Simulation?Experiment And Analysis Of Ratcheting Phenomenon In The Discontinuous Structure Of Tube-Tubesheet Joint Of A Heat Exchanger

The structure of tube-tubesheet is simplified in the paper.The ratcheting effect of the discontinuous structure is analyzed and discuss by means of uniaxial cyclic bending test and numerical simulation with advanced nonlinear kinematic hardening model.This study provides ideas and basic data accumulation for follow-up research.In addition,the research has strong theoretical significance and practical value of Engineering.First,the ratcheting effect of the discontinuous structure with uniaxial cyclic loading is experimentally studied by using Instron hydraulic servo fatigue tester and auxiliary device.The test results show that the center of the tube is the location of the maximum ratcheting strain in different working conditions.When the structure of tube-tubesheet is subjected to the cyclic positive bending moment,that is,the tube side is only subjected to cyclic tensile stress,the axial and circumferential stresses in the position of the maximum ratcheting strain is greatly influenced by the positive bending moment.Increasing the positive bending moment makes the cumulative value of the initial ratchet strain increase.It has little influence on the shear stress of the plane,which shows that the stress can be kept in a small order of magnitude and the ratcheting strain can be ignored.The ratcheting rate of structural ratcheting is inversely proportional to the positive bending moment of the structure.The ratcheting strain rate tends to zero with a certain number of cycles,which is the stability of the structure.The structure of tube-tubesheet is subjected to cyclic positive and negative bending moment,that is,the tube side is subjected to tension and pressure cyclic stress,maintaining the constant bending moment and increasing the negative bending moment mainly affect the axial and circumferential stresses.It shows that the cumulative value of initial ratcheting strain is significantly increasing,and it also has a certain degree of influence on the plane shear stress,which shows that the smaller the stress is,the more the ratcheting strain increases.The ratcheting rate of structural ratcheting is inversely proportional to the negative bending moment of the structure.The ratcheting strain rate tends to zero with a certain number of cycles,which is the stability of the structure.According to the testing data of mechanical properties of S30408 austenitic stainless-steel material,the parameters of the existing nonlinear kinematic hardening constitutive model in the finite element software ABABQUS,Armstrong-Frederic model and the development of a series of constitutive model based on A-F model is obtained.In the process of determining the parameters of the Chaboche model,basing on the literatures and a large number of simulations,I summarize the method that has a certain physical meaning to determine the parameters of the Chaboche model,namely,the parameters are more rapid determination by using finite element software and data fitting software.Finally,according to the model parameters,ratcheting tests of standard specimen under uniaxial test and uniaxial cyclic bending loads of the structures of the tube-tubesheet were simulated to analyze and compare the model of standard samples and structure prediction results by ABAQUS finite element software.It is found that all models can predict the uniaxial tensile test and stress-strain test under strain control,but only AP-OW-I model is better for uniaxial ratcheting test.For complex structures,four kinds of models can accurately predict the ratcheting strain position,but from the quantitative perspective,the four models have failed to accurately predict the ratcheting strain size and ratcheting stability rate,and the final forecasting result is too conservative.