Environmental creep intergranular damage and multisite crack evolution model for engineering alloys

Abstract

In this paper a sub-grain size finite element meshing technique has been developed to employ in a combined creep continuum damage and environmentally assisted time dependent material depletion and crack growth model. The model can cover a wide range of failures allowing crack paths for intergranular and transgranular cracking as well as surface oxidation and depletion due to corrosion. It has been customised to predict time and stress induced intergranular surface damage and cracking for isotropic metallic grain structures which undergo oxidation at the surface and are under creep loading regime. It is shown that creep crack growth predictions using a remaining ductility multiaxial failure strain constraint-based model coupled with a time-dependent environmentally assisted corrosion/oxidation rate dependent parameter can realistically assess damage and cracking rates for the engineering alloys shown. Material depletion and multiple intergranular crack paths are modelled by assuming weaker grain boundaries having realistically a reduced critical damage index of a factor of 2 less than that of the grains. A probabilistic random crack extension criterion is also proposed which allows for statistically varying random damage and surface crack growth development during each run using the same input properties. This method can take into account the level of scatter that exists in the material characterisation test data and in long term component oxide and crack damage measurements. The results for a sample rectangular block under tension using representative material creep and oxidation properties are used to predict and compare the relative extent of creep to environmental damage under different loading conditions.