Research On Low Cycle Fatigue Behavior And Damage Mechanism Of7075Aluminum Alloy

With the development of aircraft in civil and commercial fields, high requriement forintrinsic microstructure of materials is necessary due to service conditions at super highspeed and high temperature, which makes materials to be used at the limit of life andreliability.7075aluminum alloy is widely used in commercial aircraft components for thehigh specific strength and stiffness, easy to be processed, good stress corrosion resistanceand welding ability.The aluminum alloy with uniform microstructure and precipitates and the inclusionsor precipitates mainly distributed along the distinct rolling direction, where ellipsoidaltransition strengthening phase η’（MgZn₂） precipitates along the grain boundaries. Theexpression S=2532.11.1144pis obtained according to true stress-strain curves based onHollomon equation. Ramberg-Osgood formula and Manson-Coffin formula are taken to fitsome curves like cyclic stress-strain curve、cyclic stress-life curve and strain-life curve,which could be used to figure out fatigue parameters. Cyclic hardening, cyclic stability andcyclic softening occur on aluminum alloy under low cyclic loading.In generally, the fatigue crack initiates at the surface or near to the surface of thesample caused by hole and inclusion, there is radial pattern around crack initiation. Thecrack propagation regions under high stress amplitude is characterized by furrow andhollow, and obviously fine ripple vein. While lots of fatigue striations and sidesteps withthe holes and secondary cracks generated in the crack propagation regions of low stressamplitude. Primary crack is merged with multiple micro-cracks and shielded by the maincrack at the stage of fatigue propagation area. Meanwhile, the dislocation of crack tips,grain boundaries and different grain orientation will affect crack propagation path, which can make crack bend. Mixed fracture of brittle and ductile fracture is on the fatiguetransient broken area.Fatigue crack growth rate curve of aluminum alloy shows is consist of three crackgrowth stages. The first stage is micro cracks propagation, where da/dN is about10^-5mm cycle^-1as Δ K lower than6Mpa m^1/2. The second stage is macro crackspropagation, where the da/dN is linear asΔ K is about7～25Mpa m^1/2. The last stage isinstantaneous fracture stage as Δ K above25Mpa m^1/2. The metastable precipitated phase（（GPⅠ/GPⅡ）、η’） cansignificantly improve the strengthof7075aluminum alloy, butgrain boundary phase （GBP） and precipitation free zone （PFZ） will have an importantimpact on materials ductility, toughness, fatigue resistance, stress corrosion resistance. Thedislocation of aluminum alloy piped up at the grain boundaries, while the sub-boundarieswere object to hinder dislocation movement. The grain boundary dislocation formed bymicroscopic aberrations stress caused dislocation starting, proliferation and movement ofthe adjacent grains. Dislocation line had a certain direction and previewed characteristicsof slip bands without dislocation tangles along the grain boundaries.