The Ratcheting And Fatigue Behavior Of 7075 Aluminum Alloy Under Cyclic Stressing Condition

7075 aluminum alloy has been a good choice for the application of the aircraft and traffic structures for its high specific strength,stiffness,easy processingandgood resistance to stress corrosion.As the structures are always cyclic loaded,it is important to study the ratcheting behavior under asymmetrical cyclic loading with non-zeromean stressand low-cycle fatigueproperties of the 7075 material.7075-T651 alloys were examined by Scanning Electron Microscopy(SEM),X Ray Diffractometer(XRD)and Transmission Electron Microscopy(TEM)to observethe microstructures,andanalysis the phase of the as-received materials as well as asymmetrical cyclic loaded ones.At the same time,the ratchet-fatigue tests were conducted on 7075-T651 alloy to observe the character of cyclic ratcheting behaviorand micro-mechanism of fracture damage of fatigue.The results are as follows:7075-T651 aluminium alloy is mainly composed of rod like phases MgZn2 dispersed in the matrix.The coarse heterogeneous phaseslikeAl7Cu2 Fe andcompounds with Si separated outat the grain boundaries along the rolling direction.With different loads,the rapid accumulation of ratcheting strain occurs in the initial stages of the cycle process,and the ratcheting strain rate decreased rapidly.Under cyclic loading,the ratcheting strain accumulation characteristics ofthe specimens under different cyclic loading are obvious.The uniaxial ratcheting strain increases with the increase of the stress amplitude and the intermediate stress,and decreases with the decrease of the stress amplitude and the intermediate stress.With higher ?max,ratcheting strain increase but when ?min changed ratcheting strain showed irregular.In the ratcheting-fatigue experiments,the fatigue life of the specimens is negatively correlated with the final ratcheting strain,which means that at higher stress levels,the greater the ratcheting strain,the shorter the fatigue life,the shorter the steady-state stage during the ratcheting-fatigue process,and the dominance of the ratchet damage performed throughout the cyclic loading process.The stress levels could be influenced by the parameters like peak stress ?max,stress ratio |R| and loading rate.The fractures of theratcheting-fatiguespecimens was deviated from the typical fatigue ones,indicating that when loading levels increased the ratio of the crack source zone and the expansion zonewould decrease,and ratcheting damage would dominate the failure process.Analysis of uniaxialratcheting behavior based on dislocation evolution.The dislocation density increased significantly at the beginning of the cycle,and the dislocations were pinned by the precipitates,and the resistance of the dislocation increased,which led to the decrease of the ratcheting strain rate.With the cyclic loading,the dislocation density increases continuously,and the dislocation lineevolved to mild tangles.Further load led to more complex forms of dislocation like dislocation walls.Compared to the initial stage,this kind of configuration is relatively stable,so the strain rate also significantly decreased.