Initiation and short crack growth in austenitic-ferritic duplex steel-effect of positive mean stress

The effect of the mean stress on the crack initiation and short crack growth of austeniticferritic duplex steel has been studied. High mean stresses and stress amplitudes result in appreciable mean strain relaxation and long-term hardening. Mean stress produces unidirectional slip bands and slip steps that serve as nuclei for persistent slip bands and persistent slip markings. It leads to the acceleration of the crack initiation and production of a high density of cracks. Crack linkage contributes to the growth of short cracks. The concept of equivalent crack was used to describe the crack growth. The kinetics of short crack growth with positive mean stress is similar to that in symmetric loading, that is, exponential growth is observed. Positive mean stress results in earlier crack initiation and in the acceleration of the crack growth rate. Both factors contribute to the decrease of the fatigue life.