Study on the Effect of Pre-Corrosion on Crack Initiation and Fatigue Life of an Aluminum Alloy

Abstract

Corrosion leading to fatigue crack nucleation and crack growth is considered to be among the most significant degradation mechanisms in aging structures. Especially in aircrafts, widespread corrosion pits on the surface and hidden within the joints are an important cause for damage since fatigue cracks are observed to nucleate and propagate from these corrosion pits.
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 in a homogeneous material. Fatigue life estimation in stress controlled condition is done by conducting Moore test. The environmental exposure of aluminum alloys generally shifts the curve to a lower stress region and reduction in life. In several cases S-N curve of pre-corroded condition shows some lowering of the curve from the original one due to the presence of corrosion pits [15]. The effect of corrosion in fatigue life of pre-corroded 7020-T7 aluminum alloy has been studied and presented in this investigation.
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 un-corroded and pre-corroded 7020-aluminum alloy in 3.5% NaCl have been studied and compared in this project. The criteria being the S-N curve, fatigue life, endurance limit, corrosion mechanism, stress concentration factor, SEM fractograph and probable crack initiation cause and spot.
Round specimen generally used in classical fatigue tests for endurance limit 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 un-corroded specimen and in pre-corroded specimen and then results are compared. Also the spot of crack initiation is predicted by using fractographs under SEM. The pit dimensions are measured using an elliptical model and a relation between stress intensity factor and life is plotted.
The results of the experiment confirm the findings of literatures referred, along with that, the fatigue and crack initiation behavior of pre-corroded 7020-Al alloy has been found experimentally.