Shakedown Analysis And Evaluation Approaches To Structures Under Complicated Conditions

Shakedown analysis is the important content for evaluating protection against ratcheting failure of structures under cyclic loadings. But the classical shakedown theory may be time-consuming or more conservative for estimating the shakedown behavior of complicated structures and relatively more accurate material constitutive models. Considering the thermal ratcheting is the most common ratcheting failure mode in process equipments, investigations on the shakedown behaviors of the model with local discontinuity, the model coupled with ductile damage, the nonlinear hardening with creep model as well as the creep-ratcheting interaction model under cyclic thermo-mechanical loadings, were addressed. The main contents and conclusions of this paper are listed as follows:The shakedown limit loads of perforated thick-walled cylinders under cyclic thermo-mechanical loadings were studied based on the direct cycle method. The influences of, thicknesses and the axial stress states on were discussed. The results indicated that the shakedown limit loads decrease with increasing the opening radius and increase with the thickness inecreasing. Moreover, the shakedown limit loads decrease insignificantly under the axial compressive state. Two kinds of modified Bree diagrams were provided according to the calculated results, which were verified as the simple and effective methods for shakedown estimation of perforated thick-walled cylinders under various thermo-mechanical loadings.The shakedown behaviors of multilayered structures coupled with the Lemaitre ductile damage and the Armstrong-Frederick nonlinear kinematic hardening models under cyclic thermo-mechanical loadings were investigated. The conclusions presented that the shakedown limit loads reduces significantly considering the effect of ductile damage and even become less than the twice yield stress limit when the thermal load is predominant. Secondly, the ductile damage factor increases gradually to the critical damage limit, which causes the ductile crack failure, when ratcheting or excessive plastic deformation are produced. Using the coupling model to evaluate the shakedown behavior can avoid the insecurity induced by the excessive plastic deformation which may meet the requirement of the classical shakedown theorem.The shakedown behavior of multilayered systems under cyclic elastic-plastic-creep conditions were analyzed based on the direct cycle method. The influences of the nonlinear kinematic hardening and cycling hardening on the shakedown behaviors were considered. The results showed that, although the accumulated plastic strain can not be ignored, the stress relaxation gradually tends to stabilization and the multilayered systems shake down after dezons of cycles. The basic rule of shakedown behaviors for multilayered structures under the deformation-controlled loadings was obtained, which provides the basis for evaluating the integrity of multilayered structures under complicated elastic-plastic-creep conditions.According to fatigue-creep experiment of X12CrMoWVNBN 10-1-1 ferritic stainless steel, the anelastic creep recovery and its effect on the ratcheting and shakedown behaviors were discussed. The conclusions presented that when the dwell time at peak load was less than 5-min, the total strain was mainly produced by the ratcheting strain due to the anelastic creep recovery and eventually failed by ductile-fatigue damage, while the accumulated creep strain and the fatigue-creep damage were the principal deformation and failure mechanisms for a longer holding time. Furthmore, the creep-plasticity superposition model considering the effect of the anelastic creep recovery was deduced, which can simulate the creep-ratcheting behavior well.