Research On The Ratcheting-Shakedown Effect Of304Austenitic Stainless Steel Pressure Vessels

The shakedown and fatigue analysis are necessary for safety of pressure equipments under cyclic load. The traditional shakedown analysis method is time-consuming for complicated structures, and conservative for304stainless steel pressure vessels with a low ratio of yield stress to tensile strength. Thus a new shakedown analysis method adapted to engineering application is required. In this paper, investigations on304stainless steel uniaxial ratcheting-shakedown mechanism, non-cyclic method to calculate plastic shakedown load, strain-controlled method for local plastic shakedown load of complicared structures and application of ratcheting-shakedown effect in pressure vessel strain-hardening technology, are studied. The main contents and conclusions are summerized:(1) Experimental research on304stainless steel uniaxial ratcheting-shakedown mechanism. Based on the Bauschinger effect, the relationship between elastic domain and accumulated plastic strain is measured in304stainless steel uniaxial ratcheting tests at room temperature. This relationship is used to analyze the uniaxial ratcheting-shakedown mechanism of isotropic hardening model, kinematic hardening model and combined isotropic and kinematic hardening model. It can be found that only the combined isotropic and kinematic hardening model can be used to simulate ratcheting strain. Then an iterative equation of uniaxial ratcheting strain is established to simulate uniaxial ratcheting behavior. The simulation results show that the saturated ratcheting strain is decided by the elastic domain and the cyclic load range. Meanwhile, the viscoplastic strain rate is decided by elastic domain according to Perzyna and Peirce viscoplastic strain rate equation. In addition, the uniaxial ratcheting tests, the viscoplastic strain rate reduces along with the increase of the elastic domain, thus, the saturated viscoplastic strain is related to the elastic domain and the peak stress. Furthermore, the relationships among elastic domain, saturated strain and loading cases are discussed. It can be found that, the saturated strain is a function of the peak stress when the stress ratio R of valley stress versus peak stress is not less than zero, and the saturated strain is decided by the peak stress and amplitude stress when the stress ratio R is less than zero. Based on the discussion, the points of peak stress and saturated strain are tested. Eventually, a plastic shakedown constitutive curve is obtained to describe the relatinship between saturated strain and peak stress when the stress ratio R≥0by fitting these points.(2) Non-cyclic method of plastic shakedown load for typical pressure equipments. In reality, most of pressure vessels are subjected to cyclic load with the stress ratio R≥0. According to equivalent stress-equivalent strain principle, using the plastic shakedown constitutive curve, the accumulated strain in a pressure vessel subjected to cyclic internal pressure can be determined by only one elastic-plastic analysis, and without the cycle-by-cycle analysis. Moreover, physical experiments of two thin-walled pressure vessels subjected to cyclic internal pressure have been carried out to verify the feasibility and effectiveness of this non-cyclic method. It has shown that the accumulated strain evaluated by the non-cyclic method agreed well with those obtained from the experiments. So the non-cyclic method is simpler and more practical than the cycle-by-cycle method for engineering design.(3) Local strain-controlled method to calculate plastic shakedown load. For complicated pressure vessels, the effect of structure stress state must be considered if the non-cyclic method is used. From the perspective of damage, the stress state parameter TF is used to develop a local strain-controlled method. In this method, a database of plastic shakedown load for typical pressure vessels, such as cylinder joint tube, can be built efficiently, and then the engineering formulas can be obtained using mathematical fitting.(4) Application of ratcheting-shakedown effect in pressure vessel strain-hardening technology. When the ratcheting-shakedown effect is considered in pressure vessel strain-hardening techonolgy, the plastic shakedown constitutive curve can replace the uniaxial tensile constitutive curve to simulate the deformation of the strain-hardening pressure vessel, which fills a gap in the strain-hardening technology. On this basis, a new cyclic loading strain-hardening model is proposed. Comparing with the traditional constant internal pressure loading strain-hardening model, the increment of ferrite in the new cyclic loading strain-hardening model is about50%less than that in the traditional constant internal pressure loading strain-hardening model under a same strain-hardening level.(5) Fatigue analysis of strain-hardening pressure vessels. From the point view of the elastic domain, the mechanism of mean stress effecting on strain-control fatigue life is demonstrated. The prestrain304stainless steel strain-control fatigue life design curves are measured. The evaluated design fatigue life of the pressure equipments can be improved using the prestrain fatigue life design curves.