Study Of High Temperature Multi-axial Fatigue Failure Of 2A12-T4 Aluminium Alloy

With the development of Aeronautical Manufacturing technology,the requirement for lightweight and safety of aircraft has increased,which put forward higher requirements for the mechanical performance of aviation aluminium alloy.The method of simplifying complex multi-axial loading to single axle load was no longer applicable.When flying at high speed,aircraft are often subjected to thermal-force coupling due to aerodynamic friction effect.It is of great significance to reveal the multiaxial fatigue failure process of aviation aluminium alloy under thermal-mechanical coupling service environment.In this test,270 Mpa von-Mises equivalent loading was selected to study the effect of stress-amplitude ratio and phase angle on the multiaxial fatigue failure law of 2A12-T4 aluminum alloy at 175?.The following conclusions were drawn:1.To study the influence of stress-amplitude ratio and phase angle on multiaxial fatigue failure life.Under tension,the fatigue failure life was the longest.When the stress-amplitude ratio was the same,the fatigue failure life of non-proportional loading with phase angle of 90°was higher than that of proportional loading with phase angle of 0°.2.The influences of different loading paths on the initiation and propagation of multi-axial fatigue crack was studied by metallographic microscope and replica test.It was found that when the maximum shear stress amplitude existed on the material surface,the crack initiated and propagated along the plane direction of the maximum shear stress amplitude in Stage I.When there was no maximum shear stress amplitude,the crack initiated along the plane of the maximum normal stress.The crack initiation period decreased with the increase of stressamplitude ratio.During the process of crack growth,the stage I of crack propagation accounted for the majority of the fatigue life.When a long crack was formed,the stage II of crack propagated along the maximum normal stress plane,and then fast fracture failure occurred.3.The fatigue region,propagation region and final fracture region can be observed on the fracture surface.With the increase of the stress-amplitude ratio,the macroscopic morphology of the fracture gradually transitioned from the flat fracture dominated by tensile fracture to the rough fracture dominated by shear stress.The roughness of the fracture surface increased,and the torsion failure characteristic was showed.4.Under the thermal-mechanical coupling of 175?,whether it was uniaxial fatigue or multi-axial fatigue load,the honeycomb-shaped connecting holes,isometric dimple and shear dimples were found obviously in the fatigue transient area,and the main method was the fracture of transcrystalline dimples.5.In the combined tension and torsion loading,as the stress amplitude ratio increased,the final fracture region gradually transitions from tension to shear dimples,and the failure mode also changed from tension to shear;With the increase of phase angle,the propagation region was enhanced by friction,and the scratch marks were obvious.The dimples in the final fracture region also transitioned from the coexistence of isometric dimples and shear dimples to isometric dimples,and the failure mode transitioned from mixed failure mode to tensile failure dominated.