Damage Evolution And Failure Mechanism Of Pre-corroded Aeronautical Aluminum Alloy

Aerospace aluminum alloys(such as 2xxx and 7xxx)are widely used in aircraft fuselage frames,bulkheads and wing surfaces due to their high strength and light weight.However,these alloys are prone to localized corrosion during service.In order to improve the corrosion management method and ensure the safe and economical operation of the aircraft,it is of great significance to deeply understand the effects of localized corrosion on the damage evolution and crack propagation of aluminum alloys.This paper conducted the experimental analysis of the damage evolution of pre-corrosion aluminum alloy.A uniaxial tensile test is performed on aluminum alloy specimens with different degrees of initial localized corrosion.Combined with the Digital Image Correlation(DIC)and the Scanning Electron Microscope(SEM)fracture analysis,the effects of pre-corrosion on damage evolution and crack propagation of the material were studied.The DIC strain map shows the spatiotemporal evolution characteristics of crack initiation and propagation.The fracture morphology identifies the shape and size of the corrosion pit in the critical damage zone.As the experimental results shows,the pre-corrosion results in the decline of the apparent stiffness,strength and ductility of the material.The local corrosion affects the damage location,crack initiation,coalescence and fracture path.The evolution of the critical damage area dominates the entire fracture process;the critical damage area obviously has corrosion pit.Also the size of corrosion pit increases as corrosion time increases.The failure analysis shows that under the action of tensile stress,micro-crack damage occurs due to stress concentration at the local corrosion of the material,and in addition voids nucleate on the impurities and second-phase particles.Corrosion damage and void damage are further aggravated and connected,resulting in a final fracture.The effect of pre-corrosion on the macroscopic crack propagation of the material was further investigated using the above experimental method.The uniaxial tensile test of pre-cracked aluminum alloy specimens was performed under different pre-corrosion times.When the pre-corrosion time is 4 hours,the load curve of the specimen is not much different.When the corrosion time reaches 6 hours,the maximum load and the maximum displacement of the specimen have decreased significantly.The DIC results show that the pre-crack dominates the specimen fracture process for un-corroded specimens;When the pre-corrosion specimens were exposed for 4 hours,the corrosion caused deflection in the cracks,which affects the crack propagation path;when the pre-corroded specimens were exposed for 6-hour,the corrosion results in the appearance of secondary cracks.Fracture analysis shows that there are corrosion characteristics and material embrittlement characteristics in critical damage areas.The paper carried out micro-experimental study on crack initiation and propagation of pre-corroded aluminum alloy.The in-situ tensile test of aluminum alloy specimens under two different corrosion times was performed to observe the crack initiation and propagation of the material by scanning electron microscope.It was found that the micro-cracks mainly sprouted in the pits,and the longer the corrosion time,the more the number of micro-cracks.The micro-crack originated from the notch,combined with the micro-crack near the corrosion pit,forms the main crack.For the 0.5-hour corrosion specimens,due to the small number and the small size of corrosion pits,the effect of corrosion pit on the propagation of the main crack is small.The expansion of the main crack dominates the entire fracture process.For the 1-hour corrosion of the specimens,micro-cracks of corrosion pit have an important influence on the direction and path of the main crack.The propagation and combination of multiple micro-cracks lead to the final fracture of the specimens.