Characterizing Small Fatigue Cracks in Metallic Alloys

Small fatigue cracks grow faster than large fatigue cracks and below the large-crack threshold, and it is necessary to include these characteristics in many life-prediction models. Numerous micromechanics experimental measurements have been made to determine the origin of this anomalous small-crack behavior and to unify and predict small- and large-crack growth-rate behavior. Micromechanics showed that a stress-intensity factor described the crack-opening displacement (COD). Four methods for characterizing small-fatigue-crack behavior are reviewed: three based on fracture mechanics and one empirical. The three fracture mechanics-based methods are essentially complementary to each other and allow prediction of small-crack growth-rate behavior.