Fatigue Failure Analysis And Life Prediction Of Pre-corroded Aerospace Aluminum Alloys

Aluminum alloys are commonly used as aerospace materials and are widely used in airplane structures such as aircraft skin surfaces,girders,purlins,and landing gear.However,their corrosion resistance is relatively poor and localized corrosion is prone to occur.Corrosion damage can easily induce the initiation and propagation of fatigue cracks which will lead to material failure.Therefore,the study of the fatigue failure process of aluminum alloy after corrosion is of great significance.In this research,the effect of different corrosion time on the fatigue failure of aluminum alloys is studied by using digital image correlation method(DIC),scanning electron microscopy(SEM)and fatigue experiments.The spatial and temporal evolution characteristics of crack initiation and extension were obtained through DIC,and the shape and size of corrosion pit in critical damage area are further identified.Comparing with the un-corroded samples,the fatigue life of the samples corroded 48 hours and 96 hours decreased by 47.2% and 68% respectively.The results of DIC shows that corrosion leads to the shortening of the initiation time of macro crack,and the micro crack initiation cycle of 0,48 and 96 hours is about 35600,20000 and 12800 respectively.Crack initiates at the edge of the specimen,and corrosion affects the initiation position of fatigue crack.For example,the un-corroded sample cracks in the middle part of length direction of the sample,and the crack initiation position of 96 hours is far away from the middle part of the sample.The fracture analysis shows that for pre-corroded specimens,the cracks are in the pit and the size of pit increases with the increase of corrosion time.In this research,the effects of different amplitude and stress ratio on fatigue failure of pre-corroded aluminum alloy are studied.Experimental results show that for the corrosion of 96 hours,the maximum stress is 222 MPa and 278 MPa and 333 MPa,the fatigue life decreases,respectively is about 4337,119136 and 13203,stress ratio of 0.1,0.2 and 0.5 of the fatigue life of aluminum alloy increases gradually,is about 10469,13203 and 33205 respectively;The crack initiation time decreases with the increase of maximum stress and increases with the increase of stress ratio.The experimental results show that the crack initiation area is located at the edge of the specimen,and the location is determined by local stress and local corrosion.Crack propagation is divided into three types: single crack propagation;Multiple cracks compete with each other until a fracture occurs;At the same time,both sides of the cracks expand until the joint causes fatigue failure.Based on the experimental observation,the fatigue crack model of the corrosion pit is established.The pit can be equivalent to the semi-ellipse model,and the crack is thought to be generated and expanded from the pit.Then,the fatigue life is predicted by using the measured size combined with AFGROW software,and the error of the experimental results was within 1.5 times of the dispersion band.Also,the validity of the method is verified.Finally,the fatigue life of pre-corroded aluminum alloy in different working conditions is predicted by the model of pit evolution.