Feature Recognition Of Corrosion Pits And Fatigue Life Prediction For Pre-Corroded Aluminum Alloy

The aluminium alloys widely used in the aircraft are sensitive to the corrosive environment. Pitting corrosion caused fatigue crack initiation and growth is considered as a significant damage mechanism in aircraft structures. Corrosion morphology image is one of the most important features for the evaluation of the corrosion of materials. Therefore, it’s very meaningful to find the relationships between physical corrosion morphology and fatigue life of structures by using proper pitting damage characterization and crack propagation model.This paper mainly focuses on the damage characterization methods of corrosion pits based on the computer image processing technology, the statistical analyses of corrosion morphology characteristics and the pre-corroded fatigue life prediction method.(1) By using Keyence VHX-600optical microscope and white light confocal profiler Nikon ECLIPSE L150, the pictures of corrosion pits morphology of2524and7075aluminum alloys are obtained. Time domain and frequency domain filtering, wavelet transform enhancement, morphological processing and image segmentation of corrosion sites are used to enhance contrast and remove noise. Through3d reconstruction, typical corrosion morphology features are recognized, such as the fractal dimension, the depth of pits and the area of pits.(2) Statistical analyses of corrosion morphology characteristics in different pre-corrosion time are carried out.The statistical results show that the corrosion areas and the characterization parameters ab/2c2obey Gumma distribution, and the corrosion depth obeys Gumbel, normal, Weibul and log-normal distributions.(3) High frequency fatigue tests are performed to get S-N curves of pre-corroded aluminum alloy7075. Through analyzing the relationship between S-N curves and pre-corroded time, we find the influence of pre-corroded time on the following fatigue behavior of materials. The fatigue fracture are observed and the results show that the fatigue cracks generally are initiated from pits under the positive stress ratios. The geometric sizes of corrosion pits as crack source are all statistically analyzed.(4) The fatigue life of thin plate specimens with center hole is predicted by using NASGRO software based on the equivalent crack size propagation method. The error diffusion zone is within a factor of1.6times at the positive stress ratio of0.5and0.06, which indicates the validity of the equivalent crack size method used in this paper. At the negative stress ratio, the accuracy of predicted fatigue life gets worse compared with that of positive stress ratio.