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https://hdl.handle.net/2142/21332
Description
Title
Corrosion fatigue of 7005-T53 aluminum alloy
Author(s)
Segan, Ellen Georgina
Issue Date
1989
Doctoral Committee Chair(s)
Wirtz, Gerald P.
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
The process of corrosion fatigue involves mechanical and environmental influences that affect crack initiation, crack growth, and ultimately fatigue life. It is generally recognized that corrosion damage in the form of pits and the presence of humidity in the ambient environment have a deleterious effect on the fatigue properties of aluminum alloys. However, the mechanisms involved in this damage are poorly understood.
The effects of localized corrosion and ambient environment on corrosion fatigue processes in 7005-T53 aluminum alloy were characterized by investigating crack initiation from naturally occurring pits and Electrode Discharge Machine (EDM) notches and the subsequent crack growth of these cracks in environments containing humid (ambient laboratory) and dry ($<$5 ppm$\sb{\rm v}$ water vapor) air.
Corrosion pits and EDM notches were found to exert an equal effect on fatigue life of this alloy, but the mechanisms of crack initiation for the two types of stress-raisers differ. Crack initiation from pits is controlled by the presence of small tunnels in the pits, while crack initiation from EDM notches is governed by the presence of transformed regions formed during the EDM process.
Crack initiation lives and growth of small cracks are not affected by water vapor in the environment but environment, has a pronounced effect on crack growth rates as well as macroscopic fracture mode. Relatively low overall crack growth rates in dry air result in fatigue lives in dry air that are typically an order of magnitude higher than those in humid air. Crack initiation lives in humid air can account for a relatively large fraction of fatigue lives, while in dry air, crack growth processes dominate fatigue life.
The effect of humidity on fatigue of this alloy is to inhibit slip processes after cracks grow to some length. In uniaxial fatigue in dry air, predominant Mode II crack growth predominates, while in humid air, Mode I crack growth is dominant. In uniaxial fatigue in dry air, a critical stress intensity factor ($\Delta$K $\sim$ 6 MPa $\sqrt{\rm m}$) is associated with the transition from macroscopic Mode I to macroscopic Mode II crack growth.
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