied both in vacuum and LBE environment,and the effect of temperature and strain amplitude on the LCF behavior of 15-15Ti was obtained.The influence mechanism of LBE on fatigue fracture of 15-15Ti is proposed.The main contents and conclusions of this thesis are as follows:(1)The LCF mechanical behavior of 15-15Ti under vacuum at different strain amplitudes(0.3%?0.45%?0.6%?0.75%?0.9%)and temperatures(room temperature?350??450??550?)was studied.The results show that the peak stress of 15-15Ti increases first and then stabilizes during LCF loading,and the LCF life of 15-15Ti decreases with the increase of strain amplitude and temperature.Compared with 550?,the critical shear stress of 15-15Ti at room temperature is higher,resulting in less movable slip coefficient,and it is difficult for the crack tip to overcome the"obstacle" which makes the fatigue propagation path tortuous.The crack closure effect caused by rough crack surface is obvious,which reduces the driving force of crack propagation and is one of the reasons for the decrease of crack growth rate.The single slip dislocation structure and more twins are also the reasons for the low growth rate of 15-15Ti LCF cracks.(2)Based on the LCF tests in vacuum,the LCF behavior of 15-15Ti under the condition of test temperature 550? and oxygen concentration C0?5×10-6 wt.%was studied in LBE.The mechanical behavior of 15-15Ti in LBE is similar to that in vacuum,but the LCF life of 15-15Ti decreases significantly.This indicates that the influence of LBE on 15-15Ti mainly on the surface of materials and affects the initiation and propagation of fatigue cracks.Under LBE,the fatigue fracture presents quasi-dissociation fracture morphology,and the toughness trace of fatigue fracture becomes obvious with the increase of strain amplitude.(3)The feasibility of the existing liquid metal embrittlement(LME)mechanism for this study was evaluated in combination with experimental studies.A LME mechanism suitable for 15-15Ti/LBE system is proposed based on the LME mechanism of LBE leading to the reduction of inter-atomic cohesion at 550?.The mode of crack propagation is determined by the competition of several parameters,such as the theoretical cohesion strength between atoms,the actual weakened cohesion strength between atoms,and the actual positive stress acting on atoms.The difference in the number and length of micro-cracks on the surface of 15-15Ti LCF specimens under vacuum and LBE was explained by using the new LME mechanism.(4)Based on metal viscoplasticity theory and considering isotropic and follow-up hardening,a 15-15Ti fatigue mechanical constitutive model was established under the condition of strain amplitude of 0.6%at 550? in LBE.The constitutive model is used to predict and simulate the related mechanical behavior.The results shows that comparison with the experimental the model can approximate predict the fatigue mechanical behavior of 15-15Ti,which provides a theoretical basis for the design and safety assessment of fuel assembly of lead-based reactor. |
PDF Full Text Request