Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. Ferrous alloys and titanium alloys have a distinct limit, amplitude below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminum and copper do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, a number of cycles (usually 107) is chosen to represent the fatigue life of the material.
It is the highest value of the stress amplitudes in a symmetrical cycle of mechanical load variation, or the maximum stress of an asymmetrical cycle, to which a material can be subjected for an unlimited number of cycles without failure. The value of the amplitude or the maximum stress in a cycle that, when repeated for a specific number of cycles, causes fatigue failure or produces macroscopic cracks is called the fatigue strength.