A Study On The Ratcheting And Shakedown Behavior Of Pressurized Piping Components Under Cyclic Loading

Pressure vessels and piping system are subjected to internal pressure duringnuclear power station operating, cyclic thermal loadings and vibration complex loadconditions, which can result in ratcheting effect, reduce fatigue life of materials andcomponents and seriously affect the safty and reliability of pressure vessels andpiping system. In order to guarantee the safe operation of the nuclear powerpressuried piping and obtain the basic data for establishing design code of ratchetingbehavior of piping system, ratcheting effects of Z2CND18.12N austenitic stainlesssteel used for the auxiliary piping in primary collant circuit pipes of pressurized waterreactor are systematically studied through cyclic constitutive model of materials,numerical simulation and ratcheting experiments of straight pipe and elbow. The studyhas important theoretical significance and value in engineering for design andoperation of pressure pipe in nuclear power plant.In this study, ratcheting behavior of pressurized straight pipes and elbows madeof austenitic stainless steel Z2CND18.12N were experimentally studied withmultiaxial fatigue testing system. The experimental results showed that ratchetingstrain occurred mainly in the hoop direction for straight pipe, axial ratcheting strainwas less than that of hoop direction, ratcheting strain was extended along the axialdirection of straight pipe. For elbow pipe, the ratcheting strains manly occur in thehoop direction of flank, intrados and45°direction at midway between flank andintrados. Small ratcheting strain was also collected in axial direction. Ratcheting strainrate increased with the increasing of reversed bending or internal pressure for bothdifferent specimens with different loadings and same specimen with multi-steploadings. In multi-step loading, ratcheting rate suffered from the ratcheting history athigher level loading. Ratcheting boundary for straight pipe was determined with theaid of the quasi-three-point-bending apparatus.The material parameters of AF type models were determined based on theexperimental results of austenitic stainless steel Z2CND18.12N. The AF type modelswith isotropic hardening rule were implemented into user subroutine USERPL.f ofANSYS software, the predicted results of uniaxial and multiaxial ratcheting behaviorof materials and structures were largely improved. Moreover, the function p ofcyclic hardening/softening of materials and the evalution of the parameter iwereimplemented into the superposition model of Ohno-Wang model and Armstrong- Frederic model, namely a new modified model was proposed, ratcheting strain ofuniaxial and multiaxial ratcheting by the modified model was slightly improved.The ratcheting effect of straight pipe and elbow was simulated by ANSYSsoftware and elasto-plastic analysis. The predicted results of a number of models werecompared, it was shown from those results that the predicted result of Chen-Jiao-Kimmodel was in well agreement with experimental data. Taking advatanges of C-TDFmethod, ratcheting boundary of pressurized straight pipe and elbow subjected tocyclic bending loading was determined. The ratcheting boundary of straight pipe wascompared with available codes, which could divide the shakedown region well.Ratcheting strain of pressurized straight pipe and elbow with local wall thinningsubjected to cyclic bending loading was predicted by Chen-Jiao-Kim model. It wasfound that ratcheting strain was affected by the depth, axial length and hoop length oflocal wall thinning. Ratcheting strain of pressurized straight pipe and elbow subjectedto cyclic bending loading under higher temperatures was predicted by Chabochemodel. It was found that ratcheting strain increased with the increasing temperature.The ratcheting boundary under different temperatures was determined by C-TDFmethod. Moreover, a modified method was proposed based on elastic compensationmethod and limit load multiplier of lower bound theorem, which was used todetermine ratcheting boundary of pressurized straight pipe and elbow under cyclicbending. Compared with ratcheting boundary determined by non-linear elastic-plasticfinite element method, the modified method was convenient and feasible.