Research On Environmental Small Crack Behavior And Life Forecast Of Aluminum Alloy Including Pre-corrosin Damage

Aluminum alloy is a kind of light alloy.It is the most widely used non-ferrous metal structural materials in industrial,especially in the aeronautics and astronautics field.The aluminum alloy has high strength and low density,so it becomes the main material of aircraft structure which accounts for 60%?80%of aircraft structure weight.As we know,planes are used in the natural environment.Even if the atmosphere is quite a corrosive,just as rain,seawater,salt fog and other environmental etc.It can make the aircraft structure suffer corrosion.A report says that the corrosion pit has always been the basic cause of the crack initiation,and as the same with fatigue,the corrosion fatigue is also the main mechanism of the crack propagation.And in material fatigue damage process,the randomness of small fatigue crack initiation and propagation is higher than the long fatigue crack.This feature makes the fracture mechanics and traditional mechanics which are based on the continuous medium hypothesis are difficult to apply.This phenomenon is called the lack of similarity.As for the small crack behavior characteristics which is in the corrosion environment,there is no theory can be suitable for any material of the small crack behavior in any environment medium yet.Recently years,significant progress has been made in the research of new type aluminum alloy.Such as central south university,North-East university,Beihang university,621 metal research institute and so on.Dozens of universities and scientific research units have done some research on the alloy work.But There is few report on the research on environmental small crack behavior of aluminum alloy including pre-corrosin damage.System research has been taken to find the small crack growth behavior rules in corrosion environment with pre-corrosion damage by test observation in this paper.First of all,aluminum alloy's local pre-corrosion experiment was taken and the size of the pit was acquired by measurement.Secondly,the small crack fatigue experiment is operated on pre-corrosion samples in the air environment by replica.In salt water environment,the the small crack fatigue experiment is operated on pre-corrosion samples by fracture mark and backward inference method.Long crack fatigue experiment is operated on pre-corrosion samples in salt water by Online observation method.The experiment data is statistically analyzed to compare small crack evolution behavior in different environment and analyzed the influence of various factors on the corrosive small crack behavior.Thirdly,the fracture of small fatigue crack is analyzed with scanning electron microscopy,the fracture parameters of material and the fatigue damage mechanism has been studied with the method of combining quantitative and qualitative way.Through measuring the beach marks and comparing the load spectrum,the "a" and "c" have been successfully got.There is not too much difference between the fatigue fracture in air and saline,so there is almost no effect of corrosive environment on fatigue mechanism.Finally,by using nasgro software,predicting fatigue life,and contrasting with the real life,the predicted life is accurate.