Small fatigue crack growth under multiaxial 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 behaviour 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 behaviour of several steels under multiaxial states of stress. Experimental observations from solid and thin-walled tubular round specimens under various multiaxial cyclic loadings including in-phase and out-of-phase, tension–torsion and tension–tension, and with or without mean stresses are used to characterise small crack growth behaviour. 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 behaviour 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.

• Small crack growth behaviour including direction and rate under multiaxial stresses. • Modelling of crack growth rate based on physical observations of the damage process. • Verification of model with data from several steels and a wide variety of loadings.