Crystal Plasticity Based Fatigue Behavior Simulation For Weld Toe Short Cracks

As an important metal joining technology,welding procedure has been widely used in all walks of life.During the processing of welded structures,there are always small defects or crack-like notches unavoidably,and under the action of cyclic loading,short cracks gradually propagate into long cracks,and finally lead to fatigue failure,which brings uncertainty potentially dangerous to industrial production.Existing research shows that the short crack stage accounts for about 80% of the entire fatigue life and exhibits strong randomness and complex interaction.It has great limitations to study short crack behavior by using long crack fatigue theory.Therefore,it is necessary to develop new short crack fatigue behavior research methods to study the general laws of short crack initiation and propagation behavior,so as to improve the reliability of fatigue life prediction of welded structures.Aiming at BM,HAZ and WZ at the weld toe,based on the crystal plasticity theory and numerical simulation method,the initiation and propagation behavior of short cracks and several influencing factors are studied,aiming at BM,HAZ and WZ at the weld toe.The main research contents and relevant conclusions are as follows:(1)The parameter calibration method of the material crystal plasticity model is studied by using the polycrystal numerical solution software FEPX.Firstly,the crystal plastic model parameters are initially set,the batch RVE model is established,and the anisotropic elastic-plastic behavior simulation is carried out,and the stress-strain simulation curve is obtained based on the averaging method;the stress-strain test curve of the same material is obtained by combining the mechanical test data and numerical analysis method;Continuously adjust the parameters of the crystal plasticity model until the simulation curve is consistent with the experimental one.By using this method,the crystal plasticity model parameters of the material in the three regions of the weld toe of the welded joint based on Q345 B are obtained,which are important basis for the subsequent simulation work.(2)Based on the above crystal plasticity model parameters and the measured metallographic data,a two-dimensional RVE model of three regions of the weld toe is established.Based on the Mcdowell model,the fatigue process of the weld toe short crack is simulated,and the fatigue life of local short crack nucleation and early propagation is calculated The results show that compared with BM and WZ,the simulation results in HAZ are closer to the measured results,which confirms that the weld toe short cracks are more likely to initiate in HAZ.(3)For HAZ of the weld,the effects of grain micro-characteristics,dual-phase structure proportion and strain level on the fatigue behavior of weld toe short cracks are studied.The parametric simulation analysis shows that the grain size,shape distribution,strain range and average strain all have a great influence on the crack growth rate,while the grain orientation and the proportion of the dual-phase structure have a great influence on the crack growth path.