Finite Element Calculation On The Fatigue Crack Growth Of Brazed Joint Using Cohesive Zone Modeling

With the development of the nuclear industry,nuclear energy,as a clean and pollution-free energy,has received extensive attention.At present,the advanced nuclear energy system is mainly used in high temperature gas-cooled test reactors,which use the compact and efficient plate-fin heat exchangers(PFHE)to heat the nitrogen.The PFHE are of very complex structure,and its service environment is also very harsh;the brazing joint is constantly washed,and it is easy to produce fatigue crack.Therefore,it is of great engineering application value to study the fatigue crack growth law of 304/BNi-2 stainless steel brazing joint.However,the current research on the failure of brazed joints mainly focuses on the brazing process,residual stress and high temperature creep,and little research is done on fatigue failure.To solve the above problems,through the combined method of finite element analysis and experimental test,a fatigue damage theoretical model,based on the traditional cohesive zone model,is established.Through the development of the user material subroutine,the fatigue crack growth(FCG)of the brazed joints was studied to provide reference for the design of the PFHE.The main research work and the conclusions are as follows:(1)Through the uniaxial tensile test of base material,the plasticity parameters of 304 stainless steel were obtained.The load-displacement curve and the crack length-displacement curve are obtained through the peeling test of brazed joint.the parameters of the cohesive zone model are obtained by numerical calculation.(2)On the basis of bi-linear cohesive zone model,the finite element analysis of 304 stainless steel brazed joint was carried out.The crack propagation law under constant load was obtained and compared with the test.It was proved that cohesive zone model couldsimulate the crack propagation in brazed joints more accurately.The influence of the thickness of the base material on the crack propagation is discussed.The results show that increasing the thickness of the base metal has a positive effect on the crack resistance of the brazed joint,but it will promote crack propagation after cracking.(3)Based on the traditional cohesive zone model,the fatigue damage cohesive zone model was established,and the VUMAT subroutine for the brazed structure was developed.In order to accelerate the calculation speed,the method of damage extrapolation is put forward,and the validity is verified.Finally,the fatigue crack initiation and propagation of brazed joints are calculated by finite element method,and the law of FCG is obtained.(4)The influence of load and geometry on initiation and propagation of fatigue crack was discussed,and the research results provide theory and technical guidance for the brazing of 304 stainless steel plate-fin structures.Sensitivity analysis of the parameters in the damage method is carried out,and the influence of parameter changes on fatigue cracks was discussed.Finally,the reasons for the effect of various factors on the fatigue crack are analyzed.