Experimental Characterization And Modeling Analysis Of Fatigue Failure Of Pre-corroded 7075-T7651 Aluminum Alloy

Because of its series of advantages,aluminum alloy is widely used in aircraft skin,truss and other structures.However,aluminum alloy will be corroded in the service process,inducing the initiation and propagation of fatigue cracks under cycle load,failure of aircraft structures thus occurs.Therefore,the research on fatigue failure of pre-corroded aluminum alloys has far-reaching significance.In this paper,the fatigue damage evolution of AA7075-T7651 is studied by fatigue experiment with digital image correlation(DIC)technology and SEM observation.The damage accumulation,the initiation and propagation of macro cracks are described by the damage severity curve and the strain fields.The micro fracture characteristics in the macro crack initiation region are analyzed by SEM.The experimental results show that the local corrosion damage on the surface of L-T or L-S promotes the initiation of fatigue cracks,affecting the nucleation position and propagation path of cracks,and is accompanied by hydrogen embrittlement.The dispersion of local corrosion leads to the difference of damage evolution and failure process in time and space.Three typical failure modes(single crack fracture,multi-cracks competition and multi-cracks parallel propagation)were observed in the experiment,which are determined by the loading conditions and the relative position of the main crack initiation.The initiation and propagation of fatigue micro-cracks in AA7075-T7651 are analyzed by in-situ SEM fatigue test and SEM fracture observation.The experimental results show that the micro-cracks are mainly formed from inclusions in the notch and corrosion pits on the surface of specimens.The number and time required for micro-cracks initiation are affected by the severity of pre-corrosion damage and the stress level.From the phenomenon of crack deflection(0.5h corroded)and crack connection(1h corroded),it can be seen that corrosion pits play a key role in the propagation of fatigue micro-cracks.Two propagation modes,transgranular and intergranular,are observed experimentally,indicating the influence of material microstructure on crack propagation.There is inhibition between multiple microcracks,and the rapid propagation of the main crack slows the propagation of the secondary crack.Based on the experimental observation,simplified models with corrosion pit are established by ABAQUS,the stress and strain distribution of the corrosion pit is analyzed.The results show that the maximum principal stress is observed at the side or bottom of the pit and plastic deformation occurs during the tensile process.Then,the elastoplastic damage of pre corroded aluminum alloy under the fatigue load is studied by FE-SAFE,and the fatigue life is predicted,the results show that the damage achieved the largest at the location of the pit,and the predicted life is in agreement with the measured life,which verifies the validity of the model.