Breakdown of the skeletal stress technique for lifetime prediction of notched tension bars due to creep crack growth

The paper defines the range of applicability of the skeletal stress technique as a means of predicting the creep lifetime of a circumferentially notched tension bar subjected to steady load. The skeletal stress approach involves the quantification of the effective stress Σ_c and its ratio with the maximum principal tension stress Σ_l at a point, known as the skeletal point, at the throat of the notch. The stress Σ_c and stress state Σ_l/Σ_e at the skeletal point are assumed to remain constant and to determine the notched bar lifetime, which is determined by direct integration of the constitutive equations. This paper assesses the range of validity of this technique using Continuum Damage Mechanics (CDM) calculations, based on finite element analysis techniques, to predict the exact notch behaviour for a range of material conditions appropriate to nickel superalloys. It has been shown that by reducing the ductility parameter in the constitutive equations associated with the creep constrained cavitation damage variable ω₂, the CDM computations are capable of predicting two extreme types of behaviour. Firstly, widespread CDM behaviour, and secondly CDM growth on a highly localised-planar scale which models the phenomenon of Creep Crack Growth (CCG). The reference stress method is shown to be appropriate for widespread CDM behaviour, and to progressively become valid as the material ductility is reduced. This trend is accompanied by a shift from modest notch weakening to severe notch weakening. It is also shown that the stress level applied to the bar, and the strength of the dislocation softening damage mechanism, denoted by a second damage variable ω₁, also influences the above behaviour. The paper defines bounds of applicability for the skeletal stress approach to lifetime prediction, and recommends the use of complete CDM finite clement analysis for those situations where breakdown occurs.