Research On Fatigue Crack Growth Simulation Of Austenitic Stainless Steel

Owing to excellent comprehensive properties,such as ductility,weldability,high and low temperature resistance,corrosion resistance,304 austenitic stainless steel,a kind of stainless and heat resistant steel,is widely used in general chemical equipments industry,such as pressure vessel,pressure piping.With the increasing of pressure in pressure equipments,especially in inner surface of them,they tend to form a fatigue crack under the alternative load,which has become an increasingly significant problem.304 austenitic stainless steel is the research object in the thesis and is used to conduct fatigue tension test with CT specimens,as well as simulation of fatigue crack extension,which enable acquire the result of fatigue crack growth life and growth path and to some extent provide guidance for enhancing equipments's reliability and security.The finite element model of the CT specimens was built in ABAQUS for the purpose of researching the effect of different frequencies and stress ratio on fatigue crack propagation.The expression and relevant parameters of fatigue crack growth rate can be obtained by fitting crack growth rate curves,and these results will be used as subsequent XFEM simulation conditons.After fatigue crack extension test,fracture surfaces on the different stages of crack growth were observed by SEM and analyzed.Based on the XFEM theoretical model,Direct cyclic solver in ABAQUS was used to simulate fatigue crack growth process.Because this analysis is free from the constraints of mesh dependence,crack will extend along element boundaries or across the inner of elements.Accordingly,XFEM has the ability of simulation of crack growth along any direction,Based on CZM theoretical model,using VUMAT subroutine to describe constitutive behavior of crack tip during whole fatigue crack extension process,the mechanical properties of materials can be obtained.Through researching the effect of different frequencies and stress ratio on crack propagation life,the result obtained from CZM simulation can be well compared with the result obtained from expriment.It was shown that there was small error between simulation and experiment,which was in the limit of tolerance.By using extrapolation technique,adjusting mesh sizes,it will save the computing time.Hence,different magnifications and mesh sizes were used to analyze the feasibility of extrapolation and effect of mesh sizes on crack growth life.Both XFEM and CZM mehod were used in the thesis to simulate fatigue crack propagation path and propagation life respectively.It shows that XFEM method can be well used to predict crack growth path,but poor in crack growth life;CZM method can be well used to predict crack growth life,but poor in crack growth path because the crack extension path should be predefined,which therefore has its limitations.