Effect of corrosion on fatigue behaviour of aluminium alloys

• Main Author: Oh, Shan Chun
• Other Authors: School of Mechanical and Aerospace Engineering
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Aeronautical engineering::Materials of construction
• Online Access: http://hdl.handle.net/10356/60312
• Institution: Nanyang Technological University

Corrosion has always been a concern in the manufacturing industry as it shortens the lifespan of a material, and incurs huge amount of repair and replacement costs. In the area of transportation, this problem is even more detrimental, as corrosion drastically reduces the mechanical life of metal alloys, which potentially poses immense danger to the lives of the people travelling from all around the world. Due to the high strength-to-weight ratio, high strength aluminium alloys like AA7075-T6 are widely used, especially for long distance transportation vehicles such as ships and aircrafts. However, due to adverse corrosion effects, the fatigue life of such alloy is severely compromised. In this report, a study was done to analyse the effect of prior corrosion to the fatigue life of AA7075-T6 aluminium alloys. The specimens, subjected to corrosion durations of 24 hours, 120 hours and 240 hours, were put through fatigue tests with sets of under loading, and their fracture surfaces were studied under the Scanning Electron Microscope (SEM). From the observations of the marker bands on the fracture surfaces, the fatigue crack growths of the specimens were thoroughly analysed and compared. The correlation between crack growth rate of the specimens and the stress intensity factor range were also analysed to determine if corrosion affects the fatigue crack propagation of the specimens. The results show that the crack initiation of the specimens is heavily influenced by the extent of corrosion duration, and the fatigue life of the specimens has an inverse exponential relationship with the corrosion duration. This relates closely to the average pit depths of the specimens from different corrosion durations. However, the crack propagation rates of the specimens only differed slightly, with the most corroded specimens (240 hours) having marginally higher crack growth rates compared to the other specimens. The report concludes that corrosion has a significant effect on the external surfaces of the AA7075-T6 specimens. This creates corrosion pitting that greatly affects the crack initiation process of the fatigue life of the material.