Mechanics of fatigue crack propagation by crack-tip plastic blunting

Abstract

The power relation between the fatigue crack propagation rate da/ dN and the J integral range ΔJ was obtained for OFHC copper, 0.04%C steel and stainless steel (Type 304). The physical meaning of the relation was investigated based on the observation of the crack opening behavior and fractography. The striations, whose spacing was equal to da/ dN , confirm crack-tip blunting as being an operating mechanism for crack growth. da/ dN was expressed as a power function of the crack-tip opening displacement (Δ CTOD), da/ dN = A(Δ CTOD) p , where p was larger than one. The major portion of Δ CTOD contributes to crack growth at high rates, while a considerable fraction of Δ CTOD occurs behind the crack tip at low rates. Δ CTOD is correlated to ΔJ divided by the yield strength σ' Y through the equation, Δ CTOD = B(ΔJ/σ Y′) q , where q is nearly equal to one for 0.04%C steel and is larger than one for copper. The variation of q with material was explained based on the observed distribution of the crack opening displacement. The final equation for fatigue crack growth is given as da/ d N = A · B p(ΔJ/σ Y′) pq. When the shape of the crack tip opening is geometrically similar, both p and q are one. For a general case, both are larger than one, yielding the exponent of the da/ dN-ΔJ relation deviating from 1 up to 2.3.