The importance of micro-crack evolution under oxidation/creep conditions in component failure life predictions

An important issue to be examined in the life assessment of power plant components is the evolution of damage in various environmental operating conditions. Historical observations have shown that oxidation, for instance, may introduce surface hardening or in the extreme may lead to surface cracking in components operating at elevated temperatures. The mode of damage will subsequently affect the fracture behaviour of the material and shorten the lifetime of the component. In this work finite element (FE) simulations of environmental damage have been performed on uniaxial round bar and four-point bend geometries. The stress distributions and damage evolution in the four point bend model have been examined for three cases. The first case uses elastic-plastic properties assuming a uniform and uncracked surface. In the second case microcracks have been modelled in the oxide layer at the tensile surface of the four-point bend test piece. Finally these results are compared to an oxidation/creep model using a grain and grain boundary mesh structure which predicts the development of surface oxide layer cracks. The results from these simulations are discussed in terms of the significance of microcracks in assessing life times of components operating under environmental and creep damage conditions.