Creep and fracture of metals : mechanisms and mechanics

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Abstract
Modern methodologies that aim to reduce conservatism in the design of components operating at high temperatures rely on accurate predictions of materials' behaviour in conditions relevant to those experienced in service. This requirement has focussed the attention of materials engineers on developing a quantitative understanding of damage-accumulation in engineering alloys. The microstructure of metallic materials can degrade by several mechanisms at rates that depend; (i) on temperature; (ii) often on stress level or state; and (iii) sometimes on the chemistry of the surrounding fluid environment. In this paper, some recent developments in the modelling of damage processes have been reviewed; in particular, the single state variable approach has been assessed and the potential benefits of using two state variables outlined. Also, a two-bar mechanical analogue has been used to quantify certain features of creep deformation associated with grain boundary cavitation and the close agreement between theory and experiment is demonstrated. A model for the creep damage associated with the evolution of the dislocation substructure in nickel-base superalloys has been developed further and experimental support for an unusual feature of the model has been demonstrated