| Low cycle fatigue failure is one of the most important failure modes of pressure vessels.Under the action of cyclic load,microcracks will appear in the local high stress area of the pressure vessel.With the load,the microcracks continue to expand and form macro fatigue cracks,which leads to the fatigue failure of the vessel.In order to ensure the safety and reliability of pressure equipment,it is necessary to systematically study the low cycle fatigue performance and fatigue crack growth law of materials.In this paper,304 austenitic stainless steel,which is commonly used in pressure vessels,is selected as the research object,and its low cycle fatigue performance and life behavior are studied through low cycle fatigue test,Combining digital image correlation(DIC)and finite element analysis method,the fatigue crack propagation law and the plastic zone at the crack tip are studied,which provides experimental basis and theoretical model for fatigue design and life prediction of pressure vessels.The main work of this paper is as follows:(1)In this paper,low cycle fatigue tests of 304 austenitic stainless steel were carried out.The low cycle fatigue properties were obtained by analyzing the cyclic stress response,cyclic stress-strain behavior and cyclic stress-strain hysteresis curve.The fatigue life model of 304 austenitic stainless steel was established by stress,strain and strain energy density.The applicability of the model was compared and analyzed.The results show that the three parameter power function energy method and the life equation based on plastic strain energy have better prediction accuracy.(2)Fatigue test and crack growth test of 304 austenitic stainless steel were carried out.The strain distribution in the cyclic plastic zone at the crack tip was obtained by DIC analysis.The stress distribution in the cyclic plastic zone at the crack tip was calculated by combining the stress-strain field at the crack tip.The plastic strain energy in the cyclic plastic zone at the crack tip was calculated,and the relationship between the strain energy and the fatigue crack growth rate was established.The results show that the stress-strain relationship at the crack tip conforms to the stress-strain formula under low cycle fatigue;there is a nonlinear relationship between the plastic strain energy in the cyclic plastic zone of the crack tip and the fatigue crack growth rate,and a fatigue crack growth rate model based on the plastic strain energy at the crack tip is established.(3)The fatigue crack growth of 304 austenitic stainless steel under cyclic loading is simulated by finite element analysis.The stress-strain distribution at the crack tip and the total strain energy in the cyclic plastic zone are obtained.The results of finite element analysis are compared with DIC analysis.The results show that the displacement and strain fields at the crack tip obtained by finite element analysis are basically consistent with those observed by DIC,It provides an effective method for the study of the strain field and cyclic plastic zone at the crack tip.Through the analysis of the strain energy in the cyclic plastic zone at the crack tip,the linear relationship between the strain energy and the fatigue crack growth rate is obtained,and the fatigue crack growth rate model based on the crack tip strain energy is established.(4)Based on the stress-strain field at the crack tip and low cycle fatigue performance,a fatigue crack growth rate prediction model for 304 austenitic stainless steel was established,and the fatigue crack growth test data were compared and analyzed.The results show that the fatigue crack growth prediction model based on the stress-strain field at the crack tip and Manson coffin formula can predict the fatigue crack growth rate of 304 austenitic stainless steel in the crack initiation stage,stable growth stage and rapid growth stage It can accurately reflect the fatigue crack growth behavior of 304 austenitic stainless steel. |
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