A Study On The Uniaxial Ratcheting Effect And Ratcheting-fatigue Behavior Of Zircaloy-4 At High Temperature

Zircaloy-4(Zr-4) tube,used as nuclear fuel cladding,is generally subjected to alternating loads.With non-zero mean stress,ratcheting effect will occur.The material will failure as ratcheting fatigue.For the purpose of safety assessment and optimized design of Zr-4 nuclear fuel cladding,investigation of the ratcheting behavior and ratcheting fatigue at high temperature of Zr-4 is necessary.In this study,a series of uniaxial tensile tests,incremental cyclic tests,uniaxial ratcheting tests,and stress-control fatigue tests with and without mean stress have been conducted on Zircaloy-4(Zr-4)tubes used as nuclear fuel cladding at 400 oC.Combined with the isotropic hardening rule,a model on the basis of Ohno-Wang II kinetic hardening rule was used to describe the uniaxial ratcheting effect of Zr-4.On the basis of Basquin model,four mean stress models: the Goodman equation,the Gerber equation,the Smith-Watson-Topper(SWT)parameter and the Walker parameter were used to predict the fatigue life.The results showed that: this material features cyclic softening at room temperature(RT),and cyclic stable and hardening at 400 oC.The stress-strain response was rate dependent.Uniaxial ratcheting strain increases with the increase of mean stress and stress amplitude,and decrease with the increase of loading rate.Loading history has significant influence on the uniaxial ratcheting behavior.Lower stress level after a loading history with higher stress level leads to the shakedown of ratcheting.Higher loading rate after a loading history with lower loading rate brings down the ratcheting strain rate.The sequence of stress rate appears to have no effects on the final ratcheting strain accumulation.The ratcheting strain is sensitive to temperature,and the higher temperature leads to less ratcheting under same normalized stress(0.2s(28)s/ s).Combined with the isotropic hardening rule,a model on the basis of Ohno-Wang II kinetic hardening rule can describe the uniaxial tensile,strain cyclic and ratcheting effect of Zr-4 well.The ratcheting evolution of whole life can divide to primary,steady-state and tertiary stages.The final strain was increased with mean stress and stress amplitude.For fatigue cycles with mean stress,ratcheting strain leads to a shorter fatigue life than fully reversed stress cycling with the same stress amplitude.Fatigue lives were correlated using four mean stress models.It has been shown that the Goodman equation and Gerber equation do not correlate life data,while the SWT parameter and Walker parameter yield good correlation.