Effect of corrosion on fatigue behaviour of aluminium alloys

• Main Author: Tan, Ai Hua
• Other Authors: Li Pei Feng
• Format: Final Year Project
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Aeronautical engineering::Materials of construction
• Online Access: http://hdl.handle.net/10356/64007
• Institution: Nanyang Technological University

Cycles of pressurization and depressurization of the fuselage and gust disturbances are a few of the cyclic loads that aircrafts experience during flight. Aircrafts’ bodies are also subjected to corrosion as most airports are near huge seawater bodies. Aluminium alloy, the material for the skin of aircrafts, is exposed to corrosion as the chloride ions of the seawater damage the protective oxide. Frequent cyclic loadings, coupled with the corrosion, weaken the material of the aircraft and lower its fatigue life. This may jeopardise the safety of the passengers and airlines are pressurized to spend more on maintenance. An accurate analysis of the fatigue lives of the aluminium helps to save on the maintenance fees and boost the safety of passengers. In this project, the fatigue lives of non-corroded aluminium alloy 7075-T651 specimens are compared against the same material specimen but subjected to 120h of corrosion. The specimens are of a dog-bone shape with a 3mm diameter hole in the middle to increase the stress concentration. A fatigue profile has been designed using underloads to generate marker bands on the fracture surface of the specimens. The marker bands generated would be observed under the scanning electron microscope and the rate of crack growth was calculated. The effect of R ratio and the number of underload cycles were also briefly discussed. Corroded specimens generally have a weaker fatigue lives as compared to the non-corroded specimens. The specimens fail at a lower number of cycles as corrosion removes mass from the specimen and causes a reduction in the strength of the material. The cracks from the corroded specimens initiate at an earlier number of cycles and there were also multiple initiation sites on the corroded specimens. This could be due to the multiple pits present on the surface of the corroded specimens, which increase the stress intensity factor at the hole. It was found that rate of crack growth for the specimen corroded for 120h was only slightly faster than the non-corroded ones. The experiment can be improved by finding the effect of the duration of corrosion on the fatigue behaviour and fine-tuning the fatigue block profile such that the marker bands are more distinct at initiation and fast fracture regions.