Role of oxides in fatigue crack propagation

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Abstract
Oxides and oxidation play a very important role in fatigue crack propagation for many materials, not only at elevated temperatures, but also at ambient temperature. The simplest approach to modelling environmental effects on crack growth is to attempt to add a purely mechanical crack growth increment to an environmental component: the oxidation rate multiplied by the cycle time. Limited success has been achieved with models of this type, both for high temperature fatigue in air and in high pressure water environments. One significant result has been to emphasise that at temperatures up to about half the melting point of the material, it is environmental effects (and thus, frequently, oxidation) rather than creep which give rise to accelerated fatigue crack growth rates. However, the problem with these simple models is that there is no account taken of the interaction between the environmental processes taking place. There are many types of interaction between the mechanical and the chemical processes. Examples of synergies which preclude the separate consideration of mechanical and chemical mechanisms are: (i) adsorbed gas on freshly formed crack faces reducing slip reversibility and rewelding; (ii) high modulus oxide films giving rise to image forces on dislocations; (iii) a build-up of oxide as a result of fretting reducing the crack tip loading through the process of oxide induced closure; and (iv) localised oxidation causing embrittlement and a change in the crack path. The paper emphasises the importance of oxidation in fatigue crack propagation studies in gaseous environments. The effects of oxidation are reviewed briefly, with emphasis on the interaction between the fatigue and oxidation processes, highlighting areas where an improved understanding of the role of the oxide is required. MST/1162