Abstract: The present invention concerns a method of calculating the expected life of a hardened metal object in relation to a number of fatigue load cycles exerted on the hardened metal object. The method comprises determining the development of microstructural deterioration as a function of fatigue exposure time, deriving therefrom an equation for the development of a Fatigue Damage Index as a function of fatigue exposure time and relating this equation to a known critical value of the Fatigue Damage Index that leads to material failure. The present invention also provides a method of calculating the remaining life of a hardened metal object subjected to fatigue loading.