Multiaxial and non-proportional microstructure-sensitive fatigue crack nucleation

Abstract

Experimental and crystal plasticity modelling studies have been carried out to investigate non-proportionality and stress state effects in fatigue in a 316 stainless steel and nickel-based superalloy RR1000 which have substantial effects on fatigue life. Stored energy density has provided a reasonably consistent and unifying explanation for the experimental observations of fatigue life in axial, torsional, in-phase proportional tension and torsion, and non-proportional loading regimes. A single fatigue property (the critical stored energy density, equating to new surface energy) has been shown to provide good qualitative and reasonable quantitative prediction of the experimental observations of the complex loading, providing a mechanistic explanation for the fatigue behaviour. For the case where significant densities of GNDs develop (for the fine-grained nickel), the latter is found to differentiate the proportional and non-proportional fatigue lives and its contribution to the local stored energy is crucial for capturing the correct fatigue lives under the differing loadings.

Competing mechanisms of particle fracture, decohesion and slip-driven fatigue crack nucleation in a PM nickel superalloy

“…A critical stored energy density value, , was introduced at which a fatigue crack is argued to nucleate. In [50], the critical stored energy density was determined for fine grained Ni alloy RR1000 to be 404 Jm −2 . The number of cycles to nucleation is found from = ∫̇9…”

“…The form of the traction-separation relationship is argued to be of utmost importance by several researchers [e.g. 42,49,50]. The physical significance of the relationship is in many cases based on atomistic predictions, including the exponential based curve of Needleman [29], related to fundamental work on binding energies of atomistically sharp planes by Rose et al [38].…”