| A high-strength aluminum alloy, 7075-T6, was studied to quantitatively evaluate chemical pitting effects on its corrosion fatigue life. The study focused on pit nucleation, pit growth, and fatigue crack nucleation.; Pitting corrosion fatigue experiments were conducted in 3.5% NaCl aqueous solution under constant amplitude sinusoidal loading at two frequencies, 5 and 20 Hz. The smooth and unnotched specimens were used in this investigation. A video recording system was developed to allow in situ observation of the surface changes of the specimens during testing. The results indicated that pitting corrosion considerably reduces the fatigue strength by accelerating fatigue crack nucleation. The average pit nucleation life which was the number of cycles to the first observed pit at the length of about 0.02 mm on the exposed surface was about 35% of the total corrosion fatigue life. No appreciable frequency effect was observed in the pit nucleation process for the frequencies evaluated. However, a frequency effect was found in the total corrosion fatigue life. The reduced fatigue life at the lower frequency (5 Hz) can be attributed to increased exposure time allowing more corrosion to occur promoting pit growth.; The fractography of the tested specimens showed that corner corrosion pits were responsible for fatigue crack nucleation in the material due to the associated stress concentration. The pits exhibited variance of morphology. Fatigue life for the experimental conditions appeared to be strongly dependent on pitting kinetics and the crack nucleation stage.; Examination by metallographically cross-sectioning the pits showed that the pits shape might vary with growth. The growth of the pit depended strongly on the orientation with respect to the rolling direction of the material. It was also found that intergranular corrosion damage along with pitting suggests that intergranular attack might lead to fatigue cracking.; A three-dimensional finite element model was developed for calculating Mode I stress intensity factors at a quarter-elliptical corner pit by assuming the pit was a quarter-elliptical corner crack. Two different configuration parameters of the pit were modeled and the results were discussed. Based on the finite element results, the maximum stress intensity range based on a real pit size measured from the fracture surface of the tested specimen was considered as the threshold stress intensity range {dollar}(rm Delta Ksb{lcub}th{rcub}){dollar}. This value was compared with experimental data of the corrosion fatigue crack growth with the same material and environment from the literature.; Finally, a conceptual model of pitting corrosion fatigue proposed by Hoeppner was verified using the result of the finite element analysis and the experimental data. Further research to improve the accuracy of the prediction is suggested. |
