065 Small Crack Growth Path and Rate under Combined Stresses

A significant portion of the fatigue life is typically spent in growth of small cracks. In addition, the stress state in many structures and components is multiaxial. Therefore, the study of small crack growth behavior with regards to its growth path as well as growth rate under combined stresses can be of great importance in many applications. This study investigates small crack growth behavior of several steels under multiaxial states of stress. Experimental observations from solid and thinwalled tubular round specimens under various multiaxial cyclic loadings including inphase and out-of-phase, tension-torsion and tension-tension, and with or without mean stresses are used to characterize small crack growth behavior. The steels used include 1045 and 1050 medium carbon steels, 304L stainless steel, and Inconel 718. Effects of load non-proportionality, mean stresses, and friction-induced closure on small fatigue crack growth behavior are discussed. Critical plane analysis and an effective strain intensity factor are used to predict crack growth path as well as to correlate crack growth rates under various combined stress conditions.