A Study On Ratcheting Effect And Fatigue Failure Behavior Of Nuclear Power Pipeline

Ratcheting effect and fatigue failure behavior are the main contents of life-cycle damage evaluation of nuclear power pipelines.Long-term thermal aging effect is related to the urgent need of prolonging the life of nuclear power plants.At present,Z2CND18.12N,Z2CN18.10 austenitic stainless steel(French brand)and domestic new316LN austenitic stainless steel are mainly used for the primary circuit piping materials of the third generation PWR nuclear power plant.Taking Z2CND18.12N and Z2CN18.10 austenitic stainless steel elbow pipes as main objects,the ratcheting effect and fatigue failure behavior of nuclear elbow pipes were studied.Taking the new domestic 316LN austenitic stainless steel as the main object,the effect of long-term thermal aging on ratcheting boundary was studied.The ratcheting effect of Z2CN18.10 austenitic stainless steel elbow pipe under complex load was studied.The results show that the internal pressure level determines the accumulation direction of ratcheting strain.The synchronous loading of internal pressure and bending moment will promote the growth of ratcheting deformation,while the asynchronous loading of bending moment and torque is more conducive to the accumulation of plastic strain.The effect of constant torque on ratcheting strain is not significant,but alternating torque can promote the growth of ratcheting strain.The thermal aging ratcheting effect of Z2CND18.12N austenitic stainless steel elbow pipe was studied.Based on the C-TDF criterion proposed by Japan three-dimensional finite element stress evaluation committee,the ratcheting boundaries of elbow pipes before and after thermal aging were determined.The results show that with the increase of thermal aging time,the ratcheting deformation of the elbow pipe increases significantly and the ratcheting boundary decreases significantly.The ratcheting boundaries of 316LN austenitic stainless steel at room temperature,350 ~oC and before and after thermal aging were compared by efficiency diagram rule.The results show that the ratcheting boundary of 316LN at 350 ~oC is significantly higher than that at room temperature.When the secondary ratio U(the ratio of secondary stress range to primary stress)is large(U≥5),the ratcheting boundary of 316LN increases slightly with the increase of thermal aging time.When the secondary ratio U≥15,the ratcheting boundaries of materials before and after thermal aging are significantly higher than those given in ASME and RCC-MR codes.The ratcheting fatigue behavior of Z2CN18.10 austenitic stainless steel elbow pipe under internal pressure and cyclic bending moment was studied.The results show that the crown(90°)of the elbow is the main position for ratcheting fatigue evaluation.Based on the Manson-Coffin equation,the ratcheting fatigue life of elbow pipe can be better predicted by correcting the strain amplitude considering ratcheting effect,and the prediction result is better than the fatigue life evaluation method given by ASME code.The fatigue crack propagation behavior of Z2CND18.12N austenitic stainless steel elbow pipe was studied,and the propagation law of surface crack was revealed.The results show that the evolution of crack increment ratio(35)(2c)/(35)a at intrados(0°)and crown(90°)with the number of cycles conforms to a linear variation law,and the evolution of the crack growth rate with the crack driving force at these two positions can be described by the unified Paris equation.