Study On The Fatigue Strength Of Aluminum Alloy FSW Lap Joints With Tunneling Defects

As a novel solid-state welding technology,friction stir welding(FSW)solve the problem of connection of light alloys.Hook-shaped defects are inherent defects of friction stir welding lap joints and tunneling defects are common welding process defects,both defects can affect the fatigue strength of the joints.In this paper,the 6061-T6 aluminum alloy FSW lap joints with tunneling defects was taken as the research object.The constant amplitude loading and two stage variable amplitude loading fatigue tests were carried out,the fatigue performance and fatigue life were discussed by finite element simulation analysis.The main research contents of this paper are as follows:1.Fatigue testing of FSW lap joints were carrird out.The weld cross section of the intact FSW lap joints were observed,the position and shape of the hook defects and tunneling defects were recorded.Two kinds of constant amplitude loading fatigue tests were carried out on the lap joints.The effect of stress ratio on fatigue life was analyzed by using the S-N curve obtained from the test.According to the results of constant amplitude loading test,the two-stage loading fatigue tests were designed and carried out.The changes of the joints were observed in the defects during the stretching process,and the fracture position was recorded.After the fractures of specimen,the fatigue fracture was observed by scanning electron microscopy and the fracture characteristics of each region were analyzed.2.The effect of tunneling defects on the fatigue life of FSW lap joints was analyzed by finite element method.The notch stress method and linear elastic fracture mechanics method were used to predict the fatigue life of FSW lap joints respectively.Two kinds of virtual notch models were established to estimate the fatigue life respectively when using the notch stress method,and compared with the test life.A finite element model with initial cracks was established.Using the stress intensity factors obtained from the finite element analysis results,the fatigue life was estimated by using the Paris formula of crack propagation in linear elastic fracture mechanics.The effects of initial crack length and initial crack angle on the stress intensity factor were discussed in this paper.Finite element models without tunneling defects were established to estimate the fatigue life of the FSW lap joint by using the above two methods respectively.The effect of tunneling defects on the fatigue life of the FSW lap joint was analyzed.3.The residual life of the FSW lap joint under two-stage loading was predicted.The fatigue life results of FSW lap joints under high-low and low-high loading modes were analyzed.The effect of loading sequence on the cumulative damage was analyzed by using the Palmgren-Miner linear cumulative damage theory.Few factors were considered in the theory of linear cumulative damage,which led to poor prediction results.Finally,two nonlinear cumulative damage theories and respective their improved forms were used to predict the residual life in the test,the accuracy of the prediction results was significantly improved by considering the effects of loading sequence and the interaction between loads.