Generalized theory of fatigue crack propagation: Part II — Derivation of threshold and Paris regime crack growth rates

Abstract

Expressions for the growth rate of fatigue cracks by the continuous advance of the crack front normal to itself, as well as by the discontinuous ledge-controlled processes, are derived in the presence and absence of strong crystallographic constraints on crack front motion. With no crystallographic constraints, the functional trends found are consistent with the Paris law and the sharply ΔK eff- dependent threshold regime behaviors commonly observed in ductile materials. In the case where strong crystallographic constraints may be operating at all ΔK eff levels, the fatigue crack growth rates derived, based on crack advance rate controlled by the lateral motion of pre-existing ledges on the cusporiented and crystallographic crack front, are proportional to ΔK eff 4 at all ΔK eff levels.