The growth of small fatigue cracks in A286 steel

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant K_max, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σ_y. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔK_th (or ΔK _th ^eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔK_eff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK _th ^eff form and grow in statistically weak regions of the microstructure.