Abstract: In the case of those rotating components of rotary machines which are cyclically stressed as their rotational speeds are cyclically varied such as aero gas turbine engine components, the cumulative total number of stress cycles which each component receives due to such speed variations is a reliable indication of its elapsed fatigue life. The invention provides a method and a digital circuit arrangement which facilitates real-time recording of the elapsed fatigue lives of such components: a component's rotational speed is continuously monitored and each time the speed cycles through any one of a plurality of predetermined speed bands, a corresponding predetermined number of fatigue life units is added to an accumulating total of such units.