A Model of Nonlinear Fatigue-Creep (Dwell) Interactions

Abstract

A nonlinear creep/dwell interaction model is derived based on nucleation and propagation of a surface fatigue crack and its coalescence with creep/dwell damages (cavities or wedge cracks) along its path inside the material, which results in the total damage accumulation rate as given by da/dN=(1+(lc+lz)/λ){(da/dN)f+(da/dN)env}, where (da/dN)f is the pure fatigue crack growth rate, (da/dN)env is the environment-assisted crack growth rate, lc/lz is the cavity/wedge crack size, and λ is the average spacing between the internal cavities or cracks. Since wedge cracks are usually present in the form of dislocation pile-ups at low temperatures and cavitation usually occurs at high temperatures, the model attempts to reconcile the creep-/dwell-fatigue phenomena over a broad temperature range of engineering concern. In particular, the model has been used to explain the dwell fatigue of titanium alloys and high temperature creep-fatigue interactions in Ni-base superalloys under tensile cyclic creep rupture, compressive cyclic creep rupture, and tension/compression-hold strain controlled cyclic test conditions.