A review of fatigue crack growth resistance in the short crack growth regime

The resistance to the short-crack growth and the observed threshold behavior of short cracks are examined using the Unified Approach that is based on the two-parametric nature of fatigue. The fatigue crack growth is characterized by the two parameters, Kmax and ΔK. The crack growth can occur only when the crack tip driving forces meet both the thresholds, Kmax,th and ΔKth The long crack thresholds are considered as the material property representing the material resistance to crack growth for a given environment. They are independent of crack length and specimen geometry. The short cracks must also meet the same thresholds for their growth. In contrast to the long cracks, the short cracks generally grow in the presence of stress concentrations that are either pre-existing or in-situ generated by the fatigue damage. The stress concentrations exist as slip bands, intrusions, extrusions and/or dislocation pile-ups that are formed during fatigue cycling. These contribute to the internal stresses in contrast to the applied stresses since they are due to internal defects. The internal stress gradients contribute to the reduction in the crack-tip driving force, as the incipient short cracks initiated at the stress concentrations grow to become long. The arrest of a growing short crack occurs if these internal stresses decrease rapidly before the crack becomes long. Such crack-growth arrest points have been designated in the literature as the short-crack-growth thresholds. For both the short cracks and the long cracks, a growing crack can get arrested for the same reason; namely the rapid decrease of internal stress contribution. The Unified Approach accounts for both arrests, and thus unifies the behaviors of both short and long cracks using the ΔK and Kmax parameters.