<p>This paper proposes a method for calculating microdamage in metastable austenitic steel based on changes in elastic moduli associated with the formation of micropores, microcracks, and an increase in the volume fraction of strain-induced martensite under fatigue. The crack shape factor for low- and high-cycle fatigue is calculated using a micromechanical model of the relationship between microdamage and Young’s modulus. Three stages of elastic moduli change, corresponding to the rate of strain-induced martensitic transformation, are identified. It is shown that the change in the rate of elastic moduli can be used to identify the transition from the low-cycle to the high-cycle region. It was found that the relationship between microdamage and cycle ratio can be described by power-law dependence similar to the expression in the nonlinear Marco–Starkey damage accumulation model, which makes it possible to predict fatigue failure.</p>