Simulation And In-Stiu Test Of TC4-DT Alloy Electron Beam Welded Joints Fatigue Micro-Crack Initiation

The phenomenological modelsare widely used in the structure fatigue researches.However,thesefatigue modelsin most cases cannot revealphysical mechanism and process of itself,which prevent us from further research on it.In recent years,researchers on the fatigueare making their way to micro models.In the thesis,the crack initiation of TC4-DT titanium alloy electron beam welded joint is studied numerically and experimentally.The main contents of the thesis are as follows:Firstly,the cellular automaton method was used to simulate the grain growth process of welded metal(WM)and the microstructure of the WM was obtained.Secondly,Voronoi diagram method was applied to build up the microstructure ofbase metal(BM)and heat affected zone(HAZ).We found that simulated results greatly described the features of the actual microstructure after comparing actual microstructures to them.Based on previous simulated microstructure,we introduced the simulated microstructure of WM,BM and HAZ to Abaqus for further analysis.Different local coordinates and slip bands(SB)in finite element models were building up for different crystals of WM,HAZ and BM.The simulation of the crack initiation indifferentcrystalsbased on Tanaka dislocation model were conducted in Abaqus.The crack initiation behaviour of WM,HAZ,BM crystals are simulated and investigated under different conditions in order to examine the effect of microstructure,fatigue loading and orientation of crystal.We found that the orientation of crystal in WM greatly influenced the fatigue crack initiation life of itself.Finally we established a computation module in Abaqus based on crystal model of material,which can be used for simulating the crack initiation process and predicting the fatigue life.At last,we conducted in-situ fatigue test to investigate the crack initiation behaviour of TC4-DT electron beam welded joints in microscopic scale.By observing the crack evolution under SEM,we found that trigger of TC4-DT welded joint's crack intiation is the fragmentation of slip bands.Besides that,we compared the fatigue experiment data of TC4-DT welded joint with the one in other thesis.After comparing the simulated fatigue crack initiation life to the testing data under the same condition,we found that the model and simulation adopted arerational and acceptable.