Predicting fatigue resistance of nano-twinned materials: Part II - Effective threshold stress intensity factor range

The determination of ΔK_th,eff for fatigue crack growth has been a challenging task. A model is forwarded to assess this parameter in presence of nano-scale twins for Cu, Ni and Al. The model utilizes the concept of irreversibility of crack emitted dislocation glide. Incremental crack extension is formulated on the basis of force balance of continuum dislocations during cyclic flow. Peierls stresses for free and twin boundary restricted glide, computed in Part I, constitute an essential ingredient in devising crack growth threshold founded on relative positions of dislocations. Predicted ΔK_th,eff for relatively short cracks is shown to be substantially enhanced with a refinement in twin lamellar thickness and twin spacing as indicated experimentally in recent literature. The theoretical values of ΔK_th,eff in Ni, Cu and Al are in good agreement with experimental literature for longer cracks as well. Saturation effects are observed in ΔK_th,eff levels with respect to gradual increase in twin nano-dimensions as well as crack length. Mechanistic origin of these observations is discussed.