Fatigue Life Estimation of Pre-corroded Aluminium Alloy

Abstract

The effect of time and mode of corrosion in fatigue life of pre-corroded 7020-aluminum alloy has been determined and compared with that of uncorroded specimens. It is known that fatigue properties of any material depend on the homogeneity of the material mostly the surface uniformity .Any irregularity present may cause fatigue crack initiation at a stress comparably lower than that shown by the S-N curve and thus the pre-corroded specimens’ S-N curve shows some deviation from the actual curve due to the presence of corrosion pits.
As the fatigue failure process exploits the weakest links (discontinuities) within the test material, which act as nucleation sites for crack origins, the fatigue properties of uncorroded and pre-corroded 7020-aluminum alloy in aqueous solution of NaCl (3.5% NaCl , 8.2 pH) along with forced corrosion at different sweep rates have been studied and compared in this project. The properties being the S-N curve, fatigue life, endurance limit, fatigue crack growth mechanism, SEM fractograph and probable crack initiation cause and spot.
Round specimen generally used in classical fatigue tests for life estimation have been used in this experiment. S-N curve is plotted by using Moore’s rotating cantilever beam type fatigue testing machine. The tests are conducted in uncorroded specimen and in pre-corroded specimen and then results are compared. Also the mechanism and spot of crack initiation is predicted by using fractographs under SEM.