Fracture mechanics approach to fatigue crack initiation from deep notches

Abstract Fracture mechanics approach is applied to fatigue crack initiation at the tips of deep, blunt notches including those with very small notch-tip radius. The theoretical relations between the stress intensity range ΔKρ and the notch-tip radius ρ for a fixed life for crack initiation were derived based on the models of dislocation-dipole accumulation and blocked slip-band. Those are approximated by a simpler equation: ΔK ρ ΔK o = (1 + ρ/ρ0) 1 2 where ΔK0 and ρ0 are material constants which are related to the fatigue strength of smooth specimens Δρ0 as Δρ0 = 2ΔK 0 (πρ0) 1 2 . The results of experiments done with bluntly notched compact tension specimens of a structural low-carbon steel agree with the above relation between ΔK gr ΔK o and ρ/ρo. The method to predict ΔKo, ρo and Δρo from the fatigue data of cracked and smooth specimens is proposed.